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Experimental technique for multi-qubit nuclear magnetic resonance system
Pan Jian, Yu Qi, Peng Xin-Hua
Acta Physica Sinica, 2017, 66 (15): 150302
High efficiency cross-polarized wave filter for non-vacuum transmission
Li Rong-Feng, Xue Xing-Tai, Zhao Yan-Ying, Geng Yi-Xing, Lu Hai-Yang, Yan Xue-Qing, Chen Jia-Er
Acta Physica Sinica, 2017, 66 (15): 150601
Butt-joint design in a uni-traveling carrier photodiode array monolithic with an arrayed waveguide grating by the selective area growth technique
Ye Han, Han Qin, Lü Qian-Qian, Pan Pan, An Jun-Ming, Wang Yu-Bing, Liu Rong-Rui, Hou Li-Li
Acta Physica Sinica, 2017, 66 (15): 158502
Current Issue Accepts In Press Earlier Issues Top Downloaded SCI Top Cited
  Acta Physica Sinica--2017, 66 (15)   Published: 05 August 2017
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CONDENSED MATTER: STRUCTURAL, MECHANICAL, AND THERMAL PROPERTIES

Stress mechanism of Y1-xGdxBCO thin film with Gd substitution prepared by F-free metal organic deposition method

Chen Zhen-Ni, Liu Sheng-Li, Wang Hai-Yun, Cheng Jie
Acta Physica Sinica. 2017, 66 (15): 156101 doi: 10.7498/aps.66.156101
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The plasma discharge channel in three-dimensional helical shape induced by pulsed direct current (DC) discharge without external stable magnetic field is discovered experimentally. It can be observed by intensified charge-coupled device camera that a luminous plasma structure fast propagates along a helical path in the form of guided streamer (ionization wave). And the propagation of the streamer is stable and repeatable. We take this streamer which propagates along the helical discharge path as the study object, and explain its mechanism by constructing an electromagnetic model. The result shows that the helical shape plasma plumes can exhibit two different chiral characteristics (right-handed and left-handed helical pattern). While the discharge parameters such as pulse frequency, boundary condition, etc. can all affect the propagating characteristics of helical streamers. The electromagnetic radiation driven by pulsed DC power inside the dielectric tube which forms the wave mode is an important source of the poloidal electrical field. The helical steamers form when the poloidal electrical field is close to the axial electrical field. The velocities of the propagation in poloidal and axial direction are estimated respectively, and the hybrid propagation modes involving the interchangeable helical pattern and the straight-line pattern propagating plasmas are explained from the viewpoint of multi-wave interaction. Recently, the second-generation YBa2Cu3O7-δ (YBCO) high temperature superconducting materials have attracted much attention and become a hot research point. The YBCO coated conductors are widely used in transmission cables, motors, generators and magnetic energy storage systems due to their high critical current densities and high irreversible fields. To obtain high critical current, it is necessary to increase the thickness of YBCO film. However, as the thickness increases, the cracking of the film appears and the a-axis grains form, which causes the critical current density to decrease drastically, hence the critical current declines, i.e., the so called "thickness effect" appears. In order to overcome the "thickness effect", a great many of efforts have been devoted to it. It is realized gradually that the growth orientation of the c-axis can be controlled by the stress of film, which can be achieved through the substitution of Y by Gd and Sm each with a larger ionic radius. However, the systematical study of the evolution of the stress mechanism with the substitution ratio is still lacking due to the extreme complexity of the stress manipulation. Therefore, a series of Y1-xGdxBCO thin films with different substitution ratios is deposited on lanthanum aluminate substrates by the fluorine-free metal organic deposition method in order to reveal the evolution of the stress mechanism with Gd substitution. The growth orientations, microstructures and lattice vibration characteristics of the films are analyzed by X-ray diffraction, scanning electron microscopy and Raman spectroscopy. The results show that the lattice constant of the film increases and the orientation of the c-axis changes with the Gd substitution ratio for x increasing to a value less than 0.5, and the blue shift of the O(2)/O(3) mode of the Raman spectrum decreases with increasing x. For x=0.5, the blue shift of the O(2)/O(3) mode vanishes, indicating the free standing film with optimal c-axis orientation. However, with the further increase of Gd content, the film structure is deteriorated, and the performance is degraded as well. The red shift of the O(2)/O(3) mode occurs and the frequency decreases with increasing x. Our results indicate that the stress mechanism can be manipulated by controlling the content of various ionic radii in Y1-xGdxBCO films. The free standing film with optimal c-axis orientation can be obtained through adopting an appropriate substitution ratio, i.e., the ratio of m Y:Gd equaling 1:1. These results suggest that manipulation of the stress mechanism is a promising method to overcome the "thickness effect" effectively.

Simulation of spontaneous emission spectrum of degenerate Ge under large injection level

Wang Jian-Yuan, Lin Guang-Yang, Wang Jia-Qi, Li Cheng
Acta Physica Sinica. 2017, 66 (15): 156102 doi: 10.7498/aps.66.156102
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Germanium (Ge) is considered as a promising material for silicon (Si) based light source. Based on tensile strain and n-type heavy doping approaches, the light emitting efficiency of Ge can be improved. Nevertheless, due to the difficulty in introducing large tensile strain into Ge, the photoluminescence or electroluminescence of Ge is demonstrated under degenerated states currently. Traditional spontaneous emission (SE) theory deduced from Boltzmann approximation is inapplicable for this case. To accurately analyze the SE properties of Ge, the influences of strain, temperature and doping on quasi-Fermi level and subsequent SE spectrum of degenerated Ge are theoretically investigated based on Fermi-Dirac distribution model. Owing to large density of states (DOS) in heavy hole (hh) the valance band (VB) and L valley, it is found that compressive strain has a negligible effect on the quasi-Fermi level under carrier concentration of 1019-1020 cm-3, while tensile strain is of benefit to the improvement of carrier occupation levels, leading to dramatic increases of both peak and integrated intensities of SE spectra. Although the peak intensity of SE from Γ-hh transition is larger than that from Γ-1h transition regardless of strain levels in Ge, the integrated intensities of SE from Γ-hh and Γ-1h transitions are almost equal. With the increase of sample temperature, the carriers acquire lager kinetic energy, resulting in more dispersive distribution of electrons (holes) in Γ valley (VB). However, more electrons (holes) are induced into conduction (valence) band at the same time. And according to Varshini's law the energy difference between Γ and L valleys is reduced at higher temperature. Thus, both the peak and integrated intensities of the SE spectra become larger at higher temperature. It is impressive that n-type doping can greatly enhance the SE intensity compared with p-type doping irrespective of strain levels in Ge, demonstrating the significance of n-type doping in the enhancement of Ge SE. Furthermore, it is found that m factors, which can be extracted from linear fitting of log L-log Δn curves, diminish at heavier doping concentration. Under tensile strain condition, the variation of m factors for Ge SE with the sample temperature becomes less sensitive, implying that the tensile strain can effectively enhance the temperature stability of Ge SE. These results provide a significant guidance for analyzing the SE properties of degenerated Ge and other degenerated semiconductors.

Growth and carrier transport properties of highly oriented films of the semiconducting polymers via solution dip-casting

Pan Guo-Xing, Li Tian, Tang Guo-Qiang, Zhang Fa-Pei
Acta Physica Sinica. 2017, 66 (15): 156801 doi: 10.7498/aps.66.156801
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Effective control of molecular orientation and packing as well as the film texture of organic semiconductor plays a crucial role in achieving high performance of the electronic device such as high carrier mobility. Development of facile and scalable solution processing method for film deposition is one of the important routes to such a goal.
In this paper, we report on the successful preparation of the large area, macroscopically aligned film of the semiconducting polymer P(NDI2OD-T2) and PTHBDTP via an improved solution dip-coating process in which a tilted substrate is immersed in the dilute solution. Polarized optical microscopy images reveal the parallel stripe structures of both kinds of the deposited films. The chain backbones of both P(NDI2OD-T2) and PTHBDTP are highly aligned along the descending direction of solution level in the dip-coating process as indicated from polarized UV-vis spectra and X-ray diffraction measurements. Furthermore, the atomic force microscopy images of the oriented films of both kinds of polymers clearly exhibit the highly preferentially oriented nanofibril-like domains, parallel to the alignment direction of chain backbone. We elucidate the dip-coating growth process in our experiment in terms of the surface tension-and solvent evaporation-guided self-assembly of chain backbones at the substrate-solution interface near the solution surface. The influence of film texture on carrier transport property is examined by fabricating field effect transistor (FET) based on the aligned film of semiconducting polymer. The FET device of the aligned P(NDI2OD-T2) exhibits a remarkable enhancement of electron mobility by a factor of four compared with the unaligned devices, as well as a large mobility anisotropy of 19. Such a transport behavior is proposed to be attributed to the characteristic charge conducting pathways induced by chain backbone alignment in the polymeric film. In this case, fast intra-chain transport contributes to the majority of device current when the channel current is parallel to the alignment direction of the film, while charge transport will be limited severely by the inter-chain hopping within the fibrous domain and across the disordered domain boundary when the current is perpendicular to alignment direction. The facile method developed here presents a promising approach to fabricating the low-cost, high-performance organic electronic devices.

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

Electronic structure and optical absorption properties of β-AgVO3 with vacancy defects

Ren Chao, Li Xiu-Yan, Luo Quan-Wei, Liu Rui-Ping, Yang Zhi, Xu Li-Chun
Acta Physica Sinica. 2017, 66 (15): 157101 doi: 10.7498/aps.66.157101
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Semiconductor photocatalysts have received much attention due to their applications of wastewater treatment and air purification. The monoclinic β-AgVO3, which has narrow band gap (2.11 eV) and can respond to visible light, has been considered as one of the promising semiconductor photocatalysts. The vacancy defects always exist in β-AgVO3 prepared under the conventional synthesis conditions and have important influences on the structure and properties of β-AgVO3. Systematic theoretical study of the vacancy defects in β-AgVO3 is still lacking. In this paper, using density functional theory plus U (DFT+U) approach, the Ag vacancy, O vacancy and Ag-O bivacancy in β-AgVO3 are studied. The formation energy, band structure, differential charge density and optical absorption spectrum of β-AgVO3 with vacancy defects are carefully investigated. When the U values are chosen as 6 eV and 2.7 eV for the Ag-4d and V-3d electrons respectively, the reasonable lattice parameters and band gap value can be obtained for β-AgVO3. By comparing the formation energies of different Ag and O vacancies, we find that the dominating vacancy defects in β-AgVO3 are Ag3 and O1 vacancies, and the formation of Ag vacancy is much easier than that of O vacancy. The analyses of the total and partial density of states indicate that the conduction band arises mainly from V-3d orbit, and the valence band is mainly composed of Ag-4d and O-2p states for β-AgVO3. With Ag3 vacancy, O1 vacancy or Ag3-O1 bivacancy, the band gaps of β-AgVO3 all decrease in different degrees. The Ag3 vacancy behaves as p-type donor, allowing the Fermi level to shift down to the valence band maximum. However, O1 vacancy and Ag3-O1 bivacancy both act as n-type donors, and the Fermi level shifts to the conduction band minimum. The change of the Fermi level for the vacancy defect systems also means that the charge transfer occurs among the atoms around the vacancy, which is analyzed by calculating the differential charge density. The Ag3 vacancy and O1 vacancy have little effects on the light absorption of β-AgVO3 in the range of visible light, while O1 vacancy and Ag3-O1 bivacancy in β-AgVO3 cause the obvious absorption of light in the near infrared region.

