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HighLights More»   
Influences of quadratic spectral phase on characteristics of two crystal cross-polarized generation with femtosecond pulses
Geng Yi-Xing, Li Rong-Feng, Zhao Yan-Ying, Wang Da-Hui, Lu Hai-Yang, Yan Xue-Qing
Acta Physica Sinica, 2017, 66 (4): 040601
Using quaternions to analyze the trapping force of an ellipsoidal bead
Zhang Shu-He, Liang Zhen, Zhou Jin-Hua
Acta Physica Sinica, 2017, 66 (4): 048701
Effect of plasma surface treatment on embedded n-contact for GaN-based blue light-emitting diodes grown on Si substrate
Feng Bo, Deng Biao, Liu Le-Gong, Li Zeng-Cheng, Feng Mei-Xin, Zhao Han-Min, Sun Qian
Acta Physica Sinica, 2017, 66 (4): 047801
Current Issue Accepts In Press Earlier Issues Top Downloaded SCI Top Cited
  Acta Physica Sinica--2017, 66 (4)   Published: 20 February 2017
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CONDENSED MATTER: STRUCTURAL, MECHANICAL, AND THERMAL PROPERTIES

Influence of hydroxyls at interfaces on motion and friction of carbon nanotube by molecular dynamics simulation

Li Rui, Mi Jun-Xia
Acta Physica Sinica. 2017, 66 (4): 046101 doi: 10.7498/aps.66.046101
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Understanding how the groups at interface influence the friction of carbon nanotubes can provide reference for their applications. In this paper, we investigate the influences of hydroxyls on motion and friction of carbon nanotube on graphite substrate by molecular dynamics simulation. The simulation cases include the ideal vertical carbon nanotube on the ideal graphite substrate, the ideal vertical carbon nanotube on the graphite with hydroxyls on the top layer, the carbon nanotube and the graphite both with hydroxyls on the surface. The results show that the lateral force of carbon nanotube changes when hydroxyls are introduced into the interfaces. If hydroxyls are only on the graphite, the fluctuation of lateral force increases obviously. The reason can be attributed to the increase of atomic surface roughness. Moreover, due to the small contact area between vertical aligned carbon nanotube and substrate, the mean friction becomes raised with hydroxyl content increasing, which is different from the conclusion obtained from silicon tip sliding on graphene with hydrogen on the surface. In that case, owing to the large contact area, the mean friction of tip reaches a maximum value at hydrogen content in a range between 5 and 10% because of the competition between the increase in the number of hydrogen atoms and the weakening of the interlock due to the increase in separation of tip from substrate. Hydrogen bond and Coulomb force appear between interfaces when hydroxyls are both on carbon nanotube and on graphite, which significantly increases friction force on carbon nanotube. And slip interfaces translate rapidly from between carbon nanotube and graphite into between graphite layers. Like the case with hydroxyls only on the graphite, the sliding of carbon nanotube perpendicular to the initial velocity also occurs when carbon nanotube and graphite are both with hydroxyls. This phenomena can be explained as the fact that the introduction of hydroxyls breaks the equilibrium of the force on the carbon nanotube in the Y direction. Moreover, the random distribution of hydroxyls causes the random motion of the carbon nanotube.

Controlled production of double emulsion by microfluid technique

Chen Qiang, Qi Xiao-Bo, Chen Su-Fen, Liu Mei-Fang, Pan Da-Wei, Li Bo, Zhang Zhan-Wen
Acta Physica Sinica. 2017, 66 (4): 046801 doi: 10.7498/aps.66.046801
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All planned inertial confinement fusion (ICF) capsule targets except machined beryllium require plastic mandrels with tight requirements on which the ablator is built. In this paper, the fabrication of poly(α-methylstyrene) (PAMS) mandrel is studied. PAMS mandrels are produced by using microencapsulation technique. This technique involves producing a water droplet (W1) encapsulated by a flourobenzen (FB) solution of PAMS (O) with a droplet generator, and this droplet is then flushed off by external phase (W2), forming a water-in-oil-in-water (W1/O/W2) compound-emulsion droplet, which is suspended in a stirred flask filled with external phase to cure. The encapsulation process is based on a microfluid technique, which can achieve the controlled production of millimeter-scale PAMS mandrels. In this work, capillaries-based co-flowing microfluidic triple orifice generator is designed and built to fabricate W1/O/W2 droplets. Two configurations of the droplet generator:one-step device and two-step device, are employed in this experiment. In one-step device, the end of oil phase capillary is located at the same position as the end of inner water phase capillary. So the core droplet and the shell droplet break off from their capillaries ends at the same time, forming a W1/O/W2 droplet. While in the two-step device, the W1 phase capillary tip is located upstream to the W2 phase capillary tip. As a result, the core droplet and the shell droplet depart from the ends of their capillaries respectively, forming a W1/O/W2 droplet as well. Differently, the shell droplet contains only one core droplet in one-step generator, while several core droplets are contained in the shell droplet in two-step generator. In this paper, the mechanism of the droplet formation and the effect of the flow rate on the size of the droplet are studied with these two configurations. Results show that tiny difference between the two generators will lead to great differences in droplet formation mechanism and size control. In the two-step generator, the inner phase flow rate has little influence in the outer diameter of the compound-emulsion droplet. The diameters of the compound-emulsion droplets have a similar change to the diameters of the single droplets (O/W2). In one-step device, the inner phase flow rate has a significant influence on the outer diameter of the double-emulsion droplet because of the existence of W1-O interface. Finally, the compound-emulsion droplets fabricated in this experiment are cured in external phase, after which PAMS mandrels are fabricated. The diameters of the final PAMS mandrels are measured with optical microscope. The distribution of the diameters well concentrates in an area of (2000±10) μupm, which is favorable for producing the PAMS mandrels with a diameter of 2000 μupm.

Effect of doping symmetry on electron spin relaxation dynamics in (110) GaAs/AlGaAs quantum wells

Teng Li-Hua, Mu Li-Jun
Acta Physica Sinica. 2017, 66 (4): 046802 doi: 10.7498/aps.66.046802
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Considerable interest has been aroused in the study of the spin dynamics in semiconductors due to its potential applications in spintronics and quantum computation. In this paper, time-resolved circularly polarized pump-probe spectroscopy is used to study the carrier density dependences on the electron spin relaxation in approximately symmetrical and completely asymmetrical doping (110) GaAs/AlGaAs quantum wells. With the increase of the carrier density, the spin relaxation time first increases and then decrease obviously in both of the quantum wells, and the measured spin relaxation time of the approximately symmetrical doping quantum wells is always longer than that of the asymmetrical doping one. By analysis, we find that the spin relaxation is not dominated only by the Bir-Aronov-Pikus (BAP) mechanism in (110) GaAs quantum wells, that though the Dresselhaus spin-orbit coupling does not lead to any spin relaxation, the asymmetry of the doping position contributes to the asymmetry of potential energy structure, thus the built-in electric field which can induce the Rashba spin-orbit coupling to appear, and that the effective magnetic field induced by the Rashba spin-orbit coupling normal to the growth direction can lead to spin relaxation along the growth direction. Therefore, the D‘yakonov-Perel’ (DP) mechanism plays an important role in asymmetrical doping (110) GaAs/AlGaAs quantum wells. In the approximately symmetrical and completely asymmetrical doping (110) GaAs/AlGaAs quantum wells, the DP mechanism dominates the spin relaxation at low carrier density, thus the spin relaxation time increases with carrier density increasing due to the strengthening of the electron-electron scattering and the decreasing of the momentum relaxation time. However, at high carrier density, BAP mechanism plays an important role, thus the spin relaxation time decreases obviously with carrier density increrasing, but the decay rates in both of the quantum wells are slower than that in the casethat only BAP mechanism dominates, because both the DP and BAP mechanism play an important role. The strength of the Rashba spin-orbit coupling depends on the symmetry of the quantum well. The DP mechanism in a completely asymmetrical doping quantum well is stronger than that in an approximately symmetrical doping quantum wells, thus the decay rate in a completely asymmetrical doping quantum wells is always slower than that in an approximately symmetrical doping quantum wells, and the spin relaxation time in a completely asymmetrical doping quantum wells is shorter than that in an approximately symmetrical doping quantum wells.

CONDENSED MATTER: ELECTRONIC STRUCTURE, ELECTRICAL, MAGNETIC, AND OPTICAL PROPERTIES

Dynamic resistivity of Pb(Zr0.95Ti0.05)O3 depolarized ferroelectric under shock wave compression

Wu You-Cheng, Liu Gao-Min, Dai Wen-Feng, Gao Zhi-Peng, He Hong-Liang, Hao Shi-Rong, Deng Jian-Jun
Acta Physica Sinica. 2017, 66 (4): 047201 doi: 10.7498/aps.66.047201
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Explosive-driven ferroelectric generator (EDFEG) has important applications due to its excellent properties of high energy density and small volume. The output of EDFEG is based on the depolarization of ferroelectric during shock wave compression. In a “normal mode” configuration, a planar shock wave propagates in a direction perpendicular to the polarization axis. If the resulting depolarizing current passes through a large resistive load or a small capacitive load, high electric fields can be produced within the ferroelectric sample. In this case, a portion of the depolarizing charges are lost in the sample due to finite resistivity of shocked ferroelectrics during shock wave transit. But it is very difficult to accurately measure the resistivity of shocked ferroelectric during shock wave compression, due to high pressure and short duration time. In previous studies, the value of the resistivity of shocked Pb(Zr0.95Ti0.05)O3 (PZT95/5) ferroelectric was obtained from the experimental output charge difference for different large resistive loads or by fitting the experimental current histories. However, the current leakage was not observed directly in experiment in the past. Furthermore, the value of the resistivity obtained in each of all these studies was a time-averaged value. In the present work, a new experiment method is developed to investigate dynamic resistivity of PZT95/5 under shock wave compression, in which a pulse capacitor is used as an output load. The current leakage in shocked PZT95/5 is observed in the experiment at a shock stress of 3.5 GPa after the depolarization of all ferroelectrics. This current leakage is just related to the resistance of shocked PZT95/5 and the voltage applied. The experimental results show that the resistivity of shocked PZT95/5 continuously changes in a range of 2.2×104 Ω·cm-3.5×104 Ω·cm for time more than the shock transit time of the sample. Based on the experimental results, a dynamic resistance model is established to analyze the resistivity of depolarized PZT95/5 ferroelectric ceramic during shock wave transit in ferroelectric. The simulation results reveal dynamic characteristic of the resistivity of depolarized PZT95/5 ferroelectric ceramic under shock wave compression. The further analysis of experimental results shows that the resistivity continuously changes between 2.0×104 Ω·cm and 8.0×104 Ω·cm during shock transit in ferroelectrics. It is believed that dynamic characteristic of the resistivity of shocked PZT95/5 ferroelectric ceramic is related to pressure, electrical field applied and the defects in the material. The dynamic resistivity of shocked PZT95/5 obtained in this paper and its dynamic resistance model will be helpful for designing EDFEGs and their applications in the future.

Sparse inversion method of T2 spectrum based on the L1 norm for low-field nuclear magnetic resonance

Jiang Chuan-Dong, Chang Xing, Sun Jia, Li Tian-Wei, Tian Bao-Feng
Acta Physica Sinica. 2017, 66 (4): 047601 doi: 10.7498/aps.66.047601
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The technology of low-field nuclear magnetic resonance (LF-NMR) is commonly used in food, agriculture, energy and chemical sectors due to its non-destructive, non-invasive, in situ, green and other advantages. Recently, this technology played an increasingly large role in the field of food-safety supervision especially. In oil product quality testing, conventional T2 spectrum inversion methods such as the non-negative singular value decomposition (SVD) algorithm can only reflect T2 spectrum in a smooth model. However, for a sparse model, the inversion result of non-negative SVD algorithm is expected to be very glossy, leading to low resolution of T2 spectrum and inaccurate analysis of sample property. To solve this problem, we propose a sparse T2 spectrum inversion algorithm based on the L1 norm minimization constraint. In this paper, we establish the sparse model expression of NMR echo curve, and obtain the T2 sparse spectrum inversion results based on the inner truncated Newton-point method. Furthermore, the effectiveness of L1 sparse inversion algorithm is examined by the synthetic data of the smooth model and the spare model which have different peak numbers and signaltonoise ratios (SNRs). Synthetic results show that compared with the non-negative SVD algorithm, the L1 sparse algorithm is appropriate for both the smooth model and the sparse model with higher inversion accuracy. When the number of T2 peaks in a sparse model changes from a single peak to a quad peak, the L1 sparse algorithm can still obtain accurate inversion results, while the SVD algorithm results in a gradual deterioration, and cannot even determine the peak number. Under the sparse model, when the SNR of the measured NMR curve is gradually changed from 5 dB to 50 dB, the L1 sparse algorithm at 20 dB or more can obtain accurate inversion results which have less than 10% peak error and less than 5% peak position error and amplitude average error. However, the non-negative SVD algorithm cannot obtain accurate results at each SNR. Finally, multiple sets of frying oil samples are utilized to prove the accuracy and robustness of L1 sparse inversion algorithm. Inversion results of seven sets of frying oil samples show that the L1 sparse algorithm prefers the non-negative SVD algorithm. The obtained T2 spectrum by the L1 sparse algorithm shows three peaks obviously, and the T21 peak area ratio S21 and the single component relaxation time T2w are higher linear with respect to frying time than the results by non-negative SVD algorithm, which is useful for detecting the frying oil quality change. The inversion results of the T2 spectrum at different SNRs are consistent with the synthetic results, i.e., when the SNR is reduced, the T2 spectrum inversion results from the L1 sparse algorithm are better than those from the non-negative SVD algorithm when SNR is greater than 20 dB.

Effect of plasma surface treatment on embedded n-contact for GaN-based blue light-emitting diodes grown on Si substrate Hot!

Feng Bo, Deng Biao, Liu Le-Gong, Li Zeng-Cheng, Feng Mei-Xin, Zhao Han-Min, Sun Qian
Acta Physica Sinica. 2017, 66 (4): 047801 doi: 10.7498/aps.66.047801
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Unlike the finger-like n-contact that is prepared after the wafer bonding and the N-polar GaN surface roughening for GaN-based vertical structure light-emitting diodes (LEDs) grown on Si substrates, the embedded via-like n-contact is formed prior to the wafer bonding. The high temperature process of the wafer bonding often causes the electrical characteristics of the via-like embedded n-contact to degrade. In this paper, we study in detail the effect of plasma treatment of the n-GaN surface on the forward voltage of GaN-based LED grown on Si substrate. It is shown that with no plasma treatment on the n-GaN surface, the forward voltage (at 350 mA) of the 1.1 mm×1.1 mm chip with a highly reflective electrode of Cr (1.1 nm)/Al is 3.43 V, which is 0.28 V higher than that of the chip with a pure Cr-based electrode. The LED forward voltages for both kinds of n-contacts can be reduced by an O2 plasma treatment on the n-GaN surface. But the LED forward voltage with a Cr/Al-based electrode is still 0.14 V higher than that of the chips with a pure Cr-based electrode. However, after an Ar plasma treatment on the n-GaN surface, the LED forward voltage with a Cr/Al-based electrode is reduced to 2.92 V, which is equal to that of the chip with a pure Cr-based electrode. The process window of the n-GaN surface after the Ar plasma treatment is broader. X-ray photoelectron spectroscopy is used to help elucidate the mechanism. It is found that Ar plasma treatment can increase the concentration of N-vacancies (VN) at the n-GaN surface. VN acts as donors, and higher VN helps improve the thermal stability of n-contact because it alleviates the degradation of the n-contact characteristics caused by the high temperature wafer bonding process. It is also found that the O content increases slightly after the Ar plasma treatment and HCl cleaning. The O atoms are mainly present in the dielectric GaOx film before the Ar plasma treatment and the HCl cleaning, and they exist almost equivalently in the conductive GaOxN1-x film and the dielectric GaOx film after Ar treatment and HCl cleaning. The conductive GaOxN1-x film and the VN donors formed during the plasma treatment can reduce the contact resistance and the LED forward voltage.

