bluuebunny/crossref_metadata_2025 huggingface dataset:
Quantitative characterization of viscoelasticity of microbubbles in ultrasound contrast agent
Authors: Guo Ge-Pu  , Zhang Chun-Bing  , Tu Juan  , Zhang Dong  ,   Key Laboratory of Modern Acoustics (Nanjing University), Ministry of Education, Institute of Acoustics, Nanjing 210093, China;,   The Traditional Chinese Medicine Hospital of Jiangsu Province, Nanjing 210056, China
Published: December 2020

Ultrasound contrast agent (UCA) microbubbles have been commonly used in clinic to enhance the acoustic backscattering signals in ultrasound imaging diagnosis. With increasing demand for the continuous improvement of imaging resolution and sensitivity, new type UCAs (e.g., targeted microbubbles and multifunctional microbubbles) have attracted growing interest in both medical and scientific communities. Many efforts have been made to modify microbubble shell properties, which can strongly affect microbubble dynamic behaviors, so as to enable to create some new functionalities of UCAs. However, accurate characterization of the shell mechanical properties of UCAs has been recognized to be rather challenging. In previous work, microbubble’s mechanical properties are normally estimated by fitting measured dynamic response signals with coated-microbubble models. Inevitable uncertainty will be introduced in fitting results because there are more than one unknown shell parameters are adopted in these dynamic models. In the present paper, a comprehensive approach is developed to quantitatively characterize the visco-elasticity of the encapsulated microbubbles. By combining the techniques of atomic force microscopy (AFM), single particle optical sensing (SPOS), acoustic attenuation measurement, and the coated-bubble dynamics simulation, the size distribution, shell thickness, shell elasticity and viscosity of UCA microbubbles are determined one by one in sequence. To examine the validity of this approach, a kind of albumin-shelled microbubbles with diameters ranging from 1 to 5 μm are fabricated in our lab. Based on AFM technology, the microbubble effective shell stiffness and bulk elasticity modulus are measured to be 0.149±0.012 N/m and 8.31±0.667 MPa, respectively. It is noteworthy that the shell elastic property is shown to be independent of the initial size of microbubbles. Furthermore, the size distribution and acoustic attenuation measurements are also performed of these bubbles. Then, combined with microbubble dynamic model simulations, the UCA shell viscosity is calculated to be 0.374±0.003 Pa·s. Compared with previous estimation method, the current technology can be used as an effective tool to assess UCA shell visco-elasticity with improved accuracy and certainty. It is also shown that the feasibility to optimize the design and fabrication of UCAs can satisfy different requirements in ultrasound diagnostic and therapeutic applications.
Study on the droplet impact on hydrophobic surface in terms of van der Waals surface tension model
Authors: Bai Ling  , Li Da-Ming  , Li Yan-Qing  , Wang Zhi-Chao  , Li Yang-Yang  ,   State Key Laboratory of Hydraulic Engineering Simulation and Safety, Tianjin University, Tianjin 300072, China;,   Department of Civil and Environmental Engineering, the University of Adelaide, Adelaide, Australia, 5005
Published: December 2020

