bluuebunny/crossref_metadata_2025 huggingface dataset:
Progress of detecting orbital angular momentum states of optical vortices through diffraction gratings
Authors: Fu Shi-Yao  , Gao Chun-Qing  ,   School of Optics and Photonics, Beijing Institute of Technology, Beijing 100081, China
Published: December 2020

Optical vortices are a new kind of laser beam and receiving more and more attention currently.The complex amplitudes of optical vortices comprise a helical term exp (il),with l being the topological charge and the azimuthal angle.Each photon in optical vortices carries the orbital angular momentum (OAM) with a value of lħ,where ħ is the Planck's constant divided by 2.The topological charge l is the eigenvalue of optical vortices,and determines the helical wavefront distribution,thus also known as OAM state.Moreover,such an OAM state can be an infinite integer state. And vortices with various OAM states are orthogonal to each other,making it possible to be employed in high capacity data-transmission system.In addition,the above unique features contribute to their widely applications in lots of areas such as optical tweezers and spanners,rotation detection,quantum entanglement,etc.In these applications,detecting OAM states is basic,and greatly significant.Recently researchers have developed lots of approaches to detecting the OAM states,including the methods of interference,diffraction gratings,metasurface,etc.Of such approaches,the scheme of diffraction gratings is the simplest and most widely used,where one or more diffraction gratings are employed. When optical vortices propagate through such gratings,the OAM states are acquired immediately through capturing and analyzing the distinct OAM-related diffraction patterns.In this review,we focus on the techniques of detecting OAM states through diffraction gratings,which have been demonstrated by our group and other researchers.Some of the main detection gratings,including double-slit,triangle aperture and slit,angular slit,cylindrical lens,graduallychanging-period grating,annular grating,are introduced.In addition,schemes like composite fork grating,Dammann vortex grating and integrated Dammann grating,are presented to detect the OAM state for coaxial multiplexed vortices. Besides diagnosing OAM state,measuring the intensity proportion of each OAM channel,known as OAM spectrum, in multiplexed vortices is also necessary in some cases.Therefore we also introduce the ways to measure the OAM spectrum,e.g.the OAM mode sorter,the gray-scale algorithm.
Progress in Pb-free and less-Pb organic-inorganic hybrid perovskite solar cells
Authors: Chen Liang  , Zhang Li-Wei  , Chen Yong-Sheng  ,   School of Physics & Electronic Information Engineering, Henan Polytechnic University, Jiaozuo 454000, China;,   Key Laboratory of Material Physics, School of Physics and Engineering, Zhengzhou University, Zhengzhou 450052, China
Published: December 2020

The conversion efficiencies of perovskite solar cells based on organic-inorganic hybrid metal halide materials have broken through 22% in just a few years, which provides a ray of hope in solving the future energy problem, and receives great attention and research enthusiasm from the academic circle. However, what is followed is commercialization and industrialization process, which will greatly enhance the importance and urgency of the research and development of the green, non-toxic, highly-efficient, and lead-free perovskite solar cells. In order to speed up the development of these environment-friendly perovskite solar cells, we summarize the recent research progress in the perovskite solar cells from the two categories of Pb-free and less-Pb materials. In the Pb-free aspect Sn-based perovskite solar cells are emphatically introduced. A maximum efficiency of 8.12% is obtained for the solar cells based on FA0.75MA0.25SnI3, but it lags far behind the Pb-based competitors. This may be caused mainly by the oxidation of Sn2+ ions and the band mismatch with carrier transport materials, etc. So, for further improving the efficiency, it is very important to optimize the device structure and material properties, and understand the role played by Sn4+ ions in films. In addition, more attention should be paid to the inorganic halide double perovskite materials as potential solutions for the toxicity and stability issues. In the less-Pb part, Sn-doping contributes to a large reduction of lead content in the film, and a maximum efficiency of 17.6% for the (FASnI3)0.6(MAPbI3)0.4 perovskite solar cells is achieved with good long-term stability. What is even more interesting is that it can be utilized to construct tandem cells through the bandgap regulation after doping. However, it is very difficult to determine the optimum Sn-doping ratio. More systematic, rigorous and normative experiments are extremely necessary to reveal the interaction mechanism between Pb2+ and Sn2+. For other doped elements, the effects of their concentrations on the properties of thin films and the performance of solar cells are also emphatically discussed, and it is very urgent to have a further understanding of the working principles of devices and the fundamental functions of substitution elements. Thus, this review highlights the recent research efforts in the development of Pb-free and less-Pb perovskite solar cells and also provides a perspective of future development of new environment-friendly and high performance perovskite solar cells.
Sound radiation of cylinder in shallow water investigated by combined wave superposition method
Authors: Shang De-Jiang  , Qian Zhi-Wen  , He Yuan-An  , Xiao Yan  ,   Acoustic Science and Technology Laboratory, Harbin Engineering University, Harbin 150001, China;,   College of Underwater Acoustics Engineering, Harbin Engineering University, Harbin 150001, China;,   Systems Engineering Research Institute, Beijing 100036, China
Published: December 2020

