Vol. 65, No. 5 (2016)

Control of nonautonomous matter rogue waves
Zhang Jie-Fang, Dai Chao-Qing
2016, 65 (5): 050501. doi: 10.7498/aps.65.050501
Abstract +
We study a (1+1)-dimensional variable-coefficient Gross-Pitaevskii equation with parabolic potential. A similarity transformation connecting the variable-coefficient Gross-Pitaevskii equation with the standard nonlinear Schrödinger equation is constructed. According to this transformation and solutions of the standard nonlinear Schrodinger equation, we obtain exact rogue wave solutions of variable-coefficient Gross-Pitaevskii equation with parabolic potential. In this solution, a Galilean transformation is used such that the center of optical pulse is Xc = v(T-T0) while the Galilean transformation was not used in previous analysis. By the Galilean transformation, the parameter T0 is added into the solution. It is found that the parameter T0 is important to control the excitations of rogue waves. Moreover, the parameters a1 and a2 in solution are complex parameters which can modulate the behaviors of rogue waves. If they are restricted to real numbers, we can obtain some well-known rogue wave solutions. If the parameter a2 =-1/12, we can have a second-order rogue wave solution. If the parameter a2 is a complex number, the solution can describe rogue wave triplets. Here two kinds of rogue wave triplets, namely, rogue wave triplets I and II are presented. For rogue wave triplet I, at first, two first-order rogue waves on each side are excited, and then a first-order rogue wave in the middle is excited with the increase of time. On the contrary, for rogue wave triplet II, a first-order rogue wave in the middle is initially excited, and then two first-order rogue waves on each side are excited with the increase of time.#br#From these solutions, the controls for the excitations of rogue waves, such as the restraint, maintenance and postponement, are investigated in a system with an exponential-profile interaction. In this system, by modulating the relation between the maximum of accumulated time Tmax and the peak time T0 (or TI,TII), we realize the controls of rogue waves. When Tmax > T0 (or TI,TII), rogue wave is excited quickly, and the atom number of condensates increases; when Tmax = T0 (or TI,TII), rogue wave is excited to the maximum amplitude, then maintains this magnitude for a long time, and the atom number of condensates also increases; when Tmax T0 (or TI,TII), the threshold of exciting rogue wave is never reached, thus the complete excitation is restrained, and the atom number of condensates reduces. These results can be used to understand rogue waves better, that is, besides their "appearing from nowhere and disappearing without a trace", rogue waves can be controlled as discussed by a similar way in this paper. These manipulations for rogue waves give edification on theory and practical application.
Syntheses of B2O3-doped gem-diamond single crystals
Xiao Hong-Yu, Liu Li-Na, Qin Yu-Kun, Zhang Dong-Mei, Zhang Yong-Sheng, Sui Yong-Ming, Liang Zhong-Zhu
2016, 65 (5): 050701. doi: 10.7498/aps.65.050701
Abstract +
In the present paper, by the temperature gradient method, the gem-diamond single crystals with B2O3-added in the synthetic system of the FeNiMnCo-C are synthesized under 5.3-5.7 GPa and 1200-1600℃. The P-T phase diagram of diamond single crystal growing in the synthesis system of the FeNiMnCo-C-B2O3, is obtained. By B2O3-added in synthesis system, the V-shape section for the diamond growth, which is the region between the solvent/carbon eutectic melting line and diamond/graphite equilibrium line under pressure and temperature, is moved upwards. We find that the minimum pressure of diamond growing increases from 5.3 GPa to 5.4 GPa and the synthesis range of the low temperature hexahedron diamond growth becomes wider, which can be due to the chemical energy increase of the carbon depositing in the diamond surface by the additive of the B2O3. The synthetic diamond single crystal exhibits a perfect hexahedral shape or cubo-octahedral shape or octahedral shape. In the system of the FeNiMnCo-C-B2O3, we think that the catalyst activity decreases with the generation of CO2, so high-quality diamond single crystal can hardly be synthesized when the content of the B2O3 is more than 3 wt‰ and synthesis time is more than 20 h, However, when the content of the B2O3 is no more than 1 wt‰, the rate of finished products of the low temperature hexahedron diamond will increase significantly. Because the amount of the B2O3 additive is so small, in the syntheses of B2O3-added diamond single crystals, the black areas which appear when B element enters into diamond crystal lattice are not observed. The growth rate of diamond single crystal will be reduced obviously by B2O3-added in synthetic system. Under our system synthesis, when the growth time is 10 h, the growth rate of the diamond will reduce 0.22 mg/h by 2 wt‰ B2O3 added in synthetic system. When the growth time extends to 20 h, the growth rate increases to 0.47 mg/h. Moreover, with the extension of growth time, the catalyst activity decreases continuously with the product of the CO2 increasing in the reaction chamber, so the effect of additive on the growth of diamond strengthens gradually. The results of the scanning electron microscope images indicate that the surface defects of the diamond crystal increases by the addition of B2O3.
Static subminiature snapshot imaging polarimeter using spatial modulation
Cao Qi-Zhi, Zhang Jing, Edward DeHoog, Lu Yuang, Hu Bao-Qing, Li Wu-Gang, Li Jian-Ying, Fan Dong-Xin, Deng Ting, Yan Yan
2016, 65 (5): 050702. doi: 10.7498/aps.65.050702
Abstract +
The spatially modulated snapshot imaging polarimeter can acquire the image and two-dimensional state of polarization using the spatial carrier fringe to encode the full Stokes vectors in a single interference image. It can be used in space exploration, earth observation and detection of biological medicine, land surface and oceans. In an earlier publication, we demonstrated by theoretical analysis that the spatially modulated snapshot imaging polarimeter using modified Savart polariscopes (MSPSIP) is comparable in carrier frequency, signal-to-noise ratio, and spatial resolution to a spatially modulated snapshot imaging polarimeter using conventional Savart polariscopes. In this investigation, the numerical simulation is used to demonstrate theoretical analysis and the feasibility of MSPSIP. Moreover, a geometric ray model is developed to explain the principle and scheme of MSPSIP and derive the expressions of interference intensities. Moreover, a laboratory experiment is conducted to demonstrate the validity of MSPSIP. In addition, we analyze that the interference intensity varies with the direction of polarization analyzer. This investigation enriches the study on MSPSIP and provides a theoretical and practical guidance for study, design, modulation, experiment and engineering of MSPSIP. Furthermore, the MSPSIP operates based on the principle of encodeding polarization information within the spatial modulation of the image. This unique technology allows all Stokes parameters to be simultaneously recorded from each spatial position in an image with a single integration period of the imaging system. The device contains no moving parts and requires no scanning, allowing it to acquire data without the motion artifacts normally associated with scanning polarimeter. In addition to having snapshot imaging and static (no moving parts) capabilities, image processing is simple, and the device is compact, and miniature. Therefore, we believe that MSPSIP will be useful in many applications, such as remote sensing and bioscience.
Molecular structure and properties of sulfur dioxide under the external electric field
Yang Tao, Liu Dai-Jun, Chen Jian-Jun
2016, 65 (5): 053101. doi: 10.7498/aps.65.053101
Abstract +
SO2 is not only an important resource but also a notorious air pollutant, so it has attracted increasing attention nowadays. This paper focuses on the influence of external electric field on SO2. In order to obtain more reliable results, the density functional theory B3P86 method is chosen to calculate the values mentioned below. The ground states of SO2 molecule under different strong electric fields ranging from -0.04 a.u. to 0.04 a.u. are optimized by density functional theory B3P86 method with 6-311++g(3d,p) basis sets. The geometric parameters, charge distributions, total energies, dipole moments, the highest occupied molecular orbital (HOMO) energies, the lowest unoccupied molecular orbital (LUMO) energies, energy gaps of SO2 under different external electric fields are obtained, respectively. On the basis of optimized configuration, the excitation energy, transition wavelength and oscillator strength in the same intense external electric field are calculated by the time dependent density functional theory (TD-B3P86) method.#br#The calculated values for geometric parameters of SO2 without external electric field agree well with the available experimental data and other theoretical results. The geometric parameters and charge distribution of SO2 strongly depend on the intensity and direction of external electric field. The total energy of SO2 in the considered range of external electric field first increases and then decreases. On the contrary, the dipole moments of SO2 in different external electric fields ranging from -0.04 a.u. to 0.04 a.u. first decrease and then increase. When the external electric field is -0.04 a.u., the total energy and symmetry of SO2 both reach the maximum values. With the change of external electric field, the LUMO energy first increases and then decreases. The HOMO energy is found to decrease through the variation of the external field. The energy gaps of SO2 are proved to first increase, and then decrease with the variation of external electric field. Through studying the energy gaps of SO2, it is found that the external electric field can affect the chemical reactivity of SO2. The excitation energies, transition wavelengths and oscillator strengths are very complicated with the change of the external electric field. The excitation properties of SO2 molecule are seriously affected by the external electric field.
High-sensitive off-axis integrated cavity output spectroscopy and its measurement of ambient CO2 at 2 μm
Li Zhi-Bin, Ma Hong-Liang, Cao Zhen-Song, Sun Ming-Guo, Huang Yin-Bo, Zhu Wen-Yue, Liu Qiang
2016, 65 (5): 053301. doi: 10.7498/aps.65.053301
Abstract +
An off-axis integrated cavity output spectroscopy (OA-ICOS) is established by using a fiber-coupled distributed feedback diode laser operating near 2 μm. Its performances are evaluated and optimized through experimental investigation via detecting the pure CH4 absorptions at different pressures. The reflectivity of the cavity mirror is measured to be 0.99865, which results in the effective total optical pathlength of up to 407.4 m based on a 55 cm cavity. It is shown that the OA-ICOS configuration can be used to obtain very long optical pathlength, leading to pretty high sensitive monitoring of atmospheric trace gases. Based on the developed OA-ICOS, the atmospheric CO2 measurements are made and its performance is improved by using the wavelet denoising approach. The CO2 absorption line at 4993.7431 cm-1 is used for measuring the concentration. The measured results are compared with the results obtaind by a commercial H2O/CO2 analyzer. Agreement and disagreement are briefly discussed, and the results show that the OA-ICOS is reliable for detecting the atmospheric trace gases. The limitation of the developed OA-ICOS and the further steps towards the improvement in precision and accuracy are also presented.
Multiphoton ionization and dissociation dynamics of Freon-113 induced by femtosecond laser pulse
Liu Yu-Zhu, Chen Yun-Yun, Zheng Gai-Ge, Jin Feng, Gregor Knopp
2016, 65 (5): 053302. doi: 10.7498/aps.65.053302
Abstract +
The ozone layer which absorbs harmful solar UV radiation is an essential umbrella for human. However, a large number of exhausts of Freon released by human activity into the atmosphere pose a great threat to the ozone layer. The UV sunlight radiation induced Freon dissociation produces chlorine radicals, which are found to be the main culprit for destroying the atmospheric ozone. In this paper, multiphoton ionization and dissociation dynamics of Freon-113 (CF2ClCFCl2) induced by femtosecond laser pulse are studied by time-of-flight mass spectrometry coupled with velocity map imaging technique. Fragment mass spectra of Freon-113 are measured by time-of-flight mass spectrometry. No parent ions are discovered in the time-of-flight mass spectra, and all the detected ions are from the fragmentation induced by the laser pulse. Daughter ions CFCl2+, CF2Cl+, C2F3Cl2+ are found to be the three major fragmentation ions in the multi-photon ionization and dissociation. Several photodissociation channels are discussed and concluded by further analysis and calibration (via the ratio of mass to charge) of the measured time-of-flight mass spectra. Three main photodissociation mechanisms are found as follows: 1) C2F3Cl3+→C2F3Cl2++Cl with breaking C--Cl bond and directly producing the Cl radical; 2) C2F3Cl3+ →CFCl2++CF2Cl with breaking the C--C; 3) C2F3Cl3+ →CF2Cl++CFCl2 with breaking the C--C bond. Ion images of the three main fragments C2F3Cl2+, CFCl2+ and CF2Cl+ are measured by the velocity map imaging setup. The speed distributions of these three fragment ions are obtained from the velocity map imaging. The speed distribution of C2F3Cl2+ with breaking C--Cl bond can be fitted by two Gaussian distributions while the speed distributions of both CFCl2+ and CF2Cl+ with breaking the C--C bond can be well fitted by one Gaussian distribution. The different fittings reflect different production channels. The detailed photodissociation dynamics is obtained by analyzing the kinetic energy distribution and angular distribution of the fragment ions. Additionally, density functional theory calculations on high-precision level are also performed on photodissociation dynamics for further analysis and discussion. An in-depth understanding of dissociation dynamics of freon can provide theoretical reference and experimental basis for further controlling the dissociation process that can do destruction to the ozone layer.