Research on the droplet impact on a hydrophobic surface is of important theoretical significance and engineering value, both in mesoscopic fluid mechanics and interactions between microfluid and special materials. The van der Waals (vdW) equation of state relates the pressure to the temperature and the density of the fluid, and gives the long-range attractive force and short-range repulsive force between particles. The van der Waals equation of state can be used to describe the surface tension between liquid and vapor. As a pure meshless particle method, the smoothed particle hydrodynamic (SPH) method can use the vdW equation of state written in SPH form of N-S equations to describe the surface tension. The vdW surface tension mode is validated by simulating the coalescence of two equally sized static droplets in vacuum. Repellant of the hydrophobic surface is derived from a core potential. By combining the vdW surface tension and the repulsive force of the surface, the phenomenon of a liquid droplet impact with a certain initial velocity on the hydrophobic surface is studied. The SPH model is not only capable to describe the spreading of the droplet after it contacts the surface, but also clearly reproduces the springback, bouncing and secondary impact of the droplet. During the deformation of the droplet, the inertia force impels the spreading process of the droplet whilst the springback and bouncing behavior is dominated by the surface tension. The simulated results are in good agreement with the published experimental observations and VOF simulated results, indicating that the way we treat the surface tension and the repulsive force of the hydrophobic surface is effective and applicable in droplet impact surface problems. The impact velocity and liquid viscosity are considered to be two important factors that affect the deformation of the droplet after it contacts the surface. By varying the impact velocity within a certain range it is concluded that the maximum liquid-solid contact area increases as the impact velocity grows, and the bounced droplet will leave the surface when the velocity is big enough. Another comparison between different liquid viscosities shows that the maximum contact area decreases as the liquid viscosity increases because of the viscous dissipation, and the droplet barely rebound when the liquid viscosity is big enough.
Analysis on refraction of detonation wave at the explosive-metal interface
Authors: Yu Ming  , Sun Yu-Tao  , Liu Quan  ,   National Key Laboratory of Computational Physics, Institute of Applied Physics and Computational Mathematics, Beijing 100094, China
Published: December 2020

This paper analyzes theoretically and numerically the refraction phenomenon of detonation wave at the explosive-metal interface, motivated by the problem that there exist large discrepancies between the experimental results and the classical shock polar theory. After pointing out the major defects of the classical shock polar theory based on CJ model of detonation, an improved shock polar theory based on ZND model of detonation is presented to give the styles of the refraction of detonation wave and the pressure values at the interaction point between the refracted shock wave and the incident shock wave, to show the pressure values at free-surface of copper remarkably lower than the ones at the shock interaction point due to the attenuation effects from the chemical reaction expansion and the following Taylor rarefaction. A second-order cell-centered Lagrangian hydrodynamics method with high resolution based on the subcharacteristics theory is develped to solve the reactive flow equations of detonation in condensed explosive, and then to numerically simulate a representative refraction experiment about T2 explosive interacting with copper. The simulated pressure values at the interaction point agree well with the ones from the improved shock polar theory, and the simulated pressure values at free-surface of copper agree well with the experimental values, meanwhile, the refraction styles predicted by the improved shock polar theory are confirmed by the numerically simulated flowfield images. From the theoretical and numerical results, there exist three kinds of refraction styles of detonation waves at explosive-metal interface:i) the regular refraction with reflecting shock wave, ii) the irregular refraction with Mach reflection, and iii) the regular refraction without any reflecting wave; in particular, the regular refraction with no reflecting wave is a kind of refraction style unable to be predicted by the classical shock polar theory, meanwhile, the pressure values at the free-surface and the interaction point inside the shocked metal both monotonically decrease with the increase of the incident angle.
The electrical-neutrality constraint of single Langmuir probe measurement in collisional plasma
Authors: Ming Zhang-Jian  , Lan Tao  , Li Hong  , Xie Jin-Lin  , Liu A-Di  , Liu Wan-Dong  ,   CAS Key Laboratory of Basic Plasma Physics, Department of Modern Physics, University of Science and Technology of China, Hefei 230026, China
Published: December 2020

The volt-ampere (V-I) characteristic curve obtained using single probe diagnostic system for collisionless plasma will be distorted when the system is applied to collisional plasma. The V-I characteristic curves of collisionless and collision inductively coupled plasma is obtained at the gas pressure of 20 Pascal and 400 Pascal, respectively. The distortion phenomenon was studied by measuring the V-I characteristic curves at different positions of the probe and introducing interference electrode to adjust the level of overall electrical-neutrality constraint. The inferential and experimental results show that the overall electrical neutrality in collisional plasma will compose constraint on the single probe diagnostic system for collisionless plasma. Moreover, it is concluded from experiments that the plasma will adjust the potential of itself, which makes the potential of vacuum chamber wall as reference, in order to satisfy the overall electrical neutrality, and that the V-I characteristic curve obtained by single probe is not the V-I characteristic of probe sheath predicted by theory.
Flux controllable pumping of water molecules in a double-walled carbon nanotube
Authors: Cao Ping  , Luo Cheng-Lin  , Chen Gui-Hu  , Han Dian-Rong  , Zhu Xing-Feng  , Dai Ya-Fei  ,   School of Physics and Technology, Nanjing normal University, Nanjing 210023, China;,   Jiangsu Key Laboratory on Optoelectronic Technology, Nanjing 210023, China;,   School of Physics and Electronic Engineering, Jiangsu Second Normal University, Nanjing 210013, China
Published: December 2020