It can be a difficult problem to precisely predict the sound field radiated from a finite elastic structure in shallow water channel because of its strong coupling with up-down boundaries and the fluid medium, whose sound field cannot be calculated directly by current methods, such as Ray theory, normal mode theory and other different methods, which are adaptable to sound fields from idealized point sources in waveguide. So far, there is no reliable prediction method to solve this kind of problem. A combined wave superposition method is proposed for such a problem, which combines the traditional wave superposition method with the transfer function in shallow water channel and the multi-physics field coupling numerical model. This method mainly consists of three sections:1) obtaining the normal velocity on the elastic structure surface in shallow water channel by the finite element method (FEM), whose FEM model includes the up-down boundaries and the completely absorbent sound boundaries in the horizontal direction; 2) getting the equivalent point source strength by traditional wave superposition method; 3) calculating the total sound field by adding up each point source field which is obtained by normal mode method. This method is verified by numerical simulation and theoretical analysis by using an imaginary and elastic spherical sound source respectively, and the results demonstrate that the method is valid and has high precision and calculating efficiency. The acoustic radiation characteristics from elastic cylindrical shells is investigated for different acoustic radiation sources, ocean environments and measurements. The cylindrical shell material is steel, whose radius and length are 3 m and 30 m respectively. The shallow water channel is an ideal waveguide with 50 m in depth, at the upper boundary, i.e., the free surface, the lower boundary is the Neumann boundary, i.e., the normal derivative of the acoustic pressure should be zero. The analysis frequency range is from 30 Hz to 200 Hz. The results show that due to a significant coupling effect of up-down direction boundaries on the sound field, the elastic structure can be equivalent to the point source only in low frequency and far field. The spatial field directivity distribution is more obvious at high frequency. The acoustic power measured by vertical line arrayis greatly influenced by ocean boundary and the depth of target.
Calculations of energy band structure and mobility in critical bandgap strained Ge1-xSnx based on Sn component and biaxial tensile stress modulation
Authors: Di Lin-Jia  , Dai Xian-Ying  , Song Jian-Jun  , Miao Dong-Ming  , Zhao Tian-Long  , Wu Shu-Jing  , Hao Yue  ,   State Key Discipline Laboratory of Wide Bandgap Semiconductor Technologies, School of Microelectronics, Xidian University, Xi'an 710071, China
Published: December 2020

Optoelectronic integration technology which utilizes CMOS process to achieve the integration of photonic devices has the advantages of high integration, high speed and low power consumption. The Ge1-xSnx alloys have been widely used in photodetectors, light-emitting diodes, lasers and other optoelectronic integration areas because they can be converted into direct bandgap semiconductors as the Sn component increases. However, the solid solubility of Sn in Ge as well as the large lattice mismatch between Ge and Sn resulting from the Sn composition cannot be increased arbitrarily:it is limited, thereby bringing a lot of challenges to the preparation and application of direct bandgap Ge1-xSnx. Strain engineering can also modulate the band structure to convert Ge from an indirect bandgap into a direct bandgap, where the required stress is minimal under biaxial tensile strain on the (001) plane. Moreover, the carrier mobility, especially the hole mobility, is significantly enhanced. Therefore, considering the combined effect of alloying and biaxial strain on Ge, it is possible not only to reduce the required Sn composition or stress for direct bandgap transition, but also to further enhance the optical and electrical properties of Ge1-xSnx alloys. The energy band structure is the theoretical basis for studying the optical and electrical properties of strained Ge1-xSnx alloys. In this paper, according to the theory of deformation potential, the relationship between Sn component and stress at the critical point of bandgap transition is given by analyzing the bandgap transition condition of biaxial tensile strained Ge1-xSnx on the (001) plane. The energy band structure of strained Ge1-xSnx with direct bandgap at the critical state is obtained through diagonalizing an 8-level kp Hamiltonian matrix which includes the spin-orbit coupling interaction and strain effect. According to the energy band structure and scattering model, the effective mass and mobility of carriers are quantitatively calculated. The calculation results indicate that the combination of lower Sn component and stress can also obtain the direct bandgap Ge1-xSnx, and its bandgap width decreases with the increase of stress. The strained Ge1-xSnx with direct bandgap has a very high electron mobility due to the small electron effective mass, and the hole mobility is significantly improved under the effect of stress. Considering both the process realization and the material properties, a combination of 4% Sn component and 1.2 GPa stress or 3% Sn component and 1.5 GPa stress can be selected for designing the high speed devices and optoelectronic devices.
Simulation analysis of aerosol effect on shortwave infrared remote sensing detection of atmospheric CO2
Authors: Wang Qian  , Bi Yan-Meng  , Yang Zhong-Dong  ,   National Satellite Meteorological Centre, Beijing 100081, China
Published: December 2020