Total internal reflection orders in transmission grating
Wu Rong, Tian Yu-Ting, Zhao Dong-Feng, Li Da-Wei, Hua Neng, Shao Ping
2016, 65 (5): 054202. doi: 10.7498/aps.65.054202
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In a current high-power laser system, transmission grating has been used to sample, separate and focus laser beam. Under high power laser irradiation, besides the 0-order shooting beam and target sampling beam, the detrimental influence of other diffraction orders should be taken into consideration seriously. These useless orders may damage other nearby optical elements and mechanical devices, disturb and confuse the measurement of time pulse and near/far-field intensity. Especially the total internal diffraction order will lead to some series diffraction patterns, causing the above problems. First, relevant theoretical calculation and analysis are carried out for transmission grating (including beam sample grating and focusing grating), which can predict and indicate these inconspicuous diffraction orders. These orders appear on four receiving screens regularly and periodically, and the periodic distance between them is determined by ray-tracing draft. Second, the phenomenon of total internal reflection order is observed and measured by combining with anti-reflection film. The measured periodic spacings on three screens are 24 mm, 26 mm and 35 mm, respectively. Moreover, energy intensities of these redundant orders are measured finely, which shows that their contrasts or SNRs to 0-order main laser is in a range of 10-8-10-4). Finally, some appropriate and effective approaches to eliminating or avoiding total internal reflection and other useless orders are proposed and discussed, which include 1) coating by anti-reflection film with pre-deep etching; 2) optimizing the grating design to make redundant orders far from target spot; 3) placing laser scattering or absorbing devices at corresponding position to avoid being damaged by the side-leakage energy and ghost image of total internal reflection and other redundant orders.
A scheme for Sagnac effect improvement with squeezed vacuum input and homodyne detection
Chen Kun, Chen Shu-Xin, Wu De-Wei, Yang Chun-Yan, Wu Hao
2016, 65 (5): 054203. doi: 10.7498/aps.65.054203
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There has been much interest in improving gyroscope precision with quantum technology for realizing autonomous navigation. The laser light in coherent state cannot reach higher precision under shot-noise limit (SNL) caused by vacuum zero energy fluctuation, which restricts the further improvement of optical gyroscope precision. Quantum mechanics reckons that one unused port of the beam splitter (BS) is inputted with vacuum, which results in vacuum fluctuation, while another port is inputted with the laser light in optical gyroscope. In order to compress the vacuum fluctuation, we design an experimental scheme, in which squeezed vacuum light is used as another incident light into the unused port of Sagac interferometer in optical gyroscope. We analyze the physical process of this scheme theoretically and develop the quantum balanced homodyne detection technique to retrieve the relative phase information of Sagac interferometer output. There are two most important conditions that we should pay attention to. 1) We should ensure that the phase of local oscillator light arg(α L), the phase of coherent light arg (αc) and the angle of squeezed direction arg(μν) in the squeezed vacuum light satisfy the condition, i.e., arg (α L2)-arg (μν) = πup and arg (α L)-arg (αc) = 0 when we perform quantum balanced homodyne detection technique for the best sensitivity δφ = e-GδφSNL, where G denotes the squeezed degree; 2) only by deriving the fields from one common source can we ensure coherence among the squeezed vacuum, probe and local oscillator. Although the requirements for experimental settings are strict, we can meet the requirement with careful calibration. Numerical analysis shows that this proposed scheme provides much higher precision below SNL: both sensitivity detection limit and dynamic range grow with an exponential rate as the squeezed degree grows. The current technology for squeezed vacuum generation by using two consecutive crystals with the optic axes tilted allows us to reach a value as high as G ≈ 16 of squeezed degree. Only by inputting such squeezed vacuum light into the unused port of BS in the optical gyroscope, can we attain sensitivity detection limit and dynamic range with increment by 108. Our approach is a new scheme for improving optical gyroscope with current available technology.
The analytic expressions of temperature and stress in directly liquid cooled thin slab laser
Li Ce, Feng Guo-Ying, Yang Huo-Mu
2016, 65 (5): 054204. doi: 10.7498/aps.65.054204
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In this paper, based on the convective heat transfer and conduction principle, the thermal effect analysis model of the directly liquid cooled uniformly pumped thin slab laser is established. The approximate plane stress and the principle of minimum are introduced to describe thermal stress distribution in the thin slab. Firstly, the relationships between the flow velocities in different flow channel thickness values and the convection heat transfer coefficients and also the relationship between flow velocity and coolant temperature rise are studied. Moreover, the influences of different flow channel thickness values on temperature field and thermal stress distribution are analyzed. Finally, the variation trends of wave-front phase distortion with the change of heat power in the case of Zig-zag path and direct path are investigated, respectively. The results reveal that thicker flow channel can achieve stronger heat treatment effects in an appropriate range of the cooled liquid flow rate, and the thermal profile is symmetrical with respect to the center plane of slab. In addition, the longitudinal maximum temperature rise occurs in the outlet; the maximum stress distortions centralize on the both ends and partial sides of slab. It is worthy to mention that the one-dimensional temperature gradient and smaller stress form more probably for thicker flow channel., Furthermore, zig-zag path can alleviate obviously wave-front aberration due to thermo-optic effect. In this paper the thermal effect of the liquid direct cooled thin slab laser is investigated. The research results are beneficial to the design and optimization of the directly liquid cooled thin slab laser.
Characteristics of Raman spectrum from stand-off detection
Zhang Li, Zheng Hai-Yang, Wang Ying-Ping, Ding Lei, Fang Li
2016, 65 (5): 054206. doi: 10.7498/aps.65.054206
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For developing a method to detect unknown or hazardous materials beyond safe distances, an experimental standoff detection system with using Raman scattering is established in laboratory. It consists of a pulsed laser with a wavelength of 532 nm as an excitation source, an optical assembly for light collecting and focusing with a 25 mm entrance aperture, a grating monochromator for dispersing scattering light, and a photomultiplier connected to an oscillograph for signal monitoring. The angle between the direction of incident laser beam and that of the scattering light collecting assembly is less than 2°. Raman signal intensities of ammonium nitrate, potassium nitrate and sodium nitrate in solid samples in a distance range from 2 m to 10 m are measured. The results are supposed to be comparable to those obtained in a distance range from 20 m to 100 m if a telescope of 250 mm diameter is used instead to collect Raman scattering light as in a usual standoff detection system. Some characteristics of Raman spectra are investigated, such as the spectrum features, the relationships between the amplitude of the highest Raman peak of ammonium nitrate and the intensity of the excitation light, the detection distance, the concentration of the sample and the normal direction of the sample surface. The Raman spectra of ammonium nitrate, potassium nitrate and sodium nitrate look similar: each of them has a highest peak in the vicinity of 1050 cm-1, small difference can be observed, and it can serve as a "signature" for discriminating between them. The experimental results demonstrate that the intensity of the characteristic Raman spectrum of ammonium nitrate is proportional to the excitation power, with approximate quadratic relationship, and tends to be inversely proportional to the square of the detection distance except that the detection distance is too short to ignore the influence of the focal length of light collecting optics on image size. In addition, the intensity of the characteristic Raman spectrum of ammonium nitrate decays approximately at an exponential rate with the decrease of its concentration. Finally, the intensity of the Raman signal of ammonium nitrate is approximately proportional to the cosine of the angle between the direction of the incident light and the surface normal. This relationship is similar to Lambert's cosine law that the radiant intensity observed from an ideal diffusely reflecting surface is directly proportional to the cosine of the angle. The last two phenomena imply that it may be particularly difficult to detect the substances of interest in a mixture on horizontal ground surface for Raman standoff detection system.


Effects of Gd3+/Y3+ codoping on the spectral properties of Nd:CaF2 crystals
Liu Jian, Liu Jun-Fang, Su Liang-Bi, Zhang Qian, Ma Feng-Kai, Jiang Da-Peng, Xu Jun
2016, 65 (5): 054207. doi: 10.7498/aps.65.054207
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In the last few years, Nd3+ doped fluoride crystals have achieved some amazing laser performances by codoping buffer ions such as Y3+ and Gd3+ ions, which lead to the changing of local structure of Nd3+ ions. In this work, effects of doping concentration of Gd3+ and Y3+ ions on optical properties are discussed. The relationships between spectroscopic properties and the unit cells are also discussed. Nd, Y:CaF2 and Nd, Gd:CaF2 disordered crystals are grown by using temperature gradient technique (TGT). Among 0.5 at.%Nd, x at.%Gd(x=2, 5, 8, 10):CaF2 crystals, the crystal with Gd3+ of 2 at.% has the longest fluorescence lifetime (499 s). Increasing the concentration of Gd3+ up to 5 at.%, the crystal has a maximum absorption cross section of 1.910-20 cm2, and a maximum emission cross section of 1.910-20cm2. The crystal with Gd3+ of 8 at.%has a maximum emission bandwidth of 29.03 nm(FWHM). Among 0.6 at.%Nd, x at.%Y(x=2, 5, 8, 10):CaF2 crystal, the crystal with Y3+ of 5 at.%has the biggest absorption cross section (2.4110-20 cm2), and the biggest emission cross section (3.1710-20 cm2), when the concentration of Y3+ is 5 at.%. When the Y3+ concentration increases up to 10 at.%, the crystal has a longest fluorescence lifetime of 359.4 s and maximal emission bandwidth of 26 nm(FWHM).The different concentrations of codoping ions have different effects on the Nd:CaF2 crystals, for the formations of different optical centers. In order to study the effects of local structure around Nd3+ on the optical properties in a set of Nd:CaF2 single crystals with different codoping concentrations of Gd3+ and Y3+, the unit cell parameters are investigated by X-ray diffraction. With different concentrations of Gd3+ and Y3+ ions in Nd:CaF2 crystal, the local structure of Nd3+ changes, which leads to different optical properties. The relevant details will further be explained in this paper.
Design and verification of an electronically controllable ultrathin coding periodic element in Ku band
Yang Huan-Huan, Yang Fan, Xu Shen-Heng, Li Mao-Kun, Cao Xiang-Yu, Gao Jun
2016, 65 (5): 054102. doi: 10.7498/aps.65.054102
Abstract +
The coding periodic element is able to achieve coded reconfigurable electromagnetic (EM) responses by loading controllable electronic devices. In this work, an electronically controllable ultrathin planar periodic element structure in Ku band is implemented with one PIN diode. When the PIN diode turns ON or OFF by applying a proper biasing voltage, the resonant property of the element changes correspondingly, and hence a 180° phase difference between the two states is obtained. By optimizing the geometrical parameters, the reflection loss less than 0.5 dB is achieved by the proposed element. Therefore, using a proper biasing voltage control network, the PIN diodes of the proposed elements in a periodic arrangement are set at different states, which may be denoted by a binary string with "1"s or "0"s, and the whole array of elements operates as a binary coding periodic structure and exhibits controllable EM functionalities. In order to verify the coding property of the proposed element, the general principle for the biasing circuit design is given. An optimized biasing circuit is thoroughly studied using both field distribution analysis and equivalent circuit theory. Simulated results show that the specially designed biasing hardly affects the element reflection performance. Finally, a group of element prototypes are fabricated with welded PIN diodes and measured using the standard waveguide test method. The difference in mirror image between the waveguide test and the desired periodic arrangement is also discussed. The experimental results validate that the proposed element successfully achieves good coding EM performance by controlling its biasing voltage. The reflection loss of the element is very low, and well distributed phase difference between the two element states is observed. The simulation and experiment results agree well, and the deviation between them is analyzed in detail. The proposed element possesses distinctive favorable features such as coded controllable EM functionalities, simple structure and ultrathin profile, thus exhibiting the promising prospects in tunable stealth surface, agile antennas, and many other applications.