A water pumping system model has been designed based on the double-walled carbon nanotube. In this system, the inner tube is fixed as the water channel, while the exterior one can move, similar to the piston motion along the axial direction, to create a pumping force. Molecular dynamics simulations confirm that both the water flux and the water dipole orientation are sensitive to the velocity of motions of the outer tube so that a controllable unidirectional water flow can be achieved in this system by varying the velocity. Its pumping ability comes mainly from the carbon-water van der Waals driving forces of the exterior tube. The piston motion of the outer tube changes the position of the vdW balance point, which not only leads to the increase of vdW force on the water molecules already residing in the inner tube, but also enlarges their accelerated distance. Meanwhile, the orientation of water molecules inside the inner tube is strongly coupled to the water flux, the probability of +dipole states attains unity at v = 0.05 Å/ps, where the water flux reaches its maximum value (2.02 ns-1). Compared to the pump which is controlled by uniform electric field, the transmission efficiency of our mechanical pump is higher. This design may open a new way for water pumping in the field of nanodevices.
Boson peaks in doped colloid glasses
Authors: Liu Hai-Xia  , Chen Ke  , Hou Mei-Ying  ,   Department of Physics, Beijing Institute of technology, Beijing 100081, China;,   Key Laboratory of Soft Matter Physics, Beijing National Laboratory for Condensed Matter Physics, Institute of Physics, Chinese Academy of Sciences, Beijing 100190, China
Published: December 2020

We experimentally investigated the correlation between local structures and phonon modes in quasi-2D colloidal glasses. The glass samples consist of thermo-sensitive poly-N-isopropylacrylamide microgel (PNIPAM) particles, whose diameter can be tuned by small changes of sample temperature. A binary mixture of these particles is confined between two coverslips and forms a monolayer of quasi-2D glass. By changing the number ratio between large and small particles, the structure or the overall degree of disorder of the samples can be systematically tuned. We employ a video microscopy to record the motion of the colloidal particles in the sample for 11 min at a rate of 60 fps. The trajectories of individual particles are obtained by particle tracking software. Dynamical matrix is constructed using covariance matrix analysis, from which the eigenfrequency and eigenvector of vibrations are extracted. In this study, we focus on the evolution of the low-frequency quasi-localized phonon modes in glasses, as the system becomes more and more disordered from the increased dopants. To compare the results from different samples, we choose those with packing fraction of 86%, and rescale the eigenfrequencies by the median frequency of each sample. For the four doping levels investigated (2%, 9%, 29%, 61%), the density of states at low frequencies increases with the doping level, suggesting that the fraction of low-frequency modes increases with disorder, which is corroborated by the higher boson peaks at higher dopant fractions. We have measured the participation ratio of the obtained phonon modes, and find that the boson peak corresponds to quasi-localized vibration modes, or soft modes. We also examine the correlation between the soft modes and local structural parameter. Specifically, we have calculated the local orientational order parameter in our samples, and computed the correlation coefficients between the relative amplitude and the local orientational order parameter for each mode. The soft modes are found to have a significantly negative correlation with the local orientational order parameter, which implies that the soft modes are concentrated in regions with poor local order. We therefore conclude that the local disorder is probably the structural origin of soft modes in glasses.
Effect of temperature field and different walls on the wetting angle of molten silicon
Authors: Xu Duo  , Ding Jian-Ning  , Yuan Ning-Yi  , Zhang Zhong-Qiang  , Chen Guang-Gui  , Guo Li-Qiang  , Ling Zhi-Yong  ,   Micro/Nano Science and Technology Center, Jiangsu University, Zhenjiang 212013, China;,   Low-dimension Material Micro/Nano Device and System Center, Changzhou 213164, China;,   Collaborative Innovation Center of Photovolatic Science and Engineering, Changzhou University, Changzhou 213164, China;,   The breeding construction point of State Key Laboratory of Photovoltaic Engineering Science, Changzhou University, Changzhou 213164, China
Published: December 2020