INTERDISCIPLINARY PHYSICS AND RELATED AREAS OF SCIENCE AND TECHNOLOGY

Preparation and electrochemical performance of porous carbon nanosphere

Yang Xiu-Tao, Liang Zhong-Guan, Yuan Yu-Jia, Yang Jun-Liang, Xia Hui
Acta Physica Sinica. 2017, 66 (4): 048101 doi: 10.7498/aps.66.048101
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Nanostructured carbon materials possessing good mechanical properties, adsorption characteristics and electrochemical performances, are the most promising candidate for electrode materials of supercapacitors. Among all synthesis methods, hydrothermal synthesis of porous carbon nanosphere (PCNS) is mostly used. Structure-directing agent F108 (PEO132-PPO50-PEO132) has a similar function to popular agent F127(PEO106-PPO70-PEO106) and P123 (PEO20-PPO70-PEO20) used in hydrothermal synthesis, but has greater relative molecular mass and higher hydrophilic/hydrophobic volume ratio, so using block copolymer F108 as soft template will obtain PCNS with special physicochemical properties.
In this paper, PCNS is prepared by post-processing, including carbonization and subsequent KOH activation, of phenolic resin nanoparticles obtained by hydrothermal synthesis through using phenolic resin as a carbon source and block copolymer F108 as a soft template. The as-prepared PCNS sample is characterized by scanning electron microscope (SEM), transmission electron microscope (TEM), X-ray diffraction, nitrogen adsorption and FTIR, etc. The images of SEM, TEM and results of nitrogen adsorption show that the obtained PCNS has the advantages, such as uniform particle size about 120 nm, high spherical degree and large specific surface area of 1403 m2/g and also wide pore size distribution. The results show that post-processing has an important influence on the physicochemical property of PCNS sample such as specific surface area, pore size distribution, crystallinity and surface chemistry. The activation temperature plays an important role in forming pore structure as the specific area of PCNS sample increases from 519 m2·g-1 to 1008 m2·g-1 after activation at 700℃ (PCNS700), while the activation temperature changes to 900℃ (PCNS900), the specific area rises up to 1403 m2·g-1. The pore size distributions show that the peaks are at the same position, which suggests that KOH activation at high temperature makes the primary pore of PCNS deeper. PCNS900 contains more mesopores than PCNS700, so it can be concluded that at the higher activation temperature, the deeper pores inside PCNS are formed, and it is worth noting that pores near 2 nm are largely produced when the temperature arrives at 900℃. KOH processing and high temperature processing contribute greatly to structural ordering, which means that PCNS samples are greatly graphitized. Last but not least, both KOH processing and high temperature processing reduce the number of functional groups on the surface of PCNS samples. Using PCNS samples as activated material to make electrodes, we study how the different physicochemical properties of PCNS samples affect the performance of PCNS electrode. As a result, PCNS700 and PCNS900 show notably larger specific capacitance than PCNS due to their great larger surface specific areas and more structural orderings in graphitic layer stacking. However, PCNS700 shows a lager specific capacitance of 146.75 F/g than PCNS900 (132 F/g) due to its higher number of surface functional groups than PCNS900, though its lower specific surface area. The pore size distribution has a huge influence on the supercapacitor rate capability as the PCNS900 which has more mesopores and the most structural orderings in graphitic layer stacking shows excellent rate capability as well as superior long-term cycling stability (97.5% capacitance retention over 10000 cycles). In summary, PCNS obtained by hydrothermal synthesis through using block copolymer F108 as soft template shows the special physicochemical properties which make it an ideal candidate for the electrode materials of supercapacitor. Moreover, the larger the specific area, more structural orderings in graphitic layer stacking, more appropriate content of mesopores and surface functional groups, the superior performance the electrode materials of surpercapacitor exhibit.

Mechanism analysis of influence of surface-breaking orientation on magnetic leakage field distribution

Wu De-Hui, Liu Zhi-Tian, Wang Xiao-Hong, Su Ling-Xin
Acta Physica Sinica. 2017, 66 (4): 048102 doi: 10.7498/aps.66.048102
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Magnetic flux leakage (MFL) has been widely applied to the nondestructive testing (NDT) of ferromagnetic materials due to its simple operation, low cost, and steady signal. Its defects are evaluated based on the relationship between MFL signal and the geometrical characteristic of defect. In this paper, a three-dimensional (3D) mathematical model is developed for the magnetic leakage field of surface-breaking defects that are arbitrarily oriented inside ferromagnetic material. Firstly, a finite-length rectangular slot is used as a simplified and convenient representation of a surface-breaking defect. Then, the magnetic charge densities of slot walls in different surface-breaking orientations are analyzed theoretically. The distribution of the magnetic leakage field can ultimately be derived by vector synthesis. Both simulations and experiments are conducted to analyze the magnetic leakage field distributions in different magnetization orientations. The results show that with increasing the angle between the defect orientation and the magnetic field, the horizontal component of the leakage magnetic field increases as demonstrated by increasing the prominence of its single peak. At the same time, however, the vertical component shows a bimodal distribution. The proposed model can effectively describe the influence of defect orientation on MFL signals, which can offer practical guidelines for optimizing MFL detectors and improving defect assessment.

Performance analysis of polarization-space-time adaptive processing for airborne polarization array multiple-input multiple-output radar

Wang Ting, Zhao Yong-Jun, Lai Tao, Wang Jian-Tao
Acta Physica Sinica. 2017, 66 (4): 048401 doi: 10.7498/aps.66.048401
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In order to further improve the capabilities of clutter suppression and target detection in airborne multiple-input multiple-output (MIMO) radar space-time adaptive processing (STAP), the polarization-space-time adaptive processing (PSTAP) method based on polarization array MIMO radar is proposed. Firstly, by applying the novel polarization array to airborne MMO radar, the signal model of airborne polarization array MIMO radar PSTAP is established. Then based on the idea of resolution grid, the influence of clutter can be equivalent to the formation of independent point sources of clutter related to the clutter degree of freedom, and an equivalent expression for the covariance matrix in polarization array MIMO radar PSTAP is obtained. Next, combined with the equivalent covariance matrix, the signal-to-clutter-plus-noise ratio (SCNR) performance of the polarization array MIMO radar PSTAP is derived and analyzed. The effects of the polarization, spatial and temporal matching coefficients are discussed. When the target is located in the side-looking direction of the airborne radar, the normalized spatial frequency of the target is zero. Then the spatial transmit and spatial receive matching coefficients between the target and the clutter point source in the center of the space-time plane both approach to one. Meanwhile, the normalized Doppler frequency of the side-looking target is in direct proportion to the target speed. When the target speed decreases to zero, the temporal Doppler matching coefficient between the target and the central clutter source is near to one. Thus taking the spatial and temporal matching coefficients into consideration, the SCNR loss of the traditional MIMO-STAP is approximate to zero. It indicates that for traditional MIMO-STAP, its performance of detecting low-speed target is severely degraded by the clutter source, and target detection can hardly be realized just in space-time domains. However, through utilizing the additional polarization information to take advantage of the polarization matching coefficient, the polarization array MIMO radar PSTAP increases the SCNR loss and remarkably lessens the influence of the central clutter source. According to the above theoretical analysis, we can come to the conclusion that the polarization array MIMO radar PSTAP can effectively promote the capability of clutter suppression compared with the traditional MIMO-STAP, which is beneficial to the detection of the moving target with low-speed. Moreover, the improvement of output SCNR performance becomes more significant with increasing the differences between the polarization parameters of target and those of clutter. Therefore, the polarization array MIMO radar PSTAP has great application value for practical engineering. The simulation results verify the validity and superiority of the proposed polarization array MIMO radar PSTAP method.

Failure analysis of GaN-based Light-emitting diode with hole vertical structure

Fu Min, Wen Shang-Sheng, Xia Yun-Yun, Xiang Chang-Ming, Ma Bing-Xu, Fang Fang
Acta Physica Sinica. 2017, 66 (4): 048501 doi: 10.7498/aps.66.048501
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Light-emitting diode (LED) failure mechanism plays an important role in studying and manufacturing LEDs. In this paper, X-ray perspective instrument is used to carry out the non-invasive and real-time X-ray imaging detection of the representative LED packaging products purchased from 5 Chinese companies. A large number of the welded voids are founded in the thermal pad and the void ratio of thermal pad, which represents the ratio of void area to pad area, is more than 30% for all samples. 1 W warm white light LED of GaN-based vertical via structure is selected to study the mechanism of short-circuit invalidation. The method is carried out by the following steps. Firstly, the surface morphologies of failure samples are compared with those of normal samples by visual observation. Secondly, antistatic electric capacity testing instrument is used to detect the existences of the electrical parameter abnormalities of the failure of non-short-circuit samples. Thirdly, decapsulations are operated on samples by using Silica gel dissolving agent. And the microtopographies of the samples are characterized by optical microscope, energy dispersive spectrometer and scanning electron microscopy. Then the cross-sectional morphologies of failure samples are observed. The failure mechanism can be drawn from the characterizations mentioned above. The study shows that the failure mechanism of the vertical structure of GaN-based vias is that the existences of voids in the Ni-Sn alloy back gold layer and solid-crystal layer reduce the interface bonding strength and thermal conductivity of the LED chip. The heat building-up leads to thermal expansion of the air inside the voids, which increases the electrical stress and thermal stress distribution at the LED chip vias. Long-term heat accumulation and higher electrical stress in the through-hole region, where the chip current density is greatest, lead to the crack and break of GaN epitaxial layer, which is so brittle and fragile, around the through-hole region. It can eventually lead to short-circuit of PN junction and then failure of LED. Back gold layer is the heat-conductive and electric-conductive channel of LED. The concentrations of thermal stress and electrical stress caused by voids in the back gold layer further lead to the uneven current distribution on the chip. This is the main reason why GaN epitaxial layer cracks and breaks. Voids in the back gold layer and solid-crystal layer are the direct and indirect causes of LED short-circuit failure, respectively. Therefore, the packaging process should be standardized to avoid the void occurrence, based on the reasons why voids exist. It can finally improve reliability of LED.

Using quaternions to analyze the trapping force of an ellipsoidal bead Hot!

Zhang Shu-He, Liang Zhen, Zhou Jin-Hua
Acta Physica Sinica. 2017, 66 (4): 048701 doi: 10.7498/aps.66.048701
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In the ray-optics (RO) model of optical tweezers, tracing refractive and reflected rays with vectors play important roles in calculating the trapping forces. Traditional ray-tracing method with solid geometry, to some extent, is complicated in determining the orientations of those refractive and reflected rays according to spatial incident rays. It is difficult to calculate the trapping forces for irregular particles. In this paper, quaternion is proposed to rotate ray vectors for simplifying the traces of all kinds of spatial rays. Then, it is appropriate to calculate the trapping force of an ellipsoid bead. Based on the algorithm of quaternion and the convention between the interface normal and angular directions, the direction of normal always points from optically denser medium to thinner medium. The rotation axis is the cross product of the incident ray and the interface normal. And the positive angular direction can be determined by right-hand rule based on the orientation of the rotation axis. According to Snell' law, the rotation angle between the incident ray and refractive/reflected ray can be determined. The quaternion for rotation consists of rotation axis and angle. So the refractive and reflected rays are both determined by quaternions of incident ray and rotation based on rotation rules. Furthermore, the force on interface can also be calculated according to momentum changes of the photon before and after the interface refraction and reflection. The quaternion method is used to analyze the effects of coverslip position and deformation ratio on the trapping efficiency of ellipsoid particles. Our simulative results show that the lateral and axial trapping efficiencies are obviously affected by the deformation of the ellipsoid itself. No matter whether the bead deforms transversely or axially, the transverse and axial trapping efficiencies both become larger at a specific deformation. Meantime, the increase of the spherical aberration reduces the maximum axial trapping efficiency, and the equilibrium position of the bead becomes farther away from the center. Using quaternion method, the calculation of refractive lightvector can be simplified in comparison with by using the method of Euclidean geometry or transformation matrix. Theoretically, this quaternion can be used to trace rays on any irregular geometric surfaces. In conclusion, the method of quaternion can make ray tracing easier and extend the applications of RO model.

Analyses of the effect of mismatch on the performance of inverted GaInP/InxGa1-xAs/InyGa1-yAs triple-junction solar cells

Ma Da-Yan, Chen Nuo-Fu, Fu Rui, Liu Hu, Bai Yi-Ming, Mi Zhe, Chen Ji-Kun
Acta Physica Sinica. 2017, 66 (4): 048801 doi: 10.7498/aps.66.048801
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The traditional lattice matched GaInP/(In) GaAs/Ge triple-junction (3J) solar cell has no much room to enhance its practical achievable conversion efficiency because of its inappropriate ensemble of bandgap energies. According to the P-N junction formation mechanism and the close equilibrium condition, we explore a series of computational codes in the framework of MATLAB to simulate and optimize the inverted structure of series-connected 3J solar cells with a fixed top bandgap of 1.90 eV on GaAs substrate. In this paper, structural optimization is conducted in the real device design, because the realistic (QE) is closely related to a set of material parameters in the subcell, i.e., the absorbtion coefficient of material, subcell thickness, minority carrier diffusion length, surface recombination velocity, etc.
The results indicate improved inverted 3J solar cells with nearly optimized bandgaps of 1.90, 1.38, and 0.94 eV, by utilizing two independently lattice-mismatches (0.17% and 2.36% misfit respectively) to the GaAs substrate. A theoretical efficiency of 51.25% at 500 suns is demonstrated with this inverted design with the optimal thickness (4 μm GaInP top and 3.1 μm InGaAs middle). By contrast, the efficiency with the infinite thickness of subcells is reduced by 1%, which is mainly attributed to the effect of minority carrier recombination on Jsc. Exactly speaking, if photo-generated carriers make a contribution to Jsc, they must be collected effectively by the P-N junction before recombining. A new model is proposed based on the effect of dislocation on the metamorphic structure properties by regarding dislocation as minority-carrier recombination center. Our calculation indicates that threading dislocations density in the middle junction is approximate to 1.70×105 cm-2 when dislocations in the gradient buffer layer are neglected. The theoretical efficiency is increased by 0.3% compared with the inverted design containing a single metamorphic junction.
As a result, based on the two metamorphic combinations, a solar cell with an area of 30.25 mm2 is prepared. The efficiency of the designed cell with two lattice-mismatched junctions reaches 40.01% at 500 suns (AM1.5D, 38.4 W/cm2, 25℃), which is 0.4% higher than that of the single metamorphic junction 3J solar cell.

GENERAL

A method of judging a Birkhoffian to be a first integral of constrained mechanical system

Cui Jin-Chao, Liao Cui-Cui, Liu Shi-Xing, Mei Feng-Xiang
Acta Physica Sinica. 2017, 66 (4): 040201 doi: 10.7498/aps.66.040201
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As is well known, the development of analysis mechanics from Lagrangian systems to Birkhoffian systems, achieved the self-adjointness representations of the constrained mechanical systems. Based on the Cauchy-Kovalevsky theorem of the integrability conditions for partial differential equations and the converse of the Poincaré lemma, it can be proved that there exists a direct universality of Birkhoff's equations for local Newtonian system by reducing Newton's equations into a first-order form, which means that all local, analytic, regular, finite-dimensional, unconstrained or holonomic, conservative or non-conservative forms always admit, in a star-shaped neighborhood of a regular point of their variables, a representation in terms of first-order Birkhoff's equations in the coordinate and time variables of the experiment. The systems whose equations of motion are represented by the first-order Birkhoff's equations on a symplectic or a contact manifold spanned by the physical variables, are called Birkhoffian systems. The theory and method of Birkhoffian dynamics are used in hadron physics, quantum physics, relativity, rotational relativity, and fractional-order dynamics.
At present, for a given dynamical system, it is important and essential to determine whether a Birkhoffian function is the first integral of the system. Although the numerical approximation is an important method of solving the differential equations, the direct theoretical analysis is more helpful for refining the general integral method, and more consistent with the usual way of solving problems of analysis mechanics. In this paper, we study how to judge whether a given Birkhoffian dynamical function to be a first integral of Birkhoff's equations, based on the point of Birkhoffian dynamical functions carrying all the informationabout motion of the system, and use the thought of deriving the first integrals of Hamiltonian systems. In Section 2, the normal first-order form and the Birkhoff's equations of the equations of motion of holonomic systems are introduced. In Section 3, we prove that the Birkhoffian function of an autonomous Birkhoffian system must be a first integral, and the Birkhoffian function of a semi-autonomous system must not be a first integral. Moreover, the energy integral, cyclic integral and Hojman integral of the non-autonomous Birkhoffian systems are given. In Section 4, two examples are given to illustrate the applications of the results. In Section 5, the full text is summarized and the results are discussed. It is necessary to point out that the judging method is effective to determine whether a given Birkhoffian functions can be identified to be a first integral of Birkhoff's equations, but other new first integral cannot be found with this method. One possible method of covering the shortage is to obtain other equivalent Birkhoffian functions in terms of isotopic transformations of Birkhoff's equations, and then use our results to seek the new first integral. In addition, we also hope to develop a more direct method of obtaining the first integrals of Birkhoff's equations in the next study.