Analysis of Si/SiGe/Si double heterojunction band of a novelstructure of PIN electronic modulation
Feng Song, Xue Bin, Li Lian-Bi, Zhai Xue-Jun, Song Li-Xun, Zhu Chang-Jun
2016, 65 (5): 054201. doi: 10.7498/aps.65.054201
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PIN is a common structure of electrical modulation in electro-optic modulator, and the performance of the electro-optic modulator is directly affected by the carrier injection in PIN structure. In previous studies, we have invented a novel structure of PIN electronic modulation based on SOI material. In the new structure, the SiGe material is adopted in the waveguide zone, therefore the double heterojunction PIN structure is formed in the horizontal direction. The carrier injection efficiency can be enhanced in the novel structure, and the power consumption could be reduced. In order to further study the modulation mechanism of the novel structure, based on the single heterojunction band theory, the barrier heights of the double heterojunction are analyzed, and the quantitative formulas of the barrier heights of the double heterojunction are given. It is shown that the barrier heights of the double heterojunction are related to the doping concentration, the band gap of material, the temperature, and the Ge content. The bands are compared between the novel structure, SiGe-OI structure and SOI structure to analyze the reason why the carrier injection of the novel structure could be enhanced. In the same conditions, the barrier heights of Si/SiGe/Si double heterojunction are minimal values, and those of SiGe and Si materials are second minimal value and maximal value, respectively. When the PIN device is set at a forward biased voltage (P region is the anode, and N region is the cathode), the balance between the carrier diffusion and the carrier drift is broken, and the PIN device is in a non-equilibrium state. According to the quantitative formula of the barrier heights of the double heterojunction, the barrier heights of Si/SiGe/Si double heterojunction are lower than that of SiGe-OI material, and the barrier height of SiGe material is lower than that of SOI material. It is shown that the barrier heights of Si/SiGe/Si double heterojunction could be flatten at first, so its PIN structure has the higher carrier injection than those of SiGe-OI and SOI under the same conditions. Finally, the band distribution of the novel structure and the relationships between the band distribution, the modulation voltage and the carrier injection are simulated. The results show that when the modulation voltage is 1 V, the carrier density of the novel structure arrives at 8×1018 cm-3, which is 800% higher than that of SOI structure, and 340% higher than that of SiGe-OI structure. The advantages of the novel structure are further indicated, and the correctness of the theoretical analysis is also verified.
Free sink vortex Ekman suction-extraction evolution mechanism
Tan Da-Peng, Yang Tao, Zhao Jun, Ji Shi-Ming
2016, 65 (5): 054701. doi: 10.7498/aps.65.054701
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The suction-extraction phenomenon occurs in the formation process of free sink vortex (bathtub vortex), and it is a complex gas-liquid coupling matter. The Ekman layer coupling and its evolution mechanism involved in the above matter possess important scientific value and practical engineering significance. To address the above issue, a modeling and analytical method for the Ekman suction-extraction evolution mechanism of free sink vortex is proposed.#br#Based on the multiphase volume of fluid model and turbulent kinetic energy-dissipation (k-ε) model, a gas-liquid two-phase fluid dynamic model for free sink vortex Ekman suction-extraction is set up. Considering the rotating and shearing characteristics of sink vortex, a two-phase surface is reconstructed by piecewise linear interface construction method. Based on the above models, the internal relations between initial rotation velocity component, drainage capacity and Ekman suction-extraction are investigated, and the corresponding flow field profile regularities are revealed.#br#According to the results of a series of numerical instances, some regularities are obtained as follows. 1) If the initial velocity disturbance is variable, the distances of the suction and extraction holes from the container bottom both keep constant. In the suction stage, the suction hole is located at a fixed position above the container bottom surface, and the extraction hole is in the plane of the bottom surface. 2) If the initial disturbance is enhanced, the rotation velocity of the suction stage increases, and the suction and extraction heights and Ekman layer thickness become larger, while Ekman suction-extraction intensities of suction, extraction and penetration stages turn weaker. 3) If the initial disturbance is invariable, the heights of Ekman suction and extraction remain unchanged, and are independent of drainage capacity. 4) The small-scale vortexes separated from the large-scale ones in the bottom corner of container take on a phenomenon of flow around by a right-angle, which is caused by the viscosity of Ekman boundary layer and the potential flow of the sink hole. 5) Considering the stream line profiles of suction and extraction stages, the dispersion of stream lines keeps constant with the time going by, and the stream lines near the central region of vortex tend to be converged by increasing the gas-liquid coupling.#br#In general, the results can offer useful reference to the research work of free sink vortex formation mechanism, and provide technical supports for vortex suppression control of the areas of metallurgy pouring, chemistry separation and hydraulic drainage. The subsequent researches of the fractal based sink vortex evolution mechanism and lattice Boltzmann based phase surface tracing will be carried out.
An ultra-wideband absorber based on graphene
Jiang Yan-Nan, Wang Yang, Ge De-Biao, Li Si-Min, Cao Wei-Ping, Gao Xi, Yu Xin-Hua
2016, 65 (5): 054101. doi: 10.7498/aps.65.054101
Abstract +
Stealth technology is of great importance and significance in reducing the radar cross section and improving the survivability of the target aircraft. Absorber is one of the most important structures in stealth technology. However, the present investigations of absorbers mainly focus on the narrow band or multi-band. To extend the operation bandwidth, a graphene-based absorber structure is proposed in this paper. The proposed absorber has a periodic structure whose unit cell consists of a square and a circular graphene-based ring. The surface impedance of the periodic structure can be optimized to match the impedance of the free space in a very wide band by adjusting the electrostatic bias voltage. Then the operation band is significantly extended. By using the commercial software, CST Microwave Studio 2014, the performance of the proposed absorber is studied. The simulated results show that the proposed absorber can absorb electromagnetic (EM) waves in an ultra-wideband from 2.1 to 9.0 GHz, with an absorbing rate of up to 90%. Moreover, the proposed absorber is insensitive to the polarization of the incident wave due to the symmetry of the structure. We also find that the absorber can be tuned to work at any frequency in a range from 2.0 to 9.0 GHz for a fixed geometrical parameter. The equivalent circuit model (ECM) approach and interference theory (INF) are employed to investigate the physical mechanism of the proposed absorber. According to the ECM, we analyze the resonant characteristics of the square and circular graphene rings. Owing to the existence of two different graphene rings, two resonant frequencies are detected. By optimizing the structure parameters of the graphene rings, the two resonant frequencies are brought closer, resulting in the increase of the operation band. On the other hand, the real part of the input impedance of the equivalent circuit reaches up to about 300 Ω and the imaginary part is close to 0 Ω, which provides good matching to the free space, leading to high absorption rate. According to the interference theory, the amplitudes and phases of the direct reflection and the multiple reflections of EM waves are studied. It is found that the destructive interference between the direct reflection and multiple reflection makes the absorber have high performance in an ultra-wideband. The results obtained from ECM and INF are in good agreement with the simulation ones.
Analysis of characteristics of combined beam in spectral beam combining system based on multilayer dielectric grating
Wu Zhen, Zhong Zhe-Qiang, Yang Lei, Zhang Bin
2016, 65 (5): 054205. doi: 10.7498/aps.65.054205
Abstract +
Aiming at the application of multilayer dielectric gratings (MDGs) in spectral beam combining (SBC) systems, a theoretical model of rectangular MDGs is built up and a beam propagation model of SBC systems based on the rectangular MDGs is further developed. The phase modulation introduced by the rectangular MDG is composed of the optical path difference on the top surface of reliefs and that of the relief structure itself, and is affected by the MDG parameters such as the grating groove depth, the grating duty cycle, the incidence angle of the central beam, etc. By the diffraction integral method and the principle of incoherent superposition, the intensity distribution of the combined beam of the SBC system based on the rectangular MDGs is numerically calculated and analyzed. Additionally, the beam quality of the combined beam is also obtained by the intensity second-order moments method, and the effects of the MDG parameters such as the groove depth, the duty cycle, the incidence angle of the central beam, and the fabrication errors of the MDG on the characteristics of the combined beam of the SBC systems are simulated and discussed in detail. The simulation results show that the beam quality of the combined beam after passing through the SBC systems is significantly better than that of the laser array. Since the quality of the combined beam is almost the same as that of an individual laser beam, for a SBC system without fabrication error, changing the groove depth, the duty cycle of the rectangular MDG or the incidence angle of the central beam does not affect the beam quality while it has obvious influence on the energy of the combined beam. This is mainly because the diffraction efficiency of the rectangular MDG depends on both the parameters of the MDG and the incidence angle of the central beam. However, fabrication error of MDG is unavoidable, and the fabrication error has a significant effect on both the beam quality and the energy of the combined beam. Compared with the effect generated by the groove depth error on beam quality, the influence introduced by the duty cycle error is more obvious. It is worth mentioning that the theoretical model of the SBC system based on the rectangular MDG can be applied to some other high-power laser systems due to its advantages such as low absorption and high damage threshold.
A study of 100 Gb/s 2R regeneration for return-to-zero code signal
Chen Xin, Huo Li, Lou Cai-Yun, Wang Qiang, Yu Wen-Ke, Jiang Xiang-Yu, Zhao Zhi-Xi, Zhang En-Yao
2016, 65 (5): 054208. doi: 10.7498/aps.65.054208
Abstract +
Quality of optical signal can be severely degraded after a long distance optical fiber transmission, therefore all-optical 2R (re-amplification, re-shaping) regeneration is required for its low power consumption and good potential of high-speed operation. Semiconductor optical amplifier (SOA) is a very promising candidate for regeneration due to its relatively low nonlinear switching power threshold and possibility of integration. However, speeds of the existing 2R schemes based on SOAs are limited by pattern effects introduced by the long carrier recovery time. So far, most of experimental results for SOA-based regeneration schemes have been limited to 40 Gb/s, only a few demonstrations at 80 Gb/s are available. An effective method to cope with the pattern effects in the SOA is to employ cross gain compression (XGC) effect. Simultaneously injecting two signals of different wavelengths with complementary logics and balanced powers into the SOA leads to an almost constant ability to make the XGC effect intrinsically immune to the pattern effects. Previously, 2R experimental results were demonstrated at 10 Gb/s and 40 Gb/s with XGC respectively, but no further results of higher speed have been presented to date. In this study, we improve the previous two-stage configuration for XGC-based regeneration by introducing a transient cross phase modulation (T-XPM) in the first SOA for generating the high-quality logic-inverted signal, which, as we find, is essential to facilitating the high-speed XGC operation in the next SOA. Firstly, a numerical model of the photon-electron dynamics in an SOA is built with considering the ultrafast intra-band processes, amplified spontaneous emission (ASE) noise at multiple wavelengths, and device segmentation along propagation direction. The quality of the logic-inverted signal with different offsets of the filter wavelength for T-XPM is studied with the model. It is found that the appropriate blue-shift detuning of the filter wavelength greatly helps to improve the quality of the logic-inverted signal. In experiment, the influence of the filter offset on the quality of the logic-inverted signal is also investigated systematically and the best quality and the largest eye-opening of the logic-inverted signal are achieved with a blue-shift of 0.72 nm, which are consistent with the simulation result. With the best logic-inverted signal, XGC effect is deployed in the second SOA. Effective reduction of the amplitude fluctuation can be observed by comparing the eye diagrams of the input degraded with output regenerative signals. Bit error rates (BERs) are also measured for all four tributaries of the degraded and the regenerated signals and the receiving sensitivity at a BER of 10-9 is improved by 1.7-2 dB. Such results show that the XGC-based 2R regeneration scheme is effective even at a speed of as high as 100 Gb/s with the help of high-quality logic-inverted signal. Degraded signals at different wavelengths (from 1535 nm to 1555 nm) are successfully regenerated with Q-factor improvement, demonstrating the wavelength-independent regeneration capability of the XGC-based 2R regenerator.