A capillary model is developed for calculating the wetting angle of molten silicon on different walls by using the microfluidic two-phase flow level set method and studying the characteristics of the rising process. A mathematical model formulation rigorously accounts for the mass and momentum conservation by using the improved Navier-Stokes equation and considering the Marangoni effect. Compared with the experimental data, the change of the wetting angle on the chemical vapor deposition (CVD) diamond wall indicates the grids independence and the validity of the numerical algorithm. We also discuss the influence of surface tension, and Marangoni stress induced by the gradient of surface tension coefficient, and wall adhesion to the change of wetting angle for three different walls, which include SiC wall, graphite wall, and CVD diamond wall, at different temperatures (1683-1873 K). Result shows that at the same temperature, the thermal-capillary effects that induce the molten silicon to undulation are raised. The wetting angle is reduced after first being increased and finally stabilized. At the initial stage, the fluctuation of the liquid-air interface is volatile due to the large changes of the liquid-air and the wall-air surface tensions, and subsequently, the fluctuation tends to be stable while the wetting angle is close to a fixed value. It is also found that with the graphite wall, these changes are more likely to be stable. This research provides a theoretical guide to obtain a stable growth environment for silicon belt fabricated from the molten silicon.
Molecular dynamics study on tensile behavior of SiC nanofiber/C/SiC nanocomposites
Authors: Li Li-Li  , Xia Zhen-Hai  , Yang Yan-Qing  , Han Ming  ,   Department of Materials Science and Engineering, Fujian University of Technology, Fuzhou 350118, China;,   Department of Materials Science and Engineering, University of North Texas, Denton, TX 76203, USA;,   School of Materials Science and Engineering, Northwestern polytechnical University, Xi’an 710072, China
Published: December 2020

Fracture behavior and mechanical properties of SiC nanofiber (SiCNF) reinforced SiC nanocomposites as influenced by the thickness of amorphous carbon (a-C) coatings are studied via molecular dynamics simulations using Tersoff potential. To simulate the condition that a matrix crack arrives at the interface between matrix and coating, a pre-setting matrix crack is created. Results show that the tensile stress-strain curve of nanocomposites without and/or with thin a-C coatings (e.g., t≤ 0.3 nm) demonstrates an abrupt drop after achieving a maximum value, while nonlinear tails appear in the curves of nanocomposites with thick a-C coatings (e.g., t >2.0 nm). It is demonstrated that the SiCNF is penetrated by the matrix crack when it is uncoated and/or coated by a thin a-C layer (t ≤ 0.3 nm) and the nanocomposite fails in a typical brittle mode; whereas the crack deflection path changes and the SiCNF is pulled out from the matrix when the a-C coatings are thick enough (e.g., 4 nm), showing a different fracture mode in nanocomposites. Compared to nanocomposites without an a-C coating, the tensile strength of nanocomposites with a-C coating of 4.0 nm thickness is about four times higher, and the fracture energy increases around an order of magnitude. Furthermore, the average stress concentration factor for SiCNF in nanocomposites, defined as the ratio of tensile strength of single SiCNF to the average stress of the nanofiber in the composite when it is broken, is extracted and shows a decreasing trend with increasing coating thickness, indicating that a-C coating can therefore be expected to simultaneously enhance the tensile strength and fracture energy of the SiCNF/SiC nanocomposites. This work sheds light on the toughening mechanism in SiCNF/C/SiC nanocomposites where a-C coating plays a significant role, indicating that the toughening mechanism in conventional ceramic matrix composites on a microscale is still valid on a nanoscale. Simulation results suggest that coating thickness in material design is efficient for engineering SiCNF/SiC nanocomposites with high strength and toughness.
Dynamic simulation of fiber orientation in the gap flow field between two rotating cylinders
Authors: Yang Bin-Xin  , Ouyang Jie  , Zhou Wen  , Wang Fang  , Li Xue-Juan  ,   School of Applied Science, Taiyuan University of Science and Technology, Taiyuan 030024, China;,   School of Sciences, Northwestern Polytechnical University, Xi’an 710129, China;,   School of Sciences, Xi’an University of Architecture and Technology, Xi’an 710055, China
Published: December 2020

