bluuebunny/crossref_metadata_2025 huggingface dataset:
Principle of Brillouin dynamic grating and its applications in optical fiber sensing
Authors: Dong Yong-Kang  , Zhou Deng-Wang  , Teng Lei  , Jiang Tao-Fei  , Chen Xi  ,   National Key Laboratory of Science and Technology on Tunable Laser, Harbin Institute of Technology, Harbin 150001, China
Published: December 2020

Brillouin dynamic grating (BDG) has been widely studied since it was proposed for the first time to achieve optical storage in 2007. In general, when two beams of pump light (their frequency difference equal to Brillouin frequency shift of the optical fiber) with the same polarization state are injected into the fiber, the coherent acoustic wave can be excited by the stimulated Brillouin scattering effect, forming a BDG. The BDG in an optical fiber has been widely used in optical fiber sensing, characterization of optical fibers, optical storage, all-optical signal processing, microwave photonics and high-precision spectral analysis due to the advantages of all-optical generation and flexible parameter control. In this paper, we analyze the principle of BDG generation and detection, and its applications in optical fiber sensing. The simultaneous measuring of strain and temperature is achieved within a spatial resolution of 20 cm through measuring Brillouin frequency shift and birefringence-induced frequency shift in a polarization-maintaining fiber. A high-sensitivity distributed transverse load sensor based on BDG with a measurement accuracy as high as 0.810-3 N/mm is proposed and demonstrated, whose principle is to measure the transverse-load-induced birefringence change through exciting and probing a BDG in an elliptical-core polarization maintaining fiber. On the basis of the above research, a distributed measurement of hydrostatic pressure is demonstrated by using a 4-m photonics crystal fiber with a measurement error less than 0.03 MPa at a 20-cm spatial resolution, while the temperature cross-talk to the hydrostatic pressure sensing can be compensated for through measuring the temperature-induced Brillouin frequency shift changes by using Brillouin optical time-domain analysis. A system based on BDG in polarization maintaining fibers is reported to achieve a spatial resolution below one centimeter, while preserving the full accuracy on the determination of temperature and strain through measuring Brillouin frequency shift. Taking advantage of creating a long BDG in an optical fiber, an ultra-narrow bandwidth optical filter is realized by operating a BDG in a single-mode fiber, and the optical spectrometry is performed by sweeping the center wavelength of the BDG-based filter through a swept-tuned laser, where a 4 fm (0.5 MHz) spectral resolution is achieved by operating a BDG in a 400 m single-mode fiber.
Buffer layer influence on light absorption of electron intersubband transition in binary energy level systems of quantum wells
Authors: Li Qun  , Qu Yuan  , Ban Shi-Liang  ,   Department of Physical Science and Technology, Inner Mongolia University, Hohhot 010021, China
Published: December 2020

Due to the restriction of the ZnO buffer layer on the left barrier in a wurtzite asymmetric ZnO/MgxZn1-xO single quantum well (QWs) structure with finite barriers, the other factors such as the size of the well and right barrier, and Mg component, etc. will influence the critical value of the left barrier width to form a binary level energy system. By adopting a finite difference method to solve the Schrdinger equation, the eigenstates and eigenenergies of electrons in a two-dimensional electron gas are obtained, and the influences of buffer layer ZnO, size and ternary mixed crystal effects on the formation of binary energy level system in QW are discussed. In our computation, the influences of energy band bending, material doping and built-in electric fields on a realistic heterostructure potential are considered. Furthermore, based on the Fermi's golden rule, the optical absorption coefficient of electronic intersubband transition in QW and the influences of buffer layer thickness, the widths of left barrier, well and right barrier and ternary mixed crystal effects are discussed. Our results indicate that the critical width of left barrier increases with the increases of the right barrier width and buffer layer thickness for a binary energy level system of ZnO/MgxZn1-xO single quantum well with a ZnO buffer layer on the left side. However, the critical width of left barrier decreases with the increase of well width and Mg component. Besides, the buffer layer thickness, the widths of left barrier, well and right barrier and ternary mixed crystal also affect the light absorption induced by the electronic intersubband transitions. The increases of Mg component, the widths of right barrier and left barrier will increase the absorption peak and produce its blue-shift. Whereas, increasing well width will reduce the absorption peak and produce its red-shift. The above conclusions are expected to give theoretical guidance in improving the opto-electronic properties of materials and devices made of these heterostructures.
Self-reliance and independently developed high-finesse spherical ultrastable optical reference cavity
Authors: Liu Jun  , Chen Bo-Xiong  , Xu Guan-Jun  , Cui Xiao-Xu  , Bai Bo  , Zhang Lin-Bo  , Chen Long  , Jiao Dong-Dong  , Wang Tao  , Liu Tao  , Dong Rui-Fang  , Zhang Shou-Gang  ,   University of Chinese Academy of Sciences, Beijing 100049, China;,   National Time Service Center, Chinese Academy of Sciences, Time and Frequency Stardard Laboratory, Xi'an 710600, China;,   Avic Xi'an Fight Automatic Control Research Institute, Xi'an 710065, China
Published: December 2020