Theoretical studies of geometric and electronic structures of X@C20F20 (X=He, Ne, Ar, Kr)
Cao Qing-Song, Deng Kai-Ming
2016, 65 (5): 056102. doi: 10.7498/aps.65.056102
Abstract +
Several years ago, scientists could already introduced noble gas atoms (He, Ne, Ar, Kr, and Xe) into C60 and higher fullerenes. For the specific cases of He and Ne, the calculations suggested that both atoms are slightly bound inside C60 through simultaneous van der Waals interactions with all 60 carbons. The cavity in dodecahedrane is much smaller than that in C60, but the experimental study found that by bombarding dodecahedrane with fast, neutral helium atoms, He@C20H20 is formed. The structures of C20F20 and C20H20 are similar. Are noble gas atoms also stable in the C20F20? and, are there charges transferring between noble gas atoms and the carbon cage? In this paper, the generalized gradient approximation based on density functional theory is used to analyze the geometric and electronic structures of the endohedral fullerene X@C20F20 (X=He, Ne, Ar, Kr). The geometric optimization shows that the noble gas atoms X are all stable in the center of C20F20 cage. The C-C bond lengths of the X@C20F20 increase with the atomic number X increasing, while the C-F bond length is hardly changed. The inclusion energies of the X@C20F20 (X=He, Ne, Ar, Kr) are 1.359, 3.853, 11.276 and 15.783 eV respectively. These are all positive, which shows that the X@C20F20 have good thermodynamic stabilities, and the thermodynamic stabilities of the X@C20F20 are enhanced with the increase of X atomic number. The energy gaps of the X@C20F20 (X=He, Ne, Ar, Kr) are 5.179, 4.882, 5.874 and 6.205 eV respectively, which are greater than that of C20F20. It indicates that the X@C20F20 have better dynamic stabilities than C20F20. In addition, the vibration frequencies of the X@C20F20 (X=He, Ne, Ar, Kr) are all positive. These indicate that the stability of C20F20 is significantly improved when the X atom is introduced into the cage, and is gradually increasing with the increase of X atomic number. The electronic structures demonstrate that the X atom has no contribution to the occupied molecular orbitals near the Fermi level of X@C20F20, and the contribution of the X atom to the unoccupied molecular orbitals is relatively large. The calculation also shows that the atoms of He and Kr obtain 0.126 and 0.271 electrons from the carbons of the C20F20 cage, while Ar and Ne transfer 0.060 and 0.012 electrons to the carbons of the cage repectively. Thus there are electrons transferring between the X atoms and the carbons of the cage, indicating that the formed C-X bonds of the X@C20F20 are ionic bonds.
Microstructure characteristics of burning products of Ti-V-Cr fireproof titanium alloy by frictional ignition
Mi Guang-Bao, Huang Xu, Cao Jing-Xia, Wang Bao, Cao Chun-Xiao
2016, 65 (5): 056103. doi: 10.7498/aps.65.056103
Abstract +
Titanium fire in the aero-engine is a typical accident caused by ignition and burning of titanium alloy, which leads to a huge damage. Some articles wrote it as to turn pale at the mention of titanium. Fireproof titanium alloy, a new type of structural titanium alloy with fireproof function, has been developed to prevent titanium from fire hazard and to ensure safe and reliable service of aero-engine. In view of the lack of clear understanding of the microscopic mechanisms found currently for the structural functionality of fireproof titanium alloys, in this work, using frictional ignition technology in oxygen-rich environment (friction oxygen concentration method), associated with in-situ observation, SEM, EDS and XRD analyses, the microstructure characteristics of burning products of Ti-V-Cr fireproof titanium alloys are investigated and the element distribution law associated with microscopic mechanism during combustion reaction process is disclosed. Results show that Ti-V-Cr fireproof titanium alloys produce dazzling white light during combustion, with the typical flame characteristics of metal combustion. The generated products after burning are mainly TiO2, V2O5 and Cr2O3 oxides, in the form of dispersive particles and dense continuous body. The dispersive particles are in regular spheric shape, with a size of 10-50 m; the dense continuous products after burning presents divisional feature. After the combustion lasts 18 s, four distinct zones form from the alloy matrix to the combustion surface and they are in the sequence of transitional zone, heat-affected zone fusion zone, and combustion zone, with sizes of 40-50, 200-210, 60-70, and 18-21 m respectively. Further, some small granular shaped bulges exist in the transitional zone, in some fixed directions; in the heat-affected zone, a large number of V-based solid solution and some Ti-based solid solution form, and the titanium containing V-based solid solution is much higher than the needle-like precipitation phase in the matrix. In the fusion zone, there are some V-based solid solutions in most of Ti-based solid solution; while, the combustion zone mainly contains the mixed oxides of Ti, V, and Cr. The V-based solid solution in the heat-affected zone reduces the diffusion rate of titanium to the fusion zone, slowing the preferential reaction between titanium and oxygen in the combustion zone; while the generated mixed oxides of TiO2, V2O5, Cr2O3, etc. in the combustion zone and the solution of oxygen in titanium in the fusion zone jointly prevent the diffusion of oxygen to the alloy matrix, thus the Ti-V-Cr fireproof titanium alloys can have excellent fireproof functions.
Multi-scale ordered patterns in photosensitive ternary polymer mixtures
Guo Yu-Qi, Pan Jun-Xing, Zhang Jin-Jun, Sun Min-Na, Wang Bao-Feng, Wu Hai-Shun
2016, 65 (5): 056401. doi: 10.7498/aps.65.056401
Abstract +
Multi-scale ordered patterns of multi-component polymer mixtures can reveal many peculiar chemical and physical properties, which makes these systems have very important potential applications in materials engineering. Via computer simulation, we create interesting ordered multi-scale structures in photosensitive and immiscible polymer mixtures. The system that we employed comprises a ternary, molten A/B/C polymer blends and the three components are mutually immiscible. Polymer C is non-optically active, while polymers A and B can exhibit reversible chemical reaction A ⇆ B induced by light. Firstly, we investigate the phase behavior of the ternary blend guided by cross-stripy mask and light, and find that a chessboard-like ordered pattern forms in the mixture before removing the mask. In the illuminated regions, the A and C components gather into ellipsoidal core-shell structures in the uncrossed illuminated area, while the A and B components gather into star structures in the crossed stripes regions. When we remove the mask, the entire system becomes illuminated, and the reaction A ⇆ B occurs throughout the film: the ellipsoidal core-shell structures of A and C components turn to spherical structures, and the star structures of A and B components turn into concentric square ring structures. Then we show the influences of the number of cross stripes and the initial composition on the formation of structure. The average spatial volume fraction of C component first increases and then decreases with the stripy number increasing and the C component net lattices play an important role in the stability of ordered structures. Secondly, when the blend is covered by the annular mask, the C component gathers to the illuminated regions and the A and B components are in radial arrangement. As a result, the mixture forms an interesting dartboard-like pattern. However, when the mask is removed, the photochemical reactions occur in the A and B components of the whole region, the increasing of free energy induces the dartboard-like pattern to be broken and to change into dots-ring structure and then it forms a perfect concentric ring pattern and the target-like pattern. And also, we show the effects of initial composition ratio of C component, the distance between two adjacent rings D, the ring width d, and the illumination intensity on the evolution of ordered structure. The larger the initial composition ratio of C component, the more easily the ordered target-like pattern forms; the larger the distance D and the smaller the width d, the better the pinning effect of C component is. The illumination intensity has little influence on the ordered morphology of the ternary system. We provide a simple approach to creating multi-scale patterned films with long-range order, which could guide us in fabricating nanoscale devices.


Preparation and characterization of orthorhombic Fe2(MoO4)3 and first-principle study of its negative thermal expansion properties
Chai Feng-Tao, Yue Ji-Li, Qiu Wu-Jie, Guo Hai-Bo, Chen Li-Jiang, Shi Si-Qi
2016, 65 (5): 056501. doi: 10.7498/aps.65.056501
Abstract +
Monoclinic Fe2(MoO4)3 sample is synthesized by the hydrothermal method, and characterized via high temperature X-ray diffraction and thermogravimetric-differential scanning calorimetry. It is observed that the reversible phase transition between the low-temperature monoclinic and high-temperature orthorhombic phases occurs at about 510 ℃. The cell parameters at different temperatures are calculated by the Rietveld refinement method. In a temperature range from 25 ℃ to 400 ℃, the a, b and c crystallographic axes with the monoclinic phase gradually expand. On the other hand, in a temperature range from 530 ℃ to 710 ℃, the orthorhombic phase exhibits a negative thermal expansion (NTE) behavior, in which the b and c axes gradually contract but the a axis first contracts and then expands a little. Atomic and electronic structures are investigated using first-principle calculation. Results indicate that the Mo-O bonds are much stronger than the Fe-O bonds in Fe2(MoO4)_{3} and the MoO4 tetrahedrons are more rigidly than FeO6 octahedrons. To reveal the relationship between NTE and polyhedral distortion, the phonon density of state of Fe2(MoO4)3 is calculated using the ab initio method. The experimental Raman spectrum positions can be identified in the calculated dispersion of the total phonon density of states (DOS). Meanwhile, by calculating the Grneisen parameters for phonon branches at point, the optical branch with the lowest vibration frequency is believed to have the largest negative Grneisen parameter. Furthermore, we analyze the vibrational behaviors of atoms, and find that oxygen atoms have different vibrational eigenvectors from Fe or Mo atoms. and more obvious amplitudes than Fe or Mo atoms. Therefore, it is concluded that the transverse vibration of the oxygen bridge atom between the MoO4 tetrahedron and FeO6 octahedron, the soft distortion of FeO6 octahedrons, and the rigid rotation of MoO4 tetrahedrons jointly lead to the negative thermal expansion of Fe2(MoO4)3,.
Design of planar waveguide based on patterning substrate and oriented polymer film
Fang Yue-Ting, Yi Jian-Peng, Chen Jin-Shan, Wang Hong-Jie, Chi Lang, Xia Rui-Dong
2016, 65 (5): 056101. doi: 10.7498/aps.65.056101
Abstract +
Semiconducting conjugated polymersused for light emitting devices (LEDs), lasers and amplifiers have received considerable attention due to their low cost and easy fabrication through spin-coating and photochemical processing. A promising material for LED and laser applications is poly(9, 9-dioctylfluorene-co-benzothiadiazole) (F8BT). F8BT has a low stimulated emission threshold and exhibits a large net optical gain at 570 nm. It also shows liquid crystallinity and can be readily aligned into a monodomain by using an alignment layer, polyimide (PI). Oriented film of F8BT exhibits that its charge carrier mobility is increased by more than one order of magnitude compared with isotropic film. The refractive index of the material is also greatly affected by the orientation of the polymer chain. Furthermore, it has been reported that low threshold laser can be achieved by blending P3 HT or red-F solution into F8BT via energy transfer.Here we report a planar waveguide structure obtained via patterning chain oriented area on F8BT: red-F (9 : 1) blend polymer film. The blend solution is obtained by mixing the F8BT solution with red-F solution (with the same concentration, 20 mg/ml in toluene) with a ratio of 9 : 1. The designed waveguide patterns are obtained by inkjet-printing the PI solution onto the pre-cleaned quartz substrates. Thin films (150-200~nm thick) of F8BT: Red F are deposited onto PI by spin coating (2000 rpm). The chain alignment treatment is performed by the following procedure: the films are kept in N2 at 265 ℃ for 2 min, then they are cooled down to 235 ℃ at a rate of 1 ℃/min, finally they are cooled down to room temperature sharply. The PI contacted area on the film becomes anisotropic, while the area without PI keeps isotropic. The refractive index parallel (perpendicular) to the chain direction is significantly increased (reduced) in the PI contacted area compared with outside the PI area. Therefore, the waveguide confinement could be achieved without changing the thickness of the film. Experimental investigations, including AFM images, polarized microscopy images, polarized absorption, and PL spectra of the patterned samples, clearly show the difference between the aligned area and isotropic area.The large percentage of overlap between the emission spectrum of F8BT and the absorption spectrum of red-F solution leads to an efficient energy transfer from F8BT (host) to red-F solution (guest), resulting in a red emission at a wavelength between 600-670 nm from the blend. The polarized absorption and PL spectra of the aligned F8BT: red-F film demonstrate that the absorption intensity of the polarized light parallel to the aligned chain is 5.9 times that perpendicular to the aligned chain at a wavelength of 477 nm, and their ratio is 5.5 at a wavelength of 631 nm.Our demonstration suggests that patterning chain oriented area can be a promising approach to achieving planar waveguide devices by utilizing the refraction index contrast within and beyond the chain oriented region, and the substrate of polyimide (PI) could be patterned with various widths and shapes by the use of inkjet printing technology.