Magnetic entropy change and electrical transport properties of rare earth Tb doped manganites La4/3Sr5/3Mn2O7

Sun Xiao-Dong, Xu Bao, Wu Hong-Ye, Cao Feng-Ze, Zhao Jian-Jun, Lu Yi
Acta Physica Sinica. 2017, 66 (15): 157501 doi: 10.7498/aps.66.157501
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The magnetic transition process in double-layer perovskite manganites is rather different from that in the counterpart compound with standard perovskite structure. In this paper, the magnetic phases below room temperature as well as the order of magnetic phase transition in terbium (Tb) doped La4/3Sr5/3Mn2O7 are studied by analyzing the magnetization curves, including thermal hysteresis, magnetic entropy change and its universal curve. The electrical conductivities with and without applied magnetic field are also discussed.
Both the undoped and the doped samples (La1-xTbx)4/3Sr5/3Mn2O7 (x=0, 0.025) are prepared through the conventional solid-state reaction of mixed La2O3, Tb2O3, MnCO3 and SrCO3 whose purities are all higher than 99.9%. The mixture is calcined twice at 1000℃ for 12 h. Subsequently, the compactly compressed tablet of the calcined mixture is sintered in air at 1350℃ for 24 h.
The data of X-ray diffraction show that the crystallographic structures of both samples are in the Sr3Ti2O7-type tetragonal phase with the space group I4/mmm. The refinement result indicates that the smaller radius of doped Tb3+ reduces all three lattice parameters as well as the c/a ratio, which is attributed to the preferential occupation of Tb3+ on the R site in rocksalt layer instead of the P site in perovskite layer.
The temperature and field dependence of magnetization M(T, H), are recorded using the vibrating sample magnetometer of physical property measurement system (Quantum Design). Upon reducing the temperature, both samples exhibit two magnetic phase transitions from the paramagnetic phase at high temperature to the two-dimensional shortrange-ordered ferromagnetic state at the intermediate temperature, and finally the three-dimensional long-range-ordered antiferromagnetic state at low temperature. The zero-field-cooling and field-cooling curves display the characteristics of spin-glass behavior which may be due to the competition between B-site ferromagnetic and antiferromagnetic interactions associated with the randomly distributed A-site ions.
The magnetic entropy changes of the samples are obtained through analyzing the magnetization data. The maximal magnetic entropy changes under 7 T magnetic field of the two samples are -4.60 J/(kg·K) and -4.18 J/(kg·K), respectively. The doped Tb ions reduce the transition temperatures, Tc2D and Tc3D, as well as the maximal value of magnetic entropy change, and increases the transition temperature range. The re-scaling curves of magnetic entropy change at different magnetic fields do not fall into a universal one, rather disperse in a wide interval, which suggests that the system undergoes a weak first-order transition at Tc3D. This conclusion is supported by the thermal hysteresis observed in the magnetization data.
In addition, the electrical resistivity of the doped sample can be explained by using the small polaron model, which is different from three-dimensional variable-range hopping mechanism of undoped sample. On reducing temperature, the doped sample undergoes metal-insulator transition at temperature TP about 115 K, which is different from the undoped sample that shows the shoulder-shaped MI transition peaks. Under finite fields, the magnetoresistance value of intrinsic nature is about 56% near Tc3D.

Magnetic properties and exchange coupling of Nd-Ce-Fe-B nanocomposite films

Sun Ya-Chao, Zhu Ming-Gang, Shi Xiao-Ning, Song Li-Wei, Li Wei
Acta Physica Sinica. 2017, 66 (15): 157502 doi: 10.7498/aps.66.157502
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In the early 1980 s, the soft and hard magnetic nano-two-phase permanent magnet materials were developed and exchange coupling model was put forward. Moreover, the theoretical maximum magnetic energy product could reach 120 MGOe (1 Oe=79.5775 A/m). However a great many of experimental research results are always disappointing for theoretical calculation, but previous studies have shown that there exists also a strong exchange coupling in hard magnetic phase, which can improve the magnetic property of magnet.
In this paper, nanocomposite Ta(50 nm)/NdFeB(100 nm)/Ta(2 nm)/NdCeFeB(100 nm)/Ta(2 nm)/NdFeB(100 nm)/Ta(40 nm) multilayer films with Ta underlayers and coverlayers are fabricated on Si substrates by direct current sputtering. A 50 nm Ta underlayer and a 40 nm coverlayer are sputtered at room temperature to align the easy axis of the RE2Fe14B grains to the direction perpendicular to the film plane and to prevent the magnetic film from oxidizing, respectively. The 2 nm Ta spacer layer serves as suppressing the diffusion of elements between different magnetic layers. The NdFeB and NdCeFeB magnetic film are deposited at 630℃ and 610℃, respectively, and then they are followed by in situ rapid thermal annealing at 645-705℃ for 30 min. The microstructures and morphologies of the films are characterized by X-ray diffractometry with Cu Kα radiation, atomic force microscope, and magnetic force microscope. The magnetic properties of the films are measured with vibrating sample magnetometer.
The influences of annealing temperature on magnetic property and crystal structure of the film are investigated. The results show that the magnetic property of the film improves gradually with the increase of annealing temperature, but deteriorates sharply when the temperature reaches above 695℃. When the annealing temperature is 675℃, the coercivity Hci of the film reaches 10.1 kOe and the remanence 4πMr⊥ is 5.91 kG (1 G=103/(4π) A/m), with a magnetic field applied to the direction perpendicular to the plane of the Nd-Ce-Fe-B thin film. The X-ray diffraction results show that the grains of the hard magnetic phase (2:14:1 phase) grow almost along the substrate normal (c-axis direction), of course, with a certain misorientation. Through the magnetization reversal process of the Nd-Ce-Fe-B thin film, it is found that the minimum value of Mrev moves in the direction of decreasing Mirr as the applied magnetic field increases, which is similar to the domain wall bowing model. This indicates that there is a strong local domain wall pinning in the film. Moreover, the remanence curve shows that the pinning type mechanism is indeed not dominant in the magnetization reversal process of the Nd-Ce-Fe-B thin film after annealing at 685℃. In addition, Henkel plots are also investigated in the films at different annealing temperatures. It is believed that nonzero δm is due to the interaction between particles in the magnet. It can be stated based on the measuring results that there exists a strong magnetic exchange coupling effect in the Nd-Ce-Fe-B thin film.

Excited-state dynamics of m-dichlorobezene in ultrashort laser pulses

Shen Huan, Hu Chun-Long, Deng Xu-Lan
Acta Physica Sinica. 2017, 66 (15): 157801 doi: 10.7498/aps.66.157801
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The excited state dynamics of aromatic hydrocarbon has attracted a great deal of attention due to its important role in photophysics and atmosphere chemistry. With the benefit of ultra-short laser pulses, the ultrafast phenomenon can be studied in a time resolved way. In the present work, m-dichlorobenzene, a typical model of aromatic hydrocarbon, is investigated by the femtosecond time resolved time-of-flight mass spectroscopy. In order to reveal its excited state dynamics, m-dichlorobenzene is pumped to the excited state after absorbing one 200/267 nm photon, and then ionized by absorbing 800 nm photons. Time resolved mass spectra are recorded with time of flight. At 200 nm, m-dichlorobenzene is excited to a (π, π*) state. Three decay components are observed in the transient profiles of m-dichlorobenzene ions, which correspond to three competition channels in the excited states. The first channel is an ultrafast dissociation process via a repulsive state with (n, σ*) or (π, σ*) character, and the lifetime is (0.15±0.01) ps. The second channel is an internal conversion process from the populated excited state to the hot ground state, and the lifetime of the redistribution of the internal vibration in the hot ground state is (4.94±0.08) ps. The third channel is an intersystem crossing process to the triplet state, and the lifetime is (110.09±4.33) ps. Moreover, the transient profiles of C6H4Cl+/C6H4+ display similar decay tendencies to the transient profile of parent ion, except that longer lifetime constants ((127.38±29.29) ps for C6H4Cl+, and (123.76±37.12) ps for C6H4+, respectively) are observed. It is likely that the fragment ions result from the dissociative ionization of the parent molecule. At 267 nm, m-dichlorobenzene is excited to the first excited state with (n, σ*) character. Only C6H4Cl2+ and C6H4Cl+ are observed in the two-color mass spectrum. A slow decay component (~(1.06±0.05) ns) is obtained for both the parent ion and the fragment ion. It is attributed to an intersystem crossing process from the first excited state S1 to the triplet state T1. Furthermore, the transient profile of C6H4Cl+ displays other decay components, i.e., (2.48±0.09) ps, in addition to the slow decay component. This fast decay process can be attributed to an internal conversion process from the populated excited states to the hot ground states. The present study provides a more in-depth understanding of the ultrafast excited state dynamics of m-dichlorobenzene.

Particle simulation and analysis of threshold for multicarrier multipactor

Wang Xin-Bo, Zhang Xiao-Ning, Li Yun, Cui Wan-Zhao, Zhang Hong-Tai, Li Yong-Dong, Wang Hong-Guang, Zhai Yong-Gui, Liu Chun-Liang
Acta Physica Sinica. 2017, 66 (15): 157901 doi: 10.7498/aps.66.157901
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The multicarrier multipactor is a phenomenon that can be observed in vacuum environment due to the effect of secondary electron emission. Accurate analysis of the threshold of multicarrier multipactor is crucial for the long-term reliability of high-power spaceborne microwave system, and therefore it has been attracting more and more interests in fields of high-power microwave community, plasma physics and aerospace engineering. Recently, a new mechanism of multicarrier multipactor, termed “long-term” multipactor, induced by sustained accumulation of residual electrons between successive envelope periods of multicarrier signals has received much attention. Comparing with the “single-event” multipactor induced by the electron accumulation inside a single envelop period, researchers tend to believe that the threshold of the long-term discharge should be lower. However, recent experimental results show an opposite conclusion. In this work, in order to investigate the contradiction between the experimental and theoretical studies on the thresholds of multicarrier multipactors, particle simulations are used to simulate the evolution process of the multicarrier multipactor under the same conditions and judgement criterion. The behavioral characteristics and occurrence condition for multicarrier multipactors, especially the single-event ones, are analyzed based on a power scanning analysis, and the conflicting results are effectively explained. Our simulations show that if the evolution process of a multipactor can be divided into three phases, i.e., establishment phase, critical phase and saturation phase, the experimental reflection coefficient can be corresponding to the reflection coefficient simulated in the critical phase. The simulation results indicate that the type of the multipactor discharge would depend on the configuration of multicarrier signals. For multicarrier signals with relatively narrow bandwidths, single-event multicarrier multipactors could occur in the first place at a lower threshold power. Therefore, the threshold of a long-term discharge is not necessarily lower than that of a single-event one. This conclusion is important for estimating and suppressing the multicarrier multipactors in the design of high-power spaceborne microwave components.