Ultra-stable reference cavity with high finesse is a crucial component in a narrow-linewidth laser system which is widely used in time and frequency metrology, the test of Lorentz invariance, and measure of gravitational wave. In this paper, we report the recent progress of the self-made spherical reference cavity, aiming at the future space application. The main function of cavity is the reference of ultra-stable laser, which is the local reference oscillation source of space optical clock.
The diameter of the designed spherical cavity spacer made of ultra-low expansion glass is 80 mm, and the cavity length is 78 mm, flat-concave mirrors configuration, and the radius of the concave mirror is 0.5 m. The support structure is designed to have two 3.9 mm-radius spherical groves located at the poles of the sphere along the diameter direction (defined as support axis), and a 53 angle between the support axis and the optical axis. The mechanic vibration sensitivities of the cavity along and perpendicular to the optical axis are both calculated by finite element analysis method to be below 110-10/g. Five-axis linkage CNC machining sphere forming technology is applied to S80 mm spherical surface processing with spherical contour degree up to 0.02. After a three-stage surface polishing processes, the fused silicamirror substratessurface roughness is measured to be less than 0.2 nm (rms). Implementing double ion beam sputtering technique for mirror coating, the reflection of the coating achieves a reflectivity of 99.999% and a loss of 4 ppm for 698 nm laser. The coating surface roughness is measured to be 0.3 nm (rms). The cavity spacer and the mirror are bonded by dried optical contact. In order to improve the thermal noise characteristics of the cavity, an ultra low expansion ring is contacted optically to the outer surface of the mirror.
The cavity is characterized by ring-down spectroscopy, and the finesse is around 195000. With the help of a home-made 698 nm ultra narrow line-width laser, the cavity line-width is measured to be 9.8 kHz by sweeping cavity method. A 698 nm semiconductor laser is locked to this spherical cavity by PDH technology, and the cavity loss is measured to be5 ppm.
Analysis of photonic crystal transmission properties by the precise integration time domain
Authors: Yang Hong-Wei  , Meng Shan-Shan  , Gao Ran-Ran  , Peng Shuo  ,   College of Applied Sciences, Beijing University of Technology, Beijing 100124, China
Published: December 2020

Photonic crystals are materials patterned with a periodicity in the dielectric constant, which can create a range of forbidden frequencies called as a photonic band gap. The photonic band gap of the photonic crystal indicates its primary property, which is the basis of its application. In recent years, photonic crystals have been widely used to design optical waveguides, filters, microwave circuits and other functional devices. Therefore, the study on the transmission properties in photonic crystal is significantly important for constructing the optical devices. The finite difference time domain (FDTD) is a very useful numerical simulation technique for solving the transmission properties of the photonic crystals. However, as the FDTD method is based on the second order central difference algorithm, its accuracy is relatively low and the Courant stability condition must be satisfied when this method is used, which may restrict its application. To increase the accuracy and the stability, considerable scientific interest has been attracted to explore the schemes to improve the performance of the FDTD. The fourth order Ronge-Kutta (RK4) method has been applied to the FDTD method, which improves the accuracy and eliminates the influence of accumulation errors of the results, but the stability remains very poor if the time step is large. An effective time domain algorithm based on the high precision integration is proposed to solve the transmission properties of photonic crystals. The Yee cell differential technique is used to discretize the first order Maxwell equations in the spatial domain. Then the discretized Maxwell equations with the absorption boundary conditions and the expression of excitation source are rewritten in the standard form of the first order ordinary differential equation. According to the precise division of the time step and the additional theorem of exponential matrix, the high precision integration is used to obtain the homogeneous solution. To obtain the discretized electric and magnetic fields, the particular solution must be solved based on the excitation and then be added to the homogeneous solution. The transmission properties of photonic crystals are obtained by the Fourier transform. Practical calculation of photonic crystals is carried out by the precise integration time domain, and the accuracy and the stability are compared with those from the FDTD and the RK4 methods. The numerical results show that the precise integration time domain has a higher calculation precision and overcomes the restriction of stability conditions on the time step, which provides an effective analytical method of studying the transmission properties of photonic crystals.
Propagation characteristics of terahertz waves in temporally and spatially inhomogeneous plasma sheath
Authors: Chen Wei  , Guo Li-Xin  , Li Jiang-Ting  , Dan Li  ,   School of Physics and Optoelectronic Engineering, Xidian University, Xi'an 710071, China
Published: December 2020