First principles study on the H2 diffusion and desorption at the Li-doped MgH2(001) surface
Zhu Yue, Li Yong-Cheng, Wang Fu-He
2016, 65 (5): 056801. doi: 10.7498/aps.65.056801
Abstract +
As one of the most practical solutions to on-board hydrogen storage, MgH2 has attracted a lot of attention, which is mainly due to its high hydrogen capacity (7.7 wt%), high volumetric storage density(55 kg/m3) and low cost. The main obstacles for its large scale applications are the relatively low rates of hydrogen absorption and desorption in the material, which can be traced back to the slow diffusion of hydrogen into the crystal MgH2. In this work, the doping effect of Li on the release of hydrogen at the MgH2(001) surface is studied by the first-principles calculations based on the density functional theory and the climbing nudged elastic band method. Two possible diffusion and desorption paths for H atoms are designed. In path one, the two hydrogen atoms, which bond with the same substituted Mg atom in the first surface layer, climb over the nearest neighbor Mg atom to form a hydrogen molecule. In path two, the two nearest hydrogen atoms, which bond with two different Mg atoms in the first surface layer, combine directly together to form a hydrogen molecule. The calculated results show that the energy barriers for the two paths at the pure MgH2(001) surface are 2.29 and 2.50 eV, respectively. When the center Mg atom is replaced by Li atom, the corresponding energy barriers decrease to 0.31 and 0.22 eV, respectively. Compared with the pure surface, the Li-doped surface has the energy barriers that reduce almost 87% and 91%. It indicates that the formation and release of H2 at MgH2 (001) surface become easier after the surface has been doped with Li atoms. Furthermore, the doping effects are analyzed with the density of states. Compared with the pure surface, the Li-doped surface has a Fermi level that lowers from the band gap to the top of the valance band and the system is changed from insulator into conductor. At the same time, the bonds between Li and hydrogen atoms in the Li-doped system are weaker than those between the substituted Mg and the corresponding hydrogen atoms in the pure system. As a result, the doping of Li atoms makes it easier to form and release H2 at MgH2(001) surface.
Infrared laser protection of multi-wavelength with high optical switching efficiency VO2 film
Wang Ya-Qin, Yao Gang, Huang Zi-Jian, Huang Ying
2016, 65 (5): 057102. doi: 10.7498/aps.65.057102
Abstract +
Vanadium dioxide (VO2) film with nanoparticles is fabricated by reactive ion beam deposition (RIBD) technology and post-annealing method on a quartz glass substrate. RIBD can enhance the damage threshold of VO2 film and reduce its scattering at insulator-state. And post-annealing can eliminate the structure defects and residual stress. VO2 film exhibits first-order and reversible metal-to-insulator (MIT) phase transition at a temperature of 68 ℃. It also exhibits photo-induced MIT, in which process a metal-like phase of monoclinic VO2 appears. With many surprising features in heat-induced and photo-induced MIT processes, VO2 film turn to satisfy all the characteristics needed for a laser protection system. The thickness of VO2 film used in these experiments and simulations is about 100 nm. The double-frequency He-Ne laser at a wavelength of 3 m is used to perform the experiment of heat-induced MIT, with a temperature controlling system. The exact optimal annealing temperature is demonstrated to be 465 ℃, as the sample annealing at this temperature shows the sharpest transition properties and unmixed VO2 phase peaks in X-ray diffraction pattern. Drude and Drude-Lorentz dispersion models are taken to analyze the dielectric constant of VO2. Then, the complex refractive index is calculated for simulation. Simulations with the TFCale software show that the transmissions at high temperature and low temperature have high contrasts in the infrared range. MIT experiments at multi-wavelength, which cover heat-induced and photo-induced MIT phase transition, are performed to investigate the applicability of VO2 film in multi-wavelength laser protection for both continuous wave and pulsed lasers Thus the excellent performance of VO2 film for laser protection is roundly verified. The laser protection experiments on silicon photocell exhibit that the VO2 film enhances the anti-jamming capability of photocell system by about 2.6 times, demonstrating the applicability of VO2 film to laser protection system. The power density of MIT transition threshold of VO2 film with a thickness of 100 nm is 4.35 W/cm2 at room temperature, which is investigated with a continuous wave laser at a wavelength of 1.08 m with a continuous tunable system. In addition, atomic force microscope is used to observe the film surfaces, which are irradiated by lasers with different power densities for different times The experimental results demonstrate that the power density damage threshold of VO2 film becomes very high (404 W/cm2). The low MIT transition threshold and high damage threshold of VO2 film further demonstrate its applicability as a key role for a laser protection system. With the high switching efficiency ratio and high damage threshold, VO2 thin film can be used in optical switch, smart windows and photoelectric device.
Theoretical studies of the site preference, electronic and lattice vibration properties of La3Co29-xFexSi4B10
Wang Xiao-Xu, Zhao Liu-Tao, Cheng Hai-Xia, Qian Ping
2016, 65 (5): 057103. doi: 10.7498/aps.65.057103
Abstract +
In this work, the initial configuration is first optimized by the first principle and interatomic pair potentials separately, the lattice parameters of the stable structure are in good agreement with the experimental values. The site preferences of La3Co29-xFexSi4B10 compounds are studied by using the first principle with density function theory method. The calculated results show that the substitution of Fe for Co has a strong preference for the 2c site, and the substitution sequence is 2c 8j1 8i2 8j2 8i3 16k 8i1, which is in good agreement with the experimental result The lattice parameters of La3Co29-xFexSi4B10 system change little, but the magnetic moment changes obviously, when only one Co atom is substituted by Fe atoms each time. We calculate the electronic densities of states and magnetic moments of La3Co29-xFexSi4B10 compound when all the Co atoms from different sites are substituted by Fe atoms with the preferential order With the increases of Fe content values in the La3Co29-xFexSi4B10, the curves of density of states move leftwards gradually. And the magnetic moment of the La3Fe29Si4B10 is larger than that of La3Co29Si4B10. Furthermore, the lattice vibrational and thermodynamic properties are predicted by using a series of interatomic pair potentials. The Co, Fe and La atoms contribute to the lower frequency vibrations because of their heavier mass. With the increase of Fe content the cut-off frequencies of La3Co29-xFexSi4B10 first decrease and then increase, and the vibration mode induced by Si element decreases in medium frequency. The very strong B-B interaction causes higher frequency vibrations. Furthermore, the specific heat, vibrational entropy and Debye temperature are predicted based on the phonon densities of states of the La3Co29-xFexSi4B10 with the different content values of Fe. The Debye temperature rises when the Fe content is bigger than Co content in La3Co29-xFexSi4B10compound.
Numerical study of plasmonic filter based on metal-insulator-metal waveguide
Yang Yun-Ru, Guan Jian-Fei
2016, 65 (5): 057301. doi: 10.7498/aps.65.057301
Abstract +
A novel nanometeric plasmonic filter comprised of double-sided eight stub resonators side-coupled with a metal-isolator-metal waveguide is proposed and demonstrated numerically by the finite element method. The numerical results show that the four transmittance peaks in a transmission spectrum range from 400 nm to 2000 nm can be achieved due to the electromagnetically-induced-transparency-like spectral responses between every two adjacent stub resonators with detuned cavity length. Based on the magnetic field distributions from the two dimensional model, the physical origins of transmittance peaks and dips are clarified by phase analysis of Fabry-Perot resonance effect. In addition, the central wavelengths of transmittance peaks can be tuned by adjusting the cavity length of each stub resonator, which means the waveguide filter could be utilized to develop ultracompact and tunable narrowband photonic filters for high integration.
Subgroup decomposition analyses of D3h and D4h plasmonic metamolecule Fano resonance spectrum
Li Meng-Jun, Fang Hui, Li Xiao-Ming, Yuan Xiao-Cong
2016, 65 (5): 057302. doi: 10.7498/aps.65.057302
Abstract +
In recent decades, research about surface plasmon polariton (SPP) has earned its popularity in nanotechnology with many theoretical achievements, much progress in metal nanostructure manufacturing, spectral analyzing, biomedicine ultrasensing, etc. Group theory is an effective tool for analyzing the spectra of symmetrical organized multiparticles (dubbed as plasmonic metamolecule). Recently, SPP Fano resonance in nanostructure either from plasmonic metamolecules or from symmetry-breaking has attracted much attention. Regarding to the subgroup decomposition analysis of the D3h and D4h plasmonic metamolecule surface plasmon resonance spectra and the mechanism of forming the Fano resonance spectral dip, this paper proposes an explanation method based on group theory.By using a similar group theory approach to constructing the molecular vibration normal modes, the method to build the dipolar SPP symmetric modes of plasmonic metamolecules is established. It is confirmed that under the linear polarization excitation there exists only two dipolar SPP symmetric modes for a ring shaped Dnh plasmonic metamolecule, while adding the center particle will merely add an extra independent symmetric mode. For the D3h and D4h plasmonic metamolecule, it is found that there are two dominant eigenmodes i. e., one is composed by adding two symmetric modes and the other by subtracting two symmetric modes. The decomposition analysis further reveals that the negative coefficient of the symmetric mode for forming the short wavelength eigenmode for D3h tetramer plasmonic metamolecules is much smaller than that for D4h pentamer plasmonic metamolecules, thereby explaining that the Fano resonance dip of the pentamer is sharper than that of the tetramer. It is worth noting that the group theory can provide some guidance for building the symmetric modes and the SPP eigenmodes, but is unable to determine the coefficient of each symmetric mode.As for the origin of Fano resonance dip, so far there have existed two different perspectives: one is the traditional viewpoint, i.e., the Fano resonance dip is formed due to the coupling of the wideband superradiant bright mode with the narrowband subradiant dark mode, and the other is that the Fano resonance dip is formed by the destructive interference between two neighboring eigenmodes. The decomposition analysis described in this paper actually can unify these two perspectives.
Effects of Al-2N doping on the photoelectric properties of ZnO
Hou Qing-Yu, Qu Ling-Feng, Zhao Chun-Wang
2016, 65 (5): 057401. doi: 10.7498/aps.65.057401
Abstract +
In a similar range of Al-2N doping amount to that in the present paper, the absorption spectra of ZnO doped system and two kinds of experimental results have been reported in the literature. However, there is no reasonable explanation for the absorption spectra of ZnO doped system. In order to solve the problem, all calculations in the present paper are carried out by the CASTEP tool in the Materials Studio software based on the first principal ultrasoft pseudopotential of the density functional theory, and the geometric structures of ZnO, Zn0.98148Al0.01852O0.96296N0.03704 and Zn0.96875Al0.03125O0.9375N0.0625 systems are constructed by first-principal. All the models are based on the optimization of the geometry structure. And the distribution of the band structure, the density of states and the absorption spectra of the doping system are calculated by the method of GGA+U. The results indicate that in the range of the doping content restricted in the present paper, the bigger the doping amount of Al-2N, the smaller the volume of doped system is; the higher the total energy, the more the stability decreases; the higher the formation energy, the harder the doping becomes and the narrower the optical band gap of doped system. Meanwhile, the higher the Al-2N doping content, the narrower the optical bandgap of the doping system becomes, which suggests that the more significant the red shift of absorption spectrum of Al-2N doped ZnO system is. Therefore, the doped system is controlled within the doping content in experiment to obtain the narrow optical band gap and red shift in absorption spectrum in Al-2N doped ZnO, in addition to the control of lower nanoscale of Al-2N doped in ZnO. At the same time, all doping systems are p-type degenerated semiconductors. Then, the higher the Al-2N doping content, the smaller the relative concentration of free holes of doped system is; the smaller the hole effective mass, the lower the mobility is; the lower the hole conductivity, the worse the conductive property of doping system is. The calculated results are in agreement with the experimental results. The research shows that Al-2N co-doped ZnO can be a new type of semiconductor material, a functional material which is used at low temperature end of thermoelectric power generation.