Spall behavior of copper under ultra-high strain rate loading

Xi Tao, Fan Wei, Chu Gen-Bai, Shui Min, He Wei-Hua, Zhao Yong-Qiang, Xin Jian-Ting, Gu Yu-Qiu
Acta Physica Sinica. 2017, 66 (4): 040202 doi: 10.7498/aps.66.040202
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The spall behavior of copper at ultra-high strain rate is studied by molecular dynamics simulation combined with an experimental analysis of laser ablation of a bulk copper target by femtosecond laser pulses. In the molecular dynamics simulation, two-temperature model is used, shock wave and spallation characteristics of copper shock-loaded by femtosecond laser are analyzed in detail. It is concluded that the evolution of pressure indicates a triangular waveform of the shock wave, and the spall strength of copper is about 19 GPa at strain rates ranging from 109 s-1 to 1010 s-1, while higher pressure would melt the sample and the spall strength decreases to 14.89 GPa. Normally, the spallation is characterized by the sample free-surface undergoing alternately acceleration and deceleration, and the spallation mechanism could be explained by void nucleation, growth, coalescence that leads to the final fracture. An experiment is conducted to achieve high strain rate load on copper. The driving laser has a pulse width of 25 fs and central wavelength of 800 nm, the thickness values of the shocked copper foils are (502±5) nm, fabricated by electron beam sputtering deposition onto 180 μupm cover slip substrates. The driving laser beam with maximum intensity 5.5×1013 W/cm2, is focused on the front surface of the copper through the transparent substrate. Movements of the free rear surfaces of the copper foils are detected by chirped pulse spectral interferometry, and the theoretical time resolution is 1.3 ps. As a result, the free surface displacement and velocity evolution profile of the shocked area are obtained in a single measurement, and the results directly show that the maximum free surface velocity is 0.43 km/s and no alternately acceleration and deceleration appears. According to the shock wave relations, the maximum pressure near free-surface is 8.18 GPa. Meanwhile, derived from the velocity evolution profile, the strain rate is 7.3×109 s-1. Combining with the above molecular dynamics simulation results, it is concluded that there is no spallation in the copper foil. Furthermore, we recover the sample targets and observe the microstructures by using scanning electron microscope. The copper foils are peeled off, but no spall scab is observed, indicating that the internal stress is between the copper spall strength and the bonding strength of copper foil with the transparent substrate. Ripple structure on copper surface demonstrates the femtosecond pulsed laser has ablated the copper film, and the propagation of the shock in fs regime is sensitive to microscopic defects.

Influence of periodic volatility on the stability of financial market

Zhou Ruo-Wei, Li Jiang-Cheng, Dong Zhi-Wei, Li Yun-Xian, Qian Zhen-Wei
Acta Physica Sinica. 2017, 66 (4): 040501 doi: 10.7498/aps.66.040501
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Various stochastic volatility models have been designed to model the variance of the asset price. Among these various models, the Heston model, as one-factor stochastic volatility mode, is the most popular and easiest to implement. Unfortunately, recent findings indicate that existing Heston modelis not able to characterize all aspects of asset returns, such as persistence, mean reverting, and clustering. Therefore, a modified Heston model is proposed in this paper. Compared with the original Heston model, the mean-reverting Cox Ingersoll and Ross process is modified to include a cosine term with the intention of capturing the periodicity of volatility. The phenomenon that high-volatile period is interchanged with low-volatile periods can thus be better described by adding such a period term to the volatility process. In addition, the geometric Brownian motion is replaced by a random walk in the presence of a cubic nonlinearity proposed by Bonanno et al. By doing so, a financial market with two different dynamical regimes (normal activity and extreme days) can be modeled. Closed-form solution for the modified Heston model is not derived in this paper. Instead, Monte-Carlo simulation is used to generate the probability density function of log-return which is then compared with the historical probability density function of stock return. Parameters are adjusted and estimated so that the square errors can be minimized. Daily returns of all the component stocks of Dow-Jones industrial index for the period from 3 September 2007 to 31 December 2008 are used to test the proposed model, and the experimental results demonstrate that the proposed model works very well. The mean escape time and mean periodic escape rate of the proposed modified Heston model with periodic stochastic volatility are studied in this paper with two different dynamical regimes like financial markets in normal activity and extreme days. Also the theoretical results of mean escape time and mean periodic escape rate can be calculated by numerical simulation. The experimental results demonstrate that 1) larger value of rate of return, smaller long run average of variance and smaller magnitude of periodic volatility will reduce the mean periodic escape rate, and thus stabilize the market; 2) by analyzing the mean escape time, an optimal value can be identified for the magnitude of periodic volatility which will maximize the mean escape time and again stabilize the market. In addition, an optimal rate of relaxation to long-time variance, smaller frequency of the periodic volatility, larger rate of return, and stronger correlation between noises will furtherreduce the mean escape time and enhance the market stability.

Research on a six-order chaotic circuit with three memristors

Wang Wei, Zeng Yi-Cheng, Sun Rui-Ting
Acta Physica Sinica. 2017, 66 (4): 040502 doi: 10.7498/aps.66.040502
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A memristor is a nonlinear nanoscale-sized element with memory function, and it has an italic type “8” voltage-current relation curve that looks like a pinched hysteresis loop characteristic. The memristor is utilized to construct chaotic circuit, which has attracted the attention of the researchers. At present, most of studies focus on applying one or two memristors to the chaotic circuit. In order to study the multi memristor chaotic circuit, in this work we propose a six-order chaotic circuit with two flux-controlled memristors and a charge-controlled memristor. A corresponding six-order nonlinear dynamic differential equation of the circuit state variables is established. The dynamic properties of the circuit are demonstrated in detail. The analyses of equilibria and equilibrium stability show that the circuit has an equilibrium located in the three-dimensional space which is constituted by memristor internal state variables, and it is found that the equilibrium stability is determined by the circuit parameters and the initial states of three memristors. The Lyapunov exponent spectra and bifurcation diagrams of the circuit imply that it can produce two bifurcation behaviors by adjusting its parameters, which are Hopf bifurcation and anti-period doubling bifurcation. The hyperchaos, transient chaos and intermittency cycle phenomena are found in the same system. The dynamical behavior of this circuit is dependent on the initial state of memristor, showing different orbits such as chaotic oscillation, periodic oscillation and stable sink under different initial states. Finally, the simulation results indicate that some strange attractors like lotus type and superposition type are observed when voltage and electricity signal in observing chaotic attractors are generalized to power and energy signal, respectively. And the attractor production between the energy signals of the memristors are studied. Specially, when different initial conditions of three memristors are used to simulate the circuit, we can find the coexistence phenomenon of chaotic attractors with different topological structures or quasi-periodic limit cycle and chaotic attractor.
The six-order chaotic oscillating circuit is mainly composed of three parts:the parallel connection between a flux-controlled memristor and capacitor, the serial connection between a charge-controlled memristor and inductor, and another flux-controlled memristor that is alone and floating, which enriches the application of memristor in high-order chaotic circuit. Compared with most of other chaotic systems, it has many circuit parameters and very complex topological structure, which enhances the complexity of chaotic system and the randomness of the generated signal. It is more difficult to decipher the encrypted information in chaotic secure communication, and thus it has better security performance and safety performance.

Reconstruction algorithm of chaotic signal based on generalized likelihood ratio threshold-decision

Ren Zi-Liang, Qin Yong, Huang Jin-Wang, Zhao Zhi, Feng Jiu-Chao
Acta Physica Sinica. 2017, 66 (4): 040503 doi: 10.7498/aps.66.040503
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Blind signal reconstruction in sensor arrays is usually a highly nonlinear and non-Gaussian problem, and nonlinear filtering is an effective way to realize state estimation from available observations. Developing the processing problem of blind signal in wireless sensor networks (WSNs) will greatly extend the application scope. Meanwhile, it also meets great challenges such as energy and bandwidth constrained. For solving the constrained problem in WSNs, the observed signals must be quantified before sending to the fusion center, which makes the overall noise unable to be modeled accurately by simple probabilistic model.
To study the reconstruction issue of chaotic signal with unknown statistics in WSNs, a reconstructed method of chaotic signal based on a cost reference particle filter (CRPF) is proposed in this paper. The cost recerence cubature particle filter (CRCPF) algorithm adopts cubature-point transformation to enhance the accuracy of prediction particles, and cost-risk functions are defined to complete particle propagation. The effectiveness of proposed CRCPF algorithm is verified in the sensor network with a fusion center. Moreover, a generalized likelihood ratio functionis obtained by the cost increment of local reconstructed signals in the cluster-based sensor network topology model, which is used to reduce the network energy consumption by selecting working nodes. Simulation results show that compared with CPF and CRPF, the proposed algorithm CRCPF attains good performance in a WSN with unknown noise statistics. Meanwhile, the CRCPF algorithm realizes the compromise between energy consumption and reconstruction accuracy simultaneously, which indicates that the proposed CRCPF algorithm has the potential to extend other application scope.

Influences of quadratic spectral phase on characteristics of two crystal cross-polarized generation with femtosecond pulses Hot!

Geng Yi-Xing, Li Rong-Feng, Zhao Yan-Ying, Wang Da-Hui, Lu Hai-Yang, Yan Xue-Qing
Acta Physica Sinica. 2017, 66 (4): 040601 doi: 10.7498/aps.66.040601
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The rapid developments of ultra-intense and ultra-short laser offer the possibility to study laser driven ion acceleration with using solid density target. However, the prepulse and amplified spontaneous emission generated in the amplification can create preplasma at the target front by heating, melting and evaporating a portion of a solid density. The main pulse then interacts with the preplasma, which would be harmful to laser ion acceleration. Therefore, many methods have been developed to enhance the temporal contrast of high power laser system, such as saturable absorber, cross polarized wave generation (XPW) and plasma mirror. With many advantages, such as high conversion efficiency, introducing neither spatial nor spectral distortions, and easy setup compared with other mechanisms, XPW has been used to clean the femtosecond laser system. Besides that, the spectrum of the XPW pulse could be broadened by √3 times under the best condition compared with the initial spectrum. It can solve the spectrum narrowing problem during the laser amplification to obtain ultra-short femtosecond laser pulse. Here, we experimentally investigate the output power, spectrum bandwidth and center wavelength shift of the generated cross-polarized wave according to the input pulse quadratic spectral phase.
The femtosecond laser pulse in compact laser plasma accelerator system at Peking University is used to investigate the role of quadratic spectral phase in characterizing the two crystal cross-polarized generation. The Ti:Sapphire-based laser system has a central wavelength of 798 nm and bandwidth of 35.5 nm which allows the pulse to be compressed down to 40 fs duration (FWHM). Typical the input pulse energy of XPW is 150 μupJ and the laser system operates well at 1 kHz repetition rate. The quadratic spectral phase can be increased by changing the position of compressor grating.
The conversion efficiency, spectrum bandwidth and the central wavelength shift by changing the quadratic spectral phase are measured. The conversion efficiency is 17% when quadratic spectral phase φ2=0, and decreases as quadratic spectral phase increases. The rapid decrease is caused by negative quadratic spectral phase. The spectrum bandwidth is 62 nm under the optimum condition, and the broadening effect exists when quadratic spectral phase is in a range of -280 fs2 < φ2 < 1400 fs2. It is slowly blue-shifted when φ2>0 and stays at 772 nm when φ2>1000 fs2. It starts to be red-shifted when φ2<0 and stays at 806 nm finally.
In conclusion, with the increase of quadratic spectral phase, we observe the effects of conversion efficiency and spectrum bandwidth and the shift of central wavelength. Moreover, the influences of positive and negative quadratic spectral phase on characteristics of XPW are different. Our result shows that the negative quadratic spectral phaseis more effective at reducing the conversion efficiency and spectrum bandwidth than the positive one.

ATOMIC AND MOLECULAR PHYSICS

Non-dipole effects in the angular distributions of photoelectrons on sodium-like ions

Ma Kun, Xie Lu-You, Zhang Deng-Hong, Jiang Jun, Dong Chen-Zhong
Acta Physica Sinica. 2017, 66 (4): 043201 doi: 10.7498/aps.66.043201
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Photoionization processes widely exist in the astrophysical plasma and the high temperature laboratory plasma. Compared with the traditional photoelectron energy spectrum, the photoelectron angular distribution is not only related to the amplitude of the photoionization channels, but also sensitive to the phases of these channels. So the photoelectron angular distribution contains much more quantum information about the photoionization processes and is used to provide stringent tests of our understanding of basic physical processes underlying gas- and condensed-phase interaction with radiation, as well as a tool to probe physical and chemical structure in solids and surfaces. For a long time, the dipole approximation has been the basis in the study of the photoelectron angular distribution, but with the progress of light source, such as the fourth generation synchrotron facilities, more and more attention is paid to the non-dipole effect of the photoelectron angular distribution. In thispresent work, the photoionization processes of sodium-like ions (20≤Z≤92) are studied for the different incident photon energies based on the multiconfiguration Dirac-Fock method and the density matrix theory. The influences of the non-dipole terms on the photoelectron angular distributions, which arise from the multipole expansion of the electron-photon interaction, are discussed in detail. The relationship between the dipole and non-dipole parameters of the photoelectron angular distribution along with the atomic number is given. It is found that the influence of non-dipole terms on the photoelectron angular distribution is related to the incident photon energy and the atomic number of the target ion and the subshell of the ionized electron. In general, the influences of the non-dipole terms on the photoelectron angular distribution of p subshell are larger than those of the s subshell. In the electric dipole approximation, the s subshell photoelectron angular distribution is nearly independent of the photon energy and nuclear charge number, but this situation is not for the p subshell. With the increase of photon energy, an abnormal angular distribution is found for the p subshell photoelectron. However, if the non-dipole effects are included, the abnormal photoelectron angular distribution of p subshell disappears and the photoelectron distribution has maximum values respectively near 45o and 135o with respect to the polarization vector of incident light, that is, the photoelectron distribution has an obvious forward scattering characteristic.

ELECTROMAGNETISM, OPTICS, ACOUSTICS, HEAT TRANSFER, CLASSICAL MECHANICS, AND FLUID DYNAMICS

Far-field time reversal subwavelength imaging of sources based on grating structure

Gong Zhi-Shuang, Wang Bing-Zhong, Wang Ren, Zang Rui, Wang Xiao-Hua
Acta Physica Sinica. 2017, 66 (4): 044101 doi: 10.7498/aps.66.044101
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For far-field imaging applications, the imaging resolution of conventional lenses is limited by the diffraction limit because of the exponential decay of high spatial frequency waves. The key to realizing the subwavelength imaging lies in the collection of evanescent informations in far-field region. However, the collection of evanescent waves is not the only thing we need to do. The relation between target position and far-field information is also very important.
In this paper, a far-field time reversal subwavelength imaging system is constructed with the help of an evanescent-to-propagating conversion plate, i. e., a grating plate. The designed grating plate is able to convert evanescent waves into propagating waves through the modulation in space-spectrum domain. In order to clearly understand the conversion, a focusing experiment is conducted with two sources and five time reversal mirror antennas. By recording the amplitudes of the time reversal signals in the two source positions, we can see that the amplitude of the refocusing signal at the original source position is much larger than that of the other signal. Through numerical simulation and experiment, the conversion of evanescent wave into propagative wave is proved finally.
Then, according to the self-conjugation property of time reversal, the result of self-conjugation for channel response in complex environment is nearly the same as an impulse function. The image of source target can be reconstructed without exact prior knowledge of the expression of the spatial channel response. In order to exemplify the super resolution property of our designed system, experiments with simulation data and experimental data are executed with and without our designed grating plate, respectively. For imaging applications, we first record the forward signals received by the time reversal mirror antennas, and then record the refocusing field distribution on the imaging plane to obtain the image of the target. In the reconstruction process, another thing we need to notice is that the original sources should be removed. This is because in a real imaging application, we cannot know the exact position of target inadvance. The imaging results show that the resolution of our imaging system has overcome the diffraction limit.
Compared with the imaging resolution of the imaging system without the grating plate, the imaging resolution of the system with our designed grating plate is improved obviously. Since this kind of method overcomes the intrinsical diffraction limit by transmitting evanescent information to far-field region in a way of converting them into propagative waves. This kind of method offers us a promising alternative to microwave far-field subwavelength imaging applications.

Angular drift of the high current relativistic multi-beam in the hollow cylindrical waveguide

Wang Gan-Ping, Jin Xiao, Huang Hua, Liu Zhen-Bang
Acta Physica Sinica. 2017, 66 (4): 044102 doi: 10.7498/aps.66.044102
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Multi-beam klystron (MBK) is a promising high power microwave device with the traits of high power, high efficiency, high frequency, etc. For the high power relativistic MBK, the multi-beam rotation around an axis may reduce the transmission efficiency obviously due to the effect of space electromagnetic field. In previous researches, the influence of mirror-image electromagnetic field is ignored, which can play a leading role in some cases. In this study, we present a method by taking into account the mirror-image effect to analyze the angular drift of multi-beam in the hollow cylindrical waveguide. The hollow cylindrical waveguide is a part of relativistic MBK such as input cavity and transition section, which is just behind the diode. In this method, the equation of the multi-beam angular drift is deduced and analyzed quantitatively. Based on the equation, the expression of the angular velocity about the multi-beam in the waveguide is derived, meanwhile the minimum equilibrium magnetic field, called Brillouin magnetic field, is obtained. To verify the effectiveness of the method, numerical simulations are carried out by the three-dimensional (3D) code and the results show good agreement with the theoretical results. The theoretical analysis and simulation results show that the mirror-image electromagnetic field can dominate the multi-beam angular motion in some conditions, especially when the number of the multi-beams and the distance between the conducting wall and the multi-beam are both small. In this case, the mirror-image electromagnetic field can be much higher than the self-induced electromagnetic field. Nevertheless, as the the number of the multi-beams or the distance between the conducting wall and the multi-beam increases, the mirror-image electromagnetic field decreases and approaches to zero rapidly and the self-induced electromagnetic field controls the angular movement. Interestingly, in general cases, it is found that the change rate of the angular speed is not related to the number of multi-beams, nor the radius of waveguide, nor the distance between the multi-beam, nor waveguide, etc, except for the accelerating voltage. In addition, we experimentally investigate the angular drift of the multi-beam at a voltage of about 670 kV, current of about 7 kA and length of waveguide about 100 mm. The experimental results show that the multi-beam distorts obviously, which changes the beam spot shape from circle to ellipse. To solve this problem, we simultaneously investigate the multi-beam emission and transmission in simulation experiment. The analogue results not only reveal that the distortion is mainly caused by the emission of the multi-cathode rods, but also provide a new phenomenon that the angular drift distance in the accelerating gap of the diode is twice as large as that in the cylindrical hollow waveguide due to the low beam speed along the axis and high electrostatic field in the accelerating region. It is also found that the distortion is more evident as the rod radius decreases. Furthermore, we propose an optimization design to improve the relativistic multi-beam system by inclining the multi-cathode rods, which is proved to be effective by simulation. This study could provide theoretical basis for studying the relativistic MBK.