INTERDISCIPLINARY PHYSICS AND RELATED AREAS OF SCIENCE AND TECHNOLOGY

A novel algorithm of fifth-degree cubature Kalman filter for orbit determination at the lower bound approaching to the number of cubature points

Li Zhao-Ming, Yang Wen-Ge, Ding Dan, Liao Yu-Rong
Acta Physica Sinica. 2017, 66 (15): 158401 doi: 10.7498/aps.66.158401
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With more satellites launched into orbits during recent years, monitoring and cataloging of satellites play an important role in improving the utilization rate of space resource and alleviating the pressure of orbit resource. Groundbased radar, a kind of sensor in space surveillance system, does not consider the influences of the weather and other special circumstances. And it is a key technology in space target tracking by using the measurement data for real-time orbit determination. Due to the influence of orbital perturbation, the satellite orbital dynamic model is a nonlinear system. The optimal estimation of the orbital state can be achieved by means of nonlinear filtering based on the measured ranging, velocity and angle data with measurement noise, which is the essence of real time orbit determination and has important research value. The extended Kalman filter (EKF) and unscented Kalman filter (UKF) are most widely used nonlinear Kalman filters. However, the first-order Taylor expansion of nonlinear function in EKF degrades the filtering accuracy. And the weight value in UKF might be negative for the high-dimensional system, which may directly affect the filtering stability. As an important method in nonlinear filtering, cubature Kalman filter (CKF) has better accuracy and stability than UKF. However, CKF only has third-degree filtering accuracy. In order to improve the filtering accuracy further, some fifth-degree cubature Kalman filters are proposed, mainly including the fifth-degree cubature Kalman filter and the fifth-degree spherical simplex-radial cubature Kalman filter. The optimality of the radial integral cannot be guaranteed by using the moment matching method in these fifth-degree filters, so a high-degree cubature quadrature Kalman filter (HDCQKF) is proposed. The radial integral is calculated using the high-degree Gauss-Laguerre formula in HDCQKF. However, the aforementioned filtering algorithm leads to an increase in the number of cubature points, thereby improving the accuracy, and the number of cubature points increases polynomially with the increase of system dimension. Once the algorithm is applied to a high-dimensional system, or the processor has a relatively poor performance, it may impose a heavier computing burden, thus the real-time performance decreases. Therefore, it is necessary to study how to reduce the computational complexity of the fifth-degree filtering algorithm. In order to improve the real-time performance of orbit determination on condition that the accuracy of orbit determination is kept, a novel fifth-degree cubature Kalman filter for orbit determination is proposed at the lower bound approaching to the number of cubature points. The key problem in the nonlinear Kalman filter is to calculate the multidimensional integral in the form of “nonlinear function×Gaussian probability density function”, and the integral is approximated using a fifth-degree numerical cubature rule, in which the number of cubature points required is only one more than the theoretical lower bound. The abovementioned cubature rule is embedded into the nonlinear Kalman filtering framework, from which the update steps of the novel cubature Kalman filter are derived. Then, the equations of state and measurement for real-time orbit determination are obtained. The J2 perturbation and atmospheric drag perturbation are taken into account in the state equation, and the coordinate transformation is used to derive the nonlinear relationship between the orbital state and measurement element. The simulation results show that the proposed fifth-degree cubature Kalman filter can achieve a higher filtering accuracy than the CKF and the same accuracy as the existing fifth-degree filters, but has the fewest cubature points and the best real-time performance, which proves the effectiveness of the proposed algorithm.

Identifying the influence of GaN/InxGa1-xN type last quantum barrier on internal quantum efficiency for III-nitride based light-emitting diode

Shi Qiang, Li Lu-Ping, Zhang Yong-Hui, Zhang Zi-Hui, Bi Wen-Gang
Acta Physica Sinica. 2017, 66 (15): 158501 doi: 10.7498/aps.66.158501
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GaN/InxGa1-xN-type last quantum barrier (LQB) proves to be useful for Ⅲ-nitride based light-emitting diode (LED) in enhancing the internal quantum efficiency (IQE) and suppressing the efficiency droop level that often takes place especially when the injection current is high. In this work, GaN/InxGa1-xN-type LQB reported by the scientific community to enhance the IQE is first reviewed and summarized. Then, the influences of indium composition and thickness of the InxGa1-xN layer on the performance of LED incorporated with the GaN/InxGa1-xN-type LQB are studied. Through analyzing energy band diagrams calculated with APSYS, we find that the[0001] oriented LQB features an electron depletion due to the polarization induced negative charges at the GaN/InxGa1-xN interface. The electron depletion enhances the electron blocking effect and reduces the electron accumulation at the InxGa1-xN/AlGaN interface, leading to an improved IQE for the LED. In addition, increasing the indium composition of the InxGa1-xN layer will generate more negative interface charges, which result in further increased conduction band barrier height for the electrons and reduced electron leakage. On the other hand, for the GaN/InxGa1-xN-type LQB with a fixed indium composition, there exists an optimum thickness for the InxGa1-xN layer in maximizing the improvement of IQE for the LED, mainly because the interaction between two mechanisms co-exists when varying the thickness of the InxGa1-xN layer, i.e., the initial increase in the InxGa1-xN layer thickness will lead to an increased conduction band barrier height, which prevents electrons from leaking into the InxGa1-xN layer. However, further increasing the InxGa1-xN layer thickness to a certain value, tunneling effect will kick in as a result of the simultaneously reduced GaN thickness-the electrons will tunnel through the thin GaN layer in the LQB from the quantum wells to the InxGa1-xN layer. This will cause electrons to increase in the InxGa1-xN layer. Therefore, as a result of the interaction between the above-mentioned two mechanisms, there is an optimum thickness for the InxGa1-xN layer such that the electrons in the InxGa1-xN layer will reach a minimal value, which in turn will lead to a maximized conduction band barrier height for the AlGaN electron blocking layer and facilitate the performance of LEDs.

Butt-joint design in a uni-traveling carrier photodiode array monolithic with an arrayed waveguide grating by the selective area growth technique Hot!

Ye Han, Han Qin, Lü Qian-Qian, Pan Pan, An Jun-Ming, Wang Yu-Bing, Liu Rong-Rui, Hou Li-Li
Acta Physica Sinica. 2017, 66 (15): 158502 doi: 10.7498/aps.66.158502
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Monolithic integration of an InP-based O-band 4-channel arrayed waveguide grating (AWG) to a uni-traveling carrier photodiode (UTC-PD) array is realized by the selective area growth (SAG) technique. The passive-active buttjoint design is introduced and experimentally proved to ensure both good compatibility between the PD fabrication process and the SAG technique, and high photodiode quantum efficiency under the complex butt-joint geometry. An extended coupling layer is adopted between the AWG output waveguides and the PD mesa. The extended coupling layer length, the regrowth boundary edge position and the AWG etching edge position relative to the heterogeneous butt-joint boundary, and the refractive indices of the PD collector and coupling layer are optically simulated and optimized by a finite-difference time-domain method.
It is found that the extended coupling layer, compared with the un-extended situation, ensures a good matched optical field from AWG to PD and could reduce nearly 30% quantum efficiency loss when connecting seamlessly to the regrown InP AWG top cladding layer. A stable high efficiency around 80% is maintained within an extended layer length from 7.5 μm to 15.0 μm. The regrowth boundary edge into the coupling region will cause a drastic efficiency oscillation up to 20% period with the increase of distance. The efficiency drop is also attributed to the light scattering at the regrowth boundary edge, caused by the optical field mismatch, while the oscillation comes from the alternative light power concentration between the coupling layer and the core layer, for the light scattering is only obvious when the light power is well concentrated in the coupling layer. The AWG etching edge position deviation from the butt-joint boundary, however, exerts little influence on the PD quantum efficiency, which is believed not to bring obvious coupling loss during device fabrication.
The higher UTC-PD collector refractive index is proved to be crucial for further better optical coupling from the coupling layer to the PD, with quantum efficiency rapidly increasing from around 0.1 to 0.8 when the index is increased from 3.20 to 3.42. By comparison, the efficiency is little affected by the coupling layer refractive index from 3.34 to 3.42.All things considered, we select a 10 μm extended coupling layer, the refractive indices of both PD collector and the coupling layer to be 3.42, and align both the regrowth boundary edge and the AWG etching edge to the heterogeneous butt-joint boundary, and a PD quantum efficiency of 80% is expected.
Owing to the extended coupling layer at the butt-joint, the SAG technique facilitates the PD fabrication process. The overgrown AWG top cladding layer ridge stretches out 4.67 μm toward the PD, but not over the mesa yet, hence has little influence on the PD fabrication accuracy. The monolithic chip presents a uniform photodiode quantum efficiency of 76%, which accords well with theoretical value and confirms the butt-joint design. Central wavelengths for the four channels are 1347.0 nm, 1325.0 nm, 1308.0 nm, and 1286.5 nm, respectively. The low crosstalk level (below -22 dB) also indicates a good de-multiplexer performance.

Shape transformations of opening-up vesicles with one hole

Liang Yue-Feng, Zhang Shao-Guang
Acta Physica Sinica. 2017, 66 (15): 158701 doi: 10.7498/aps.66.158701
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So far two kinds of solutions to the problem of opening-up vesicles with one hole have been found. One is cup-like shape found by Umeda and Suezaki (2005 Phys. Rev. E 71 011913), the other is dumbbell shape with one hole, found by our group. As seen in the context of the bilayer coupling (BC) model, the former corresponds to relatively small reduced area difference Δa, and the latter corresponds to relatively large value of Δa. The relationship between these two kinds of shapes is not clear. Viewing from the angle of the cup-like shape, whether one can obtain the dumbbell shape by increasing Δa is not known. In this paper, we try to clarify this problem by solving the shape equations for free vesicles and adhesive vesicles based on the BC model. Firstly, we solve the set of Euler-Lagrange shape equations that satisfy certain boundary conditions for free vesicles. A branch of solution with an inward hole is found with the reduced area difference Δa slightly greater than 1. It is verified that the solution named cuplike vesicles, which was found by Umeda and Suezaki, belongs to another solution branch (Δa < 1) with an outward hole near Δa=1. According to this result, we make a detailed study of these two solution branches for free vesicles and vesicles with adhesion energy. We find that there is a gap near Δa=1 between the two solution branches. For Δa in this gap, there is no opening-up solution. For adhesive vesicles, the gap will move towards the right side slowly with increasing adhesive radius. In order to check whether the two solution branches can evolve into closed shapes, we also make a calculation for closed vesicles. For free closed vesicles, we find that there is only the sphere solution when Δa is exactly equal to 1 for Δp=0 (in order to comply with the opening-up vesicle, no volume constraint is imposed on it), while for adhesive vesicles there exist closed solutions in a region of Δa without volume constraint. Both studies for free vesicles and adhesive vesicles show that these two kinds of opening-up vesicles belong to different solution branches. They cannot evolve from one to the other with continuous parameter changing. And strictly speaking, they cannot evolve into the closed vesicles. With increasing Δa, the opening-up branch on the right side of the gap can evolve into an opening-up dumbbell shape with one hole via the self-intersection intermediate shapes. Another interesting result is that for adhesive opening-up vesicles, in the Δa parametric space, the solutions are folded for a solution branch, which means that there exist several shapes corresponding to the same Δa value in the folding domain. This phenomenon has never occurred in previous study of the closed vesicles under the BC model. The influences of Δa on the shape and energy of the free vesicles and adhesive vesicles are also studied.