The gap flow field formed by two rotating cylinders and the fiber orientation in the gap flow field are studied numerically. The finite volume method on the collocated body fitted grid is used for solving the field. On the assumption that there is no relative motion between the fibers and the fluid, the motion of the fibers is determined. The velocities of fibers are calculated by bi-linear interpolation method. The orientation of fibers is obtained by solving the Jeffery equation. Periodic boundary conditions are used for the fiber motion to ensure that the fibers keep staying in the computational area. Two cases i. e., two cylinders rotate in the opposite directions with the same speed and only the mandrel cylinder rotates, are considered. Physical quantities, such as velocity and pressure, for each case are obtained. For the first case, the velocity and pressure are completely symmetric about the mid-line of the computational area and the absolute values of the maximum and minimum velocity are equal due to the fact that both the casing and mandrel cylinders rotate at the same speed. The absolute values of the maximum and minimum pressure are not equal because the radii of the two cylinders are different. For the second case that only the mandrel cylinder rotates, the symmetries of the velocity and pressure about the mid-line of the computational area can also be found although the absolute values of the maximum and minimum velocity are not equal because of the different velocities of the two cylinders. Fiber motions and orientations at different times for both cases are captured. The twisting of fibers (matrix) can be observed vividly. For the case that the casing and the mandrel cylinders rotate in the opposite directions, fibers move and orientate in a two-layer structure. While for the case that only the mandrel cylinder rotates, fibers move and orientation in a single-layered structure. For both cases, the fibers have a strong tendency to align along the stream lines of the field. The influence of the slenderness ratio of fibers on fiber orientation is also studied. A stronger tendency to align along the stream lines of the field can be found as the slenderness ratio of fibers increases.
Optical properties of (Mg0.97, Fe0.03)O ferropericlase under the pressure of the Earth’s lower mantle
Authors: Gao Min  , Shu Wen-Lu  , Ye Qiang  , He Lin  , Zhu Wen-Jun  ,   Institute of Solid State Physics, College of Physics and Electronic Engineering, Sichuan Normal University, Chengdu 610068, China;,   National Key Laboratory for Shock Wave and Detonation Physics Research, Institute of Fluid Physics, China Academy of Engineering Physics, Mianyang 621900, China
Published: December 2020