The plasma sheath is produced by high-temperature heating during the reentry of a hypersonic vehicle to the Earth atmosphere. Temperature around the vehicle rises rapidly because of severe friction with air. The vehicle temperature behind friction is high enough to excite various real gas effects including chemical reactions of air, which contains ablation particles of vehicle, free electrons, and ions. The plasma sheath greatly affects the transmission of electromagnetic waves and has very strong interference on the communication signals, which results in interrupt between the target and the ground station, namely, blackout. The electron density of plasma sheath surrounding the aircraft is inhomogeneous and varies with time. Temperature and pressure will also change at different altitudes. Therefore, it is meaningful to investigate the propagation characteristics of electromagnetic waves in temporally and spatially inhomogeneous plasma sheath. The temporally and spatially inhomogeneous plasma sheath model is introduced and the electron density data of the National Aeronautics and Space Administration (NASA) reentry vehicle is employed. The relationships among temperature, pressure, and collision frequency are obtained with the empirical formula of collision frequency. Then, the reflection coefficient and transmission coefficient of time-varying single layer plasma are calculated with the shift operator finite-difference time-domain (SO-FDTD) method. These results are compared to verify the correctness of the proposed method. Finally, the LTJEC-FDTD method is used to calculate the reflection coefficient, transmission coefficient and absorptivity at different relaxation time, temperature, and pressure in the terahertz (THz) band. The results show that the higher temperature and pressure will enable the electromagnetic wave to penetrate the plasma sheath at high relaxation time of electron density. If the incident wave frequency is lower than the cut-off frequency of plasma, the reflection of electromagnetic wave will be more obvious. However, when the incident wave frequency is in the THz band, the effects of temperature and pressure on the propagation of electromagnetic wave are obviously weakened. The absorption of electromagnetic wave by plasma will be more obvious when the relaxation time, temperature, and pressure decrease. If the relaxation time of electron density is shorter than or equal to the period of THz wave, more energy of electromagnetic wave will be absorbed by the plasma sheath. Contrarily, if the relaxation time of electron density is much longer than the period of THz wave, the absorption of electromagnetic energy will decrease. This study gives some insight into the temporally and spatially inhomogeneous plasma sheath, and provides a theoretical basis for solving the blackout problem.
A method to diverge reflected beam uniformly using cube-corner retroreflector array with dihedral angle tolerances
Authors: Zhou Xiao-Feng  , Qi Zu-Min  , Luo Xiang-Qian  , Liu Chang-An  , Zhu Jian-Hui  , Wang Ze-Hua  , Zhang Yi  , Zi Yan-Yong  ,   Northwest Institute of Nuclear Technology, Xi'an 710024, China
Published: December 2020

The cube-corner retroreflector (CCR) is widely applied in the electro-optical tracking, satellite communication, interferometers and adjust-free solid state laser. In some applications, the incident beam emitted by a laser is reflected back by the CCR to a photoelectric detector. The distance between the photoelectric detector and the laser source on the ground is much larger than the diffraction-limited spot. Meanwhile, the attitude angle of the CCR would randomly vary for the jitter of the platform. Therefore, the reflected beam should be diverged uniformly at far-field, whereas the normal CCR cannot achieve the divergence on the reflected beam. The investigation indicated that six sub-spots are generated by a CCR with dihedral angle tolerances at far-field. According the characteristics of the CCR with dihedral angle tolerances, a structure and its design method are proposed to diverge the reflected beam with a CCR array. The azimuthal angles of the every CCR of the array should be specially designed to generate an annular and uniform pattern. Due to the propagation distance is much larger than the size of the CCR array, the feasibility of the method is analyzed by the wave theory. A CCR array with a divergence half-angle of 0.5 mrad is designed, in which the dihedral angle tolerance of every CCR is 20. The influences of the beam and structure parameters on the diffraction characteristics of the reflected beam are investigated. The numerical results indicate the divergence half-angle of the CCR array varies quasi-linearly with the change of the dihedral angle tolerance, and the intensity distribution of the incident beam does not influence the divergence half-angle. The propagation distance does not affect the uniformity of the reflected beam when the CCR array satisfies the point source condition. When the number of the array element increases to a certain value, the increase of the number can strengthen the intensity and hardly influences the uniformity of the reflected beam. For the restriction of the machining and assembling technics, the dihedral angle tolerance of every CCR is hardly identical and the assembling azimuthal angles of the array element can not be identical with the design result. Therefore, the influence of the assemblage azimuth error and machining accuracy of the dihedral angle are studied. It reveals that the assemblage azimuth error does not remarkably the reflection pattern, whereas the machining accuracy can observably affect the uniformity of the reflection pattern, which can be resolved by the growth of the number of array element.
2.5 kW average power, two-channel spectral-beam-combined output based on transmitting volume Bragg grating
Authors: Zhou Tai-Dou  , Liang Xiao-Bao  , Li Chao  , Huang Zhi-Hua  , Feng Jian-Sheng  , Zhao Lei  , Wang Jian-Jun  , Jing Feng  ,   Research Center of Laser Fusion, China Academy of Engineering Physics, Mianyang 621900, China
Published: December 2020