Carrier selective contacts:a selection of high efficiency silicon solar cells

Xiao You-Peng, Gao Chao, Wang Tao, Zhou Lang
Acta Physica Sinica. 2017, 66 (15): 158801 doi: 10.7498/aps.66.158801
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Solar cell has two basic units:the photon absorption layer and the contact layer. The contact layer is a region between the highly recombination-active metal interface and the photon absorption layer. It is vital to reduce the recombination loss between the photon absorption layer and the contact layer in pursuit of the higher conversion efficiency of silicon solar cell. In recent years, carrier selective contact is arousing research interest in photovoltaic industry because it is deemed as one of the last remaining obstacles in approaching to the theoretical efficiency limit of silicon solar cell. In this paper, three different types of carrier selective contacts are analyzed, which includes:1) sandwiching a heavily doped thin layer between the photon absorption layer and the metal interface, which is the so-called emitter or back surface field; 2) aligning the conduction bands or the valence bands of two materials; 3) inducing the band bending through a high work function metal oxide contacting crystalline silicon. Based on one-dimensional solar cell simulation software wxAMPS, three different silicon solar cell structures are numerically simulated, which includes:1) diffused homojunction silicon solar cell[(p+)c-Si/(n)c-Si/(n+)c-Si]; 2) silicon heterojunction solar cell with amorphous silicon thin films[(p+)a-Si/(i)a-Si/(n)c-Si/(i)a-Si/(n+)a-Si]; 3) silicon heterojunction solar cell with metal oxide thin films[(n)MoOx/(n)c-Si/(n)TiOx], then the energy band structures and the spatial distributions of carrier concentrations of solar cells in the dark are discussed. The simulation results show that the key factor of carrier selective contacts is the asymmetric spatial distribution of the carrier concentrations, i.e. the asymmetric conductivities of electrons and holes. This leads to the formation of high resistance to electrons and low resistance to holes, or high resistance to holes and low resistance to electrons, so the holes will go through the contact easily and the electrons will be blocked simultaneously, or the electrons will go through the contact easily and the holes will be blocked simultaneously. Therefore a hole selective contact or a electron selective contact is formed, respectively.

REVIEW

Recent progress of high-coherence ultrafast electron sources

Luo Duan, Hui Dan-Dan, Wen Wen-Long, Liu Rong, Wang Xing, Tian Jin-Shou
Acta Physica Sinica. 2017, 66 (15): 152901 doi: 10.7498/aps.66.152901
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Microscopic dynamic process of material structure which determines the inherent property of substance takes place on a molecular and atomic scale. Understanding the underlying mechanisms of the various fundamental processes has always been the goal of chemistry, physics, biology and materials science. With Ahmed Zewail's pioneering work in the field of femtoscience, the time-resolved electron diffraction, combining the pump-probe and electron diffraction technique, has become an excellent tool with sufficient temporal precision to directly deliver insights into ultrafast phenomena on an atomic level. Central to this method is the ultrashort electron pulses generated from a metal photocathode.
However, up to now, owing to the initial size, effective temperature, energy dispersion and inherent coulomb repulsion of electron source, the state-of-the-art transverse coherence of conventional planar cathode photoemission source is still insufficient to resolve the complex chemical and biological organic molecules. Hence, in recent years, many efforts have focused on developing high-coherence ultrashort electron sources. The main methods include minimizing the initial beam size, weakening the space charge, reducing the effective temperature, and matching the photon energy of laser with the work function of cathode material.
In this review, we firstly summarize the history and advantages of the electron probe, secondly sketch out the figure of merit of the electron source. And then taking coherence as the main line, we review recent progress in common planar photoemission sources, and discuss the latest development of tip-based electron sources and cold atom electron sources in terms of their generation mechanisms, unique properties and research progress. Finally, the development and future applications of the diffraction technique are prospected. In general, the high-coherence length of photoelectric surface source is often at the expense of the current. The needle source can obtain the highest coherence length, but it is similar to femtosecond single-electron pulse, which must be less than one electron per pulse to eliminate the electron-electron coulomb interaction. Thus, a diffraction pattern can only be formed by accumulating millions of shots. The cold atom electron source, which has a transverse coherence greater than 15 nm and a peak brightness similar to conventional electron source's, is sufficient for some molecular systems in biochemistry. In short, with the improvement of coherence and the emergence of new electron sources, it is possible to reveal complex organic and inorganic structures, especially the dynamic behaviors of protein, and promote the understanding of nanoscale energy transport, solid-liquid and solid-gas interfacial dynamics and chemical reaction and so on. High-coherence electron sources not only serve in the diffraction experiments, but also play a key role in developing ultrafast electron microscopy, coherent diffraction imaging and ptychography.

GENERAL

Modelling and simulation of DNA hydrogel with a coarse-grained model

Wang Xi, Li Ming, Ye Fang-Fu, Zhou Xin
Acta Physica Sinica. 2017, 66 (15): 150201 doi: 10.7498/aps.66.150201
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Recently supramolecular hydrogels have become a hot research point in the field of hydrogels. As promising building block for supramolecular hydrogel, DNA has received considerable attention for its designability and excellent mechanical strength, and DNA hydrogel has shown great potential applications in biological and medical areas. To better understand the structure and property of DNA hydrogel, computational simulation is a very powerful tool to complement experimental study. However, owing to the large size of DNA hydrogel system and long time scale of self-assembly process, it is practically unachievable to simulate the system directly at an all-atom level. Coarse-grained simulations should be developed. In this article, we propose a highly coarse-grained model to investigate the mesoscopic structure of well-designed pure DNA hydrogel constructed by Y-shape DNA blocks and linear DNA linkers with sticky ends. In this model, we ignore almost all the atomic details of the building blocks and only give a coarse-grained description of their shapes, and carefully design the Lennard-Jones (LJ) interaction between coarse-grained particles in order to take into account the fact that any of the three arms of a Y block can only interact with a single linker (i.e., the bond is saturated). To design a suitable interaction, here we use a combination of LJ repulsive potential between like particles and LJ attracting potential between unlike particles. Our simulation results show that the hydrogel has two states, namely, homogeneous liquid-like state at high temperature and spongy gel-like state at low temperature. State of this system is related to the degree of cross-linking which is described by average cross-linking pair number per Y-scaffold here. We find that the pair number per Y-scaffold is positively correlated with the concentration of hydrogel blocks, which is consistent with experimental results. We also investigate the distribution of local structure by using voronoi cells, then predict the hole size of the hydrogel network. By the micro-rheology method, we then determine more precisely the value of the transition temperature to be 0.06ε/kB-0.10ε/kB, which is also consistent with experimental result. The quantitative relation between transition temperature and binding energy of sticky ends can hopefully provide guidance for the optimal design of DNA hydrogels. The qualitative and even semi-quantitative agreement between our simulation results and experimental results indicates that our coarse-grained model is a suitable and effective one for this pure DNA hydrogel system. The basic ideas of our model can be generalized to more complicated DNA hydrogel systems.

Geometrical optimization of Cu-Au-Pd clusters based on the construction of inner cores

Wu Xia, Wei Zheng
Acta Physica Sinica. 2017, 66 (15): 150202 doi: 10.7498/aps.66.150202
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The trimetallic cluster has become a hot topic in the field of basic scientific research due to its special catalytic, magnetic and chemical activities. It is very important to determine the stable structures of clusters. In order to optimize the stable structure of large size Cu-Au-Pd cluster, a modification algorithm of adaptive immune optimization algorithm based on the construction of inner cores, called AIOA-IC algorithm, is proposed. The only difference between AIOA and AIOA-IC lies in their starting structures. Instead of generating the starting structure randomly in AIOA, an inner core in the AIOA-IC method is used for generating the starting structure. Several motifs, such as decahedron, icosahedron, face centered cubic, six-fold pancake structure, and Leary tetrahedron, are randomly selected as the inner cores. The size of the inner core is determined according to the cluster size. The Gupta potential based on the second moment approximation of tight binding potential is used to describe the interatomic interaction between Cu-Au-Pd clusters, and the corresponding potential parameters, such as the cohesive energy, lattice constants, and elastic constants are obtained by fitting the experimental values. To test the efficiency of the proposed algorithm, the stable structure of Ag-Pd-Pt cluster with 60 atoms is optimized. The results show that the new structure has lower energy than the cluster reported in the literature. It can be seen that the AIOA-IC algorithm has a stronger ability to search for the potential energy surface of the Gupta potential. Furthermore, the proposed algorithm is used to optimize the stable structures of 38-atom and 55-atom Cu-Au-Pd clusters. The structures of the investigated Cu6AunPd32-n, CunAu6Pd32-n and CunAu32-nPd6 (n=1-31) clusters can be categorized into three types:five-fold, six-fold, and truncated octahedron. Moreover, it is found that the compositions of Cu, Au and Pd atoms in the trimetallic clusters affect the structural type of the cluster. However, the Cu13AunPd42-n, CunAu13Pd42-n, and CunAu42-nPd13 (n=1-41) clusters each have a structure of complete Mackay icosahedron. Furthermore, the order parameter results show that Cu, Au and Pd atoms each have a significant segregation phenomenon. For the 147-atom Cu12Au93Pd42 cluster, the structure is also of an icosahedron. The central atom is Au, and the inner shell and sub-outer shell are occupied by 12 Cu and 42 Pd atoms, respectively. The outer shell is filled with 92 Au atoms. The results show that the Cu, Pd and Au atoms tend to be distributed in the inner shell, sub-outer shell, and outer shell, respectively. This can be further explained by the results of the atomic radius and the surface energy.

Influence of atmosphere attenuation on quantum interferometric radar

Wang Shu, Ren Yi-Chong, Rao Rui-Zhong, Miao Xi-Kui
Acta Physica Sinica. 2017, 66 (15): 150301 doi: 10.7498/aps.66.150301
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There has been aroused much interest in quantum metrology such as quantum radar, due to its applications in sub-Raleigh ranging and remote sensing. For quantum radar, the atmospheric absorption and diffraction rapidly degrade any actively transmitted quantum states of light, such as N00N and M&M' states. Thus for the high-loss condition, the optimal strategy is to transmit coherent state of light, which can only provide sensitivity at the shot-noise limit but suffer no worse loss than the linear Beer's law for classical radar attenuation.
In this paper, the target detection theory of quantum interferometric radar in the presence of photon loss is thoroughly investigated with the model of Mach-Zehnder interferometer, and the dynamic evolution of the quantum light field in the detecting process is also investigated. We utilize the parity operator to detect the return signal of quantum interferometric radar with coherent-state source. Then we compare the detection result of quantum radar with that of classical radar, which proves that the quantum radar scheme that employs coherent radiation sources and parity operator detection can provide an N-fold super-resolution, which is much below the Rayleigh diffraction limit; besides, the sensitivity of this scheme can also achieve the shot-noise-limit.
Also, we analyze the effect of atmospheric attenuation on the performance of quantum radar, and find that the sensitivity is seriously influenced by atmospheric attenuation:only when the reference beam and the detection beam have the same transmissivity, will the sensitivity increase monotonically with increasing the photon number per pulse N, otherwise it first increases and then decreases with increasing N. Further, the sensivity is directly proportional to 1/√N for the first case.
In conclusion, we investigate the effects of atmospheric absorption on the resolution and sensitivity of quantum radar, and find that one can overcome the harmful effects of atmospheric attenuation by adjusting the transmissivity of reference beam to the atmospheric transmittance.