The optical-absorption and refractive-index properties of (Mg0.97, Fe0.03)O ferropericlase crystals without and with Mg and O ionic divacancy point-defect under the pressure of the Earth’s lower mantle are investigated using the first-principles calculations. Optical-absorption data show that the perfect-crystal results are similar to the predictions from the crystal-field theory:the pressure-induced spin transition of iron in ferropericlase causes a large blue-shift in its optical-absorption spectrum, leaving the near-infrared region transparent. However, when there are point defects in ferropericlase, the calculated optical-absorption results are completely inconsistent with predictions from the crystal-field theory, the spin transition causes the enhancement in the optical absorption in the near-infrared region. Refractive-index data of defect crystal indicate that the effects of pressure, wavenumber, and spin-transition on the high-pressure refractive-index of (Mg0.97, Fe0.03)O ferropericlase are obvious, but perfect-crystal results show that those effects should be relatively weak. The ～15%-20% iron-bearing ferropericlase is currently considered as an important mineral in the Earth’s lower mantle. Due to similar characteristics of the observed high-pressure optical-absorption spectrum in ferropericlase with different iron content, we suggest that:(1) the above-mentioned calculated results is conducive to the understanding of high-pressure optical properties of lower-mantle ferropericlase and the exploring of the origin of discrepancies in its high-pressure optical-absorption spectrum between experiment and crystal-field theory; (2) the high-pressure optical-absorption spectrum measurements may be a good approach for probing iron spin state.
Period-adding bifurcations in a discontinuous system with a variable gap
Authors: Yang Ke-Li  ,   Institute of Theoretical and Computational Physics, School of Physics and Information Technology, Shaanxi Normal University, Xi’an 710062, China;,   Nonlinear Research Institute, Baoji University of Arts and Sciences, Baoji 721016, China
Published: December 2020

The period-adding bifurcations in a discontinuous system with a variable gap are observed for two control parameters. Various period-adding bifurcations are found by simulations. The bifurcation diagram can be divided into two different zones: chaos and period. The period attractor takes up a considerable part of the parameter space, and all of them show stable period attractors. The periodic zone can also be divided into three different zones: stable period-5 attractor, period-adding bifurcations on the right side of period-5 attractor, and period-adding bifurcations on the right side of period-5 attractor. We choose various control parameters to plot the cobweb of period attractor, and find that it will exhibit a border-collision bifurcation and the period orbit loses its stability, once the position of iteration reaches discontinuous boundary. The discontinuous system has two kinds of border-collision bifurcations: one comes from the gap on the right side, and the other from the gap on the left side. The results show that the period-adding phenomena are due to the border-collision bifurcation at two boundaries of the forbidden area. In order to determine the condition of the period orbit existence, we also choose various control parameters to plot the cobweb of period attractor. The results show that the iteration sequence of period trajectory has a certain sequence with different iteration units. The period trajectory of period-adding bifurcation on the left side of period-5 attractor consists of period-4 and period-5 iteration units, forming period-9, period-13 and period-14 attractor. The period trajectory of period-adding bifurcation on the right side of period-5 attractor consists of period-6 and period-5 iteration units, forming period-11, period-16 and period-21 attractor. All attractors can be easily shown analytically, owing to the piecewise linear characteristics of the map. We analyze its underlying mechanisms from the viewpoint of border-collision bifurcations. The result shows that the period attractor can be determined by two border-collision bifurcations and the condition of stability. Based on the theoretical and iteration unit, the border-collision bifurcations, two border collision bifurcation curves are obtained analytically. The result shows that the theoretical and numerical results are in excellent agreement.
Bit error rate analysis of X-ray communication system
Authors: Wang Lü-Qiang  , Su Tong  , Zhao Bao-Sheng  , Sheng Li-Zhi  , Liu Yong-An  , Liu Duo  ,   Xi’an Institute of Optics and Precision Mechanics, Chinese Academy of Science, Xi’an 710119, China;,   University of Chinese Academy of Sciences, Beijing 100049, China
Published: December 2020