Effect of Na substitution on the electronic structure and ion diffusion in Li2MnSiO4
Jia Ming-Zhen, Wang Hong-Yan, Chen Yuan-Zheng, Ma Cun-Liang
2016, 65 (5): 057101. doi: 10.7498/aps.65.057101
Abstract +
With the developments of electric vehicles, the portable electronics and the large-scale storage systems, the research of the Li-ion rechargeable battery has focused on its high gravimetric and volumetric capacity. As a potential cathode, the Li2MnSiO4 structure has been intensively studied, in which two lithium ions of per formula unit (f.u.) can be extracted, and it exhibits a high theoretical capacity of about 330 mAh/g. However the low intrinsic electron conductivity and the slow lithium diffusion prevent its further development. In this paper, we build three structures with different Na+ doping concentrations in Pmn21 symmetric Li2MnSiO4, the electronic properties and Li+ ion diffusion behavior are studied by using the first principle and considering the transition barrier of the Mn-3d. Within the GGA+U scheme, the pure Li2MnSiO4 structure is semiconducting with a large band gap (3.28 eV), which is primarily derived from Mn-3d and O-2p states. Because lithium and sodium ions in the same main group have similar chemical properties, all the doped Li2-xNaxMnSiO4 (x= 0.125, 0.25, 0.5) are still semiconducting with the analogous densities of state (DOSs) to the pure Li2MnSiO4, however the band gaps reduce to 3.23 eV, 3.19 eV and 3.08 eV, respectively. Thus Na+ substitution can improve the electron conductivity. In Li2MnSiO4, the Li+ ions have two major diffusion channels predicted by the climbing image-nudged elastic band (CI-NEB) method. Channel A is along the a-direction [100], and channel B is in the bc plane with a zigzag trajectory. In the migration process, each of all the structures has only one migration pathway of Li ions. In the doped structures, the volumes of the crystal structures are increased by 1.40%, 2.65% and 5.25% for Li2-xNaxMnSiO4 (x= 0.125, 0.25, 0.5), and thus enlarge the hopping distances. Along channel A, the longer Li-O bond makes the ionic diffusion channel wider, therefore Li2-xNaxMnSiO4 (x= 0.125, 0.25, 0.5) have lower activation barriers of 0.48, 0.52 and 0.55 eV than the pure Li2MnSiO4 (0.64 eV). However, in channel B, the strong Li-O bonds increase the activation barriers of Li ion migration. When the doping concentration is x=0.125, the Li+ ion migration effect is strongest. For the Li+ ion migration pathways, it is easier for Li ion to hop into the site near Na ion. It means that the crystal structures are stabler at the short Li-O bond site. Therefore, doping Na+ ions would be a feasible method to improve the electron conductivity and Li+ ion migration rate in Li2MnSiO4 of Pmn21 phase.
Measurement and study of low-frequency noise in TMR magnetic field sensor
Cao Jiang-Wei, Wang Rui, Wang Ying, Bai Jian-Min, Wei Fu-Lin
2016, 65 (5): 057501. doi: 10.7498/aps.65.057501
Abstract +
The magnetic field sensor based on tunnel magnetoresistance (TMR) effect has potential applications in various fields due to its very high field sensitivity and low power comsuption. However, the resolution of magnetic sensor depends on not only field sensitivity, but also intrinsic noise level. The intrinsic noise of an electronic device is normally frequency-dependent and increases in low frequency range. In a magnetic tunneling system, thermal magnetization fluctuation in the magnetic layer can couple to the resistance through the spin-dependent tunneling effect and create low-frequency magnetic noise. In addition, the charge trapping effect in the oxide barrier may also contribute to the external low-frequency noise. Therefore, the depression of the noise in TMR magnetic field sensor, especially the low-frequency noise, is extremely important for the application with high resolution requirement. In this work, a low-frequency noise measurement system for TMR magnetic sensor is built by using a highaccuracy data acquisition card and a low noise preamplifier. After subtracting the circuit noise from the measured noise, the noise spectral patterns of TMR magnetic field sensor with a full Wheatstone bridge structure are obtained under various bias currents and external magnetic fields. It is found that the noise spectra of the TMR sensor exhibit a clear 1/f character in the low frequency region and the noise power spectral intensity is proportional to the square of the bias current. By fitting the power spectral density of the noise versus frequency in the TMR sensor, the Hooge parameters are obtained, which remain unchanged in the measurement. The noise intensity increases abruptly in the magnetization switching region of the free layer in magnetic tunnel junction, suggesting that the 1/f noise mostly comes from the magnetic noise. In a magnetic hysteresis loop, this noise power is strongly field-dependent, which is due to thermal magnetization fluctuations in magnetic layers. We attribute this magnetic fluctuation to thermally excited hopping of the magnetic domain wall between the pinning sites. Finally, according to the R-H transfer curves and the measured noise spectra of the TMR sensor, the detectable minimum magnetic fields of the sensor are 9 nT and 1.3 nT at 100 Hz and 4 kHz with 1 V input voltage, respectively. These results pave a way for optimizing the noise properties of TMR magnetic sensors.
Effect of SiO2 on the Stark splitting enlargement of Yb3+ in phosphate glass
Wang Peng, Wang Chao, Hu Li-Li, Zhang Li-Yan
2016, 65 (5): 057801. doi: 10.7498/aps.65.057801
Abstract +
Up to now, high-energy large-scale Yb3+ laser has still faced to the fact that no proper gain media are available, though researchers made a more than ten year effort for it. Yb: fluorophosphate glass is the only material used in a laser system, reaching an output of more than 10 TW. As is well known, the thermal blocking of Yb3+ laser is a bottleneck in laser operation, which is closely related to the Stark splitting of Yb3+ in a gain medium. Conventional Yb: phosphate glass has the advantages in Yb3+ concentration, lifetime and cross section over silicate glass, however, its small Stark splitting is the main drawback that induces difficulties in realizing laser output at room temperature. Yb: phosphate glass will be a good gain medium for high power Yb3+ laser if the Stark splitting is improved. This study focuses on the enlargement of the Yb3+ Stark splitting in phosphate glass by introducing SiO2, thereby achieving a large Stark splitting property compared with the phosphate glass. The glass 60P2O5-7.5Al2O3-15K2O-17.5BaO-1Yb2O3 is used as the base glass, and the modified glass denoted as PS is obtained by doping a certain amount of SiO2. Such a glass is prepared by the conventional melting-quenching method. Lorentz fitting is performed to the absorption and fluorescence spectra for determining the Stark splitting scheme. Raman spectrum is used for the auxiliary judgment of the attributions of the different spectroscopic bands. Then the results are confirmed by the barycenter law of Yb3+ ion. Investigations show that the addition of SiO2 can enlarge the Stark splitting obviously from the original 670 to 771 cm-1 in PS3. Scalar crystal field parameter N_{J} and asymmetry degree around Yb3+ are also increased due to SiO2 incorporation. Meanwhile, spectroscopic properties of PS series glasses, such as fluorescence effective line width (Δλeff) and fluorescence lifetime (τf), are moderately enhanced. The glass transition temperature is improved greatly, which is very valuable for high power Yb3+ laser. These results suggest that the introduction of a second network former is an effective way to enlarge the Stark splitting of Yb3+ in phosphate glass. Next, our investigation will focus on preparing the high-homogeneity SiO2 modified phosphate glasses and the corresponding laser experiments.
Structural and photoluminescence characteristics of ZnCdO/ZnO single quantum well
Yi You-Gen, Wang Yu-Ying, Hu Qi-Feng, Zhang Yan-Bin, Peng Yong-Yi, Lei Hong-Wen, Peng Li-Ping, Wang Xue-Min, Wu Wei-Dong
2016, 65 (5): 057802. doi: 10.7498/aps.65.057802
Abstract +
Zn1-xCdxO/ZnO single quantum well is grown by laser molecular beam epitaxy on Al2O3(0001) substrate. Single quantum well samples respectively with the well-widths of 1.0 nm, 1.5 nm, 4 nm are obtained by controlling the epitaxial temperature and oxygen pressure in the vacuum chamber. The chemical compositions, surface morphologies, crystal structures of the samples are carefully studied, and the results show that the Zn0.98Cd0.02O single quantum wells are of high quality with very smooth surface (with the root mean square value of 0.6 nm in 20 μm×20 μm area) and good crystal structure. Quite a strong photoluminescence emission is obtained at 3.158-3.219 eV from the ZnCdO single quantum well at 4 K under a 325 nm He-Cd laser by tuning quantum well-width. The full width of half maximum of the photoluminescence emission peak of the 1.0 nm quantum well reaches 60 meV, which indicates a strong quantum confinement effect.
A novel Y2O3-Gd2O3-HfO2 impregnated W base direct-heated cathode in magnetron tube
Qi Shi-Kai, Wang Xiao-Xia, Luo Ji-Run, Zhao Qing-Lan, Li Yun
2016, 65 (5): 057901. doi: 10.7498/aps.65.057901
Abstract +
As the heart of a magnetron, cathode plays an important role in the device. At present, the pure W cathode is mainly used in high-power continuous wave magnetron tube. However, the pure W cathode has low thermionic emission capability and secondary electron emission yield (1.25-1.50), which result in the cathode operating at a high temperature (2450-2700 K). The higher the operating temperature of the cathode, the faster the evaporation of its surface is, which can shorten the cathode lifetime. In order to enhance the emission current, reduce the operation temperature and prolong the lifetime of the pure W cathode, a novel refractory Y2O3-Gd2O3-HfO2 impregnated W base direct-heated cathode (Y-Gd-Hf-O impregnated cathode) is developed in this paper. The present investigation mainly focuses on the thermionic emission, work function, lifetime, emission mechanism, and anti-bombing property. The direct current (dc) emission properties of the Y-Gd-Hf-O impregnated cathode are investigated, showing that it can provide more than 0.4, 1, 4.0, 7.74, 10.5 A/cm2 current density for the space charge limitation (SCL) at 1300, 1400, 1500, 1600, 1700 ℃ respectively. Absolute zero work function for the cathode is only 1.68 eV obtained by the Richardson line method. The effective work function for the cathode is in a range of 2.6-3.1 eV obtained by the Richardson-Dushman formula. The lifetime for the cathode is more than 3600 h with an initial load of 1.5 A/cm2 at 1600 ℃. The surface microstructure, element composition and content of the Y-Gd-Hf-O impregnated cathode are analyzed by the scanning electron microscope (SEM), Auger electron spectroscopy (AES), and energy disperse spectroscopy (EDS). The analysis results show that the surface of the cathode contains the Y2O3-x semiconductor layer, which causes an improvement of the electro-conductivity during the activation. The work function of the cathode can also be reduced due to the improvement of the electro-conductivity. Besides, the addition of the transition-metal oxide HfO2 changes the internal lattice energy level, which can further reduce the work function. Therefore, the Y-Gd-Hf-O impregnated cathode has good thermionic emission capability. In addition, the anti-bombing performance of the cathode is also studied, which shows that the dc emission current density decreases linearly from the initial current density of 1.5 A/cm2 to 0.4 A/cm2 after 150-h continuous electron bombing at 10 W/cm2. In the future research, we will focus on enhancing the anti-bombing property for the Y-Gd-Hf-O impregnated cathode by using Y-Gd-Hf-O doped W base direct-heated cathode.