Experimental technique for multi-qubit nuclear magnetic resonance system Hot!

Pan Jian, Yu Qi, Peng Xin-Hua
Acta Physica Sinica. 2017, 66 (15): 150302 doi: 10.7498/aps.66.150302
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With the development of quantum information and quantum computation science, quantum information processor has been widely used in different areas such as quantum simulation, quantum computation and quantum metrology and so on. To make quantum computer come true, we need to increase the number of controllable qubits of the system and improve the controllability to perform complex quantum manipulation. As a good experimental testbed for quantum information processing, nuclear magnetic resonance (NMR) spin system provides rich and sophisticated quantum control methods. In recent years a lot of multi-qubit experiments have been performed on the platform and a series of experimental technologies have been developed. In this paper, we firstly explain the difficulties of multi-qubit NMR experiments. Then by focusing on the experiment of 7-qubit labelled pseudo-pure state preparation and other relevant experiments, we review the technologies in multi-qubit experiments. Using the radio frequency selective method, the inhomogeneities of the radio frequency pulses are reduced and the spectral resolution is improved. After performing 1/2 spin selective sequence, we can regard the three methyl protons in the sample of crotonic acid as a single 1/2 spin nucleus and treat the whole molecule as a 7-qubit quantum information processor. We utilize Gauss pulses, Hermite pulses, composite pulses and gradient ascent pulse engineering (GRAPE) pulses to implement basic π/2 and π rotation operations. The GRAPE pulses are calculated by subspace GRAPE program to speed up the computation greatly. The errors of the basic pulses caused by chemical shift and J-coupling evolution can be estimated by the program of pulse compilation. It divides the errors of the pulses into a series of post-errors and pre-errors. A program of sequence compilation is used to eliminate the accumulated error of the whole pulse sequence, reduce the number of pulses and optimize the experimental duration. A variety of methods of quantum state tomography have been proposed to improve the efficiency of reading out information about quantum state. As an experimental example, we combine the above experimental technologies and perform the experiment of 7-qubit labelled pseudo-pure state preparation by using the method of cat state preparation. The sequence of cat state preparation consists of three steps:encoding procedure, phase cycling and decoding procedure. We use 14 experiments to realize the phase cycling and acquire the final 7-qubit labelled pseudo-pure state. The total duration of experimental sequence is about 132 ms. All the readout spectra have the similar shapes to the theoretical expectations. Finally we give an outlook for further research in this direction.

High efficiency cross-polarized wave filter for non-vacuum transmission Hot!

Li Rong-Feng, Xue Xing-Tai, Zhao Yan-Ying, Geng Yi-Xing, Lu Hai-Yang, Yan Xue-Qing, Chen Jia-Er
Acta Physica Sinica. 2017, 66 (15): 150601 doi: 10.7498/aps.66.150601
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Development of high-peak power laser system encounters difficulties in producing the pulses with high temporal contrast. To increase the pulse temporal contrast ratio, a nonlinear filter based on crossed-polarized wave (XPW) generation is proposed. The XPW generation relies on a third-order nonlinear process occurring in a nonlinear medium, such as barium fluorite (BaF2) crystal. The XPW process is quite straightforward:a linearly polarized laser pulse is focused on BaF2 crystal positioned between two orthogonally polarizers, high power main pulses due to nonlinear polarization rotation can pass through the second polarizer, while low power unconverted pre-and post-pulses are filtered by the second polarizer. With the XPW technique, pulse contrast can be enhanced by several orders of magnitude. Furthermore, XPW spectrum can be broaden by a factor with respect to the initial spectrum. This efficient pulse cleaner presents many advantages and has proved to be a simple and reliable pulse filter operating in a double chirped pulse amplification system.
Most of previous XPW experiments utilize short focal systems or work off focus due to an intensity limit in the crystal (BaF2). These drawbacks result in a lower conversion efficiency (lower than 10%) when using a single crystal. Dual crystal setup is capable of achieving efficiency more than 20%, yet the configuration restricts the crystal separation to a millimeter level. The use of long focus lens in the XPW device is capable of reaching higher efficiency, with BaF2 crystal positioned in the focal plane. Hence for milljoule pulses, the setup distance increases to tens of meters, resulting in a complicated system and cumbersome configuration.
Considering these limitations, a compact, highly efficient and stable XPW generation using dual-lens system suitable for non-vacuum transmission is presented. The measured nonlinear accumulated phase shows little deterioration of pulse quality. With a compact dual lens system, we realize an excellent XPW conversion of above 22% (internal efficiency of 30%) with using double BaF2 crystals, while a femtosecond laser pulse can experience a spectrum broadening up to a factor of 1.78. The dual-lens configuration overcomes the crystal separation limit, and conversion efficiency exceeds 20% for a crystal separation from 13 cm to 22 cm, which is conducible to flexibility and robustness. The stability for the setup to generate shorter pulses with very high contrast or compensate for spectral gain narrowing in the preamplifier is ensured due to the dual-lens focusing system.

Design of varying f/number of cooled infrared detectors based on spherical reflecting warm shield

Chang Song-Tao, Tian Qi-Jie, He Feng-Yun, Yu Yi, Li Zhou
Acta Physica Sinica. 2017, 66 (15): 150701 doi: 10.7498/aps.66.150701
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As is well known, the f/number of a cooled infrared detector is determined by the aperture and position of the internal cold shield. Moreover, the f/number can be changed by inserting a warm shield in front of the detector. In order to reduce the stray radiation introduced by an ordinary planar warm shield, we propose a method of varying f/number of the infrared detector based on a well-designed spherical reflecting warm shield in this paper. First, an infrared radiation model is established in order to analyze the influence of the stray radiation introduced by the ordinary planar warm shield. Then the design principle of the spherical reflecting warm shield is put forward. By changing the surface shape and emission characteristics, the stray radiation introduced by the ordinary planar warm shield can be obviously reduced. Hence it is beneficial to maintain the performance of the detector effectively while the f/number is changed. To validate the proposed method, a spherical reflecting warm shield and an ordinary planar warm shield are designed to vary the f/number of a cooled infrared detector respectively. To compare the influences of the two warm shields on the cooled infrared detector, radiometric calibration experiments are conducted in a high-low-temperature test chamber. The analyses and experimental results show that the stray radiation of spherical reflecting warm shield is far less than that of the ordinary planar warm shield. Moreover, the noise equivalent temperature difference introduced by the designed spherical reflecting warm shield is lower. Therefore it is indeed better than an ordinary planar warm shield in ensuring the performance of an infrared imaging system.

ATOMIC AND MOLECULAR PHYSICS

Quantum trajectory simulation for nonadiabatic molecular dynamics

Li Xiao-Ke, Feng Wei
Acta Physica Sinica. 2017, 66 (15): 153101 doi: 10.7498/aps.66.153101
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The mixed quantum-classical (MQC) molecular dynamics (MD) approaches are extremely important in practice since, with the increase of atomic degrees of freedom, a full quantum mechanical evaluation for molecular dynamics would quickly become intractable. Moreover, in some cases, the nonadiabatic effects are of crucial importance in the proximity of conical intersection of potential energy surfaces (PESs), where the energy separation between different PESs becomes comparable to the nonadiabatic coupling. In the past decades, there has been great interest in developing and improving various nonadiabatic MQC-MD protocols. The widely known nonadiabatic MD proposals include the so-called “Ehrenfest” or “time-dependent-Hartree mean-field” approach, the “trajectory surface-hopping” method, and their mixed scheme. Among the trajectory-based surface hopping methods, the most popular one is Tully's fewest switches surface hopping approach. In this approach, the nonadiabatic dynamics is treated by allowing hops from one PES to another, with the hopping probability determined by a certain artificial hopping algorithm.
In our present work, we extend the study of a recent work on the nonadiabatic MQC-MD scheme, which is based on a view that the nonadiabatic MQC-MD actually implies an effective quantum measurement on the electronic states by the classical motion of atoms. The new protocol, say, the quantum trajectory (QT) approach, provides a natural interface between the separate quantum and classical treatments, without invoking artificial surface hopping algorithm. Moreover, it also connects two widely adopted nonadiabatic dynamics methods, the Ehrenfest mean-field theory and the trajectory surface-hopping method. In our present study, we implement further the QT approach to simulate several typical potential-surface models, i.e., including the single avoided crossing, dual avoided crossing, extended coupling, dumbbell and double arch potentials. In particular, we simulate and compare three decoherence rates, which are from different physical considerations, i.e., the frozen Gaussian approximation, energy discrimination and force discrimination. We also design simulation algorithms to properly account for the energy conservation and force direction change associated with the surface hopping. In most cases, we find that the QT results are in good agreement with those from the full quantum dynamics, which is insensitive to the specific form of the decoherence rate. But for the model involving strong quantum interference, like other nonadiabatic MQC-MD schemes, the QT approach cannot give desirable results. Developing better method should be useful for future investigations in this research area.

2000 eV X-ray laser transparent mechanism of neon atom

Feng Lei, Jiang Gang
Acta Physica Sinica. 2017, 66 (15): 153201 doi: 10.7498/aps.66.153201
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X-ray transparency occurs during the interaction of X-ray free electron laser with matter. The study of the mechanism of X-ray transparency is of great value for understanding the interaction between X-ray free electron laser and matter. In this paper, the main ionization modes from neutral neon atom till bare nucleus at different flux densities are determined based on the 2000 eV photoionization cross sections and the Auger decay rates of various neon atoms (ions), calculated by the Flexible Atomic Code program. By establishing and solving the rate equations, the formulas of the proportions of various electronic configurations of neon in the main ionization mode are obtained. The proportions of electron configurations in the main ionization modes and the atomic average photoionization cross sections at flux densities of 2000 and 10000 Å-2·fs-1 are calculated by using the formulas. The ratios of the number of hollow atoms to that of complete valence electrons at any time under different flux density laser irradiations are calculated. It is found that both the bare nuclei and the hollow atoms cause X-ray transparency, and a relatively high ratio of the number of hollow atoms to that of complete valence electrons can be achieved by choosing appropriate flux density and pulse duration.