The research on the illumination distribution law of the first-order scattered light in the focal plane of transmission optical system

Tan Nai-Yue, Xu Zhong-Jie, Wei Ke, Zhang Yue, Wang Rui
Acta Physica Sinica. 2017, 66 (4): 044201 doi: 10.7498/aps.66.044201
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With the advantages of simple structure, low-cost, large field of view, and high image quality, the transmission optical system is widely used in detection system, microscope, telescope, etc. However, the research on the illumination distribution law in the focal plane of transmission optical system is rarely reported. In this paper, this issue is studied. During the study on the first-order scattered light distribution law in the focal plane of the transmission optical system, the limitations of the two-parameter Harvey bi-directional scatter distribution function (BSDF) scattering theory are found, namely in the condition of small scattering angle, the two-parameter Harvey BSDF theory cannot accurately describe the scattering properties of the optical surface material. So the scattering model of the transmission optical system under small scattering angle is established by introducing parameter l, and the accuracy of the new theoretical model is verified experimentally. This model complements the two-parameter Harvey BSDF scattering model and broadens the application scope of the Harvey BSDF scattering model so that it can better explain the imaging law of scattered light spot in the focal plane of transmission optical system. At a small scattering angle, the conclusions can be drawn from the new theoretical model as follows. 1) The irradiance of the final image plane increases linearly with the increase of the incident optical power. 2) In the transmission optical system, the contribution of each optical surface with the same scattering properties to scattered spot irradiance in the final image plane can be expressed by a Gaussian function. 3) The irradiance of scattered spot in the final image plane can be expressed as the superposition of n Gaussian functions, where n is the number of optical surfaces with different scattering properties in the transmission optical system.

Identity authentication based on two-beam interference and nonlinear correlation

He Jiang-Tao, He Wen-Qi, Liao Mei-Hua, Lu Da-Jiang, Peng Xiang
Acta Physica Sinica. 2017, 66 (4): 044202 doi: 10.7498/aps.66.044202
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In this paper, a new approach to identity authentication is proposed, which takes advantage of the two-beam interference setup and the nonlinear correlation technique. According to the traditional two-beam interference encryption/decryption structure, we design a modified iterative phase retrieval algorithm (MIPRA), which takes the random binary amplitudes as the constraints at the input plane to encode different images (standard reference images) into a set of sparse phase distributions. In the MIPRA, a given random phase distribution serves as a system lock, and it is placed at one of the arms of the two-beam interference setup and keeps unchanged in the whole iterative phase retrieval algorithm but equivalently provides a fixed shifting vector toward the output complex amplitude field. While the peak-to-correlation value (between the output intensity and the original image) reaches a presetting threshold value, or the iterative numer of time reaches a presetting maximum value, the MIPRA stops. Here, the phase lock is assumed to be the same for all the users and thus it is placed and fixed in the system, while the calculated phase distributions vary from the MIPRA to different binary constraints, which are related to different users. Meanwhile, we also study an extension version of the proposed method. By using a superposition multiplexing technique and a nonlinear correlation technique, we can realize a function of hierarchical authentication for various kinds of users through a similar but more smart decision strategy. For example, we adopt the MIPRA four times with different constraints (random binary amplitude distribution) to obtain four phase distributions, the sum of them will be regarded as a final phase key and is designed to the user with the highest privilege. He is then able to pass all the authentication process for each standard reference image with his multiplexed phase key, that is to say, there are obvious peaks in all the nonlinear correlation maps between all the output images and the corresponding standard reference images. In a similar way, the user with the lowest privilege can only pass one authentication process. Compared with the previous identity authentication methods in the optical security area, the phase key for each user, no matter what level he belongs to, is easy to be stored and transmitted because its distinguishing feature of sparsity. It is worthwhile to note that the cross-talk between different output images are very low and will has no effect on the authentication decision since we deliberately assemble all the binary distributions, which act as constraints at the input plane in the MIPRA. Moreover, the output results are all noise-like distributions, which makes it nearly impossible for any potential intruders to find any clues of the original standard reference images. However, on the other hand, with the nonlinear correlation technique, we can easily extract enough information from these noise-like output results to authorize any users, usually we can obtain an obvious peak at the center of the correlation results but there is no peak if we adopt the traditional correlation algorithms. This feature helps reduce the risk of information leakage, thereby providing an additional protection layer. Also, weinvestigate the robustness properties by taking the sparsity ratio, Gaussian noise, and shear/occluded attack into consideration. Some previous tests alsoindicated that our scheme can resist the attack employing incorrect random phase keys. Theoretical analysis and a series simulation results are provided to verify the feasibility and effectiveness of the proposed scheme.

Generation of Bessel beam by manipulating Pancharatnam-Berry phase

Chen Huan, Ling Xiao-Hui, He Wu-Guang, Li Qian-Guang, Yi Xu-Nong
Acta Physica Sinica. 2017, 66 (4): 044203 doi: 10.7498/aps.66.044203
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Bessel beam is one of diffraction-free beams and has some peculiar properties. Varieties of its applications have been found, such as microparticle manipulating, material processing and biological studies. In this work, we propose a method of creating a Bessel beam by manipulating Pancharatnam-Berry phase. Using femtosecond laser, nano waveplatelets are written on a fused silicon glass to form a metasurface. The optical axis of waveplatelets rotating in the radial direction can produce the space-varying Pancharatnam-Berry phase. The designed metasurface acts as a planar axicon to generate Bessel beams by replacing the traditional one. A Jones calculation is employed to analyze the transformation of the metasurface. The theoretical results indicate that a left-handed circularly polarized light passing through the planar axicon is convergent, while a right-handed circularly polarized one is divergent. The intrinsic physical reason is that Pancharatnam-Berry phase is spin-dependent. Therefore, Bessel beams are generated by the planar axicon only when a left-handed circularly polarized light inputs the system. It is notable that the maximum nondiffracting distance is determined by the rate of rotation of the metasurface microstructure. By reducing the rate of rotation, we can easily obtain a longer nondiffracting distance, thus avoiding the problem that the base angle of the traditional axicon is too small to fabricate. According to the Fresnel diffraction integral, we simulate the propagation of the field emerging from the planar axicon and obtain the intensity distributions behind the planar axicon with different distances. The results show that the intensity pattern remains unchanged in the propagating process and possesses the propagation properties of Bessel beam. It implies that approximate nondiffraction Bessel beams can be achieved by employing the planar axicon with metasurface. Finally, we set up an experimental system with the Pancharatnam-Berry phase metasurface with period d=1000 μupm to verify the theoretical analysis. Theoretically, the maximum nondiffraction distance is 7.9 m. In the shaded region, we measure the intensity distributions at different distances. The experimental results are in good agreement with the simulation results, so the planar axicon based on Pancharatnam-Berry phase can be an effective Bessel beam generator. We believe that these results are helpful for developing more spin-dependent photonic devices.

A new technique for measuring single-shot ultrashort laser pulse

Xia Yan-Wen, Shen Miao, Sun Zhi-Hong, Peng Zhi-Tao, Lu Zong-Gui, Zhou Song, Zhang Bo, Su Jing-Qin
Acta Physica Sinica. 2017, 66 (4): 044204 doi: 10.7498/aps.66.044204
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A new triple correlation technique for measuring intensity profile of single-shot ultrashort laser pulse is described. The technique uses two consecutive second-order nonlinear interactions of replicas of the pulse for generating a two-coordinate output intensity distribution that corresponds to a third-order correlation function and offers advantages over the previously techniques such as frequency-resolved optical gating, self-referencing spectral phase interferometry for direct field reconstruction because it requires no additional spectral information to profile the pulses. This intensity distribution is recorded, and the pulse profile can be obtained by analytical calculation. Combining the reconstructed intensity profile with its corresponding optical spectrum, the exact phase variation in time can be recovered with Gerchberg-Saxton algorithm through an iterative calculation.

Optimazation of broadband third-harmonic UV generation in highly nonlinear photonic crystal fiber

Teng Huan, Chai Lu, Wang Qing-Yue, Hu Ming-Lie
Acta Physica Sinica. 2017, 66 (4): 044205 doi: 10.7498/aps.66.044205
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The generation of pulse radiation with different frequency based on nonlinear optical frequency conversion technology is an effective method to produce lasers with the wavelength in the visible light or ultraviolet (UV) light range. In recent years, the developments of photonic crystal fiber (PCF) technology and ultra-short pulse technology have brought new solutions to the problems that the system needs great maintenance work, has low frequency conversion rate and much difficulty in popularizing, which the traditional frequency conversion system based on nonlinear crystal is confronting. Research on UV pulse radiation has been consistently attracting much attention of many academics. Particularly, narrowband and broadband UV pulse radiation sources are complementary, each having its own characteristics and scope of applications. The generation of narrowband UV pulse radiation of high sensitivity and high resolution through third harmonic generation (THG) in PCF has already been reported. However, the frequency conversion rate of narrowband UV pulse radiation is relatively low and the tunable ability of the spectrum is limited. These imperfections can be exactly completed by broadband UV pulse radiation. Broadband UV pulse radiation based on THG in PCF can be realized efficiently in PCF. This means that the conversion of UV light increases substantially, and simultaneously, the narrowband UV radiation of any wavelength in a certain range can be acquired more easily and the tunable ability of narrowband UV pulse radiation can be enhanced further. In this paper, the femtosecond pulse with a central wavelength of 1035 nm at a pulse repetition rate of 50 MHz is coupled into a highly nonlinear photonic crystal fiber with an appropriate length. The Raman self-frequency shift soliton produced from the ultra-short input pulse acts as a pump resource of third harmonic, transmitting through fundamental mode in PCF. Phase-matching between the fundamental mode and the high order modes is achieved and the third harmonic transmitted by specific high order modes (such as HE13) at deep UV wavelength is acquired effectively. Besides, the very high order UV mode (HOUVM) transmitting third harmonic with shorter wavelength is stimulated when intentionally inputting the ultra-short pulse into the PCF in the direction of a certain angle deviating from the axis of fiber core. Broadband deep UV (320-360 nm) pulse radiation with a UV light conversion rate of 3.6% can be acquired effectively in nonlinear PCF by stimulating a number of adjacent HOUVMs and achieving phase matching between the modes. Good agreement between theoretical results and experimental results is achieved.

Compression of correlation time of chirped biphotons by binary phase modulation

Li Bai-Hong, Wang Dou-Dou, Pang Hua-Feng, Zhang Tao, Xie You, Gao Feng, Dong Rui-Fang, Li Yong-Fang, Zhang Shou-Gang
Acta Physica Sinica. 2017, 66 (4): 044206 doi: 10.7498/aps.66.044206
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Chirped biphotons generated via spontaneous parametric down-conversion in chirped quasi-phase-matched nonlinear crystals have ultrabroadband frequency spectra. However, the presence of quadratic frequency phase factor restricts their applications in quantum metrology and quantum lithography due to simultaneously lengthening the correlation times of biphotons. The key point to improve the temporal correlation of chirped biphotons is how to compensate for or remove the quadratic frequency phase factor. Phase compensation methods have been demonstrated to solve this problem in earlier reports. But the compressed efficiencies of these methods are strongly dependent on the length of the utilized dispersive medium and decreased by the higher-order dispersion of the dispersive medium. In this paper, based on the phase transform of a lens for a light field in spatial domain, we theoretically propose a method of the equivalent removal of the quadratic phase by realizing a Fresnel-zone lens-like modulation on the biphotons spectrum in frequency domain, thereby compressing the correlation time of chirped biphotons to the Fourier-transform limited width. By analogy to the idea of Fresnel wave zone plate, this lens-like modulation can be realized by dividing the biphoton spectrum into Fresnel frequency zones and applying only binary spectral phase (0, π) sequentially to these zones. The theoretical results show that the correlation time width of chirped biphotons can be reduced, and the correlation signal intensity can be increased compared with the original one, by a factor about 100 and 30, respectively. The physical reason is that these Fresnel frequency zones under binary spectral phase modulation will lead to constructive interference at zero delay and destructive interference elsewhere. This method can significantly enhance biphoton time correlation without biphoton signal loss and avoids the limitations of phase compensation methods. Therefore, we can obtain biphotons with both ultra-broad bandwidth and ultra-short correlation times by using our proposed method. The attainable compression efficiency is constrained by the division resolution of the Fresnel frequency zones and the precision of applied binary phase modulations. It should be noted that a constraint condition about crystal length, chirp parameter and the number of frequency zones is summarized in designing the experimental parameters for the desired compression goal. Since binary spectral phase π and 0 are easy to obtain and calibrate in practice, we thus believe that our proposed method is feasible to implement experimentally. Moreover, the proposed method can also be generalized to other fields relating to the quadratic phase factor, such as two-photon absorption, second-harmonic generation and chirped pulse compression.

Simulating scattering properties of nonspherical aerosol particles using multiresolution timedomain method

Hu Shuai, Gao Tai-Chang, Li Hao, Yang Bo, Jiang Zhi-Dong, Chen Ming, Li Shu-Lei
Acta Physica Sinica. 2017, 66 (4): 044207 doi: 10.7498/aps.66.044207
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Scattering process of aerosol particles plays an important role in atmospheric radiative transfer since it can modify the transmission, reflection and absorption ability of atmospheric system. Owning to the uncertainty of aerosol particles' scattering properties, which results from their complicated geometries and inhomogeneous compositions, there still exists a considerable uncertainty in the radiative transfer numerical simulation, and simulating the scattering properties of aerosol with irregular shapes has become a hotspot in meteorological study. To this end, a new aerosol scattering model is developed based on multi-resolution time-domain (MRTD), by which the scattering processes of nonspherical and inhomogeneous particles can be simulated. In this model, the near electromagnetic field is calculated by MRTD technique. Considering the particularity of aerosol medium, a transformation technique from near field to far field is derived based on volume integration method, and then the scattering amplitude matrix and Müeller matrix can be calculated by the obtained far electric field as well. The models for particle extinction and absorption cross section are derived from Maxwell's curl equations in the frequency domain, by which the integration scattering properties can be simulated accurately. The MRTD scattering model is validated by comparing with Mie theory and T matrix method for spherical particle, ellipsoidal particle and cylindrical particle, and the influence of grid size on the simulation accuracy is analyzed subsequently. In the last part, the efficiency of the MRTD scattering model is quantitatively discussed. The simulation results show that the relative errors of scattering phase function simulated by our model are less than 8%, and the errors in forward scattering direction are much smaller, which are less than 4%. The precisions for extinction and absorption efficiency are much higher than the results from the scattering phase function, and the relative errors can reduce to 0.1% for particles with their radii comparable to the wavelength of incident light. The gird size has a significant influence on model precision; to achieve the same accuracy, the grid size first increases with increasing particle radius, and then decreases as a function of particle size for particles with size parameter less than 20. In the next step, we will try to establish the scattering property database of nonspherical particles based on the MRTD scattering model developed here.