Relaxation behavior simulation of power lithium-ion battery in high-rate charging-discharging process
Tang Yi-Wei, Ai Liang, Cheng Yun, Wang An-An, Li Shu-Guo, Jia Ming
2016, 65 (5): 058201. doi: 10.7498/aps.65.058201
Abstract +
The relaxation behaviors of a power lithium-ion battery significantly affect its performance, and these properties are greatly affected by temperature. This study presents a validated electrochemical-thermal model battery model covering the conservations of charge, mass, and energy and the electrochemical reaction kinetics, and considering the effect of heat on electrochemical performance of a battery. Using this battery model, the relaxation behavior of power lithium-ion battery in high-rate charging-discharging process and the effect of difference among charge-discharge systems are investigated. It is found that ohmic polarization is the main reason for voltage change in charging-discharging process. Constant-current-constant-voltage charging mode can effectively remit ohmic polarization and then avoid changing the voltage rapidly. In the shelving process after constant-current-constant-voltage charging, voltage change is smaller and the time for it to take to reach open circuit potential is shorter than in the shelving process after constant-current charging. In charging-discharging process, the values of polarization at positive and negative electrode are almost the same. Power lithium-ion battery can be charged into more energy by constant-current-constant-voltage charging modes, meaning that it is beneficial to battery performance. Because active material particles in electrodes have certain sizes, in discharging process, there is some gradient between the surface and center of solid particle, and the electrodes each have a certain thickness, different place of electrode has a different lithium-ion concentration. In the shelving process after discharging, there is no outer current, so the gradient of lithium-ion concentration disappears due to the effect of diffusion process. The relaxation time of lithium-ion concentration in solid phase is longer than in liquid phase. The ratio between characteristic time of solid diffusion and that of liquid diffusion increases constantly near the end of the discharge, thus the polarization due to solid diffusion cannot be neglected in the whole discharging process.
Investigation of photo-chromic properties of remote phosphor film and white light emitting diode mixed with TiO2 particles
Zhuo Ning-Ze, Zhang Na, Li Bo-Chao, Li Wen-Quan, He Qing-Yang, Shi Feng-Hua, Zhu Yue-Hua, Xing Hai-Dong, Wang Hai-Bo
2016, 65 (5): 058501. doi: 10.7498/aps.65.058501
Abstract +
Based on the hot pressing method, the remote phosphor films are prepared by adding TiO2 particles into YAG:Ce and silicon binder, and then they are packaged into white light emitting diode (WLED) device with chip on board (COB) blue light source. The photo-chromic properties and mechanism are studied and calculated. Based on Mie theory and Henyey-Greenstein function, forward scattering is the main light scattering form of YAG:Ce phosphor powder, while the forward scattering intensity is close to the back scattering intensity of TiO2 particles. The emission spectral intensity and relative luminance of remote phosphor film change with increasing the concentration of TiO2 particles, and the optimum concentration is 0.966 g/cm3. Forward transmission intensity and back reflection intensity are calculated and analyzed, when the concentration of TiO2 is low, the forward transmission intensity of blue light is stronger than that of yellow light and the main transmission form is forward transmission, while the forward and backward intensity of yellow light are similar because of isotropy. With increasing the concentration of TiO2, the forward intensity of blue light gradually decreases, and the transmission intensity is lower than that of yellow light. The forward and backward intensity of yellow light reach their maxima when the TiO2 concentration is 0.966 g/cm3. The main reason for this phenomenon is that the increasing of the utilization ratio between blue light and transmission of yellow light is affected by the strong scattering ability of TiO2. Finally the WLEDs are packaged by remote phosphor films and COB blue light source, the luminous flux of WLED reaches 415.28 lm (at 300 mA and 9.3 V) at a concentration of 0.966 g/cm3, which is increased by 8.15% compared with the concentration in the case of no TiO2 mixing. Besides, the correlated color temperature changes from cool white 6900 K to warm white 3832 K gradually. Consequently, the adding of TiO2 particles can not only improve the emission intensity of remote phosphor film and the luminous flux of WLED, but also regulate the correlated color temperature.
Quantitative analysis of the field of view for X-ray differential phase contrast imaging
Du Yang, Liu Xin, Lei Yao-Hu, Huang Jian-Heng, Zhao Zhi-Gang, Lin Dan-Ying, Guo Jin-Chuan, Li Ji, Niu Han-Ben
2016, 65 (5): 058701. doi: 10.7498/aps.65.058701
Abstract +
Grating-based X-ray differential phase contrast imaging provides excellent image contrast for low-Z objects that cannot be acquired by conventional X-ray imaging, which has great potential applications in the early diagnosis of cancer and non-destructive detections of low-Z materials and devices. Large field of view imaging is a crucial factor for this technology from the laboratory to practical application. For the objective need of large field of view, on the basis of the Fresnel diffraction theory and the structure characteristics of gratings, we establish a quantitative physical model to analyze the factors that affect the imaging field of view and give a feasible way for large imaging field of view. This work provides a theoretical basis for the large field of view grating-based X-ray differential phase contrast imaging in the future.
Core-periphery structure in heterogeneous adaptive network and its inhibiting effect on epidemic spreading
Yang Hui, Tang Ming, Cai Shi-Min, Zhou Tao
2016, 65 (5): 058901. doi: 10.7498/aps.65.058901
Abstract +
The study of epidemic spreading in node-property heterogeneous adaptive network shows that node-property heterogeneity can greatly increase the epidemic threshold, and the initial network can adaptively self-organize into a more robust degree heterogeneous network structure. The difference in epidemic spreading between homogeneous and heterogeneous node-property adaptive networks is of great importance for understanding the threshold increasing in the heterogeneous node-property adaptive network. In this paper, we study the transient spreading process in the heterogeneous node-property adaptive network. In order to capture the core-periphery structure in the network, we define two hierarchical structure indicators. When both indicators are of large values in the network, not only is the network of strong core-periphery property, but also less susceptible nodes are more likely to be in the core area of the network. By combining them with various network structure properties, such as the average degree ratio and static threshold of transient network, we analyze the evolution of network structure and show the self-organizing formation process of robust degree heterogeneous structure by numerical simulations. We find that the threshold increase is basically due to the formation of core-periphery structure, where the less susceptible nodes are more likely to be reallocated to the core area of the network by rewiring. In view of this, we propose a new preference rewiring strategy. The results show that the new strategy can increase the epidemic threshold by faciliating the formation of core-periphery structure, which verifies the correctness of the transient network structure analysis. It not only helps to deeply understand the epidemic spreading process in the node-property heterogeneous adaptive network, but also provides new ideas for putting forward the strategy of controlling epidemics.
Rapid preparations of Bi1-xLaxFeO3± δ thin films and their ferroelectric properties
Shi Yu-Jun, Zhang Xu, Qin Lei, Jin Kui, Yuan Jie, Zhu Bei-Yi, Zhu Yun
2016, 65 (5): 058101. doi: 10.7498/aps.65.058101
Abstract +
Multiferroic materials exhibiting the features of ferroelectricity, ferromagnetism and even ferroelasticity simultaneously have attracted much attention because of their vast potential applications in multifunctional devices as well as their interesting physical connotations. BiFeO3 (BFO) is the multiferroic material most studied because it has only single phase of multiferroic oxide with giant remanent polarization above room temperature. Although BFO has many excellent advantages, the large leakage current is a chief obstacle for its practical application in some devices. As is well known, the leakage current of BFO is due to the valence transformation from Fe3+ to Fe2+ which results in the oxygen vacancy defect and low ferroelectric properties. Some experiments have confirmed that substituting some cations at A site (Bi) or B site (Fe) can improve the multiferroic property of BFO. In addition, we can reduce the leakage current by increasing the oxygen pressure to compensate for the vacancy defect during annealing. In the present work, we employ the sol-gel method which has been widely used in industries to prepare lanthanum doped BFO thin films (La =0, 5%, 10%, 15%, 20% and 25%) (BLFO) and Bi0.75La0.25FeO3± δ thin films separately in air and pure oxygen annealing atmosphere. And we are to achieve the optimal ferroelectric properties of BFO thin films. The traditional trial-and-error method which is used to check the value of a certain parameter one by one always takes rather long time. The high throughput methodology can screen the parameters simultaneously, which greatly reduces the optimizing time. Employing the high throughput methodology, we successfully realize a faster optimizing process to achieve the strongest ferroelectric property in La-doping BFO thin film. We analyze the structures and the ferroelectric properties of the samples grown in different conditions, such as the annealing temperature, the concentration of La-doping and the annealing atmosphere, etc. Results are as follows. 1) The optimal annealing temperature for achieving a single phase thin film is around 560℃. X-ray diffraction (XRD) patterns show that all the samples, including La-doping thin films with different concentrations, are of perfect single phase. Bi0.75La0.25FeO3± δ thin films are prepared separately in air and pure oxygen annealing atmosphere. 2) We calculate the lattice constants for all the doping samples of BLFO. With the increase of La-doping concentration, both a and b values reach the largest lattice constants of a=b=5.59~Å at La=15%. 3) Among all the doping samples, the sample with a La-doping concentration of 15% has the largest polarization 26.7 μC/cm2, which is consistent with its largest lattice constants. 4) The degrees of crystallinity and the ferroelectric properties of Bi0.75La0.25FeO3±δ thin films annealed in pure oxygen atmosphere are much better than those in air. The high throughput method is successfully used in the present work, and it plays an important role in exploring new materials in high-efficiency, speediness and objectivity. Therefore, it can be extended to many other materials for optimizing the grow conditions.
Effects of different coil combinations on the optimal design of a 25 T superconducting magnet
Zhu Guang, Liu Jian-Hua, Cheng Jun-Sheng, Feng Zhong-Kui, Dai Yin-Min, Wang Qiu-Liang
2016, 65 (5): 058401. doi: 10.7498/aps.65.058401
Abstract +
High field above 20 T is required in diverse physical programs and nuclear magnetic resonance (NMR) systems. For intended science program requirements, as a demonstration of the development in high field superconducting magnet technology, a 25 T (4.2 K) 52 mm cold-bore all-superconducting magnet consisting of a 10 T high-temperature superconducting insert magnet and a 15 T low-temperature superconducting background magnet, is being developed at the Institute of Electrical Engineering, Chinese Academy of Sciences. The development of such a magnet requires its optimization, and the choosing the number and type of coils is crucial to the final optimal design. However there are few researches focusing on the effect of coil combinations. To study the relationship between the number of coils and the magnet parameters, we first discuss the magnet optimization. The objective function of the optimization is defined as the weighted function of coil volume according to the costs of different superconductors, and the following constraint conditions are taken into considerations: center field, YBCO conductor characterization, hoop stress in Nb3Sn coils, and the critical performances of these wires. All those constraint conditions are taken in the analytical form, and the magnetic field, stress results are verified with the finite element method. To guarantee the reliability of the optimal results, in addition to consider the constraint conditions, a method of combining global optimization and local optimization is adopted. 20 different coil combinations are selected according to the investigation of superconducting wires, and their optimal results are calculated. The following conclusions are drawn from the analyses of these results. Firstly, in the design of high field magnet, the number of coils and magnet cost demonstrate a "V"-shaped relationship, that is, there exist an optimal number of coils. Secondly, when the objective function demonstrates good values, Nb3Sn coils generate fields in a range of 6-7 T, whereas NbTi coils generate fields in a range of 8-9 T. Finally, the objective functions under two different situations, i.e., Nb3Sn coils and NbTi coils are powered together and separately, are calculated. From the comparisons we find that the effect of reducing one power supply is acceptable when the number of coils is not too big.