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

Generation of ultrafast broadband small angle hundreds MeV electron bunches from laser wakefield acceleration

Li Rong-Feng, Gao Shu-Chao, Xiao Chao-Fan, Xu Zhi-Yi, Xue Xing-Tai, Liu Jian-Bo, Zhao Yan-Ying, Chen Jia-Er, Lu Hai-Yang, Yan Xue-Qing
Acta Physica Sinica. 2017, 66 (15): 154101 doi: 10.7498/aps.66.154101
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Electrons can be accelerated to a GeV level in centimeters by plasma wakefield driven by laser. With the development of chirped pulse amplification technique, the accelerating field can reach 100 GV/m. The laser driven wakefield acceleration experiments with ionization injection are carried out using 68 TW (1.7 J, 25 fs) laser and a mixture gas of 99% He and 1% N2. In experiment, the output electron beam has broadband spectrum with a maximum cut-off energy of about 290 MeV and a maximum output energy is quite stable in a certain range of laser focal positions. Two-dimensional particle-in-cell simulation is carried out. It is found that the longitudinal phase space is occupied by the continuously injected electrons and the phase space distribution is quite stable after the laser has propagated several millimeters inside plasma. This acceleration process can lead to quite stable maximum output energy of electron beam. These experiments reveal the physical nature of continuous ionization injection, which is very important for improving the performance of ionization injection.

Inversion and reconstruction of the macroscopic photometric characterization model for on-orbit space object

Hou Qing-Yu, Gong Jin-Nan, Fan Zhi-Peng, Wang Yi-Hui
Acta Physica Sinica. 2017, 66 (15): 154201 doi: 10.7498/aps.66.154201
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In the field of space object optical situational awareness by space-based optics, the current research focuses on the detecting of long distance point target, the predicting and confirming of target trajectory. It is very important to analyze the on-orbit operation status and basic physical attribution of the space object by remote imaging without any structure or texture information. The analysis method can be used effectively to support the space object status discrimination and the related decision for on-orbit maintenance. In recent years, the number of three-axis stabilization space objects in orbit has increased dramatically. In order to retrieve the optical characteristic parameters of the three-axis stabilization space object surface in a long on-orbit distance, a new method is proposed to reconstruct the macroscopic photometric characterization based on analyzing visible photometric sequence signal. Firstly, based on the principle that solar panel can receive the maximum solar radiation energy, a directing model of solar panel is proposed. Considering the structural characteristics of space object, surface material characteristics, directing characteristics of the solar panels, illumination-observation geometry and the optical system characteristics, the photometric modeling method of the space object oriented to space-based observation is improved. Secondly, the photometric model is equivalent to a two-facet model, then multi-level fusion model of bidirectional reflectance distribution function (BRDF) is used to characterize optical reflection characteristics of complex material surfaces of the space object, and the product of area and reflection corresponding to multi-level BRDF is taken as the parameter of the reconstruction model. Finally, the minimum error between the measured result and reconstruction model of photometric signal is used as the optimization goal, and the linear optimization method is established to realize the inversion of the model parameters. As the space object and the observing satellite are in the same orbit or near orbit condition, the simulating experiment of photometric sequence signal and reconstructing experiment of macroscopic characterization model of optical characteristics are carried out. The simulation result shows that the proposed photometric model can describe the dynamic characteristics of on-orbit space object more comprehensively. And using the on-orbit reconstruction method of the macroscopic photometric model, the photometric signal reconstruction accuracy achieves more than 97% in the near orbit condition. So it is demonstrated that the on-orbit reconstruction method is correct. The method can provide a solution of optical situational awareness for space object on space-based platform, and provides technical support for inversing the attitude and shape of space object.

Nanoscale surface topography imaging using phase-resolved spectral domain optical coherence tomography

Wang Yi, Guo Zhe, Zhu Li-Da, Zhou Hong-Xian, Ma Zhen-He
Acta Physica Sinica. 2017, 66 (15): 154202 doi: 10.7498/aps.66.154202
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Microscopic surface topography plays an important role in studying the functions and properties of materials. Microscopic surface topography measurement has been widely used in many areas, such as machine manufacturing, electronic industry and biotechnology. Optical interferometry is a popular technique for surface topography measurement with an axial resolution up to nanoscale. However, the application of this technique is hampered by phase wrapping, which results in a limited measurement range for this technique. Various digital algorithms for phase unwrapping have been proposed based on the phase continuity between two adjacent points. However, several significant challenges still exist in recovering correct phase with this technique. Optical coherence tomography (OCT) is a non-contact three-dimensional imaging modality with high spatial resolution, and it has been widely used for imaging the biological tissues. In this paper, we demonstrate a method for nanoscale imaging of surface topography by using common-path phase-resolved spectral domain OCT to reduce the influence of phase wrapping. The system includes a superluminescent diode with a central wavelength of 1310 nm and a spectral bandwidth of 62 nm, an optical fiber circulator, a home-made spectrometer, and a reference arm and a sample arm in common-path arrangement. The reference mirror and the sample under investigation are positioned on a same stage in order to further reduce the influence of ambient vibration. The phase difference between two adjacent points is calculated by performing Fourier transform on the measured interferometric spectrum. The phase difference distribution of the surface is obtained first. And then, the surface topography of the sample is constructed by integrating the phase difference distribution. In the traditional methods, phase wrapping occurs if the absolute value of the measured phase is greater than π. However, in the present method, phase wrapping occurs if the absolute value of the phase difference between two adjacent points is greater than π. The maximal detectable absolute value of the phase difference between two adjacent points increases from π for the traditional methods to 2π for the present method. The experimental results indicate that the present system has a high stability and the maximum fluctuation is less than 0.3 nm without averaging. The accuracy of the system is tested with a piezo stage, and the mean absolute deviation of the measured results is 0.62 nm. The performance of the present system is also demonstrated by the surface topography imaging of an optical resolution test target and a roughness comparison specimen. The experimental result shows that the present system is a potential powerful tool for surface topography imaging with an axial resolution better than 1 nm.

Single photon transport properties in the system of coupled cavity array nonlocally coupled to a Λ-type three-level atom

Hai Lian, Zhang Sha, Li Wei-Yin, Tan Lei
Acta Physica Sinica. 2017, 66 (15): 154203 doi: 10.7498/aps.66.154203
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In this paper, we discuss the transport properties of a single photon, which is in a coupled cavity array system where the two nearest cavities nonlocally couple to a Λ-type three-level atom, under the condition of ideal and dissipation, respectively. By employing the quasi-boson picture, the transmission amplitude of the single photon in an open system is investigated analytically. The system where the coupled cavity array nonlocally couples with the three-level atom demonstrates several advantages. Compared with other systems, this system has many parameters to manipulate the single photon transport properties. Moreover, the system of the coupled cavity array that nonlocally couples with the three-level atom may have a wider range of application because the single photon transmission spectrum in this system has three peaks. Furthermore, it has characteristics of its own. At the same value of Rabi frequency Ω, changing the coupling strength between the atom and one cavity of the coupled cavity array shows that there exists an fixed point where the transmission rate is always 1, and the point is corresponding to the frequency of the photon ωc-Ω. In the nonideal case, it is shown that the dissipations of the cavity and the atom affect distinctively the transmission of photons in the coupled cavity arrays. When considering only the dissipation of the atom, the atomic dissipation increases the dips of the single photon transport spectrum, while the peaks have no observable changes. When considering only the dissipation of the cavity, the peaks of the single photon transmission amplitude are diminished deeply, while the cavity dissipation does not have any effect on the dips. In addition, with both the cavity dissipation rate and the number of the cavity increasing, the photon transmission spectrum peaks decrease. A comparison of the dissipative cavity case with the dissipative atom case shows that the incomplete reflect near the peak is mostly caused by the cavity dissipation, and that the incomplete reflect near the dip is mostly caused by the three-level atom dissipation. Specifically, when considering both the atom and the cavity dissipation at the same time, the dips of the single photon transport spectrum are affected by both the atomic and the cavity dissipation. Instead, with the cavity dissipation rate increasing, the photon transmission spectrum dips are reduced. But for the peaks of the single photon transport spectrum, the dips are always determined by the cavity dissipation rate and the number of the cavity, while the atomic dissipation has no significant influence on them.

Spectral and laser properties of Nd:YSAG single crystal

Lu Wan-Cheng, Zhang Qing-Li, Luo Jian-Qiao, Ding Shou-Jun, Dou Ren-Qin, Peng Fang, Zhang Hui-Li, Wang Xiao-Fei, Sun Gui-Hua, Sun Dun-Lu
Acta Physica Sinica. 2017, 66 (15): 154204 doi: 10.7498/aps.66.154204
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In order to solve the problem of low segregation coefficient of Nd3+ ions in YAG crystal, Sc3+ ions are used to replace some Al3+ ions in YAG, so that the YAG cell becomes bigger, thereby the segregation coefficient of Nd3+ ion increases. In this paper, the spectra and laser characteristics of Nd:YSAG single crystal are studied. The (1.5 at.%) Nd3+:Y3Sc2Al3O12 (YSAG) laser crystal is grown by Czochralski method, and X-ray rocking curve shows that the crystal quality is good. The concentration of ions in Nd:YSAG is measured by an electronic probe. The segregation coefficient of Nd3+ ion is calculated to be 0.35, which is approximately twice as much as that of Nd3+ in YAG. The absorption and emission spectra are measured, which indicates that it can be pumped by an 808 nm-laser diode (LD). The strongest emission from the transition 4F3/24I11/2 of Nd:YSAG peaks at 1059 nm with an emission cross section of 1.03×10-19 cm2, and the lifetime of 4F3/2 is about 253 μs, which indicates that Nd:YSAG has roughly an efficiency equal to that of Nd:YAG, but the laser upper level lifetime is longer than that of Nd:YAG, which is more favorable for Q-switched laser operation. The 808 nm-LD is used to pump Nd:YSAG rod of 2 mm×2 mm×6 mm, the laser operation with a threshold of 0.85 W and a maximum output power of 1.1 W is realized:the laser slope efficiency is 21.1%, and the optical conversion efficiency is 18.3%. All of the above results show that Nd:YSAG single crystal is a good solid state laser material, which is more favorable for Q-switched laser operation.