The research of carbon dioxide (CO2) sources and sinks within the carbon cycle is significant for enhancing our understanding of global climate change. Space based measurement of CO2 concentration in lower atmosphere by reflected sunlight in near infrared (NIR) band has become a hot research topic at present. The global characteristic of atmospheric CO2 retrieval from NIR is studied using the expected measurement performance of Tansat (Tan Satellite) mission. With the development of CO2 retrieval algorithms, the light-path modification due to multiple scattering by aerosol is identified as a major source of error when retrieving CO2 from high resolution near-infrared spectrum. The present study focuses on atmospheric CO2 retrieval sensitivity to aerosol properties such as aerosol types, aerosol modes, and profiles aiming at the demands for retrieval accuracy of CO2 no larger than 0.3%-0.5% on a regional scale. Here, we carry out the aerosol scattering effects analysis on retrieving atmospheric CO2 near 1610 nm using the simulated nadir observation for Tansat based on CALIPSO aerosol profile products and SCIATRAN radiative transfer model. The results show that light path modification due to aerosol scattering is closely related to their types, modes and vertical distributions. For aerosol types, on the one hand, urban aerosol has the most significant influence on the measured radiance, followed by maritime aerosols, and has a much smaller influence for rural aerosol, which will lead to overestimated CO2 concentration for the typical surface albedo. On the other hand, the measured radiance will decrease with the increase of aerosol optical thickness (AOT) for urban and rural aerosols, but exactly the opposite to maritime aerosols. For aerosol modes and vertical distributions, aerosols in accumulation mode, the continental aerosols with multilayer aerosol vertical distribution and maritime aerosols with AOT less than 0.3 will bring about less than 5% of negative radiance changes, and will cause positive changes with the increase of AOT. However, aerosols in coarse mode will always cause negative changes of radiance regardless of aerosol vertical distribution, and thus resulting in an overestimation of CO2. In addition, the higher the aerosol layer distributed, the smaller the negative radiance change is. If aerosol profiles can be successfully retrieved as a state vector, then it can be expected that satellite measurement can lead to tremendous improvement in CO2 retrieval precision. This study provides important information about estimations of the influence of aerosol property on CO2 retrieval algorithm. All these results can contribute to improving the accuracy of CO2 retrieval.
A new method of measuring boundary value of atmospheric extinction coefficient
Authors: Sun Guo-Dong  , Qin Lai-An  , Zhang Si-Long  , He Feng  , Tan Feng-Fu  , Jing Xu  , Hou Zai-Hong  ,   Key Laboratory of Atmospheric Optics, Anhui Institute of Optics and Fine Mechanics, Chinese Academy of Sciences, Hefei 230031, China;,   Graduate School, University of Science and Technology of China, Hefei 230026, China
Published: December 2020

We construct a nonlinear equation between the return signal and the boundary value of extinction coefficient according to the lidar equation. And according to the nonlinear equation, we put forward a new method to solve the nonlinear equation by using Broyden algorithm. The Broyden algorithm is a concrete application of the quasi-Newton method. It has faster convergence and less iteration times, and does not need to calculate the derivative value. After choosing a suitable initial value, the boundary value can be obtained through the algorithm. A 532 nm single-band Mie scattering imaging lidar system is developed in Hefei, Southern China, for real-time atmospheric aerosol/particle remote sensing. Atmospheric measurement has been performed in Science Island during night time, and the time-range distribution of atmospheric backscattering signal was recorded on April 6, 2017, by employing the imaging lidar system. Then, the boundary values are achieved based on the Broyden algorithm and the least square algorithm. It adopts the Klett backward integration method to retrieve the horizontal distribution of extinction coefficients in a range of 1 km after the acquisition of the signal by changing the distance, then the horizontal atmospheric transmittance can be achieved based on the path integral. We also conduct a contrast experiment with the one-way transmission of the horizontal light near the ground within the range of 1 km at the same time. The initial site is situated in the experimental room besides the Dongpu reservoir and the end site is located on the second floor of our office building. The important things in this experiment are that the light reaching the target surface must be fully received and the laser power should be monitored at the double-end. Then we can obtain the transmittance by the direct method. By comparing the transmittance from the direct method with the transmittance from imaging lidar between the two different ways, i.e., Broyden algorithm and least square algorithm, then the correlation coefficients are obtained to be both over 0.95 in the period. And the method introduced in this paper is a little better than the least square algorithm with a value of 0.968. Besides, the average relative errors between the two inverse methods and the direct method are 4.66% and 9.10%, respectively. The average relative errors obtained by using the least square algorithm is about twice that by using the Broyden algorithm. It can be concluded that the algorithm introduced in this paper is effective and has certain advantages for the inverse problem.
Novel materials and devices bring new opportunities for holographic display
Authors: Peng Wei-Ting  , Liu Juan  , Li Xin  , Xue Gao-Lei  , Han Jian  , Hu Bin  , Wang Yong-Tian  ,   Beijing Engineering Research Center for Mixed Reality and Advanced Display, School of Optics and Photonics, Beijing Institute of Technology, Beijing 100081, China;,   School of Electrical and Electronic Engineering, Nanyang Technological University, Singapore 639798, Singapore;,   Key Laboratory of Information System and Technology, Beijing Institute of Control and Electronic Technology, Beijing 100038, China
Published: December 2020