Application of cone-cylinder combined fiber probe to surface enhanced Raman scattering

Guo Xu-Dong, Tang Jun, Liu Wen-Yao, Guo Hao, Fang Guo-Cheng, Zhao Miao-Miao, Wang Lei, Xia Mei-Jing, Liu Jun
Acta Physica Sinica. 2017, 66 (4): 044208 doi: 10.7498/aps.66.044208
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Owing to increasingly severe environmental pollution, food safety and other problems, higher and higher requirements for the detecting technique of poisonous and harmful biochemical molecules have been put forward. The conventional biochemical detector has the disadvantages of large size, high cost and inability to realize far-end and in-situ detection functions. Based on the requirements of the biochemical molecular detection technology for high sensitivity, miniaturization, far-end detection, insitu detection, real-time analysis and the like, a detection method using a fiber surface-enhanced Raman scattering (SERS) probe to carry out Raman signal detection has been put forward in recent years. The detection method not only realizes far-end and insitu detection functions, but also has a relatively high sensitivity. In this paper, a taper and cylinder combination type fiber probe is made by adopting a simple tube corrosion method, Under the situation of fixed temperature, cone-cylinder combined fiber probes with different diameters are obtained by controlling the corrosion time, and silver nanoparticles are bound to the surface of a silanized silicon dioxide fiber probe through electrostatic forces. Then, the sizes and morphologies of silver nanoparticles on the surface of the fiber probe are observed under a scanning electron microscope. Besides, the detection limit of a rhodamine 6G (R6G) solution is used to manifest both the activity and the sensitivity of the fiber probe, and the self-assembly time of the silver nanoparticles are further optimized to be 30 min and the diameter of the fiber probe to be 62 μupm. When the concentration of a silver sol solution is constant, a high-sensitivity fiber SERS probe can be prepared. Through far-end detection, the detection limit of the R6G can reach 10-14 mol/L, and the enhancement factor is 1.36×104. This work can serve as an experimental basis for a novel fiber surface-enhanced Raman scattering sensor in such aspects as high sensitivity and low cost. The studies of this paper are expected to provide an appropriate detection technique for rapid quantitative detection of biochemical molecules, and further provide a reference for various application fields of environmental monitoring and food safety analysis in future in terms of realizing rapid and accurate in-situ detection. Therefore, the fiber SERS probe has large application foreground in molecular detection.

Acoustic manipulation of particles by a resonant one-dimensional grating in air

Huang Xian-Yu, Cai Fei-Yan, Li Wen-Cheng, Zheng Hai-Rong, He Zhao-Jian, Deng Ke, Zhao He-Ping
Acta Physica Sinica. 2017, 66 (4): 044301 doi: 10.7498/aps.66.044301
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It is well known that acoustic wave carries momentum and energy. An object in a sound field, which absorbs or reflects sound energy, can be subjected to the acoustic radiation force (ARF), and thus can be manipulated in the contactless and noninvasive manners. This effect has potential applications in the fields of environment monitoring, microbiology, food quality control, etc. Obtaining a tunable trapping or pushing ARF should enable the design of an incident beam profile. However, the conventional acoustic manipulation system with plane wave, standing waves or Gaussian beams, which is usually generated directly by acoustic transducer, cannot be redesigned easily, nor can the corresponding ARF be modulated efficiently. Phononic crystals, which are artificial periodic structure materials, exhibit great advantages in modulating the propagation and distribution of acoustic wave compared with conventional materials, and thus have potential applications in tunable particle manipulation. Here, we present a theoretical study of the ARFs exerted on a cylindrical polystyrene foam particle near the surface of a one-dimensional (1D) grating in air. By using the finite element method (FEM) to investigate the transmission spectra and field distribution of the 1D grating and the FEM combined with momentum-flux tensor to obtain the ARF on the particle, we find that there are two resonance modes in the 1D grating, which origin from the coupling between the diffractive waves excited from the export of periodic apertures and the Fabry-Perot resonance mode inside the apertures. In addition, it can be seen from field distribution that in the first resonant mode, the resonance wavelength is approximate to the period of grating, and the enhanced spatial confinement of acoustic wave is located at the surface of the plate besides in the aperture. In the second resonant mode, the corresponding wavelength is more than twice the period of grating, and the enhanced spatial confinement of acoustic wave is mainly located in the aperture. Moreover, due to the gradient field distribution at the surface of slits and plate in these resonance modes, particles at the surface can be under the action of tunable negative ARFs. In the first resonance mode, the particle can be trapped on the surface of grating. While in the second resonance mode, the particle can be trapped in the aperture, and the amplitude of ARF of this mode is far smaller than that of the first mode. Thus, this system in the first resonance mode may have potential applications in air acoustic manipulation, aligning, and sorting micro-particles.

Non-orthogonal multiple-relaxation-time lattice Boltzmann method for axisymmetric thermal flows

Wang Zuo, Zhang Jia-Zhong, Wang Heng
Acta Physica Sinica. 2017, 66 (4): 044701 doi: 10.7498/aps.66.044701
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Axisymmetric thermal flows in cylindrical systems are widely encountered in engineering practices. Typically, axisymmetric thermal flows belong in three-dimensional (3D) problems. However, taking advantage of the axisymmetric condition, the 3D axisymmetric flows can be reduced to quasi two-dimensional (2D) problems in the meridian plane, which significantly reduces the computational requirements and avoids treating the curved boundary. In recent years, various 2D lattice Boltzmann (LB) models, including single relaxation time LB (SRT-LB, or LBGK) and multiple relaxation time LB (MRT-LB) models, for axisymmetric thermal flows have been proposed. In the LB community, it is well accepted that the MRT-LB is superior to the LBGK in terms of numerical stability. The existing MRT-LB model for axisymmetric thermal flows are developed based on orthogonal basis vectors obtained from the combination of the lattice velocity components, i.e., the transform matrix in the existing MRT-LB is an orthogonal one. Unlike the existing MRT-LB model, in this paper, a non-orthogonal multiple-relaxation-time lattice Boltzmann (MRT-LB) method of simulating axisymmetric thermal flows is proposed. In the proposed MRT-LB method, the velocity field is solved by a D2Q9 discrete velocity set while the temperature by a D2Q5 discrete velocity set. The main advantage of the present MRT-LB model is that the transform matrix of the model is a non-orthogonal one, which is comprised of some proper non-orthogonal basis vectors obtained from the combination of the lattice velocity components. The non-orthogonal transform matrix of the present MRT-LB model contains more zero elements than the classical orthogonal transform matrix, and thus the present MRT-LB model is expected to be more efficient than the existing orthogonal-based MRT-LB model. The equilibrium velocity and temperature moments of the present MRT-LB model are expressed by mapping the equilibrium distribution functions onto their moment spaces through using the non-orthogonal transformation matrix. Also the vectors in the forcing term are modified according to the matrix mapping. Through the Chapman-Enskog analysis, it is demonstrated that the macroscopic governing equations in the cylindrical coordinate can be recovered from the present MRT-LB model. Then several numerical tests, including thermal Womersley flow, Rayleigh-Bénard convection in a vertical cylinder and natural convection in a vertical annulus, are conducted to validate the present model. It is found that the present numerical results are in good agreement with the analytical solutions and/or other numerical results reported in the literature. Numerical stability is also tested, and the results suggest that the present MRT model shows better numerical stability than its LBGK counterpart. Moreover, the numerical results also indicate that the present MRT-LB model is more computationally efficient than the existing MRT-LB model for axisymmetric thermal flow. These findings indicate that the present MRT-LB model can serve as a powerful method of computing the axisymmetric thermal flows.

Formation mechanism of water jets induced by the interaction between bubble and free surface

Zheng Jian, Zhang Duo, Jiang Bang-Hai, Lu Fang-Yun
Acta Physica Sinica. 2017, 66 (4): 044702 doi: 10.7498/aps.66.044702
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Explosion in shallow water or small depth water will generate upward water jet, mainly because bubbles generated by explosion will interact with the surface of water. Different underwater depths can result in upward water jets with different kinds of shapes, such as water column, water plume, jet, spall dome, splash, spike, etc. To reveal the formation mechanisms of different types of water jets, a spark bubble experiment platform is set up, and the motions of bubble and free surface are studied experimentally by high-speed photography. The dynamic images for the formation process of the water jets under different initial depths of bubble are obtained. Through theoretical analysis and direct observation of the experimental data, the interaction process between the oscillating bubble and free surface are clarified, and the evolution rule of water jets is obtained. It is found that the key factor affecting the formation of different shapes of the water jets is the superposition of the disturbance of the second bubble pulse and the simple-shape jet induced by the first bubble pulse. Five types of the superpositions are summarized:1) All-fit type, with a large depth of initial bubble, the first and the second bubble impulse fit well to form a smooth and slightly arched water dome; 2) partial-fit type, with a less large depth of initial bubble, higher arched water dome is formed due to the raising effects of second bubble pulse partially fit the initial water dome shape; 3) catch-up type, with a mediate depth of initial bubble, the free-surface jet caused by first bubble pulse will be caught up from the bottom by the second pulse, and form a thin and high velocity jet; 4) run-after type, with a smaller depth of initial bubble, the free-surface jet caused by first bubble pulse will be raised from the bottom by the second pulse, and form a jet with thin head and thick pedestal, sometimes form a crown-type splash; 5) non-superposition type, the depth of initial bubble is so small that the bubble will break up, and no superposition will happen. In summary, the ratio of the initial depth to the maximum radius of bubble is found to be a decisive factor of the superposition type. The initial bubble is described by a dimensionless distance. These conclusions well explain the phenomena observed in experiment, and can provide a new vision and reference to the understanding of the formation mechanism of water jets induced by the interaction between bubble and free surface.

Effect of micro-structure array on the liquid flow behaviors of near-surface layer

Qiao Xiao-Xi, Zhang Xiang-Jun, Tian Yu, Meng Yong-Gang
Acta Physica Sinica. 2017, 66 (4): 044703 doi: 10.7498/aps.66.044703
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Study of the liquid flowing behavior through the micro-structure array has aroused the significant interest due to its key roles in the fields of microfluidics, micro-mixers, micro-heat exchangers, tribology, etc. Micro-structure array can significantly affect the liquid flowing characteristics of the near-surface layer and the solid-liquid interfacial properties, like adhesion, surface wetting, shear viscous resistance, interfacial slip, etc. The researches indicate that the stripe- and square-patterned electrodes can improve the storage properties of the lithium-ion battery due to its ability to promote the diffusion of the liquid electrolyte. Micro-structure array patterned micro-channel can reduce the friction drag of liquid flowing through it. And the surface fabricated with lotus-leaf-like dual-scale structure array can achieve the super-hydrophobicity.
For a micro-structure array, its influences on the liquid flowing behaviors greatly depend on the shape and size of the micro-structure, and the porosity, arrangement and size of the array. Here, we mainly focus on the influences of the micro-structure shape and surface topography on the liquid flowing behaviors, by adopting the same array porosity, arrangement and size, and the same feature size of the micro-structure. In the present paper, we prepare three different surfaces, which are the micro-pillar array surfaces, micro-hole array surface, and dual-scale micro-pillar array surface (i.e., micro-pillar with rough top surface), respectively. Their influences on the liquid flowing characteristics of the near-surface layer are investigated by quartz crystal microbalance (QCM). The QCM is a powerful and promising technique in studying the solid/liquid interfacial behaviors. Its main output parameters are frequency shift and half-bandwidth variation, which are closely related to the rheological properties and flow characteristics of the near-surface liquid layer. When the QCM chip is patterned with micro-structure array, it will inevitably influence the liquid motion and makes it more complicated, like the generation of non-laminar motion, the trapping of liquid in the gap, and the conversion of the in-plane surface motion into the surface-normal liquid motion. The experimental results show that for the same tested liquid, the frequency shift caused by the micro-hole array is higher than that by the micro-pillar array with the same feature size. And the dual-scale micro-pillar array surface results in a higher half-bandwidth variation than the micro-pillar array surface with the same feature size. It demonstrates that micro-hole tends to confine the liquid motion and make the trapped liquid oscillate with the substrate like a rigid film, thus resulting in a higher frequency shift. The dual-scale micro-structure will render the flow behavior of the near-surface layer more chaotic, thus showing a larger half-bandwidth variation. This study provides an experimental basis for selecting the type of micro-structure used in the microfluidic chip to better control the liquid flowing and mixing.

Numerical simulation of complex immersed boundary flow by a radial basis function ghost cell method

Xin Jian-Jian, Shi Fu-Long, Jin Qiu
Acta Physica Sinica. 2017, 66 (4): 044704 doi: 10.7498/aps.66.044704
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A radial basis function ghost cell immersed boundary method of simulating flows around arbitrary complex or multiple immersed boundaries is proposed in this paper. In this method, incompressible Navier-Stokes equations are discretized on fixed Cartesian staggered gridby the finite difference method. A fractional step method is used for time integration, together with third order Runge-Kutta scheme. A high-order TVD MUSCL (total variation diminishing monotonic upstream-centered scheme for conservation law) scheme is used to discretize convective terms. Two salient features are emphasized in the present study. First, boundary conditions at the immersed interface are enforced by a continuous ghost cell method to consider the influence of immersed boundary on the flow field. The immersed bodies are treated as virtual boundaries immersed in the flow. And Navier-Stokes equations are solved in the entire computation domain, including solid domain. Therefore, programming complexity is greatly reduced and the treatment of immersed boundaries is simplified. Second, a polynomial and radial basis function is introduced to implicitly represent and reconstruct arbitrary complex immersed boundaries. Iso-surface distance functions about interface geometries are fitted with some sampling points of body surfaces. It is flexible and robust. Moreover, the information about interface positions on the background grid can be easily identified by the signed distance functions. Based on our in-house developed immersed boundary method solver, typical test cases are simulated to validate the proposed method. The flows around a cylinder at Reynolds numbers of 40, 100 and 200 are first simulated and a grid resolution study is carried out. Good agreement is achieved by comparing with previous numerical results, which shows that this method is accurate and reliable. In the second case of flow around airfoil, the good agreement with previous study shows that the present method has the ability to simulate complex immersed boundary flow. In the last case of flow around array of thirteen cylinders, the ability of present method for multiple immersed boundaries is well proved. And hydrodynamic interaction among multiple bodies is briefly analyzed.

PHYSICS OF GASES, PLASMAS, AND ELECTRIC DISCHARGES

Effect of high power microwave injection on tropospheric freon

Ran Mao-Yi, Hu Yao-Gai, Zhao Zheng-Yu, Zhang Yuan-Nong
Acta Physica Sinica. 2017, 66 (4): 045101 doi: 10.7498/aps.66.045101
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High power microwave injection into the troposphere is a feasible approach to the decomposition of chlorofluorocarbon (CFC). However, in existing researches, there are only basic principles which lack quantitative tests. Hence, in this article we introduce the finite-difference time-domain method to quantitatively analyze the decomposition of CFC under high power pulses. We first investigate the principal chemical reactions of CFC decomposition induced by high power microwave injection and find that dissociation attachment is a dominant process of the microwave discharge decomposition of CFC. We use an empirical formula to calculate the decomposition efficiency of CFC. The result shows that 20% of the initial content of CFC molecules will be dissociated over 100 microseconds where we assume the electron number density to be 1013 cm-3. Then according to Maxwell's equations and the current density equation, we adopt the finite difference time domain method to simulate the generation process of a large number of free electrons induced by injecting the high power microwaves into the troposphere. The ionized electron generated by the high power microwave in troposphere is in favor of CFC decomposition since the electron affinity of CFC is larger than dissociation energy of CFC molecules. The simulation results indicate that the number density of electrons grows up to 1017 cm-3 exponentially with the injection time and will grow faster at higher height (<10 km) or by the larger field intensity. During the pulse, the higher electron energy corresponds to a smaller dissociative attachment coefficient. Thus, most of the CFC molecules are decomposed during the electron-decay phase. During the relaxation period, the electron energy will return to the natural state within 0.01 ns. The number density of electrons decreases slower than the electron energy and it will take 1 ms to reach the natural state. From the results we can also see that the decay rates of the electron energy and number density decrease with the increase of the height. In this paper, two methods of calculating the CFC decomposition rate are utilized. One method is from the chemical reaction and the other method is based on an empirical formula which is mentioned before. It is shown that the results of these two methods present obvious consistency. The simulation results demonstrate that the CFC decomposition rate will increase with larger microwave intensity or higher frequency and can approach up to 6%. In conclusion, this study gives the quantitative analyses of the CFC decomposition induced by high power microwave injection in the troposphere for the first time.

Effect of standing wave on the uniformity of a low magnetic field helicon plasma

Niu Chen, Liu Zhong-Wei, Yang Li-Zhen, Chen Qiang
Acta Physica Sinica. 2017, 66 (4): 045201 doi: 10.7498/aps.66.045201
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Helicon wave discharge has higher coupling efficiency than capactively coupled and inductively coupled discharge in low static magnetic field. In the wave sustained mode, a large volume and large area plasma can be produced at lower pressure by using comparable discharge power, and thus it expands the helicon wave plasma applications in material surface modification, thin film deposition, dry etching and thruster usage. However, the application of helicon wave source still faces challenges, such as the controversial power coupling mechanism, operation stability and the plasma distribution uniformity in the experiment. The wave mode existing in bounded helicon wave plasma column generally consists of helicon and Trivelpiece-Gould (TG) components, and their mode transitions and different transverse wave field distribution regions, and the propagating characteristic of the helicon wave are directly related to the power coupling and plasma density distribution in the source region, then affect the uniformity of material processing and film deposition in the diffusion chamber. In this paper, the plasma azimuthal non-uniformity, with using Doubble Saddle antenna, 100 G static magnetic field in helicon wave plasma source, is studied by electrical characteristic (power-current) curve, intensified charge coupled device (ICCD) image and magnetic probe measurements. The electrical characteristic curve indicates two discharge stages with different effective resistances. Meanwhile, in the second stage, the higher effective resistance would result in higher coupling efficiency and higher plasma density. But the ICCD image demonstrates the azimuthal non-uniformity of plasma, indicating that the main heating points at the diagonal edge are linked to the stationary transverse electrical field line pattern of azimuthal mode number m=+1 helicon wave, and the magnetic probe is used to measure the helicon wave magnetic field Bz component along the quartz source tube axially. The magnetic probe results show that the standing wave appearing below the antenna even though in the upper region of the antenna is characteristic of the traveling wave. Furthermore, at the plasma boundary, the standing wave can be coupled to the TG wave, and not like travelling wave it has no angular rotation of the electric field and may cause the non-uniform coupling between the helicon and TG components. The TG wave then has azimuthal non-uniform electron heating. Therefore, the standing helicon wave below the antenna is the key factor to the plasma non-uniformity problem. Changing the propagating characteristics of the helicon wave further in the plasma column will be of positive significance for optimizing the discharge efficiency of the plasma source and controlling the plasma distribution uniformity, stability and other operations as well.