Tuning characteristics of fluorescent light source by dye-doped liquid crystal filled hollow fiber

Lü Yue-Lan, Yin Xiang-Bao, Yang Yue, Liu Yong-Jun, Yuan Li-Bo
Acta Physica Sinica. 2017, 66 (15): 154205 doi: 10.7498/aps.66.154205
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The fluorescent fiber light source has been widely used in many areas, such as optical fiber communication and medical imaging, owing to its low cost and wide optical spectrum. The temperature-sensitive refractive index of liquid crystal makes it a suitable filling material used in the fluorescent light source. The existing work has investigated the filling of liquid crystal into the air holes in cladding of photonic crystal fiber. However, the photonic crystal fiber has the disadvantages of complicated craft and high cost. As is well known, the hollow fiber has the advantages of the easy preparation and low cost, but the filling of liquid crystal into the hollow fiber of fluorescent light source is rarely investigated. In this paper, we investigate that a tunable hollow fiber of fluorescent light source is filled with dye doped liquid crystals. The transmission characteristics of the fluorescent light source are theoretically analyzed. The variation in property of the B4400 fluorescent spectrum is numerically discussed with the dye molecule energy level structure theory. The numerical simulation results show that the relative refractive index is dependent on temperature. It first increases linearly with the increase of temperature and then exponentially increases rapidly till clearing point 61.9 C, finally decreases slowly to a saturated value. In order to find an optimum doping concentration, the doping-concentration-dependent fluorescence output intensity is analyzed by using the super continuum spectrum of YAG pump with a wavelength of 1064 nm. The fluorescence light intensities are amplified at three different selective dye doping concentrations, namely 0.2 wt%, 1 wt% and 2 wt% in the experiment, respectively. The highest output is obtained at the 1 wt% doping concentration, which verifies the selective fluorescence amplification property of the fluorescent source. It is also demonstrated that the transmission characteristics and the tunable range of the liquid crystal filled fluorescent light source can be adjusted by modulating the temperature in experiment. And the temperature-dependence of the fluorescence source is experimentally demonstrated by using the 1 wt% doping concentration dye-doped liquid crystal. Using a pulsed YAG pump with a wavelength of 532 nm, tunable characteristics of the fluorescent light source composed of a dye-doped liquid crystal filled hollow fiber, are studied and show that the central wavelength increases from 590 nm to 605 nm and the spectral width broadens from 228 nm to 236 nm with the increase of the temperature. The proposed fluorescent light source can be controlled by adjusting the temperature within limits. These findings will give a guidance for the practical applications of the dye doped liquid crystal based fluorescent light source, and offer a theoretical foundation for the further study of the liquid crystal filled fluorescent fiber light source.

Method and experiment of path rainfall intensity inversion using a microwave link based on nonspherical rain-induced model

Song Kun, Gao Tai-Chang, Liu Xi-Chuan, Yin Min, Xue Yang
Acta Physica Sinica. 2017, 66 (15): 154301 doi: 10.7498/aps.66.154301
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It is important to measure rainfall accurately with high spatial and temporal resolution in meteorology, hydrology, agriculture industry, environment conservation, flood warning and weather forecasting. The use of attenuated information about microwave propagation in rainfall areas to acquire surface precipitation intensity has been shown to be a practical approach to measuring rainfall in recent years. However, the inversion of a single-frequency link is based on the assumption of rainfall attenuation under a certain frequency condition. Further, obtaining parameters that comply with all rainfall events for the rainfall attenuation model is a challenge, often leading to an overestimation of the rainfall intensity. Therefore, based on extended boundary condition method and Gamma raindrop size distribution, an inversion method of the path rainfall intensity by using a microwave link rain-induced attenuation is proposed in order to improve the accuracy of rainfall measurement by microwave rain-induced attenuation. In this paper, we use the characteristics of an atmospheric attenuation model to eliminate the influence of non-rainfall-caused attenuation on the process of rainfall inversion. On the basis of scattering theory and by utilizing the Gamma raindrop size drop, we use the extended boundary condition method to calculate the characteristics of microwave attenuation for Pruppacher-Beard raindrop shape model. The correction model of rainfall effective attenuation and rainfall inversion model of line-of-sight microwave links are proposed, based on the microwave rain attenuation characteristics and raindrop size distribution statistics. In this paper, we propose 15-20 GHz inversion model of path-average rainfall intensity based on nonspherical rain-induced model by using Levenberg-Marquardt optimization algorithm. Meanwhile, we analyze the variations of parameters of rain-induced model under the condition of different temperatures. Besides, we design a line-of-sight microwave experimental system for measuring the rainfall, and the path average rain rate is inversed by rainfall inversion model, which is compared with an OTT disdrometer. The results show that the correlation coefficient of rain rate inversed by microwave link and that of disdrometer are both higher than 0.6 mostly, and the maximum value is 0.96; the error of accumulated rain amount is less than 2.47 mm, the minimum value is 0.28 mm; the relative error of accumulated rain amount is less than 1.84%, the minimum value is 0.44%. The experimental results validate the feasibility and accuracy of rainfall inversion method proposed in this paper. In addition, the experimental result reflects that rainfall intensity retrieved method based on nonspherical raindrop model has advantages over the method based on spherical raindrop model.

Acoustic focusing by thermoacoustic phased array

Liu Chen, Sun Hong-Xiang, Yuan Shou-Qi, Xia Jian-Ping, Qian Jiao
Acta Physica Sinica. 2017, 66 (15): 154302 doi: 10.7498/aps.66.154302
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Broadband acoustic focusing effect based on a thermoacoustic phased array is studied. In this work, according to the relationship between the sound velocity and the temperature, a new type of a thermoacoustic phase control unit is designed by using air with different temperatures surrounded by rigid insulated boundaries and thermal insulation films. The acoustic wave velocity could be adjusted by changing the temperature of air in the unit, and the transmitted and reflected acoustic phase delays can cover the whole 2π interval. On the basis of this thermoacoustic phased array, we design four different types of acoustic focusing lenses. By using eight or two kinds of such units, we realize the transmitted and reflected acoustic focusing effect, respectively. The results show that the thermoacoustic phased array lens has a good focusing performance in a frequency range of 4.0-15.0 kHz. In addition, the center intensity of the focal spot is much greater in the focusing lens with eight phase units, and the design method is simpler and more robust in the focusing lens with two phase units. Compared with other types of focusing lenses, the proposed focusing lens based on the thermoacoustic phased array has the advantages of broad bandwidth, high focusing performance and simple designed method. The results provide a theoretical basis and experimental reference for designing the broadband thermoacoustic phased array devices and new types of acoustic focusing lenses.

Vibro-acoustic stimulating ultrasonic guided waves in long bone

Liu Zhen-Li, Song Liang-Hua, Bai Liang, Xu Kai-Liang, Ta De-An
Acta Physica Sinica. 2017, 66 (15): 154303 doi: 10.7498/aps.66.154303
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Ultrasonic guided wave is sensitive to waveguide microstructure and material property, which has great potential applications in long cortical bone evaluation. Due to the multimodal dispersion effect, low-frequency guided wave is usually used to avoid multimode overlapping and simplify the signal processing. However, the traditional low-frequency ultrasound transducer is usually designed on a large-scale (around several millimeters), leading to relatively low-spatial resolution. In response to such a technique limit, an ultrasound-stimulated vibro-acoustic method is introduced to excite low-frequency ultrasonic guided waves. There are two excitation ways of the ultrasound-stimulated vibro-acoustic method, i.e., a single amplitude-modulated (AM) beam and confocal beam excitation. In the case of the single beam excitation, a high-frequency signal is modulated by using a low-frequency amplitude. In addition, low-frequency vibration can also be produced by a confocal transducer, where two beams are close to the center frequency and focus on a small region. In this way, the frequency difference between two beams can be selected to generate the arbitrary low-frequency excitation in a given bandwidth on the focus point. In this paper, we first introduce the theory of ultrasonic guided wave in the plate and the basic principle of ultrasound-stimulated acoustic emission. Second, the three-dimensional finite element method is used to simulate the phenomena of the low-frequency ultrasonic guided waves excited by the ultrasound-stimulated vibro-acoustic method. Two Gaussian-function enveloped tone-burst signals close to the center frequencies of 5 MHz are used to excite 150 kHz low-frequency guided wave in a 3 mm-thick bone plate. An ex-vivo bovine bone plate is involved in the experiments to test the feasibility of the proposed method. The axial transmission ultrasonic guided waves are recorded at eight different propagation distances. The time-frequency representation method is used to analyze the dispersive guided waves. The results indicate that both the two confocal beams and the single AM beam are capable of stimulating low-frequency ultrasonic guided waves in the bone plate. The first two fundamental guided wave modes, i.e., symmetrical S0 and asymmetrical A0 are observed in the bone plate. Similar spectrum can be obtained in the two different excitation ways. In the simulation and experiment, two wave packets can be separated in the distance-time diagram of the received signals. Good agreement can be found between the results of time-frequency representation and the theoretical group dispersion curves. This study can enhance the spatial resolution of measuring ultrasonic guided wave in long bone, and improve the flexibility of excitation with arbitrary frequency in a given bandwidth. The study can be helpful for developing the new clinical techniques of using low-frequency guided waves for long cortical bone assessment.

Dynamic modeling method of flexible bodies with contact/impact based on interactive mode

Wang Jian-Yao, Liu Zhu-Yong, Hong Jia-Zhen
Acta Physica Sinica. 2017, 66 (15): 154501 doi: 10.7498/aps.66.154501
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To solve the contact/impact problem of flexible bodies in arbitrary shape, the finite element method is widely used to discretize the contact bodies. In the finite element method, two contact models are mainly used to compute the contact force, i.e., penalty function method and additional constraint method, which are different in constraint imposition strategy. The penalty function method regards the contact effect as a force function of local penetration at the contact point and its rate. This method has gained significant popularity because it does not bring extra dimensions to the dynamic equations and does not need to solve constraint equations either. However, as the non-penetration constraint is not precisely satisfied in the contact process when using the penalty function method, the accuracy of the numerical simulation depends on the choice of the penalty parameter. On the other hand, the additional constraint method can strictly satisfy the contact constraint condition by introducing the Lagrange multipliers into the dynamic equations, but the method poses some numerical difficulties due to the additional effort required to solve the multipliers. Considering the advantages and disadvantages of the two contact methods, the interactive mode method is proposed. This method divides the whole model into local static module and main dynamics module. The static module establishes a local finite element model of the contact region to compute the contact force, and the main dynamics module is used to obtain the kinematic variables of the whole body. In the simulation, the two modules are coupled by exchanging displacements and forces in each time step. In the current integration step, the main dynamics module provides the displacements of the boundaries of the local contact region at first, the values are transferred to the local finite model to compute the contact force next, and then the contact force is fed back to the dynamics module for the calculation of the next step. The proposed method combines the advantages of both the additional constraint method and the penalty function method, in which not only the artificial selection of penalty parameter is avoided, but also the non-penetration constraint of local contact region is satisfied and the numerical solution is convenient. The validity of the proposed method is verified by the comparison between simulation results and experimental results of a rod-plate impact case. Furthermore, a multi-point impact problem of a slider sliding in the gap chute is presented to validate the proposed method of dealing with the general impact problem.