Three-dimentional (3D) display is one of the effective ways to obtain visual information feeling like actual environment. Since holographic technique can provide full depth information for human eyes, it is considered to be an ideal 3D display technique. However, it is limited by the features of display elements and devices, such as the time-space (time and space) bandwidth product, massive data processing speed and low image quality and so on. To improve the display quality, expand the time-space bandwidth product, improve the performance of the system, and overcome the limitation, optical elements and devices made from novel materials are introduced, such as metamaterials, metasurfaces and two-dimensional (2D) materials, and thus bringing new challenges and opportunities to holographic display. Meta-atom structure whose unit size is much less than wavelength is designed and fabricated specially, and it can realize the isotropical or anisotropical manipulation of the amplitude and phase of the light wave. By encoding the meta-atom structures into the hologram, the 2D or 3D images can be achieved. The development of a refreshable metamaterials and their applications in dynamic holographic display will be one of the most important topics in the future. Though the 2D or 3D holographic displays based on the elements and devices made from novel materials still have some basic problems, it is expected that they would bring new impetus and promising perspective for the future display market.
Improvement of phase modulation laser Doppler shift measurement method
Authors: Du Jun  , Yang Na  , Li Jun-Ling  , Qu Yan-Chen  , Li Shi-Ming  , Ding Yun-Hong  , Li Rui  ,   College of Computer Science and Information Engineering, Harbin Normal University, Harbin 150052, China;,   College of Electrical and Information Engineering, Heilongjiang Institute of Technology, Harbin 150050, China;,   National Key Laboratory of Science and Technology on Tunable Laser, Harbin Institute of Technology, Harbin 150080, China
Published: December 2020

Sinusoidal phase-modulated signal light through the Fabry-Perot interferometer can produce a beat signal. Moreover, its amplitude monotonically changes with the signal light frequency. So the beat signal amplitude can be used to measure laser-Doppler-shift. In addition to the beat signal, the phase-modulated signal also contains a direct current (DC) signal, and it still contains a large amount of Doppler-shift information, but the information is not utilized, resulting in the waste of Doppler information. In this paper, this kind of phase-modulated laser-Doppler-shift measurement method is improved to simultaneously utilize the useful information in the DC and beat signal for the Doppler-shift measurement. The specific method is to use the ratio of beat signal amplitude to DC signal amplitude to define a new parameter used in Doppler-shift measurement. The signal light intensity terms in DC and beat signal can be eliminated, so the improved phase-modulated laser-Doppler-shift measurement method does not need to measure the signal light intensity, which makes its structure further simplified and a noise channel eliminated. By comparing the frequency change curves between the newly defined parameter and the beat signal amplitude theoretically, we find that they have the same distribution rule. This theoretical result shows that the improved phase-modulated laser-Doppler-shift measurement method will keep the same working mode as un-improved one, and can inherit its advantages. In theory, by comparing the measurement sensitivity curves, it is proved that the improved phase-modulated laser-Doppler-shift measurement method has higher measurement sensitivity and dynamic range than the un-improved one. The useful information included in the DC signal is the modulated signal light intensity transmittance of Fabry-Perot interferometer. So the improvement is essential to introduce the advantages of edge-technique laser-Doppler-shift measurement method based on the Fabry-Perot interferometer into the phase-modulated method for achieving higher performance. Two phase-modulated laser-Doppler-shift measurement methods before and after improvement are separately used to measure the frequency-shifted controllable signal light reflected by a hard object. The experimental results are in accordance with the theoretical analysis results very well. The comparison of experimental result between the two methods shows that the improved phase-modulated laser-Doppler-shift measurement method can approximately double the measurement dynamic range and reduce about 35% measurement standard deviation compared with the un-improved one.
Multi-pulse laser ranging method for pre-detection with high frequency resonance
Authors: Huang Min-Shuang  , Ma Peng  , Liu Xiao-Chen  ,   Opto-Mechatronic Equipment Technology Beijing Area Major Laboratory, Beijing Institute of Petrochemical Technology, Beijing 102617, China;,   Beijing University of Chemical Industry, Mechanical and Electrical Engineering College, Beijing 100029, China
Published: December 2020