GEOPHYSICS, ASTRONOMY, AND ASTROPHYSICS

Spatial distribution characteristics of nearly extremely anomalous temperature events in summer and winter in China

Qian Zhong-Hua, Cao Chun-Hong, Feng Guo-Lin
Acta Physica Sinica. 2017, 66 (4): 049201 doi: 10.7498/aps.66.049201
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The events near their extreme values are termed nearly extreme events. The generalized density of states is proposed that is defined by a probability density function. The rate of nearly-extreme events to the total sample size at a given point is the crowding of nearly extreme events, which is an important index used in many fields. Based on the estimation of the generalized state density of nearly extreme events, the parameters of the generalized state density of nearly-extreme anomalous temperature events are constructed with the temperature daily maximum data in summer and daily minimum data recorded in China in winter in 1961-2013. The daily maximum and minimum temperatures recorded at 174 observed stations in 1961-2013 are selected based on the requirement of data continuity from the climate dataset over China, released by the China Meteorological Administration. According to the analysis of the single station Nanjing, the maximum probability density of occurrence about nearly extremely anomalous temperature is marked as ρmax and the corresponding r of ρmax is marked as rp, which indicates that when the difference between nearly extremely anomalous events and extremely anomalous events is rp, the probability of occurrence is maximum. Then rp is defined as the most probable intensity of nearly extremely anomalous temperature events. ρmax and rp can show the crowding degree characteristics about nearly extremely anomalous temperature events and can carry significant physical meanings in the practical application. So the spatial distribution characteristics of ρmax and rp about nearly extremely anomalous temperature events in China in summer and winter are analyzed respectively. In summer, in the west part of Northwest China, South China and south part of Southwest China easily happen the extremely warming events when the most probable intensity of nearly extremely warming temperature event rp values are 1.0℃ and 2.8℃ and the maximum probability density of occurrence about nearly extremely warming temperature ρmax is up to 44%. In South China, south part of Southwest China and Xizang easily occur the extremely cooling events when the most probable intensity of nearly extremely cooling temperature event rp values are 0.5℃ and 2.5℃ and the maximum probability density of occurrence about nearly extremely cooling temperature ρmax is up to 34%. In winter, the warning information about extremely warming events should give to Southwest China when the most probable intensity of nearly extremely warming temperature events rp values are 1℃ and 2℃ and the maximum probability density of occurrence about nearly extremely warming temperature ρmax is up to 32%. The warning information about extremely cooling events should give to Southwest China, South China and south part of the Yangtze River when the most probable intensity of nearly extremely cooling temperature events rp are 1.0℃ and 4.0℃. Therefore, the maximum probability density of occurrence ρmax and the most probable intensity rp of nearly extremely anomalous temperature events can give some early warning information about the coming extremely anomalous temperature events.

Study on incoherent scatter theory of high density dusty plasma

Xu Bin, Li Hui, Wang Zhan-Ge, Xu Zheng-Wen, Wu Jian
Acta Physica Sinica. 2017, 66 (4): 049401 doi: 10.7498/aps.66.049401
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Incoherent scatter radar is one of the most important detection instruments of the space plasma. But because of the low dust density in natural space plasma, the contribution of charged dust to incoherent scatter spectrum can be completely ignored, therefore the incoherent scattering theory has not appeared in dusty plasma. In the solid rocket plume, the propellant combustion can form a large number of nanometer- and micronmeter-sized dusty particles, and produce a high electron density from high temperature ionization, which makes considerable contributionto charged dusty particles with the high density. Therefore, we develop the incoherent scattering theory of dusty plasma in order to calculate the scattering characteristics of high density dusty plasma produced by rocket plume, for example. The theoretical model including electrons, ions and dusty particles is established by combining effects of charged dusty particles. The incoherent scatter spectral lines of ion resonance region and dust resonance regionare calculated. The effects of dusty particle radius, temperature and density on spectral line structure are discussed. With the increases of dusty particle radius and density, the amplitude of power spectrum increases. With the increase of dust temperature, the amplitude of power spectrum decreases. In the dust resonance region, the control mechanism of dust in spectrum is similar to that of the ions. With the increase of particle size (mass) and decrease of the temperature, the spectrum width narrows, and amplitude and area increase with the increase of density. But in the ion resonance region, the dust control mechanism is completely different, and the influence of the dust on ion line is in the way of attracting ions. So with the increase of dust density, ion line characteristics do not show that the area increases, and dust controls ions by adjusting the Debye radius or electrostatic shielding ball size. By comparing the ion lines with and without dust under the same parameters conditions, the amplitude of the ion line with dust is much larger than that without dust, and the resonance frequency of the ion line is greatly changed. With the dust particles of a relatively high density, one can enhance the ion line, hence the incoherent scattering phenomenon can be more easily observed in rocket plume. On the other hand, due to significant changes of frequency and amplitude in the ion line spectrum, the incoherent scattering inversion method based on the traditional theory will cause a large error in the inversion parameter, even a failure of parameter retrieval. The incoherent scattering theory and relevant physical laws of dusty plasma are presented, which are of great significance for establishing the incoherent scattering theory system and studying the rocket plume parameters.

Spectral radiant characteristic of airborne optoelectronic system detecting aerial maneuver target

Kou Tian, Yu Lei, Zhou Zhong-Liang, Wang Hai-Yan, Ruan Cheng-Wei, Liu Hong-Qiang
Acta Physica Sinica. 2017, 66 (4): 049501 doi: 10.7498/aps.66.049501
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Spectral radiation detection in deep space background is an important fundamental research in the field of infrared target detection and identification. Based on the spectral radiation and scattering theory, the spatial distribution model of aerial target reflecting background radiation under complex environment is first built. Then the horizontal and pitch spectral radiation models of target are built based on target skin temperature distribution caused by aerodynamic heating. Combining the target motion equation and relative rotation matrix between target matrix and detector matrix, the process-oriented characteristic of spectral response signal with spatiotemporal variation is emphatically analyzed. The simulation results indicate that different target maneuver modes cause different characteristics of spectral response signal, which shows that a remarkable mapping relationship exists between the target maneuver mode and spectral response signal characteristic. Thus using the spectral response signal to identify target maneuver mode provides a feasible method, and the target posture and relative position are the main factors to affect the spectral response signal characteristic.

Acta Physica Sinica
Accepts
Note: The papers published below will continue to be available from this page until they are assigned to an issue. To see an article, click its [PDF] link. To review many abstracts, check the boxes to the left of the titles you want, and click the 'Selected articles' button. To see one abstract at a time, click its [Abstract] link.
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The Effect of Collision Parameter on a Magnetized Electronegative Plasma Sheath Structure

null
Accept: 2016-10-11
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The structure of an electronegative plasma sheath in an oblique magnetic field is investigated. More over, the collisions between positive ions and neutral particles are taken into account. It is assumed that the system consists of hot electrons, hot negative ions and cold positive ions. Also the negative ions and the electrons are assumed to be described by the Boltzmann distributions of their own temperatures, and the accelerated positive ions are treated by means of the continuity and momentum balance equations through the sheath region. In addition, the assumption that the collision cross section has a power law dependence on the positive velocity is introduced. After theoretical derivation, an exact of sheath criterion is obtained. The numerical simulation results include the distributions of the positive ions density for different invariable ion Mach number satisfying Bohm criterion, the comparison of net space charge distributions for variable and invariable ion Mach number. Furthermore, three species of charged particles density, the net space charge and the spatial electric potential in the sheath are studied numerically for different collision parameters under the condition of the fixed ion Mach number. The results show that the ion Mach number has not only the lower limit but also the upper limit. The ion Mach number affects the sheath structure by influencing the distribution of the positive ion density, and different conclusions can be obtained because ion Mach number is adopted as variable or invariable value while discussing the effects of the other variables which can result in the variety of the ion Mach number on the sheath formation. The reason is the actual sheath structure modification brought on by the variation of a parameter can be resolved into two parts. One is the sheath formation change caused directly by the variation of the parameter, the other is the sheath formation change caused by the Bohm criterion modification which the variation of the parameter results in. Therefore, an identical ion Mach number should be adopted when researching the direct effects of a parameter variety on plasma sheath structure. In addition, it is concluded that the collisions between positive ions and neutral particles make positive ions density curve higher and electrons’ lower than the case without collisions. Negative ions density does not alter significantly whether there exists collision or not. Besides there is a peak in the profile of the net space charge while in the presence of ion-neutral collision and the net space charge peak moves toward the sheath edge. The spatial potential increases and the sheath thickness decreases on account of the presence of the collisions between ions and neutral particles.
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Calculation of Hamilton energy function of dynamical systems by using Helmholtz theorem

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Accept: 2016-10-11
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The Helmholtz theorem confirmed that any vector field could be decomposed of gradient and rotational field. The supply and transmission of energy occur during the propagation of electromagnetic wave accompanied by variation of electromagnetic field, thus the dynamical oscillators and neurons can absorb and release energy in presence of complex electromagnetic condition. Indeed, the energy in nonlinear circuit is often time-varying when the capacitor is in charged or discharged, and occurrence of electromagnetic induction is available. Those nonlinear oscillating circuits can be mapped into dynamical systems by using scale transformation. Based on mean field theory, the energy exchange and transmission between electronic field and magnetic field could be estimated by appropriate nonlinear dynamical equations for oscillating circuits. In this paper, it investigates the calculation of Hamilton energy for a class of dimensionless dynamical systems based on Helmholtz’s theorem. Furthermore, scale transformation could be used to develop dynamical equations from the realistic nonlinear oscillating circuit, so the Hamilton energy function could be approached effectively. These results could be much useful for self-adaptive control of dynamical systems.
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Ballistic thermal rectification in the three-terminal graphene nanojunction with asymmetric connection angles

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Accept: 2016-10-11
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By using the nonequilibrium Green’s function method, the ballistic thermal rectification in the three-terminal graphene nanojunction is studied. The dynamics of atoms are described by the interatomic fourth-nearest neighbor force-constant model. The nanojunction has a Y-shaped structure, created by a combination of a straight graphene nanoribbon and a leaning branch as the control terminal holding a fixed temperature. No heat flux flows through the control terminal. There exists a temperature bias between the two ends of the graphene nanoribbon served as the left and right terminals, respectively. The primary goal of this paper is to demonstrate that the ballistic thermal rectification can be introduced by the asymmetric structure with different connection angles between terminals. The control terminal has a smaller connection angle with respect to the left terminal than to the right terminal. The forward direction is defined as being from the left terminal to the right terminal. The results demonstrate that, given the same control temperature and absolute temperature bias, the heat flux in the graphene nanoribbon tends to run preferentially along the forward direction. When the difference between the connection angles increases, the rectification ratio rises. Compared to the zigzag graphene nanoribbon, the rectification ratio of the armchair nanoribbon is more sensitive to the direction the control terminal. However, the greatest rectification ratio is found in the zigzag graphene nanoribbon which has a connection angle of 30 degrees with respect to the armchair branch. In addition, the direction of the control terminal can be adjusted to raise more than 50% of the rectification ratio of the graphene thermal recti?er based on the width discrepancy between the left and right terminals. The mechanism of the ballistic thermal recti?cation is also discussed. In the three-terminal graphene nanojunction, a smaller connection angle with respect to the control terminal leads to more phonon scattering. The confirmation of this conclusion comes from a comparison of phonon transmission between different couples of terminals, which shows that, in most of the frequency spectrum, the phonon transmission between the control terminal and the left terminal is smaller than that between the control terminal and the right terminal. Given the same control terminal temperature and temperature bias, the asymmetric connection angles therefore will introduce a higher average temperature of the left and right terminals, and a larger heat flux in the forward process. Moreover, the average temperature difference between in the forward process and in the reverse process is found to be proportional to the temperature bias, and the proportionality coefficient will get bigger if the asymmetry is strengthened.
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The Propagation Properties of Vortex Beams in a Ring Photonic Crystal Fiber

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Accept: 2016-10-11
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In the last decade, the vortex beams have received lots of attention for their orbital angular momentum. When they are applied to optical fiber communication field, the data channels will increase and information propagation speed will be effectively improved. Recently, researchers have shown the capability of long length stably propagation, nonlinear frequency conversion and mode division multiplexing of vortex modes in a ring fiber. Due to the photonic crystal fiber (PCF) has very flexible design degrees of freedom, it will enable a wide range of propagation properties. In this paper, A SiO2 air-holes ring PCF is proposed for separation and propagation of optical vortex modes. By using COMSOL Multiphysics software, the vortex modes(TE01, HE_21^± and TM01) are simulated and calculated. The differences of the effective refractive index between them are 4.59×〖10〗^(-4) and 3.62×〖10〗^(-4) respectively. One can analyze the propagation properties of vortex beams in the ring PCF by changing the size of first layer air holes’ radius and air hole pitch. When the incident light wavelength of TE01 mode ranges from 1650 nm to 1950 nm, this ring PCF can achieve a total dispersion variation between 44.18 to 45.83 ps?nm^(-1)?km^(-1), which is tend to be flat. When incident light wavelength is 1550 nm, the nonlinear coefficient of TE01 mode vortex light is 1.37 W^(-1)?km^(-1); Due to the long wavelength light is easier to leakage through the cladding than the short wavelength light, the confinement loss increases with the wavelength. When incident light wavelength is 2000 nm, there is still an eight-orders-of-magnitude of the low confinement loss. Theoretically, flat dispersion and low loss vortex beams in this fiber can be beneficial to propagate stably, and the vortex modes lay the foundation for long distance propagation in the optical fiber. In the future, this ring PCF will be used in optical fiber communication field and application in aspects such as continuous spectrum research, which can make it have immense advantage to traditional fibers.
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Penta-decomposition of instantaneous field in spanwise-rotating turbulent plane Couette flow

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Accept: 2016-10-11
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Spanwise-rotating turbulent plane Couette flow (RPCF) is one of the fundamental prototypes for wall-bounded turbulent flows in the rotating reference frames. In this turbulent problem, there are large-scale roll cells, which are widely studied. In this paper, a penta-decomposition method is proposed to separate the instantaneous velocity and the total kinetic energy into five parts, including a mean part, a streamwise part and a cross-flow part of the secondary flow, and a streamwise part and a cross-flow part of the residual field, aimed to explore the energy balance and transfer among different shares of the turbulent kinetic energy in RPCF at Reynolds number Rew=Uwh/ν=1300 (here, Uw is the half the wall velocity difference, and h is half channel-height) and rotation number Ro=2Ωzh/Uw (Ωz is the constant angular velocity in the spanwise direction) in the range of 0≤Ro≤0.9. The results show that the energy is transferred between streamwise part (cross-flow part) of secondary flows and residual field through the correlation between the vorticity of secondary flows and shear stress of residual field. The rotation term acts as a bridge to transfer the energy between streamwise part and cross-flow part of secondary flows (residual field). Moreover, pressure-strain redistribution term also plays an important role in the energy transfer between streamwise part and cross-flow part in residual field. For the streamwise part of residual field, in certain rotate rates, the energy obtained from the streamwise part of secondary flows is larger than that got from mean flow, implying that the streamwise motions of secondary flows have a significant impact on the streamwise motions of residual field.
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A fast particle simulation method for calculating the multipactor threshold based on the frequency domain solutions in microwave devices

null
Accept: 2016-10-11
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In order to compute the multipactor thresholds of microwave devices with high ef?ciency and precision, a novel fast particle-in-cell (PIC) method is proposed, which takes advantages of the frequency-domain (FD) electromagnetic field solver of CST Microwave Studio (MWS). At the initial stage of multipactor (when there are not many electrons in the devices), the self-consistent field generated by the electrons is much smaller than the applied electromagnetic field. Therefore it can be ignored in calculating the multipactor threshold and this will significantly reduce the computation burden. During simulations of multipactor processes, the FD fields pre-calculated by CST MWS are converted into time-domain (TD) scaling with the square root of the input power. Then the electrons are advanced by Boris algorithm. When the electrons hit the boundaries of the simulation region, where triangular facets from CST are used for discretization, the secondary electrons would be emitted. After series of simulations with variable input powers, the multipactor threshold is determined according to time evolutions of the electron number. As verifications, the multipactor thresholds in a parallel plate and a coaxial transmission line are investigated. Compared with the results of CST Particle Studio (PS), the fast method obtains almost the same thresholds, while the computational efficiency is improved more than 1 order of magnitude. Since the self-consistent field generated by the electrons is ignored in the fast method and it is considered in CST PS, the results validate that the self-consistent field can be ignored in calculating the multipactor threshold. Finally, taking a parallel plate transmission line and a stepped impedance transformer as examples, we studied the effects of the number of initial macro-particles on the calculation precision. When the initial particles are so few that it can hardly reflect the randomness of the multipactor process, it results in a higher calculated value. With the increase of the number of initial macro-particles, the calculated multipactor threshold is lower and more accurate. It is convergent when the number reaches about 2000 for the parallel plate transmission line and 4000 for the stepped impedance transformer, respectively. Taking into account other microwave devices with more complex electromagnetic field distribution, in order to ensure precision, it is recommended to select the number of initial macro-particles 8000. In addition, although CST MWS was used to obtain the electromagnetic fields and boundary information in this paper, of course, other electromagnetic software (such as HFSS) can also be adopted as an alternation.
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The effect of linear bubble vibration on wave propagation in unsaturated porous media containing air bubbles