Spectral beam combination based on volume Bragg gratings is an effective approach to obtaining high power laser output. In spectral beam combining system, spectral channel spacing will affect the number of non-combined sub-beams and the overall combined output power due to the finite available gain bandwidth. Based on coupled wave theory, a two-channel high power spectral beam combining model is proposed. By appropriately relaxing the requirements for the spectral channel spacing and line-width of sub-beams, the higher combined output power can be obtained but the spectral density does not significantly decrease. In this work, a 2-channel spectral beam combining system is demonstrated to present a 2.5 kW combined power with combining efficiency 85% by employing a transmitting volume Bragg grating. The combining system has a high spectral density of 0.51 kW/nm with 5 nm spectral spacing between channels. The output can keep a good beam quality when the combined power is less than 1 kW, while the significant degradation of combined beam quality occurs when output power is 1.5 kW and is restricted mainly by the dispersion properties and thermal effects of volume Bragg gratings. During this 2-channel beam combining process, no special active cooling measure is used. Interactions between laser radiation and the grating are verified. Thermal absorption of high power laser radiation in the grating will cause the temperature to remarkably increase, resulting in the thermal expansion of the grating period, which leads to the degradations of diffraction efficiency and the spectral selectivity. Research is also focused on the surface distortion, and the results indicate that the thermal-induced wave-front aberrations of the non-combined sub-beams lead to the deterioration of beam quality. Transmitted and diffracted beams experience wave-front aberrations to different degrees, leading to distinct beam deterioration.
Evolution mechanism of vortices in a supersonic mixing layer controlled by the pulsed forcing
Authors: Guo Guang-Ming  , Liu Hong  , Zhang Bin  , Zhang Qing-Bing  ,   School of Aeronautics and Astronautics, Shanghai Jiao Tong University, Shanghai 200240, China;,   Institute No. 2, China Aerospace Science and Industry Corporation, Beijing 100854, China
Published: December 2020

Pulsed actuation is one of the most fundamental control types to study regularity of flow structures in supersonic mixing layers, which helps to predict the aero-optical effects caused by the supersonic mixing layer where the different-sized vortices dominate the flow field. However, the knowledge about the evolution mechanism of vortices in the supersonic mixing layer which is controlled by the pulsed forcing is limited. Based on the large eddy simulation (LES), the visualized flow field of a supersonic mixing layer controlled by the pulsed forcing is presented and the unique growth mechanism of the vortices in such a case is revealed clearly. The method of position extraction of the vortex core in the supersonic mixing layer, which is a quantitative technique to obtain the instantaneous location of a vortex in flow field, is employed to calculate the dynamic characteristics (e.g., instantaneous convective speed and size) of the vortices quantitatively. The pulsed forcings of different frequencies are imposed on the same supersonic mixing layer respectively, and the instantaneous convective speed and size of the vortices for each pulse frequency considered in this study are then computed. By comparing the dynamic characteristics of the vortices between cases, the evolution mechanism of the vortices in the supersonic mixing layer controlled by the pulsed forcing is revealed.as follows. 1) Growth of the vortices in the supersonic mixing layer controlled by the pulsed forcing no longer depends on the pairing nor merging between adjacent vortices, which is just the growth mechanism of vortices in a free supersonic mixing layer. Actually, the size of a vortex in the controlled supersonic mixing layer is dominated by the imposed pulse frequency, so the size of each vortex in such a flow field is approximately identical. 2) The number of vortices in the controlled supersonic mixing layer is proportional to the pulse frequency, whereas the size of vortex is inversely proportional to the pulse frequency. That is, the higher the pulse frequency, the bigger the number of vortices in the controlled flow field is and the smaller the size of every vortex. 3) The average convective speed of vortices in the controlled supersonic mixing layer gradually decreases with pulse frequency increasing because the pulsed forcing essentially drags on the movement of vortices in flow field. Finally, an equation which describes the quantitative relationship between the dynamic characteristics of a vortex and the pulsed forcing frequency is derived, that is, the product of the average convective speed of vortices in the controlled supersonic mixing layer and the imposed pulse period is approximately equal to the mean diameter of vortices in the flow field.
Numerical solution procedure for Hall electric field of the hypersonic magnetohydrodynamic heat shield system
Authors: Li Kai  , Liu Jun  , Liu Wei-Qiang  ,   College of Aerospace Science and Engineering, National University of Defense Technology, Changsha 410073, China
Published: December 2020