Based on the measurement principle of pulse time-of-flight, non-cooperative target ranging technology using a pulsed laser diode (LD) as a light source has received widespread attention in recent years. Using leading edge timing method to directly detect pulses, its measuring range is about a few tens of meters and only a cm-level single-shot accuracy could be reached due to the limitations of its pulse width and eye-safe laser power of the LD, which cannot meet the needs of most applications. Especially, in order to increase its receiver channel bandwidth from hundreds of MHz to even a few GHz to reduce its work error, its distance measurement accuracy and ranging distance are significantly degraded as its signal-to-noise ratio (SNR) decreases. When a target is out of its measuring range, the back diffused laser pulse signal with an SNR of much less than 1 will be too weak to be extracted even with digital correlation processing technology.In this paper, using a pre-detection with high frequency resonance and multi-pulse correlation processing, a new ranging method to solve long ranging targets with high precision is proposed for the first time. Through the pre-detection circuit with high frequency resonance, a pulsed photocurrent signal is amplified and filtered, and then converted into a bipolar attenuation oscillation signal. Thereafter, its SNR is further improved by a new pulse function constructed through multi-pulse correlation processing. The peak of the new pulse is constant and its zero crossing point is found to be the timing point to calculate the target distance. The method has a better SNR and a high timing accuracy. And the detected ranging distance could be increased over one thousand meters or more. Theoretical calculation results show that the minimum detectable peak current of light pulse is around 17 nA in the method. Comparing with the direct pulse detection method, its SNR can increase 60 times. When a received peak of a photocurrent pulse is within a dynamic range of 1:10000, its work error is less than 0.1 ps. A pulsed laser rangefinder is developed based on the principle. And its average laser emission power is about 1 mW. Its measurement ranging without cooperative target is greater than 2000 m. Its distance measurement accuracy increases up to ± (3 mm+2 ppm) in a range of 1.5-300 m. For a long ranging target, its distance measurement accuracy is ± (10 mm+10 ppm). The rangefinder system is used in a total station product and can be used to measure large-scale engineering structures (such as roads, bridges, dams, tunnels, subways, etc.), building structures and industrial sites.
Fidelity susceptibility and entanglement entropy in S=1 quantum spin chain with three-site interactions
Authors: Ren Jie  , Gu Li-Ping  , You Wen-Long  ,   Department of Physics, Changshu Institute of Technology, Changshu 215500, China;,   College of Physics, Optoelectronics and Energy, Soochow University, Suzhou 215006, China
Published: December 2020

In the present work, we study the fidelity susceptibility and the entanglement entropy in an antiferromagnetic spin-1 chain with additional next-nearest neighbor interactions and three-site interactions, which are given by H=(J1SiSi+1+ J2SiSi+2)+[J3(SiSi+1)(Si+1Si+2)+ h.c.]. By using the density matrix renormalization group method, the ground-state properties of the system are calculated with very high accuracy. We investigate the effect of the three-site interaction J3 on the fidelity susceptibility numerically, and then analyze its relation with the quantum phase transition (QPT). The fidelity measures the similarity between two states, and the fidelity susceptibility describes the associated changing rate. The QPT is intuitively accompanied by an abrupt change in the structure of the ground-state wave function, so generally a peak of the fidelity susceptibility indicates a QPT and the location of the peak denotes the critical point. For the case of J2=0, a peak of the fidelity susceptibility is found by varying J3, and the height of the peak grows as the system size increases. The location of the peak shifts to a slightly lower J3 up to a particular value as the system size increases. Through a finite size scaling, the critical point J3c=0.111 of the QPT from the Haldane spin liquid to the dimerized phase is identified. We also study the effect of the three-site interaction on the entanglement entropy between the right half part and the rest. It is noted that the peak of the entanglement entropy does not coincide with the critical point. Instead, the critical point is determined by the position at which the first-order derivative of the entanglement entropy takes its minimum, since a second-order QPT is signaled by the first derivative of density matrix element. Moreover, the entanglement entropy disappears when J3=1/6, which corresponds to the size-independent Majumdar-Ghosh point. The positions of quantum critical points extracted from these two quantum information observables agree well with those obtained by the string order parameters, which characterizes the topological order in the Haldane phase. Secondly, we also study the case of J20, and obtain the critical points by both the fidelity susceptibility and the entanglement entropy. Finally we provide a ground-state phase diagram of the system. To sum up, the quantum information observables are effective tools for detecting diverse QPTs in spin-1 models.
Electronic structure and optical properties of Cu-O co-doped AlN
Authors: Cheng Li  , Wang De-Xing  , Zhang Yang  , Su Li-Ping  , Chen Shu-Yan  , Wang Xiao-Feng  , Sun Peng  , Yi Chong-Gui  ,   Key Laboratory of Integrated Fiber Optics, Ministry of Education, Harbin Engineering University, Harbin 150001, China
Published: December 2020

The geometry parameters, band structure, electronic density of states, and optical properties of AlN before and after being co-doped by Cu and O are investigated by the ultra-soft pseudo-potential plane wave based the density functional theory. The results show that the lattice volume increases and the total energy of the system decreases after doping. The Cu doping system makes Cu 3d electrons hybridize with its nearest neighbor N 2p electrons strongly. In the Cu-O co-doped system, Cu and O attract each other to overcome the repelling of acceptor Cu atoms, thereby increasing the doping concentration of Cu atoms and the stability of the system. Dielectric function calculation results show that Cu-O co-doping can improve the optical transition characteristics in low energy area of AlN electrons, and thus enhancing the optical transition of electrons in visible area. The complex refractive index calculation results indicate that Cu-O co-doped system increases the absorption of low frequency electromagnetic wave.
Influence of secondary electron yield of material on two-sided multipactor discharge in cavity
Authors: Dong Ye  , Liu Qing-Xiang  , Pang Jian  , Zhou Hai-Jing  , Dong Zhi-Wei  ,   School of Physical Science and Technology, Southwest Jiaotong University, Chengdu 610031, China;,   Institute of Applied Physics and Computational Mathematics, Beijing 100094, China;,   Institute of Fluid Physics, China Academy of Engineering Physics, Mianyang 621900, China
Published: December 2020