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Accept: 2016-10-11
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Biot model is widely applied in geophysics, petroleum engineering, civil engineering and ocean engineering since it has been presented. This leads to a considerable development of the research on the wave propagation in saturated porous medium. However, fully saturated porous medium is rarely found in nature, almost all the rocks or soils contain two kinds of fluid, such as gas and petroleum. So many researches has been done on the wave propagation in unsaturated porous medium by domestic and abroad scholars. It is well known that the presence of a small volume of gas bubbles in a liquid can greatly alter the velocity and attenuation of acoustic waves in the liquid. Evidence is beginning to accumulate that the velocity and attenuation of acoustic waves in a saturated marine sediment can be affected by the presence of gas bubbles in the saturating liquid. To investigate the sound propagation in porous media when the pore water contains a small amount of air bubbles, this paper integrates the volume vibration of bubbles in pore water into the continuity equation of pore-fluid filtration in porous media based on Biot theory, so as to obtain the continuity equation of pore-fluid filtration with bubble volume vibration. On this basis, according to the relationship between the instantaneous radius of bubbles and the background pressure of the medium under the linear vibration of bubbles, as well as the equations of motion of the fluid medium and porous medium, a new displacement vector wave equation of porous media under the influence of bubbles is derived, which establishes the model for the sound velocity dispersion and attenuation prediction under the unsaturated porous media. The presence of air bubbles increases the compressibility of pore fluid, which leads to the decrease in the sound velocity of the bubbly saturated porous media. When the wave frequency equals to the resonance frequency of the bubbles, the bubbles in pore water will produce resonance; the medium will present to be highly dispersive and the velocity can greatly exceed the gas-free velocity, but these have not been measured in field data; and the absorption cross section of the air bubble can reach the maximum, which leads to the maximum attenuation of the porous media. It should be noted that the attenuation coefficient calculated with this model is related to the damping of bubble motion(radiation, thermal and internal friction) and the dissipation of the relative motion between the pore water and porous solid frame. The obtained numerical analysis is consistent with the above conclusions, which indicates that the volume concentration, the bubble size and the excitation frequency of sound field are important parameters affecting the sound wave propagation in the saturated porous media containing few bubbles.
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Ferroelectric phase transition of perovskite SnTiO3 based on first principles

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Accept: 2016-10-11
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Due to their spontaneous polarization, ferroelectric materials have excellent dielectric, piezoelectric, pyroelectric and other properties, which enable them to be used in many applications, such as capacitors, filters, sensors, detectors, and transducers, among others. In this paper, we employ a first-principles-based effective Hamiltonian method to investigate perovskite SnTiO$_3$, obtaining essential coefficients for the effective Hamiltonian via ab initio computations, which are used in subsequent Monte-Carlo simulations to predict the phase transition temperature of SnTiO$_3$, and different structural phases involved in such phase transition.
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Nonlocal Symmetries and Interaction Solutions of the (2+1)-dimensional Higher Order Broer-Kaup System

xiangpeng xin Hanze Liu Xi-qiang LIU
Accept: 2016-10-11
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The (2+1)-dimensional higher-order Broer-Kaup (HBK) system is studied by nonlocal symmetry method and consistent tanh expansion (CTE) method. In this paper, via the localization of the residual symmetries, the nonlocal symmetries are localized to Lie point symmetries and symmetry groups are also obtained. Many types of soliton solutions and interaction solutions among different nonlinear excitations such as solitons, periodic waves etc. are constructed. In order to study their dynamic behaviors, corresponding images are explicitly given.
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Development of a intranuclear-cascade code CBIM applicable to the nuclear reaction with incident particle energy above 45MeV

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Accept: 2016-10-11
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The Monte Carlo intra-nuclear cascade program CBIM has been developed for describing nuclear reactions involving protons, neutrons and pions on complex nuclei. In order to describe cascade process, several simplifications have been made in the following: firstly, neither reaction, reflection, refraction, nor ionization will be taken into account before the incident particle enters the target nucleus; secondly, target nucleus is regarded as spherical and the atom number should be greater than 2; thirdly, the knocked nucleon is determined by cross section sampling; last, in the center-of-mass frame, the scattering angle is sampled based on differential cross section distribution.. The basis physics model bases on the above assumptions and Bertini intra-nuclear cascade model; meanwhile, nucleon-nucleon angle differential distributions of INCL in the center-of-mass frame have been introduced to overcome the shortage of Bertini model. The interactions between nucleon and nucleon or between nucleon and pion, for example, elastic scattering, pion production and charge exchange, are simulated in the code. In the particles collision, the nucleon density changes with the target nucleus radius; and the interaction cross sections refer to 22 kinds of experimental cross sections in Bertini model. The intra-nuclear cascades induced by 45MeV~3500MeV neutron, proton or pion below 2500MeV can be simulated by this code. Finally, comparisons with experiment on reaction cross section over the energy range 60~378MeV, and some simulation results by MCNPX, GEANT4 and PHITS over the energy range 65~3000MeV, the CBIM results are in reasonable agreement with them over the broad energy range considered.
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Omnidirectional photonic bandgap of the one-dimensional plasma photonic crystal based on a novel Fibonacci quasiperiodic structure

Accept: 2016-10-11
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Take the binary one-dimensional plasma photonic crystal based on Fibonacci quasiperiodic structure as an object, on the basis of the photonic bandgap characteristics of the structure with different initial sequence and number of period, a novel structure of one-dimensional plasma photonic crystal is proposed in this paper to enlarge the omnidirectional photonic bandgap (OPBG). Compared with previously reported structures in literatures, this structure is simpler in configuration with fewer layers and materials, and its OPBG width is wider. The influence of the parameters of the plasma material, such as the thickness, plasma frequency and collision frequency, on the OPBG characteristics of this structure is systematically discussed and compared with that of the structure in literatures. The research results can provide important theoretical guidance for the design of novel omnidirectional reflectors.
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Quantum secure direct communication protocol based on the mixture of Bell state particles and single photons

Zheng-Wen CAO
Accept: 2016-10-11
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By studying the properties of the mixture of Bell state particles and single photons, the paper designs a quantum code scheme with high coding capacity, and proposes a novel quantum secure direct communication protocol with high transmission efficiency. Alice prepares Bell state particles and single photons, and divides Bell state particles into two sequences $S_A$ and $S_B$. $S_B$ is sent to Bob for the first security check using quantum correlation properties of particles. When the check result shows that the quantum channel is safe, using designed quantum code scheme, Alice encodes her classical message on the mixed quantum state sequence of Bell sequence $S_A$ and single photon sequence $S_S$. Then, some single photons that are used for security check are re-inserted randomly into the encoded sequence, and the order of particles is rearranged to ensure to check Eve's attack. Alice sends the new sequence to Bob. Bob delays and receives it. And then, the quantum channel is conducted security check for the second time. The transmission error rate is calculated, if the error rate is lower than the tolerance threshold, the channel is safe. Bob decodes and reads Alice's message. The first security check is to determine whether quantum channel is safe. The second security check could test whether there are eavesdroppers during information transmission. Safety analysis is done by using quantum information theory to the proposed protocol. The error rate introduced by Eve and the amount of information by Eve are calculated. It is showed that this protocol can effectively resist measurement-resend attack, intercept-resend attack, auxiliary particle attack, denial of service attack and Trojan attack. Among them, auxiliary particle attack is analyzed in details. The transmission efficiency and coding capacity are also analyzed. The transmission efficiency is 2, the quantum bit rate is 1, and the coding capacity is that a quantum state can encode three bits of classical messages. We also compare the proposed protocol to many existing popular protocols in terms of efficiency, e.g., Ping-Pong protocol, Deng,F.G. et al.'s Two-step and One-pad-time quantum secure direct communication protocol, Wang,J. et al.'s quantum secure direct communication protocol based on entanglement swapping and Quan,D.X. et al.'s one-way quantum secure direct communication protocol based on single photon. It is proved that this proposed protocol has higher transmission efficiency. In addition, complex U operation and entanglement swapping are not used, and implementation process is simplified. However, this protocol is devoted to theoretical research of quantum secure direct communication. There are still some difficulties in the practical application. For example, the storage technology of quantum states is not mature at present. It is not easy to prepare and measure Bell state particles and combine them with single photons, and so on. The implementation of this protocol depends on the development of quantum technology in the future.
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The Relationship between Dielectric Properties and Nanoparticle Dispersion of Nano- SiO2/Epoxy Composites

null
Accept: 2016-10-11
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Nano-SiO2 was modified by silane coupling agent and modified nano-SiO2 powder and nano-SiO2 dispersing liquid was obtained. Unmodified and modified nano-SiO2/Epoxy composites made by “mechanical mixing method”, and modified namo-Silica/Epoxy composites made by “bubble mixing method” were prepared, respectively. The content of nano-SiO2 in the composite is 2wt%, 3wt%, 4wt%, 5wt% and 6wt%. Breakdown strength and corona-resistance characteristics of the composites were tested. The results show that, with the increase of nano-SiO2 loading, the breakdown strength and corona-resistance of nano-SiO2/Epoxy composites increase. The maximum breakdown strength of namo-Silica/Epoxy composites was appeared when the nano-Silica content is 5wt%. The SEM images of 5wt% nano-Silica loading composites were analyzed by Software Image J, and the Morisita’s Index method was used to evaluate the dispersion of nano-Silica particles in the matrix quantitatively. The best dispersion was found in the composites made by “bubble mixing method”. The relationship between dielectric properties and nano-particle dispersions of nano-Silica/Epoxy composites was discussed.
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Combined noise source identification method based on spherical microphone array with random unifrom distribution of elements

null
Accept: 2016-10-11
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As the developing of techlology, noise controlling is paied wide attention in recent years. Noise source identification is the key step for noise controlling. Spherical microphone array, which can located the noise source of arbitrary direction in three dimensional space, is widely used for noise source identification in recent years. Conventional methods for noise source localization include spherical near field acoustic holography and spherical focused beamforming. The acoustic quantities are reconstructed by using spherical near field acoustic holography method to realize the noise source identification, while the noise source can also be located by using focused beamforming based on spherical harmonic wave decomposition. However, both these methods have their own limitations while being used in noise source identification. Spherical near field acoustic holography has low resolution in high frequency with far distance from noise source to measurement array for noise source identification, whereas the spherical focused beamforming has low localization resolution in low frequency. Noise source identification is discussed here and a 64-element microphone spherical array with randomly uniform distribution of elements is designed. The combination methods of noise source identification by using spherical near field acoustic holography and mode decomposition focused beamforming are researched. The performance of the proposed combination methods is simulated, and an experiment of noise source identification is carried out based on the designed spherical microphone array to test the validity of proposed method. The dividing frequency point is when selecting noise source identification methods between near field acoustic holography of spherical wave decomposition by using the spherical array designed in this paper. Research results show that high resolution of noise source identification can be obtained by using near field acoustic holography when reconstruction frequency is with a distance from noise source to the center of spherical array, while high resolution of noise source localization can be achieved by using spherical wave decomposition beamforming when signal’s frequency is with a distance from noise source to the center of spherical array. Spherical array with random uniform distribution of elements maintains stable identification ability in all bearing. Spherical near field acoustic holography has high resolution distinguish ability in near field and low frequency, while focused beamforming method has high resolution distinguish ability in far field and high frequency. Therefore the noise source can be efficiently identified by using the proposed combined method of near field holography and focused beamforming with less elements and small aperture spherical microphone array.
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Optimization design of a Gamma-to-Electron spectrometer for high energy gammas induced by fusion

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Accept: 2016-10-11
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Apart from neutrons, the fusion core produces gamma rays during one fusion reaction. The spectrum of gamma ray can provide very important information for fusion diagnosis. However, due to the gamma energy and yield in one fusion pulse, the gamma spectrometer used should have high detection efficiency and energy resolution. The concept of a Gamma-to-Electron magnetic spectrometer GEMS provides the idea to build up such a spectrometer to meet this requirement. Based on this concept design, four important parts of this facility are investigated. The first part is the gamma-electron converter. The main physics processes include Compton scattering of gamma ray with converter material generating electron, the electron Multiple Coulomb scattering (MCS) inside the converter and the electron attenuation. Affected by the thickness of convector, these processes gives a complex influence on the detection efficiency and angular-energy distribution of the electrons which are emitted from the downstream face of the convector. The Monte Carlo code Geant4 is employed to investigated the functions of Compton scattering, MCS and converter thick on the angular-energy distribution. The second one is the collimation. The collimation is used to select the forward direction election, the performance of cutoff angle of the collimator on the detection efficiency and resolutions, as well as the correlation between electron transportation direction and energy, are also studied using Geant4 code. The third part is the dipole magnetic field. There are several parameters of geometric and magnetic, therefore, a multi-thread parallelized Genetic algorithm is developed to get the best result. Both the irregular geometry (shape) and dipole magnetic field strength are optimized to achieve the best energy resolution and detection efficiency. The obtained magnetic field has intensity less than 100 Gauss, and its performance on gathering elections is also verified by Geant4 code. The last one is the location of electron detectors. The study shows that all the electron detectors should be located according to not a straight line but a quadratic curve. Then the optimized spectrometer is simulated by Geant4 to get the responses of gamma rays with various energies. For the gammas provided by fusion reaction, the simulation shows that when the neutron yield is about 2.5×1015 and 1.2×1016, the energy resolution reaches 0.5 MeV and 0.25 MeV, respectively, provided that different thick Be converters are employed. All in all, this optimized GEMS can be employed to measure the spectrum of gamma rays generated by the fusion reaction.
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Influnence of Nonspherical Effects on the Secondary Bjerknes Force in a Strong Acoustic Field

null
Accept: 2016-10-11
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The secondary Bjerknes force between bubbles in an acoustic field is a well-known acoustic phenomenon. The major theoretically researches of the secondary Bjerknes force were owing to two spherical bubbles. The secondary Bjerknes force between two spherical bubbles which is calculated based on the linear equations is very small and negligible, therefore these theoretically researches did not give a well explanation for the phenomenon, such as “streamer formation” and Multi-bubble sonoluminescence (MBSL). Experiments of sonoluminescence (SL) show that bubbles in a sound field are not entirely spherical bubbles. Nonspherical effects have an important influence on the secondary Bjerknes force when two bubbles come close to each other in a strong acoustic field (>1.0×〖10〗^5 Pa). How does the shape distortion of a nonspherical bubble cause the change of the secondary Bjerknes force between two bubbles, and the secondary Bjerknes force how to affect the oscillation and movement of bubbles are major problems which we wish to solve. The of the secondary Bjerknes force between a nonspherical bubble and a spherical bubble is obtained by considering the shape oscillation of a nonspherical bubble. We numerical simulate the secondary Bjerknes force between a nonspherical bubble and a spherical bubble based on the nonlinear oscillation equations of two bubbles, and compare the secondary Bjerknes force between a nonspherical bubble and a spherical bubble to the secondary Bjerknes force between two spherical bubbles in the same condition. We discuss the influence of nonspherical effects on the secondary Bjerknes force between two bubbles. The results show that when the amplitude of driving pressure is greater than the Blake threshold of a nonspherical bubble and makes the bubble oscillate stably, the secondary Bjerknes force between this nonspherical bubble and a spherical bubble is different to the secondary Bjerknes force between two spherical bubbles in direction and magnitude. The secondary Bjerknes force between a nonspherical bubble and a spherical bubble is much bigger than that of two spherical bubbles. The interactional distance of the secondary Bjerknes force between a nonspherical bubble and a spherical bubble is further than that of two spherical bubbles. The secondary Bjerknes force between a spherical bubble and a nonspherical bubble depends on the radii of two bubbles, distance between two bubbles, shape mode of the nonspherical bubble and the amplitude of driving pressure. Our research is more close to the actual bubbles in liquid. We also prove that big mutual interaction between bubbles is mainly cause for the formation of a stable structure between bubbles. For bubbles, big mutual interaction causes the cavitation become easier. These results are important to explain the phenomenon in an acoustic field, such as “streamer formation” and Multi-bubble sonoluminescence (MBSL).
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The Principle and Application of Diagonal Reducing Method in the Complex Noise Fields