X-ray communication, which was firstly introduced by Keithe Gendreau in 2007, is potential to compete with conventional communication methods, such as microwave and laser communication, against space surroundings. As a result, a great deal of time and effort has been devoted to making the initial idea into reality in recent years. Eventually, the X-ray communication demonstration system based on the grid-controlled X-ray source and microchannel plate detector can deliver both audio and video information in a 6-meter vacuum tunnel. The point is how to evaluate this space X-ray demonstration system in a typical experimental way. The method is to design a specific board to measure the relationship between bit-error-rate and emitting power against various communicating distances. In addition, the data should be compared with the calculation and simulation results to estimate the referred theoretical model. The concept of using X-ray as signal carriers is confirmed by our first generation X-ray communication demonstration system. Specifically, the method is to use grid-controlled emission source as a transceiver while implementing the photon counting detector which can be regarded as an important orientation of future deep-space X-ray communication applications. As the key specification of any given communication system, bit-error-rate level should be informed first. In addition, the theoretical analysis by using Poisson noise model also has been implemented to support this novel communication concept. Previous experimental results indicated that the X-ray audio demonstration system requires a 10-4 bit-error-rate level with 25 kbps communication rate. The system bit-error-rate based on on-off keying (OOK) modulation is calculated and measured, which corresponds to the theoretical calculation commendably. Another point that should be taken into consideration is the emitting energy, which is the main restriction of current X-ray communication system. The designed experiment shows that the detected X-ray energy is 7×10-5 mW/m2. This relatively low power level not only restricts the bit rate of transceiver, but also increases the error fraction to some extent. Obviously, OOK modulation can meet the high communication rate and relatively low bit-error-rate requirement of current audio demo system. Current restriction has been pointed out and the potential improvement is also presented.
Transmission characteristics of photonic crystal fibers based on filling different kinds of liquid crystals
Authors: Wang Jia-Lu  , Du Mu-Qing  , Zhang Ling-Li  , Liu Yong-Jun  , Sun Wei-Min  ,   College of Automation, Harbin Engineering University, Harbin 150001, China;,   Key Lab of In-fiber Integrated Optics, Ministry Education of China, Harbin Engineering University, Harbin 150001, China;,   Department of Physics, Harbin Institute of Technology, Harbin 150001, China
Published: December 2020

The transmission characteristics of full-filled photonic liquid crystal fibers (PLCFs) which are filled with five kinds of liquid crystals (LCs) are experimentally studied and theoretically analyzed. The influences of temperature and external electric field on the transmission characteristics of PLCFs are also discussed in this paper. The transmission spectra of PLCFs show obvious bandgaps, and the number and the central wavelengths of the bandgaps depend on the average value of the refractive indices of LCs. By changing the temperature from 20 ℃ to 80 ℃, a blue shift in the bandgap is observed, and the maximum tuning range of the bandgap is 41 nm. Then, with the voltage turning from 0 V to 250 V, the output power of the transmission spectrum decreases, while the central wavelength of the bandgap is almost unchanged. Finally, the transmission spectrum keeps a good stability, even if the polarization state of the input light changes.
Laser induced thermal grating spectroscopy thermometry technique
Authors: Qu Pu-Bo  , Guan Xiao-Wei  , Zhang Zhen-Rong  , Wang Sheng  , Li Guo-Hua  , Ye Jing-Feng  , Hu Zhi-Yun  ,   State Key Laboratory of Laser Interaction with Matter, Northwest Institute of Nuclear Technology, Xi’an 710024, China
Published: December 2020

In this paper the laser induced thermal grating spectroscopy thermometry technique is investigated. Two coherent, pulsed pump lasers are crossed in NO2/N2 mixture to induce an interference pattern, owing to the resonant absorption and the subsequently quenching effect. The heat released into the bulk gas can modulate the local refractive index (temperature grating). Simultaneously, the sound wave induced by the electric field forms the standing wave (acoustic grating). These two effects mentioned above produce a thermal grating, and a continuous probe laser satisfying the Bragg scattering condition, generates a coherent signal in the crossed region. The spatial and spectral filtering signal is detected with a photomultiplier tube, and displayed with a digital oscilloscope. The signal carries plenty of flow field information. The gas temperature is obtained through frequency analysis. In order to increase the precision of temperature measurement, we calibrate the grating spacing at a known temperature in a pressurized gas cell. Then the temperature in a range of 300-500 K is measured by the laser induced thermal grating spectroscopy technique, and the thermocouple temperatures are recorded at the same detecting point simultaneously. Both of them agree well with each other, though some discrepancies are still existent. The difference is explained according to the heat radiation loss. We also use this technique to measure the gas sound speed directly, which is crucial to studying the gas behaviors at high pressures and the interaction between molecules. In a certain temperature range, the measurement result and the theoretical curve are nearly consistent, which shows the high precision and multi-parameter measurement ability of laser induced thermal grating spectroscopy. The factors influencing the signal waveform are analyzed, too, and the results demonstrate that the signal duration, the signal intensity, and the oscillation peaks increase with pressure increasing. As a consequent, the accuracy of measurement can be improved. Also, other gas dynamic parameters, such as the thermal diffusion rate and the heat conductivity, can also be measured by using this technique. The unique advantage of laser induced thermal grating spectroscopy thermometry technique provides us with a powerful diagnostic tool used in high pressure condition.
Inversion of regional range-dependent evaporation duct from radar sea clutter
Authors: Zhang Jin-Peng  , Zhang Yu-Shi  , Wu Zhen-Sen  , Zhang Yu-Sheng  , Hu Rong-Xu  ,   China Research Institute of Radiowave Propagation, Qingdao 266107, China;,   School of Physics and Optoelectronic Engineering, Xidian University, Xi’an 710071, China
Published: December 2020