The influences of secondary electron yield (SEY) of material on the transient and saturation characteristics of two-sided multipactor discharge in cavity are numerically investigated by using particle-in-cell and Monte-Carlo methods. The numerical results indicate that as the SEY increases, the rate of electron number increases and the average value and magnitude of steady electron number also increase. The oscillation start-time of discharge current is shortened, and the steady value of discharge current increases and tends to be saturated. Both the average value and magnitude of steady discharge power increase and tend to be saturated. Both the time-delay and pulse width of deposited power waveform increase and also tend to be saturated. Under the circumstances of higher and lower value of SEY, the physical images of electron phase space, charge density, average impact energy, average SEY, electron number and discharge current are in detail shown in particle-in-cell simulation. The results can be concluded as follows. Under the circumstances of lower value of SEY, the saturation characteristics is determined by both debunching and reverse field of space charge effects. But under the circumstances of higher value of SEY, the multipactor mechanism tends to be one-sided mode in the steady stage which can be obviously determined by reverse-field of space charge effect.
Molecular dynamics simulation of freezing process of water droplets impinging on cold surface
Authors: Dong Qi-Qi  , Hu Hai-Bao  , Chen Shao-Qiang  , He Qiang  , Bao Lu-Yao  ,   College of Marine, Northwestern Polytechnical University, Xi'an 710072, China;,   Research and Development Institute of Northwestern Polytechnical University in Shenzhen, Shenzhen 518057, China;,   Xi'an Precision Machinery Research Institute, Xi'an 710077, China
Published: December 2020

The freezing of water droplet is a ubiquitous phenomenon in nature. Although the freezing process of water droplet impacting on cold surfaces is widely observed on a macroscopic scale, the study of freezing process on a micro-scale is still deficient, and it is definitely difficult to study micro-droplets and nano-droplets using experimental methods due to the obstacles in both generation and observation. For these reasons, simulation methods using molecular dynamics (MD) have been proposed to study micro-droplets and nano-droplets, as molecular dynamics can trace each atom, count up the collective behavior of a group of atoms and describe the detail interaction between atoms. In this paper, a three-dimensional model is established by molecular dynamics simulation to study the freezing process of water droplets impinging on a cold solid surface on a nanoscale. We select the micro-canonical ensemble (NVE) as a statistical system and the TIP4P/ice model as a potential energy function to simulate oxygen atoms, hydrogen atoms and water molecules. The LJ/126 model is used to simulate the interaction between water molecules and solid atoms. Different wettability walls are simulated by adjusting the potential energy parameters. For all the simulations, the velocity-rescale method is used to keep the temperature constant and the Verlet algorithm is adopted to solve the Newton equations. In the velocity-rescale method, the temperature is calculated by using the profile-unbiased thermostat. The freezing process inside the water droplet is determined by the temperature distribution of water molecules along the vertical direction, which is more concise than by the location coordinates of the microscopic atoms. Through the numerical experimentations, we find that when the surface temperature decreases, the completely freezing time of drops is reduced; meanwhile, the time required for water temperature to drop down to the wall temperature is increased. Moreover, the heat transfer inside the water droplet slows down with the decreasing of wall hydrophilicity while the total freezing time is prolonged.
Phase transformation kinetics of zirconium under ramp wave loading with different windows
Authors: Chong Tao  , Wang Gui-Ji  , Tan Fu-Li  , Zhao Jian-Heng  , Tang Zhi-Ping  ,   Institute of Fluid Physics, China Academy of Engineering Physics, Mianyang 621900, China;,   Department of Modern Mechanics, University of Science and Technology of China, Hefei 230027, China
Published: December 2020