Magnetohydrodynamic (MHD) heat shield system is a novel-concept thermal protection technique for hypersonic vehicles, which has been proved by lots of researchers with both numerical and experimental methods. Most of researchers neglect the Hall effect in their researches. However, in the hypersonic reentry process, the Hall effect is sometimes so significant that the electric current distribution in the shock layer can be changed by the induced electric field. Consequently, the Lorentz force as well as the Joule heat is varied, and thus the efficiency of the MHD heat shield system is affected.In order to analyze the influence of Hall effect, the induced electric field must be taken into consideration. According to the weakly-ionized characteristics of hypersonic flow post bow shock, the magneto-Reynolds number is assumed to be small. Therefore, the Maxwell equations are simplified with the generalized Ohm's law, and the induced electric field is governed by the potential Possion equation. Numerical methods are hence established to solve the Hall electric field equations in the thermochemical nonequilibrium flow field. The electric potential Poisson equation is of significant rigidity and difficult to solve for two reasons. One is that the coefficient matrix may not be diagonally dominant when the Hall parameter is large in the shock layer, and the other is that this matrix including the electric conductivity is discontinuous across the shock. In this paper, a virtual stepping factor is included to strengthen the diagonal dominance and improve the computational stability. Moreover, approximate factor and alternating direction implicit method are employed for further improving the stability. With these methods, a FORTRAN code is written and validated by comparing the numerical results with the analytical ones as well as results available from previous references. After that, relation between the convergence property and the virtual stepping factor is revealed by theoretical analysis and numerical simulations. Based on these work, a local variable stepping factor method is proposed to accelerate the iterating process. Results show that the convergence property is closely related to the mesh density and Hall parameter, and there exists a best stepping factor for a particular mesh as well as a particular Hall parameter. Since the best stepping factor varies a lot for different meshes and different Hall parameter, its appropriate value is hard to choose. The best value of stepping factor coefficient still exists in the local step factor method, but its value range is relatively smaller. More importantly, the local stepping factor method yields better convergence property than the regular constant one when employing a locally refined mesh.
First-principles study on adsorption mechanism of hydrogen on tungsten trioxide surface
Authors: Jiang Ping-Guo  , Wang Zheng-Bing  , Yan Yong-Bo  ,   School of Metallurgy and Chemical Engineering, Jiangxi University of Science and Technology, Ganzhou 341000, China
Published: December 2020

With the development of modern industrial technology, tungsten products prepared from normal tungsten powder cannot meet the demands of industry. The tungsten product produced from ultra-fine tungsten powder exhibits high strength, high toughness, and low metal plasticity-brittleness transition temperature, which greatly improves the performance of materials. Hence, it is necessary to carry out theoretical research on the micro adsorption dynamics during hydrogen reduction of tungsten trioxide to prepare ultra fine tungsten powder. In order to understand crystal characteristics of WO3 and WO3(001) surface characteristics, and to provide beneficial theoretical support for reaction law of hydrogen reduction on the WO3(001) surface, the mechanisms of H atom adsorption on cubic WO3 and WO3(001) surface are studied by the first-principles calculation based on the density functional theory (DFT) plane wave pseudo-potential method. The results show that theoretically calculated band gap of the cubic crystalline WO3 is 0.587 eV. There are two kinds of WO3(001) surfaces, WO-terminated (001) surface and O-terminated (001) surface. The W-O bond length and the bond angle of W-O-W structure change after the geometric optimization of the surface, and thus the surface relaxation is realized. The WO-terminated (001) surface shows n-type semiconductor characteristics while the O-terminated (001) surface shows p-type semiconductor characteristics. Four adsorption configurations of H atoms on the WO-terminated (001) surface and the O-terminated (001) surface, including H-O2c-H, H-O2 cH-O2c, H-O1c-H, and H-O1cH-O1c, are calculated. Among them, the adsorption energy of the H-O1c-H configuration is the smallest (-3.684 eV) with the shortest bond length of H-O bond (0.0968 nm), and hydrogen atoms lose the most of electrons (0.55e), which indicates that the H-O1c-H adsorption configuration is the most stable one. The band gap of the H-O1c-H configuration increases from 0.624 eV to 1.004 eV after adsorption, while the bandwidth of valence band is almost unchanged. The results about the density of states (DOS) reveal that 1s state of the H atom interacts with 2p and 2s states of the O atom. Strong isolated electron peaks are formed to be at about -8 and -20 eV. The outermost O1c atoms of O-terminated (001) surface contain an unsaturated bond, facilitating the bonding between two H atoms and one O1c atom. Thus, two H atoms and one O1c atom form chemical bonds respectively, and an H2O molecule is generated, leaving an oxygen vacancy on the surface after adsorption reaction. By combining experimental observations with simulation results, the mechanism of hydrogen reducing tungsten trioxide can be elaborated profoundly from a micro view.
Simulations of the effects of electric field and temperature on space charge traps in polymer
Authors: Li Li-Li  , Zhang Xiao-Hong  , Wang Yu-Long  , Guo Jia-Hui  ,   Key Laboratory of Engineering Dielectrics and Its Application, Ministry of Education, College of Electrical and Electronic Engineering, Harbin University of Science and Technology, Harbin 150080, China;,   College of Rongcheng, Harbin University of Science and Technology, Rongcheng 264300, China
Published: December 2020