Experimental study on the influence of granular shear deformation on sound propagation

Zhou Zhi-Gang, Zong Jin, Wang Wen-Guang, Hou Mei-Ying
Acta Physica Sinica. 2017, 66 (15): 154502 doi: 10.7498/aps.66.154502
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Effective medium theory (EMT) predicts a scaling relation between sound velocity c and pressure P as c∝ (φZ)1/3 (P/E0)1/6, where φ and Z are respectively the packing fraction and the mean coordination number of granular material. In this relation, the granular contact network is represented via two simple parameters φ and Z stemming theoretically from a strong approximation that microscopic and macroscopic granular displacements remain affine. This hypothesis simplifies tremendous computations for sound wave in a granular system, however some experimental results show that the scaling relation is recovered only for the case of very high pressure confinement (larger than 106 Pa for a glass bead system), but for the lower pressure case (less than 106 Pa) the relation does not hold. Owing to the fact that the change of microscopic granular displacement relates to the contact network variation of granular sample, and for better understanding the effect of the variation of contact network on the sound propagation in granular system, we conduct uniaxial shear experiments, in which the granular solid sample, composed of 0.28-0.44 mm glass beads, is cyclically compressed under a series of axial loadings (denoted as Pcomp). After these axial loadings, different contact networks of the sample are formed. Ultrasonic waves are then measured in the granular sample with these different contact networks under a constant axial pressure (denoted as Pobse). It is found that the axial deformation of the granular sample apparently affects the incoherent part of ultrasonic wave, but not the coherent part. A resemblant parameter is introduced to quantitatively discuss the variations of incoherent parts of sound waves in different axial deformations. In this paper, we also compare the frequency and the energy spectra of the sound waves, and find that the tendencies of their varying with the increase of axial deformation are nearly the same. This indicates that during the sound wave propagation in the granular solid sample, the processes of wave scattering and dissipation on particle contacted occur at the same time and the energy dissipation of sound wave in the air among particles can be neglected. In our experiments, compressional wave velocities based on time-of-flight method are also explored. The experimental results show that the velocity increases rapidly at the beginning of the axial deformation, and then tends to a steady value which is predicted by EMT. These illuminate that the variation of contact networks of granular sample may contribute to the deviation of velocity-pressure exponent from the prediction of EMT in low confining pressure.

PHYSICS OF GASES, PLASMAS, AND ELECTRIC DISCHARGES

Characteristics of gain in Ne-like Ar 69.8 nm laser pumped by capillary discharge

Liu Tao, Zhao Yong-Peng, Ding Yu-Jie, Li Xiao-Qiang, Cui Huai-Yu, Jiang Shan
Acta Physica Sinica. 2017, 66 (15): 155201 doi: 10.7498/aps.66.155201
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In this paper, the theoretical calculation model of the gain coefficient of Ne-like Ar 69.8 nm laser is established. With the collisional-radiative model, the rate equations for the 46.9 nm and 69.8 nm lasers are built by considering the 4 levels of the 2s2p6 1S0, 2p53p 1S0, 2p53p 3P2, and 2p53s 1P1. The gain coefficients per ion density of 46.9 nm and 69.8 nm lasers are calculated on the basis of the rate equations. The results show that the 46.9 nm laser has potential of higher gain than the 69.8 nm laser at an electron temperature of 200 eV. The gain coefficients per ion density at different electron temperatures are also calculated. Under the same electron density, the higher electron temperature is favorable for increasing the gain coefficients per ion density of the 69.8 nm laser. Meanwhile there is also an optimal electron density corresponding to the maximum gain coefficient per ion density of the 69.8 nm laser at a given electron temperature. Then a one-dimensional cylindrical symmetry Lagrangian magneto-hydrodynamics (MHD) code is utilized to simulate the Z-pinch process. The radial distributions of the electron temperatures, the electron densities and the Ne-like Ar ion densities are calculated with the MHD code at the different initial pressures. According to the rate equations for the 69.8 nm laser and the simulation results of the MHD code, the gain coefficient distribution of 69.8 nm laser in the radial direction of the plasma can be determined when the plasma is compressed to a minimum radius. According to the experimental parameters, the maximum gain coefficient of 69.8 nm laser is calculated to be 0.32 cm-1 when the main pulse current is 12 kA. The relationship between the radial distribution of gain coefficient of 69.8 nm laser and the initial pressure is also simulated. The theoretical results show that the optimal initial pressure is in a range of 12-14 Pa, in which the amplitude of gain coefficient is maximum. The experiments about 69.8 nm laser are conducted with Al2O3 capillary which has an inner diameter of 3.2 mm and a length of 35 cm. A main current of 12 kA with a rise time of 32 ns is produced by the main pulse generator, which consists of a Marx generator and a Blumlein line filled with de-ionized water. The Blumlein line is pulse-charged by a ten-stage Marx generator and discharges through the capillary by a self-breakdown main switch pressurized with N2 gas. To reduce the amplitude of main current, we reduce the charging voltage of the Marx generator and increase the conducting inductance of the main switch. Prior to the operation of the main current pulse, the capillary filled with Ar is predischarged by a current of~20 A. The 69.8 nm laser intensity as a function of initial pressure is measured by a 1-m grazing incidence Rowland spectrograph. The experimental results show that the optimum pressure is 16 Pa which is similar to the theoretical result. In addition, the gain coefficient (0.4 cm-1) measured in experiment is slightly higher than that (0.32 cm-1) of the theoretical calculation.

Plasma characteristics of helical streamers induced by pulsed discharges

Zou Dan-Dan, Cai Zhi-Chao, Wu Peng, Li Chun-Hua, Zeng Han, Zhang Hong-Li, Cui Chun-Mei
Acta Physica Sinica. 2017, 66 (15): 155202 doi: 10.7498/aps.66.155202
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The plasma discharge channel in three-dimensional helical shape induced by pulsed direct current (DC) discharge without external stable magnetic field is discovered experimentally. It can be observed by intensified charge-coupled device camera that a luminous plasma structure fast propagates along a helical path in the form of guided streamer (ionization wave). And the propagation of the streamer is stable and repeatable. We take this streamer which propagates along the helical discharge path as the study object, and explain its mechanism by constructing an electromagnetic model. The result shows that the helical shape plasma plumes can exhibit two different chiral characteristics (right-handed and left-handed helical pattern). While the discharge parameters such as pulse frequency, boundary condition, etc. can all affect the propagating characteristics of helical streamers. The electromagnetic radiation driven by pulsed DC power inside the dielectric tube which forms the wave mode is an important source of the poloidal electrical field. The helical steamers form when the poloidal electrical field is close to the axial electrical field. The velocities of the propagation in poloidal and axial direction are estimated respectively, and the hybrid propagation modes involving the interchangeable helical pattern and the straight-line pattern propagating plasmas are explained from the viewpoint of multi-wave interaction.

Acta Physica Sinica
Accepts
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A fast particle simulation method for calculating the multipactor threshold based on the frequency domain solutions in microwave devices

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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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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

null
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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Influnence of Nonspherical Effects on the Secondary Bjerknes Force in a Strong Acoustic Field

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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 Effect of Collision Parameter on a Magnetized Electronegative Plasma Sheath Structure

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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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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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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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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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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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The Principle and Application of Diagonal Reducing Method in the Complex Noise Fields

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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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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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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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Spatial Correlation of Underwater Bubble clouds Based on Acoustic Scattering

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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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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 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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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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Penta-decomposition of instantaneous field in spanwise-rotating turbulent plane Couette flow

null
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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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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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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A super-resolution infrared microscopy based on a doughnut pump beam

null
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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Optimization design of a Gamma-to-Electron spectrometer for high energy gammas induced by fusion

null
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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Combined noise source identification method based on spherical microphone array with random unifrom distribution of elements

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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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Ferroelectric phase transition of perovskite SnTiO3 based on first principles

null
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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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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Development of a intranuclear-cascade code CBIM applicable to the nuclear reaction with incident particle energy above 45MeV

null
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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Experimental observation and study of two sets of four-wave mixing processes in a single-zero-dispersion microstructured fiber by the same pump

null
Accept: 2016-08-18
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A highly nonlinear microstructured fiber with single-zero-dispersion wavelength was designed and drawn by reducing the core area in order to observe two groups of four-wave mixing by a single pump. The foundational material of the fiber was silica and its cladding was consisted of seven layer air holes. The air holes were arranged in a hexagonal lattice and the lattice pitch was Λ=2.5 μm, the radius of the air holes were r=1.03 μm. There was just one zero-dispersion wavelength in our considerable wavelength range for the microstructured fiber and the corresponding wavelength was near λ_D=0.85 μm. The basic properties of the fiber including effective refractive index, dispersion coefficient, and nonlinear coefficient were calculated by finite element method. The effective mode area was 4.4〖 μm〗^2 and the nonlinear coefficient was 0.057 m^(-1) W^(-1) for the foundation mode near the wavelength of 0.8 μm, and the nonlinear coefficient reached 0.053 m^(-1) W^(-1) near the zero dispersion wavelength of 0.85 μm. In short, the optical fiber had a stable and high nonlinear coefficient in the whole experimental band (0.80 μm~0.83 μm) which provided an important guarantee for the occurrence of four-wave mixing double parameter gain process. In addition, the phase mismatch curve was simulated by using the four-wave mixing phase mismatch formulation. Numerical simulation shown that two sets of four-wave mixing processes could occur in the designed fiber. At the normal dispersion wavelengths of 0.800 μm, 0.810 μm and 0.820 μm with different powers, the experiment result shown a significant feature of four gain wavebands located at both sides of the pump wavelength. By comparing experimental data with the phase mismatch curve, we found that the band generation meet four-wave mixing phase matching condition, thus, the simultaneous occurrence of two groups of four-wave mixing processes observed in the experiment was explained in theory. The experimental results agreed well with the theoretical predictions. This also proved the theoretical predictions that two sets of parametric gain processes and two pairs of signal and idle frequency waves can be generated in PCF. The four-wave mixing effect occurred at the normal dispersion region could be attributed to the contribution of negative fourth-order dispersion to the phase matching process. The present work can provide valuable reference to the design of microstructure fibers and the development of multi-wavelength conversion technology based on four-wave mixing effect. At the same time, this work could also supply guidance for the development of uncommon waveband lasers and broadband light sources.
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Experimental study of diode-pumped Nd,Y:CaF2 amplifier for ICF Laser Drivers

null
Accept: 2016-08-18
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In this paper Nd,Y:CaF2 crystal is taken as the gain medium for the laser amplifier for the first time. A laser-diode plane-array five-direction horizontal-side-pumped Nd,Y: CaF2 laser amplifier φ5 mm × 70 mm is developed and an experimental study has been carried out; the absorption spectrum and emission spectrum of Nd,Y:CaF2 crystal and the fluorescence distribution of the amplifier have been measured. And the small signal gain of Nd,Y:CaF2 and Nd:glass has been measured working under repetition frequency 10Hz and 1Hz respectively under the same pump power. The small signal gain of Nd,Y:CaF2 amplifier reaches 6.12 under pump power 9.63kW, which is 1.5 times of Nd:glass amplifier’s. Through measuring on the spectrum of the seed beam and the one out of Nd,Y:CaF2 amplifier, no change has been found before and after amplification.
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The Rydberg state excitation and double ionization of different atoms in strong femtosecond laser field

Lei Zhao Hang Lv Haifeng Xu
Accept: 2016-08-18
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Using the mass resolved pulsed electric field ionization method combined with the time of flight mass spectrometer, we investigated the Rydberg state excitation of noble gas atoms (He, Ar and Xe) at 800 nm femtosecond laser field, and compared it with the atomic nonsequential double ionization. We discuss the difference between the excitation and the nonsequential double ionization. The results shown that both the yields of the Rydberg state excitation and non-sequential double ionization increase as the atomic number growing, and the dependence on laser ellipticity get weaker. We point out that the probability of atomic NSDI is lower than that of RSE. The results should add our knowladge on the atomic Rydberg state excitation in strong laser field, and will be very helpful in further study on atomic strong field physics.
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Accept: 2016-08-18
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