The effect of window acoustic impedance on the wave profile of phase transition of zirconium under ramp wave compression is investigated in experiment and simulation. In the experiments, a ramp wave driven by magnetic pressure is applied to the zirconium samples backed windows with different acoustic impedances such as LiF, sapphire and free surface based on the compact pulsed power generator CQ-4. The experimental wave profiles measured by an advanced laser interference velocimeter show that the characteristic particle velocity of the onset phase transition from to is about 331.0 m/s in the conditions of LiF widow and free surface with low acoustic impedance, and it is approximately 301.9 m/s for the sapphire window with higher acoustic impedance. The corresponding onset pressure of phase transition varies from about 9.14 GPa to 8.27 GPa. The result shows that this onset pressure of phase transition, which is affected by diverse factors, is not the inherent value of phase transition belonging to the material properties. In order to describe these dynamic responses in experiment well, the numerical simulation of phase transition dynamics of zirconium is conducted in one-dimensional hydrodynamic code, in which included are the muti-phase equation of state based on Helmholtz free energy, the equation of non-equilibrium phase transition dynamics, and Steinberg constitutive relationship. The simulated results show that they can reflect the physical processes of elasto-plastic transition and - phase transition as well, which are excellently consistent with the experimental data. The relaxation times of - phase transition in three different acoustic impedance experiments are nearly the same (30 ns), and their finishing times of phase transition are all about 100 ns. The calculated quasi-isentrope of zirconium below 20 GPa in the pressure-volume and temperature-pressure thermodynamic planes shows that the isentrope and shock adiabat exhibit tiny difference before phase transition, and then separate gradually with the increase of pressure. The isentrope lies below the shock adiabat after the onset of phase transition. At about 20 GPa, the temperature of zirconium under ramp wave loading is bout 100 K lower than that under shock loading. Meanwhile, the abrupt change of volume at phase transition causes the Lagrange sound speed to reduce about 7% and then comes back to the bulk sound speed again after the phase transition has been finished.
Theoretical model and correction method of range walk error for single-photon laser ranging
Authors: Huang Ke  , Li Song  , Ma Yue  , Tian Xin  , Zhou Hui  , Zhang Zhi-Yu  ,   School of Electronic Information, Wuhan University, Wuhan 430072, China
Published: December 2020

Single-photon laser ranging is a new generation of lidar which represents the future lidar development trend.It uses the single photon detector as the receiving device.Due to the fact that single-photon detector possesses the ultra-high sensitivity,the single-photon laser ranging is much easier to achieve the high density as well as the high coverage target sampling.However,the existence of the range work error in single-photon laser ranging,resulting from the fluctuation in the number of signal photoelectrons restricts the improvement of the ranging accuracy.In this paper,the range walk error model based on the lidar equation and the statistical property of single-photon detector is established.Then the relation between the range walk error and the number of signal photoelectrons is also derived.The range walk error of single-photon laser ranging is predicted and the corresponding compensation for the original result is obtained,with the derived function and the detection probability model of single-photon laser ranging.The experiment for its proof is also carried out.In the experiment,the number of signal photoelectrons is changed by the different attenuators for the same target and at the same distance.When the attenuator is changed,the pulse width of echo signal changes very little (about 3.2 ns).However,the average number of signal photoelectrons varies between 0.03 counts and 4.3 counts.So the range walk error,resulting from the fluctuation in the number of signal photoelectrons cannot be ignored.For example, when using an attenuation of 1/10 pass rate,the average number of signal photoelectrons is about 4.3 counts and the range walk error is almost 46 cm,which is the main factor of the range error.The reduction of the range walk error is achieved by applying the correction of the range walk error in this paper.After correction,the standard deviation of the range walk error decreases significantly from 15.17 cm to 1.16 cm.The mean absolute error is also reduced from 11.56 cm to 0.99 cm.Generally,the range walk error has an unnegligible influence on the ranging accuracy.The experimental result confirms that the theoretical model is accurate.It also shows that the bigger the number of the received signal photoelectrons,the greater the range walk error is,and the accuracy of single-photon laser ranging is improved by applying the technique proposed in this paper.Briefly,this paper presents the technical method of optimizing the design and evaluating the performance of single-photon laser ranging.
Conformation of disordered peptides modulated by distributions of charged residues: Case study of random peptides composed of arginines and aspartic acids
Authors: Kang Wen-Bin  , Wang Jun  , Wang Wei  ,   School of Physics, Nanjing University, Nanjing 210093, China;,   Department of Mathematics and Physics, Hubei University of Medicine, Shiyan 442000, China;,   Bio-X Research Center, Hubei University of Medicine, Shiyan 442000, China
Published: December 2020

The relationship between the sequential and structural features of intrinsically disordered peptides (IDPs) has attracted much attention during the recent decade. One essential problem relating to sequence-structure relationship is how the distribution of charged residues affects the structure of IDP. In this work, we address this problem with simulations on a series of random peptides composed of arginine and aspartic acids. With the ABSINTH implicit solvation model, the structural ensembles are generated with Markov Chain Monte Carlo method and replica-exchange sampling. The relations between various structural features (including the gyration radius, the tail distance, the distance between residues, and asphericity) and the distribution of charged residues are analyzed. Several limit cases (with parts of interactions switched off) are also calculated for comparison. The conversion from extended conformations to compact structures is observed, following the demixing of negatively and positively charged residues along the sequence. For the cases with well-mixed charges, the intra-chain electrostatic repulsions and attractions are balanced, which results in a generic Flory random coil-like conformation. Differently, for the case with well-separated charged residues, the electrostatic attraction between residues distant along the sequence induces a semi-compact hairpin-like conformation. This is consistent with the observations of Pappu group. Our results suggest that the structural dependence on charge distribution would not be sensitive to the selection of amino acid, and is determined by the patterns of charges, which demonstrates the robustness of the mechanism that the charge distribution modulates the structural features in the IDP system. Our results may broaden our understanding of the sequence-structure relation of IDP system.
Structural broadband absorbing metamaterial based on three-dimensional printing technology
Authors: Xiong Yi-Jun  , Wang Yan  , Wang Qiang  , Wang Chun-Qi  , Huang Xiao-Zhong  , Zhang Fen  , Zhou Ding  ,   Institute of Aeronautics and Astronautics, Central South University, Changsha 410012, China;,   Hunan Key Laboratory of Advanced Fibers and Composites, Changsha 410012, China;,   Institute of Physical and Electronic, Central South University, Changsha 410012, China
Published: December 2020