The simulations of the structure and behavior of the molecule in the simulation software are an effective way to analyze the microscopic mechanism associated with performance change of space charge trap in the polymer. To achieve this, in this paper we first present the polyethylene molecular model which is developed by using the simulation software Materials Studio (MS). Then, the microstructure and property of space charge trap are analyzed by the changes with the energy and the free volume in the polyethylene due to the chain segment motion under the universal force field (UFF), respectively. Some important findings are extracted from simulation results. First, in the process of the temperature gradually increasing from 298 K to 363 K, the phenomena of slippage and diffusion of the molecule due to the enhanced thermal motion of molecules are observed. These phenomena lead to the free volume increasing and the space charge trap energy level decreasing gradually, whose maximum value is 1542.073 and the minimum value is 0.66 eV when the temperature is 363 K. Second, when an electrostatic field of 0.0007 Hartree/Bohr is applied to the polymer, molecular chain segments are oriented by the Maxwell stress that is generated by the electric effect. Molecular chain segment orientations induce the van der Waals interaction energy to increase to -360.18 kcal/mol (1 kcal/mol = 4.18 kJ/mol), the free volume to decrease by 279.77 3, and the space charge trap energy level to decrease by 0.45 eV. Third, by comparing the cases of applying the temperature field and the electric field to the polyethylene, it is found that the electric field has stronger effect on charge trap. Specifically, the space charge trap energy level of the polyethylene associated with 0.0007 Hartree/Bohr electric field is reduced by 0.17 eV compared with that associated with the temperature of 363 K. Moreover, simulation results and measured results are compared with each other and they are well consistent. Finally, it is concluded that using electric effect and molecular thermodynamic movement is an very effective way to analyze the microscopic mechanism of changes with free volume and van der Waals interaction energy. This analysis confirms that molecular motion changes the microstructure of the polyethylene and generates charge traps. In addition, it confirms that the influence of the electric field on the polyethylene generates the lower level of space charge trap than the effect of the temperature field.
Interface electronic structure and the Schottky barrier at Al-diamond interface: hybrid density functional theory HSE06 investigation
Authors: Wu Kong-Ping  , Sun Chang-Xu  , Ma Wen-Fei  , Wang Jie  , Wei Wei  , Cai Jun  , Chen Chang-Zhao  , Ren Bin  , Sang Li-Wen  , Liao Mei-Yong  ,   School of Electrical and Information Engineering, Anhui University of Science and Technology, Huainan 232001, China;,   Wide Band-Gap Semi-Conductor Research Group National Institute for Materials Science (NIMS), Tsukub 305-0044, Japan
Published: December 2020

Diamond is regarded as one of the most promising semiconductor materials used for high power devices because of its superior physical and electrical properties, such as wide bandgap, high breakdown electric field, high mobility, and high thermal conductivity. Highpower diamond devices are now receiving much attention. In particular, Schottky diode based on a metal/diamond junction has promising applications, and high breakdown voltage has been achieved, though unfortunately its forward resistance is high. In this paper, the first principles calculations are performed to study the electronic structure of interface and the Schottky barrier height of Al-diamond interface. The projection of the density of states on the atomic orbitals of the interface atoms reveals that the typical Al-induced gap states are associated with a smooth density of states in the bulk diamond band gap region, and these gap states are found to be localized within three atom layers. At the same time, electronic charge transfer makes the Fermi level upgrade on the side of diamond. Besides, the typical Al-induced gap state model gives a simple picture about what determines Schottky barrier height at Al-diamond interface, by assuming an ideal, defect-free and laterally homogeneous Schottky interface in which the only interaction comes from the decay of the electron wave function from the metal into the semiconductor, which in turn induces electronic charges to be rearranged in the region close to the interface. As for the electronic charge transfer, this potential shift can be extracted by subtracting the superimposed planar or macroscopically averaged electrostatic potentials of the Al and diamond surfaces (at frozen atomic positions), from the planar or macroscopically averaged potential of the relaxed Al-diamond interface. The electronic charge transfer suggests that the formation of an interface should be associated with the formation of new chemical bonds and substantial rearrangements of the electron charge density. Especially, we obtain the Schottky barrier height of 1.03 by the first principle, which is in good agreement with the results from phenomenological model and experiment. The research results in this paper can provide a theoretical basis for the research of the metal diamond Schottky junction diode, and can also give a theoretical reference for the research of the metal-semiconductor highpower device based on diamond material.
Radical anion based liquid electrode materials
Authors: Yu Jue-Zhi  , Hu Yong-Sheng  , Li Hong  , Huang Xue-Jie  , Chen Li-Quan  ,   Institute of Physics, Chinese Academy of Sciences, Beijing 100190, China;,   College of Physics, University of Chinese Academy of Sciences, Beijing 100049, China
Published: December 2020

Electrochemical batteries for wind and solar renewable energy storage have attracted world-wide attention, due to their merits of flexibility, modularity and being environmental friendly. Nowadays, new rechargeable battery systems are highly desired for large-scale electrical energy storage. Here in this paper, we report that alkali metal can be dissolved into aromatic compound-ether solvent to obtain a dark blue solution with high conductivity. This solution consists of alkali metal cation and radical anion generated by electron transfer reaction between alkali metal and aromatic compounds. For instance, sodium and biphenyl can be dissolved into 1,2-dimethoxyethane to obtain a dark blue solution which exhibits high electronic conductivity (8.410-3Scm-1), high ionic conductivity (3.6 10-3Scm-1), low potential 0.09 V vs. Na/Na+ and low cost. Using this solution as the anode, we demonstrate a new rechargeable battery with quinone liquid cathode. It is found that the battery with anthraquinone (AQ) liquid cathode displays long cycle ability, low cost properties. This work proposes a new strategy for designing the electrode materials and rechargeable battery systems. Furthermore, this kind of liquid may possess other unique physical properties and might be used in other devices, like thermoelectric battery.
Wavelet transform in the application of three-dimensional terahertz imaging for internal defect detection
Authors: Dai Bing  , Wang Peng  , Zhou Yu  , You Cheng-Wu  , Hu Jiang-Sheng  , Yang Zhen-Gang  , Wang Ke-Jia  , Liu Jin-Song  ,   Wuhan National Laboratory for Optoelectronics, Huazhong University of Science and Technology, Wuhan 430074, China;,   College of Mechanical Science and Engineering, Huazhong University of Science and Technology, Wuhan 430074, China
Published: December 2020