Progress of new carbon material research in perovskite solar cells
Wang Jun-Xia, Bi Zhuo-Neng, Liang Zhu-Rong, Xu Xue-Qing
2016, 65 (5): 058801. doi: 10.7498/aps.65.058801
Abstract +
A photoelectric conversion efficiency of 3.8% was achieved based on organic-inorganic hybrid perovskites CH3NH3PbBr3 and CH3NH3PbI3 in 2009, and their efficiencies have leaped to 20.1% in the past five years, which are comparable to Cu(In,Ga) Se2 solar cells. The researchers mainly focused on appropriate materials and device structures, high-quality film depositions, careful interface designs and controllable carrier properties. Even so, it is still a long-term work to develop the low-priced, stable, environmental-friendly and highly-efficient perovskite solar cells, for example, the hole transport material spiro-OMeTAD is complicated and expensive, the electron transport material TiO2 must be processed by high temperature annealing and the Au electrode is extensively used, all of which are not conducible to the commercialized application. On this occasion, new carbon materials, such as graphene oxide, carbon nanotubes, fullerene, graphdiyne, etc. have become another highlight of perovskite solar cells due to their excellent thermal, mechanical, electrical and optical performances. Carbon materials are low-cost and highly available industrial materials, which have been applied to highly efficient counter electrodes for dye-sensitized solar cell and quantum dot-sensitized solar cells. The approximate 5.0 eV work function makes carbon material the ideal counter electrode material for perovskite solar cell. Carbon material is endowed with remarkably high charge mobility and electronic conductivity, which has been identified as one of the strongest materials for electron transport in perovskite solar cell. Similarly, a perovskite solar cell using hole transport materials incorporating carbon material shows an improved power conversion efficiency due to enhanced electrical conductivity and carrier mobility because the low electrical conductivity of hole transport material such as spiro-OMeTAD is considered to be an impediment to further enhancement of the power conversion efficiency and a hole transport material with higher conductivity should reduce the series resistance and increase the fill factor, thereby enhancing the power conversion efficiency of perovskite solar cell. In this paper, the research progress of new carbon materials for counter electrode, electron transport materials, hole transport materials in perovskite solar cells are summarized. The power efficiency of perovskite solar cell is enhanced greatly because of the introduction of new carbon materials, which provides a new idea for the further application of new carbon materials and device design of perovskite solar cells.
Spherical Dirac equation on the lattice and the problem of the spurious states
Zhao Bin
2016, 65 (5): 052401. doi: 10.7498/aps.65.052401
Abstract +
With the development of radioactive ion beam facilities, the study of exotic nuclei with unusual N/Z ratio has attracted much attention. Compared with the stable nuclei, the exotic nuclei have many novel features, such as the halo phenomenon. In order to describe the halo phenomenon with the diffused density distribution, the correct asymptotic behaviors of wave functions should be treated properly. The relativistic continuum Hartree-Bogoliubov (RCHB) theory which provides a unified and self-consistent description of mean field, pair correlation and continuum has achieved great success in describing the spherical exotic nuclei. In order to study the halo phenomenon in deformed nuclei, it is necessary to extend RCHB theory to the deformed case. However, solving the relativistic Hartree-Bogoliubov equation in space is extremely difficult and time consuming. Imaginary time step method is an efficient method to solve differential equations in coordinate space. It has been used extensively in the nonrelativistic case. For Dirac equation, it is very challenging to use the imaginary time step method due to the Dirac sea. This problem can be solved by the inverse Hamiltonian method. However, the problem of spurious states comes out. In this paper, we solve the radial Dirac equation by the imaginary time step method in coordinate space and study the problem of spurious states. It can be proved that for any potential, when using the three-point differential formula to discretize the first-order derivative operator, the energies of the single-particle states respectively with quantum numbers and - are identical. One of them is a physical state and the other is a spurious state. Although they have the same energies, their wave functions have different behaviors. The wave function of physical state is smooth in space while that of spurious state fluctuates dramatically. Following the method in lattice quantum chromodynamics calculation, the spurious state in radial Dirac equation can be removed by introducing the Wilson term. Taking Woods-Saxon potential for example, the imaginary time step method with the Wilson term is implanted successfully and provides the same results as those from the shooting method, which demonstrates its future application to solving the Dirac equation in coordinate space.
JMCT Monte Carlo analysis of BEAVRS benchmark: hot zero power results
Li Gang, Deng Li, Zhang Bao-Yin, Li Rui, Shi Dun-Fu, Shangguan Dan-Hua, Hu Ze-Hua, Fu Yuan-Guang, Ma Yan
2016, 65 (5): 052801. doi: 10.7498/aps.65.052801
Abstract +
J Monte Carlo transport code (JMCT), a new three-dimentional (3 D) Monte Carlo transport code, is introduced in this paper. The code is developed on the basis of 3 D geometry infrastructure JCOGIN and composed of multilayer modules. JMCT is capable of simulating the collision of particles with multi-group energy or providing energy data libraries. Two forms of parallelism supported in JMCT are domain decomposition and domain replication. The code has very good expansibility. JMCT Monte Carlo results have been compared with hot zero power (HZP) measurements of BEAVRS benchmark model from the MIT Computational Reactor Physics Group. Included in the comparisons are the eigenvalues, control rod bank worths, isothermal temperature coefficients, axially integrated full core detector measurements, axial detector profiles, etc. The eigenvalues for the HZP condition with different control rods positions and boron concentrations are calculated and the error is less than 0.2% compared with the theoretical error 1.000. The results of JMCT for isothermal temperature coefficients are also listed together with MC21 results and measured data. Each calculation for the eigenvalue is run by 1000 cycles in total, discarding 600 cycles, tracking 4 million neutrons each cycle. It takes 5.3-5.7 hours to run on 200 CPU cores. The JMCT results of axially integrated radial detector relative power distribution (RPD) and axial normalized detector signal are compared with the measured data. Power depressions from grid spacers are clearly seen in the JMCT results and accord with the measured data. The JMCT results of axially integrated assembly RPD power distribution are in good agreement with MC21 results, the maximum difference being 3.173% for 193 assemblies. So is the result of pin power RPD relative power at the axial elevation of peak power; the minimum relative power RPD 0.278 of JMCT is comparable to 0.283 of MC21, and the max relative power RPD 2.422 of JMCT is comparable to 2.452 of MC21. The calculation for RPD is run with 3000 inactive cycles and 5000 active cycles, tracking 4 million particles each cycle. It takes about 4.8 days to run on 200 CPU cores. Shannon entropy is used to demonstrate that the fission source distribution is converged after 3000 inactive cycles. With the development of computers and parallel computing, the Monte Carlo method can be used in reactor design instead of benchmarking other calculated results.
Seed electron production from O- detachment in high power microwave air breakdown
Wei Jin-Jin, Zhou Dong-Fang, Yu Dao-Jie, Hu Tao, Hou De-Ting, Zhang De-Wei, Lei Xue, Hu Jun-Jie
2016, 65 (5): 055202. doi: 10.7498/aps.65.055202
Abstract +
The existence of seed electrons is the precondition of air breakdown induced by high power microwave (HPM). Seed electrons are usually assumed to exist in background atmosphere when simulating the air breakdown triggered by HPM. However, this assumption may lead to some large errors especially in lower atmosphere where the number of electrons is very small. We establish a physical model of seed electron production from O- detachment collision with air molecules using the Monte Carlo method. A three-dimensional Monte Carlo program is developed to simulate this process. The average energies of O- and the average generation time of seed electrons under different electric intensities, frequencies, air pressures and breakdown volumes are obtained through simulation. The simulations show that the average generation time of seed electrons becomes longer with the increase of air pressure or the HPM frequency. The average seed electron generation time becomes shorter with the increase of electric intensity or breakdown volume. Finally, we simulate the processes of O- detachment collision with air molecules under the same experimental conditions. The comparative results show that the seed electron generation from O- detachment can explain the experimental results when the HPM frequency is low, while at higher frequencies, the average seed electron generation time becomes so long that it cannot correspond to the experimental value. Therefore some other mechanisms should be considered in the higher frequency case.
Evolution of electron energy distribution function in capacitively coupled argon plasma driven by very high frequency
Wang Jun, Wang Tao, Tang Cheng-Shuang, Xin Yu
2016, 65 (5): 055203. doi: 10.7498/aps.65.055203
Abstract +
Capacitively coupled plasma driven by a very high frequency power has attracted much attention due to its rather independent control of ion flux and energy. In this paper, Langmuir probe diagnostic technique is used to observe the evolution of plasma properties such as electron energy distribution function, electron temperature and density, etc. Our experiment is performed in capacitively coupled argon plasma driven by a 40.68 MHz frequency. Experimental results show that the electron energy probability function changes from bi-Maxwellian type to single-Maxwellian type and then to Druyvesteyn type with the increase of the discharge pressure. At a low gas pressure, the electron collisionless heating in bulk plasma leads to bi-Maxwellian type in electron energy possibility function (EEPF), which has a double temperatures structure in EEPF. As the gas pressure increases, the electrons with low energy are able to collide with the neutral species more frequently, thus they gain energies through collisional heating. Therefore, these electrons can overcome the dc ambipolar potential and the collisional heating becomes a main electron heating mechanism. Increasing the input power enhances the electron population with low energy. From the discharge center to the edge, electron population with low energy decreases clearly due to the dc ambipolar potential, and they are unable to reach an oscillating sheath where collisionless heating occurs. However, electron population with high energy is slightly increased. The result indicates that more uniform plasma can be achieved at a high gas pressure. Additionally, EEPFs are measured for different discharge gaps between electrodes. The change of electrode gap for the plasma leads to a transition of electron heating mode along the axial direction. In order to characterize the electron behavior further, we introduce the ratio of the cold electron density to hot electron density (α) and the ratio of cold electron temperature to hot electron temperature (β). The ratios also show the proportional distributions of the cold and hot electron populations. The electrode gap has a great influences on α while little influence on β. When the discharge gap between electrodes varies from 20 to 40 mm, α changes from 0.2 to 0.5 while β has the same trend. Spatial distributions of electron density and temperature with low and high energy are also discussed.
Femtosecond laser ablation of an aluminum target in air
Kang Xiao-Wei, Chen Long, Chen Jie, Sheng Zheng-Ming
2016, 65 (5): 055204. doi: 10.7498/aps.65.055204
Abstract +
The dynamics of laser ablation of solid target with ultrashort intense laser pulses is not only fundamentally interesting, but also relevant to a few important applications such as microfabrication, laser propulsion, laser induced breakdown spectroscopy, etc. By use of time-resolved pump-probe shadowgraphic imaging technology, we study the dynamic process of laser ablation of a planar aluminum target in air. The incident laser pulses are from a Ti: sapphire femtosecond laser amplifier system with a duration of 50 fs, central wavelength of 800 nm, pulse energy varying between 4 mJ and 7 mJ. Time-resolved shadowgraphs of material ejection from the aluminum target are recorded at the time delay up to a few nanoseconds after laser irradiation. By changing the distance between the target and the focal point of the incident laser, we obtain the shadowgraphs of the target ejection under different laser spot sizes. When the laser spot size is relatively large say, over 1 mm, the irradiated target surface is ablated in the form of a planar shock. However, when the laser spot size is relatively small, the ejection appears in the form of a hemispherical blast wave. It is found that the hemispherical blast wave well conforms to the Sedov's blast wave theory. When the laser energy is relatively large, it is found that ionization of air has a great effect on laser ablation. Additional small ejections appear as columnar and hemispherical structures near the laser axis, which are superimposed on the large planar shock. These can be attributed to the following processes. Firstly, as the ionization of air occurs near the laser axis, effective heat transfer from air plasma to the aluminium target leads to enhanced target temperature. This leads to the formation of a columnar structure on a picosecond time scale. Secondly, the columnar ejection and air plasma expansion near the laser axis result in the decrease of air density and pressure, which leads to the formation of the hemispherical structure on a nanosecond time scale.
Experimental study of the communication performance of electromagnetic wave in time-varying and magnetized plasma channel
Bo Yong, Zhao Qing, Luo Xian-Gang, Fan Jia, Liu Ying, Liu Jian-Wei
2016, 65 (5): 055201. doi: 10.7498/aps.65.055201
Abstract +
In this paper the influences of the time-varying plasma and magnetized time-varying plasma on the communication performance are investigated. Using a 5.8 GHz microwave source, the electron density and collision frequency of the time-varying glow discharge plasma are measured. An experimental platform is set up to test the bit error rates (BERs) of a variety of the modulation signals after going though the time-varying plasma channel. The experimental results show that the binary phase shift keying (BPSK) modulation signal has a minimal communication BER. Meanwhile, the variations of L-band BPSK and S-band QPSK (quadrature phase shift keying) signal's eye diagram and the constellation diagram, and the variation of energy after a magnetized plasma are observed. Compared with the un-magnetized situation, the magnetized plasma communication performance is greatly improved and the BER becomes much lower. The results prove that the magnetic field can effectively relieve the amplitude modulation and phase modulation caused by the plasma channel.