In order to verify the feasibility of three-dimensional (3D) printing technology in preparing the metamaterial absorbers with complex structure, a three-layer broadband absorbing metamaterial is designed and fabricated by 3D printing technology. The surface layer and middle layer of the metamaterial are composed of periodic arrays with different unit dimensions and the bottom layer of a slab structure. The optimized thickness of the metamaterial is 4.7 mm. A composite absorbent which consists of carbonyl iron powder and nylon is used to fabricate the absorber. In experiment, the obtained absorber is vertically irradiated by an electromagnetic (EM) wave. Two strong absorption peaks at 5.3 GHz and 14.1 GHz are achieved, with the reflection losses of -15.1 dB and -12.5 dB, respectively. The superposition of the two absorption peaks results in a reflection loss below -10 dB in a range from 4 to 18 GHz. The effective EM parameters of the surface layer and the middle layer are calculated by the S parameter inversion method. An effective model of the three-layer structure absorber is proposed and its reflectivity is calculated by using a multilayer structure reflectivity formula. The calculated reflectivity agrees well with the measured one. The absorbing and resonance mechanisms of the two absorption peaks are investigated by analyzing the dynamic distributions of power density loss, electric field and magnetic field. It can be clearly confirmed that the reflection losses at 5.3 GHz and 14.1 GHz are primarily concentrated on the bottom layer and surface layer, and the broadband absorption performance can be derived from the superposition of broadband absorptions of the three absorbing layers. Meanwhile, the strong electric coupling effect between the adjacent units in the surface layer is demonstrated by analyzing the electric-field distributions, which indicates that the strong reflection loss at 14.1 GHz is mainly caused by the electric response. The multiple scattering effects among the three layers are also considered according to the magnetic field distributions at two resonance frequencies. It is shown that there are two magnetic responses at 5.3 GHz and 14.1 GHz, respectively, and the multiple scattering contributes to increasing the EM wave propagation distance and enhancing the power loss. The designed absorbing metamaterials in this paper achieve good broadband absorption performances, particularly in the low frequency band. Combined with 3D printing rapid technology, a promising route to constructing 3D absorbing metamaterials with complex structures is proposed, which would be of great significance and broad practical prospect.
Evaluation methods of node importance in undirected weighted networks based on complex network dynamics models
Authors: Kong Jiang-Tao  , Huang Jian  , Gong Jian-Xing  , Li Er-Yu  ,   College of Mechatronics Engineering and Automation, National University of Defense Technology, Changsha 410073, China
Published: December 2020

Identifying the most important nodes is significant for investigating the robustness and vulnerability of complex network. A lot of methods based on network structure have been proposed, such as degree, K-shell and betweenness, etc. In order to identify the important nodes in a more reasonable way, both the network topologies and the characteristics of nodes should be taken into account. Even at the same location, the nodes with different characteristics have different importance. The topological structures and the characteristics of the nodes are considered in the complex network dynamics model. However, such methods are rarely explored and their applications are restricted. In order to identify the important nodes in undirected weighted networks, in this paper we propose a method based on dynamics model. Firstly, we introduce a way to construct the corresponding dynamics model for any undirected weighted network, and the constructed model can be flexibly adjusted according to the actual situation. It is proved that the constructed model is globally asymptotic stable. To measure the changes of the dynamic model state, the mean deviation and the variance are presented, which are the criteria to evaluate the importance of the nodes. Finally, disturbance test and destructive test are proposed for identifying the most important nodes. Each node is tested in turn, and then the important nodes are identified. If the tested node can recover from the damaged state, the disturbance test is used. If the tested node is destroyed completely, the destructive test is used. The method proposed in this paper is based on the dynamics model. The node importance is influenced by the network topologies and the characteristics of nodes in these two methods. In addition, the disturbance test and destructive test are used in different situations, forming a complementary advantage. So the method can be used to analyze the node importance in a more comprehensive way. Experiments are performed on the advanced research project agency networks, the undirected networks with symmetric structures, the social network, the Dobbs-Watts-Sabel networks and the Barrat-Barthelemy-Vespignani networks. If the nodes in the network have the same dynamic model, the network is considered to be the homogeneous network; otherwise, the network is heterogeneous network. And experiments can be divided into four categories, namely, the disturbance test, the destructive test on the homogeneous network, the disturbance test and the destructive test on the heterogeneous network. The experimental results show that the methods proposed in this paper are effective and credible.