Spatial resolution and spectral contrast are two major bottlenecks for non-destructive testing of complex samples with current imaging technologies. We use a three-dimensional terahertz (THz) imaging system to obtain the internal structure of the sample, and exploit the wavelet transform algorithm to improve the spatial resolution and the spectral contrast. With this method, the longitudinal resolution of terahertz imaging system can be improved to the wavelength comparable thickness, while the x-y plane resolution can be as high as 0.2 mm0.2 mm, which benefits from the point-to-point scanning on the x-y plane. In this three-dimensional terahertz imaging system, the Syn View Head 300 with light source/detector frequency of 0.3 THz is used for two-dimensional scanning (x-y direction) of the sample, and the linear frequency modulated continuous wave technique is used to obtain the reflected terahertz light intensity at different depths (z axis) of the sample. When the sample is thin, the upper and lower interface reflection peaks are difficult to distinguish due to broad peak width of the THz source. To solve this problem efficiently, continuous wavelet transform (CWT) is used. In recent years, CWT is applied widely because of its particular mathematical properties in the feature signal recognition. Since the Gaus2 wavelet basis is better to highlight the peak signal, we choose it for CWT. After CWT, one scale of the wavelet coefficients is chosen for three-dimensional data reconstruction, for which the widths of the reflection peaks are narrower and the noise signals are weaker. That means if we reconstruct the three-dimensional wavelet coefficient data on the chosen scale, the three-dimensional image of the tested sample will be enhanced. In order to demonstrate that, the three-dimensional images reconstructed by wavelet coefficients are compared with those by original data. The tested sample has holes inside with different depths. Based on the original three-dimensional THz image, it is hard to locate the top of 4 mm deep hole (1 mm deep photosensitive material plate), while the top of the inner 4 mm deep holes (the bottom of the 1 mm deep photosensitive material plate) can be distinctly located and the noises are greatly reduced based on the three-dimensional images reconstructed by wavelet coefficients. With this method, the longitudinal resolution of terahertz detection systems can be improved to 1 mm that is comparable to the wavelength, which demonstrates advantages of this method.
A direct algorithm with square wave perturbation for calculating phase response curve
Authors: Xie Yong  , Cheng Jian-Hui  ,   State Key Laboratory for Strength and Vibration of Mechanical Structures, School of Aerospace, Xi'an Jiaotong University, Xi'an 710049, China
Published: December 2020

Neuron is a typical dynamic system, therefore, it is quite natural to study the firing behaviors of neurons by using the dynamical system theory. Two kinds of firing patterns, i.e., the periodic spiking and the periodic bursting, are the limit cycle oscillators from the point of view of nonlinear dynamics. The simplest way to describe the limit cycle is to use the phase of the oscillator. A complex state space model can be mapped into a one-dimensional phase model by phase transformation, which is helpful for obtaining the analytical solution of the oscillator system. The response characteristics of the oscillator system in the motion state of the limit cycle to the external stimuli can be characterized by the phase response curve. A phase response curve illustrates the transient change in the cycle period of an oscillation induced by a perturbation as a function of the phase at which it is received. Now it is widely believed that the phase response curve provides a new way to study the behavior of the neuron. Existing studies have shown that the phase response curve of the periodic spiking can be divided into two types, which are closely related to the bifurcation mechanism of neurons from rest to repetitive firing. However, there are few studies on the relationship between the phase response curve and the bifurcation type of the periodic bursting. Clearly, the first prerequisite to understand this relationship is to calculate the phase response curve of the periodic bursting. The existing algorithms for computing the phase response curve are often unsuccessful in the periodic bursting. In this paper, we present a method of calculating the phase response curve, namely the direct algorithm with square wave perturbation. The phase response curves of periodic spiking and periodic bursting can be obtained by making use of the direct algorithm, which is verified in the four neuron models of the Hodgkin-Huxley, FitzHugh-Nagumo, Morris-Lecar and Hindmarsh-Rose. This algorithm overcomes the limitations to other algorithms in the application. The calculation results show that the phase response curve of the periodic spiking is determined by the bifurcation type. We find a suprathreshold periodic oscillation starting from a Hopf bifurcation and terminating at a saddle homoclinic orbit bifurcation as a function of the applied current strength in the Morris-Lecar model, and its phase response curve belongs to Type II. A large amount of calculation indicates that the relative size of the phase response and its positive or negative value depend only on the time of imposing perturbation, and the phase response curve of periodic bursting is more complicated than that of periodic spiking.
Shack-Hartmann optimum threshold estimation for the point source
Authors: Zhou Rui  , Wei Ling  , Li Xin-Yang  , Wang Cai-Xia  , Li Mei  , Shen Feng  ,   Key Laboratory on Adaptive Optics, Chinese Academy of Sciences, Chengdu 610209, China;,   Institute of Optics and Electronics, Chinese Academy of Sciences, Chengdu 610209, China;,   University of Chinese Academy of Sciences, Beijing 100049, China
Published: December 2020