null
Accept: 2016-10-11
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Acoustic environment has low signal-to-noise ratio (SNR); hence, array signal processing is always used for noise reduction and signal enhancement. Because the delay-and-sum beamforming method performs robust, so it is almost widely used, but the array gain is limited by the array aperture. The actual underwater ambient noise is complex, which includes uncorrelated noise and correlated noise. The noise power of each array element is unequal. The noise covariance matrix is not a scaled identity matrix. Consequently, the performance of array signal processing method decreases obviously. Aiming at these two problems, the diagonal reducing method of the covariance matrix in the complex noise fields is proposed. Firstly, a reducing matrix, which is defined as a diagonal matrix with unequal diagonal elements, is subtracted from the covariance matrix so as to reduce the noise, and a new matrix is obtained. Secondly, the delay-and-sum beamforming is done by using the new matrix to obtain the beaming output. The analytic solution and approximate solution of reducing matrix are obtained under the constraint condition that the output SNR attains its maximum. Thirdly, the estimation of the reducing matrix is determined by minimizing the function that is defined as the error between the covariance matrix and the estimated covariance matrix. This minimization problem is accomplished in an iterative method. Fourthly, if the noise is uniform white noise or the nonuniform white noise, this proposed method performs well. While, under the complex noise field the performance of the proposed method may be deteriorated. So the effects of the correlation of the noise field and the input SNR on the estimated error is analyzed. In fact, the weaker the correlation is, or the smaller the input SNR is, the smaller the estimated error is. Lastly, the simulation experiment and the lake trial are implemented. The simulation results show that the diagonal reducing method of the covariance matrix reduces some ambient noise, the noise output power is decreased, the output SNR is increased, and the proposed method improves performance of array signal processing. The experimental results show that the output SNR of the target using the proposed method is increased by about 14 dB. The diagonal reducing method of covariance matrix has definite value to engineering application, and is computationally attractive.
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Moving target compressive imaging based on improved row scanning measurement matrices

null
Accept: 2016-10-11
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Abstract: Moving target imaging (MTI) plays an important role in practical applications. How to capture dynamic images of the targets with high quality is a front-burner issue in the field of MTI. In order to improve the reconstruction quality, a new MTI model based on compressed sensing (CS) is proposed here, applying a sampling protocol of the row-scanning together with a motion measurement matrix constructed by our own. It is proved by the simulation and the experimental results that a relatively higher quality can be achieved through this approach. Furthermore, an evaluation criterion of reconstructed images is introduced to analyze the relationship between the imaging quality and the moving speed of the target. By contrast, the performance of our algorithm is much better than that of traditional CS algorithms under the same moving speed condition. As a result, it suggests that our imaging method may have a great application prospect in the earth observation of unmanned aerial vehicles, video monitoring in the product line and other fields.
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Spatial Correlation of Underwater Bubble clouds Based on Acoustic Scattering

null
Accept: 2016-10-11
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Using effective medium theory to describe acoustic scattering from bubble clouds, one of the underlying assumptions shows that the probability of an individual bubble being located at some position in space is independent of the locations of other bubbles. However, bubbles within naturally occurring clouds are usually influenced by the motion of the fluids which makes they become preferentially concentrated or clustered. According to Weber’s method, it is a useful way to importing spatial correlation function to describe this phenomenon in bubble clouds. The spatial correlation function is contained in acoustic scattering and it is important to notice that the spatial correlation should be dependent of the position and radius of each bubble due to the ‘‘hole correction’’ or the effect of the dynamics of the fluids. Because of these reasons, it is hard to invert the spatial distribution of bubble clouds using spatial correlation function in acoustic scattering. A method is described here in which bubble clouds are separated into many small subareas and the conception called effective spatial correlation function which is the statistic of spatial correlation function used to describe the correlation between each subarea of bubble clouds. Since the effective spatial correlation function is independent of bubbles’ radius and positions, the bubble clouds’ distribution and the trend of clustering can be inverted by using this function. The result of simulation indicates that the effective spatial correlation function can precisely track the position of the clustering center, even the clustering center covered by other bubble clouds can be detected. Using multi-bean sonar measuring the bubbly ship wake generated by a small trial vessel, the method is used to invert the spatial distribution and clustering centers of bubble field in the ship wake. The results show that effective spatial correlation function accurately inverts the distribution and clustering centers of bubbles in ship wake. Furthermore, the method presented in this paper could distinguish the bubble clouds caused by different reasons and detect upper ocean bubble clouds covered by other bubbles generated by wave breaking.
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Uncertainty Quantification in the Calculation of keff Using Sensitity and Stochastic Sampling method

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Accept: 2016-10-11
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In the neutronics simulation of nuclear reactor, the uncertainties associated to the integral parameters due to the uncertainties in nuclear data are usually quantified using the sensitivity and uncertainty (S/U) analysis method based on the perturbation theory. S/U analysis method is only applicable to the linear model, moreover neutronics code generally can not be directly used in sensitivity analysis. Sampling approach, which evaluating the uncertainties by performing a set of stochastic simulations, is easy to implement and the uncertainties quantified is close to exact. The function of uncertainty quantification based on sampling approach have been added to uncertainty analysis code SURE. Before applying the sampling method to the uncertainty quantification in the simulation of complex problems, it is necessary to carry out a careful verification. The uncertainties of the calculated effective neutron multiplication factor keff for two selected simple critical benchmark experimental model are quantified using SU method and sampling method respectively. The keff uncertainties due to all nuclides and reaction types nuclear data quantified by two methods are in good agreement, and the correctness of the sampling function of SURE code is verified. The keffs distributions from sampling method obey normal distribution, which embodies a linear relation between input nuclear data and output keff in the range of the uncertainty range of nuclear data, and sensitivity analysis method is adaptable to quantify uncertainty of calculated keff.
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A super-resolution infrared microscopy based on a doughnut pump beam

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Accept: 2016-10-11
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This paper presents an approach to break through the diffraction limitation in infrared microscopies. In this method, instead of Gaussian pump beam, an intensive vortex beam is firstly focused on the sample, leading to saturation absorption of the peripheral molecules in the point spread function (PSF). The vortex beam is followed by a Gaussian beam with the same wavelength, which can only be absorbed by the molecules near the center, resulting in shrunken PSF which means higher resolution. Furthermore, the PSF of a system based on this approach is numerically simulated. With an 100 nJ pulse energy vortex beam and a 0.1 nJ pulse energy probe beam, the theoretical resolution (full width at half maximum, FWHM) is measured to be about 236 nm which is 14 times better than that of the traditional infrared microscopy.
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Fast Bayesian Blind Restoration for Single Defocus Image with Iterative Joint Bilateral Filters

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Accept: 2016-10-11
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It is significant to realize effective defocus image restoration for acquiring clear image in military and geological examination field. Most of existing algorithms have the problems of large computational cost, ringing and noise sensitivity, hence a novel approach by iterative joint bilateral filtering under Bayesian framework is proposed. Firstly, it utilizes defocus image depth estimation to compute the point spread function in the Bayesian framework. Then a minimum optimization problem is built to represent the blind restoration problem. After inferring the solution procedure of the minimum optimization problem, we find that the joint bilateral filters can be used to search the optimal solution, which not only simplify the searching procedure but also reduce the computational cost. Finally, an iterative joint bilateral filtering was designed to realize the image restoration. That means the original restored image obtained from the bilateral filtering is used to design the guide image for the joint bilateral filters, and the guide image will serve as the input of the optimization problem for acquiring the better optimal result. This procedure was repeated until convergence. The experiment results indicate that this method can yield the ringing, reduce the computational cost and remove the noise. Generally speaking, the average pixel error of 85% images is under 0.03, which has improved 19% comparing with the same error rang of existing algorithms. And 78% shorter than those of compared algorithms. It can be used in the engineering practice of blind restoration for single defocus image.
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First-principles study on the thermodynamic stabilities and electronic structures of long-period stacking ordered phases in the Mg-Y-Cu alloys

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Accept: 2016-10-11
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A first-principles method based on density functional theory has been used to investigate thermodynamic stability and electronic characteristics of long-period stacking ordered (LPSO) phases 14H and 18R (18R(m),18R(t)) in Mg–Y–Cu alloys. The present calculations are performed using Vienna Ab-initio Simulation Package (VASP) with projector augmented plane wave pseudopotential, and generalized gradient approximation is used to treat with and describe the exchange-correlation interaction. The plane wave cutoff energy is set to 360 eV, the forces on all the atoms is less than 0.02 eV/?. The calculated negative enthalpies of formation show that both 14H and 18R can exist in Mg–Y–Cu system, 14H and 18R are stable with respect to the Mg, Cu and Y elements, the reaction energies indicate that 14H is more stable than 18R. The density of states (DOS) of these phases reveals that the main bonding peaks of 14H is located at energy range between -6.82 eV and 2.09 eV, those of 18R(m) at energy range between -6.82 eV and 2.02 eV, and 18R(t) at energy range between -6.82 eV and 1.98 eV. The Cu 3d orbits, Y 4d orbits, Mg 3s and Mg 2p orbits are broadly distributed in the entire region, while Cu 4s orbits, Y 4s and Y 4p orbits are very weak in whole region. For 14H,18R(m) and 18R(t) phases, the bonding originates mainly from the valence electrons of Mg 3s, Mg2p, Cu 3d and Y 4d orbits. The presence of pseudogap indicates that the bonds in 14H and 18R phases are noticeable covalent. In addition, the charge density on (0 0 0 1) plane of 14H and 18R phases are analyzed, and the results indicate that the Cu-Y bonds exhibits covalent feature in 14H and 18R, the covalent bonding of 14H phase is stronger than that of 18R phase.
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A broadband low-frequency sound insulation structure based on two-dimensionally inbuilt Helmholtz resonators

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Accept: 2016-10-11
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A man-made acoustic structure with broadband low-frequency sound insulation property is designed based on circularly inbuilt Helmholtz resonators. Beyond this structure, a two-dimensional quiet zone can be created. Being the same as the simulating model, an experimental structure is fabricated. Experiments are carried out to study its sound insulation properties. The experimental results are very coincident with the simulating one, which show that this structure has an excellent sound insulation effect in the frequency band of 680-1050Hz, and the maximum insulation sound pressure level can reach 41dB. Meanwhile, the distribution of the two-dimensional sound field above this structure is measured. The results point out that the range of the insulation area can be changed with the change of the incident frequency. In addition, the sound insulation effect is sensitive to the resonant state of the Helmholtz resonators. This work will be of help for designing new sound protection devices.
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Total Dose Dependence of Hot Carrier Injection Effect in the NMOS Devices

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Accept: 2016-10-11
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The equipments and devices which were long-time running in space were affected by space radiation effects and hot carrier injection effects at the same time which would reduce their optional times. Normally, the single mechanism test simulation method was used on the ground simulation test but the multi-mechanism effects was affected the space equipments and devices, included total irradiation dose effect, hot carrier injection effect, and so on. The total dose dependence of hot carrier injection (HCI) effect in the 0.35μm NMOS Devices was studied in this paper. Three samples were test with different conditions (sample 1# with TID and HCI test, sample 2# with TID, annealing and HCI test, sample 3# only with HCI test). The results shows that threshold voltage of NMOS devices with 5000s HCI test after 100krad (Si) total dose radiation shift negatively then positively during total dose irradiation test and HCI test,and it was more than the devices without radiation test. But the threshold voltage shift of NMOS devices with 5000s HCI test and 200hours annealing test after TID test was more than the devices without radiation test and lower than the devices without annealing test. That was, the parameters of NMOS device varied faster with the association effects (included total dose irradiation effect and HCI effect) than with single mechanism effect. It was indicated that the hot electrons were trapped by the oxide trap charges induced by irradiation effect and then became recombination centre. And then the oxide trap charges induced by irradiation effect reduced and became to negative electronic. The interface trap charges induced by irradiation effect were reduced and then increased and it was because that the electrons of hole-electron pairs in the Si-SiO2 interface were recombined by oxide traps in the oxide during the forepart of HCI test but then the electrons were trapped by interface traps in the Si-SiO2 interface because the electrons from source area were injected to interface during the HCI test. So the threshold voltage shift was positively due to the negative oxide trap charges and interface trap charges. The association effect was attributed to the reduction of oxide traps induced by recombination with hot electrons and the increase of the interface traps induced by irradiating trapped with hot electrons.
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Optoelectronic properties of N/B doped graphene

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Accept: 2016-10-11
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Since its discovery in 2004, graphene has attracted great attention because of its unique chemical bonding structure, which has excellent chemical, thermal, mechanical, electrical and optical properties. Due to the zero band gap material, graphene has limited its development in the field of Nano Electronics. Only expanding the band gap of the graphene can promote the application of graphene in Nano Electronics. In this paper, we constructed three models of intrinsic graphene, N-doped graphene and B-doped graphene. The energy band structures, electronic density of states and optical properties of N/B doped graphene with intrinsic graphene and different doping concentrations were studied. The absorption spectra, the reflection spectra, the refractive index, the conductivity and the dielectric function were studied. The study shows that the electronic states near the Fermi level of N/B doped graphene are mainly composed of C-2p and N-2p/B-2p orbitals, and N/B doping can induce the change of the Fermi level and the photoelectric properties of graphene. The conclusion of this paper can provide a theoretical basis for the application of graphene in optoelectronic devices.
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The study of the dynamic of the slow electrons transmitted through straight glass capillary and tapered glass capillary

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Accept: 2016-08-18
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It was found that the transmission rate of the electrons through insulating capillaries as a function of the time/incident charge is not the same as that for the ions. The question arouse that, by using the electrons, if the negative charge patches can be formed to facilitate the transmission of the followed electrons, substantiating that the so-called guiding effect works also for electrons. This study aims to observe the time evolution of the transmission of electrons through a straight glass tube and a tapered glass capillary. This would reveal the details that how and/or if the negative charge patches can be formed when the electron are being transported through them. In this work, a set of MCP/phosphor two-dimensional detection system based on Labview platform was developed to obtain the time evolution of the angular distribution of the transmitted electrons. The pulsed electron beams through a small hole with the diameter of 0.5 mm was obtained to test our detection system. The time evolution of the angular profile of 1.5 keV electrons transmitted through the glass tube/capillary was observed. The transmitted electrons are observed on the detector for a very short time and disappear for a time and then back again for both the glass tube and tapered glass capillary, leading to an oscillation. The positive charge patches are formed in the insulating glass tube and tapered glass capillary since the secondary electron emission coefficient for the incident energy is larger than 1. It is due to the fast discharge of the deposited charge, leading to an increase of the transmission rate, while the fast blocking of the incident electrons due to the deposited positive charge, leads to a decrease of the transmission rate. The geometrical configuration of the taper glass capillary tends to make the secondary electrons deposited at the exit part to form the negative patches that facilitate transmission of electrons, similar to the guiding of positive charged ions. This suggests that if the stable transmission needs to be reached for the production of the electron micro-beam by using tapered glass capillaries, the steps has to be taken to have the proper grounding and shielding of the glass capillaries and tubes. Our results show a difference for electrons in transmission through the insulating capillary from that of highly charged ions.
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Accept: 2016-08-18
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internuclear-distance-dependent ionization of H$_2^+$ in strong laser fields in a classical perspective

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Accept: 2016-08-18
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The enhanced ionization of H$_2^+$ in strong laser fields is studied by numerically simulating the classical Hamiltonian equation with the fix-nuclei approximation. The classical trajectory of the electron shows the electron gains energy from the laser field by circulating one electron, then passes through the interatomic barrier and move around the other nucleus before ionization. The ionization probability is maximum when the energy difference between the ground state and the the higher value of the interatomic barrier and outatomic Coulomb barrier is minimum. The classical calculation offers a perspective to inspect the intriguing phenomena in quantum systems.
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Partition and growth of convection patterns in Poiseuille-Rayleigh-Benard flow

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Accept: 2016-08-18
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In this paper, the Simple algorithm is used to numerically simulate the two-dimensional fully hydrodynamic equations. Partition of convection pattern , growth and the effect of horizontal flow on the characteristical parameters of different patterns in Poiseuille-Rayleigh-Benard flow are studied. The result indicated that flow zone is divided into three zones by the upper and lower critical Reynolds numbers , such as traveling wave zone, localized traveling wave zone, horizontal flow zone.and increase with reduced Rayleigh number. In the growth stage of the convection pattern, the growth process of three kinds of patterns with time is different, but the convection rolls grow all from downstream; Variation of characteristic parameters with time is also different, maximum vertical velocity and Nusselt number of traveling wave and localized traveling wave enter into the stable stage of the cycle variation after the exponential growth stage;and of horizontal flow pattern down to a stable constant after slow growth. and of three types of patterns decrease with increasing Reynold number, there are different rules in the different pattern areas. In this paper, formulas on variation ofandwith and formulas on variation ofandwithin different convection patterns are suggested.
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