The Shack-Hartmann wavefront sensor (SHWFS) is an optical detection device based on the measurements of wavefront slopes. It is widely used in an adaptive optics system due to its simple structure and strong environment adaptability. The measuring accuracy of the SHWFS depends mainly on the accuracy of the spot image centroid in each sub-aperture. There are many centroid algorithms including the center of gravity algorithm, Gauss fitting algorithm, and correlation algorithm. As to the simplicity, robustness, high accuracy and stability, the center of gravity algorithm is more widely used. However, the accuracy of gravity algorithm is sensitive to the noise including discretization, aliasing, photon noise, readout noise, stray light, and direct current bias. To improve the accuracy of centroid, the output signals of SHWFS must be pre-processed to suppress the noise effect by using the method of thresholding in general. Many threshold methods have been presented to reduce the error of centroid and there theoretically exists an optimum threshold which causes the minimum error of centroid based on the characteristics of SHWFS and noise. However, it is difficult to separate the signals from the noises, and the optimum threshold cannot be estimated accurately in real time in the SHWFS systems. In this paper aiming at noises in SHWFS, which vary with time and space rapidly, a method based on the noise weighted function of the mean value of pixels and the local gradient direction of image signals in the moving windows is presented according to the characteristics of the Gaussian spot and noise distributions. Moreover, the theory and parameters determination of the method are analyzed. The method utilizes the probability that the pixels in the moving windows belong to the noise, and the probability is inversely proportional to the mean value of pixels and the local gradient direction of image signals, and so the monotonically reducing probability function of pixels is constructed. Finally, the standard deviation and mean value of noise can be obtained, and the estimation value of optimum threshold is equal to the mean value of noise plus three times the standard deviation of noise. To investigate the effects of the optimum threshold estimation with the different spot sizes, spot strengths and noise levels, the proposed algorithm is compared with traditional methods. The simulation and experimental results show that the proposed method could achieve higher accuracy, and the error between the threshold obtained by the method presented in this paper and theoretical optimum threshold is less than 10%, which is less than those from the traditional methods.
A quasi-cyclic compressed sensing delay estimation algorithm based on progressive edge-growth
Authors: Leng Xue-Dong  , Wang Da-Ming  , Ba Bin  , Wang Jian-Hui  ,   The PLA Information Engineering University, Zhengzhou 450001, China
Published: December 2020

Time delay estimation (TDE) is a hot research topic in wireless location technology. Compressed sensing (CS) theory has been widely applied to image reconstruction and direction of arrival estimation since it was proposed in 2004. The sparse model can be constructed in time domain for estimating the time delay by using the CS theory. The measurement matrix plays a crucial role in the processing of signal reconstruction which is the core problem of CS theory. Therefore the research in the measurement matrix has becomes a hotspot in recent years. The existing measurement matrix is mainly divided into two categories, i.e., random measurement matrix and deterministic measurement matrix. The performance of random measurement matrix has bottlenecks. Firstly, because of the redundant measurement matrix data, the generation and storage of the random number put forward a high requirement for hardware. Secondly the random matrix can only satisfy the restricted isometry property in a statistical sense. The research of the deterministic measurement matrix is of great value under this background. The parity check matrix of low density parity check (LDPC) code has good performance in CS theory. However, the method of randomly selecting non-zero element position has a certain probability to generate a measurement matrix with a short loop structure during generating LDPC code measurement matrix. The robustness of the reconstruction performance decreases with the increase of iteration times.
A novel quasi-cyclic CS algorithm based on progressive edge-growth is constructed to estimate the time delay. The purpose of this article is to deal with the need to store a large number of data in existing measurement matrix during time delay, by using the CS theory. The algorithm presented here can achieve TDE in a high precision. First, the theoretical bridge between CS and the maximum likelihood decoding is established. And the design criterion of measurement matrix based on the LDPC code is derived. The sparse measurement matrix with quasi-cyclic structure is constructed by introducing the idea of progressive edge-growth. Finally, the orthogonal matching pursuit algorithm is used to estimate the time delay. Furthermore, the computational complexity of the algorithm and the data storage of the measurement matrix are analyzed theoretically. Simulations show that the correct reconstruction probability of the proposed approach is higher than those of the Gauss random matrix and random LDPC matrix under the same dimension. Compared with the random LDPC matrix, the proposed method can improve performance at the expense of less complexity under the condition of the same data storage.