Because the weather conditions in different sea areas are different, the evaporation duct occurring over a large sea surface is normally regional and range-dependent. This property results in the fact that the radio wave propagation within the environment of this type is distinct from that within the range-independent evaporation duct environment. Therefore, it is meaningful to perform the regional range-dependent evaporation duct inversion for accurately predicting radio wave propagation and improving radar performance. From among the variety of ways of detecting evaporation duct in practical application, we adopt the regional modified refractivity profile of evaporation duct predicted by the mesoscale numerical weather model MM5 as the prior information, and propose a posterior probability estimation model of the regional range-dependent evaporation duct on the basis of the radar sea clutter power. First, in this model we use the principal component analysis method to model the range-dependent property of evaporation duct, and on this basis, establish the inversion procedure of the range-dependent evaporation duct by using the radar sea clutter. Then, we obtain the relationship among prior probability distribution, posterior probability distribution, and likelihood function of the parameters of the modified refractivity profile by using the Bayesian theory, and finally realize the maximum posterior probability estimation of the evaporation duct parameters. By estimating the real regional range-dependent evaporation duct over East China Sea, it is indicated that the proposed model can perform the inversion of regional range-dependent evaporation duct with a higher precision.
Influence of left-handed material on the resonant frequency of resonant cavity
Authors: Li Pei  , Wang Fu-Zhong  , Zhang Li-Zhu  , Zhang Guang-Lu  ,   College of Sciences, Tianjin Polytechnic University, Tianjin 300387, China;,   College of Sciences, Tianjin Vocational Technology Normal University, Tianjin 300222, China
Published: December 2020

The quality factor and the resonant frequency of a resonant cavity are the key factors that need to be considered in the process of resonator design. The wall of cavity is composed of conductor materials which are effective tools to generate high-frequency oscillation. The microwave cavity is widely used. From the perspective of the circuit, it has almost all the properties of LC resonance unit, such as mode selection. Therefore, it is widely used in filters, matching circuits, and antenna design. In industrial applications, the demand for high-frequency resonant cavity is relatively large. A traditional method can increase the resonant frequency of the resonant cavity by reducing the size of the cavity or using the high-order modes. However, as both approaches have their limitations, the design results are not ideal. By combining theoretical calculation and simulation, the factors that affect the resonant frequency of the resonator are analyzed. The results show the relationship between material properties of the filling medium and the resonant frequency of the cavity. Theoretical calculations show that when the left-handed materials are used as filling materials in the cavity, the resonant frequency can be increased without changing the size of the cavity. The results of high frequency structure simulator also prove the above result. Therefore, the resonant frequency of the resonator cannot be limited by the cavity size. It can be seen from the data that compared with reducing the size of the resonator or using high-order modes, filling left-handed materials can improve resonant frequency to a greater extent. The obtained conclusion shows a further progress compared with the traditional theory and provides a theoretical basis for the exploration and design of novel resonators.
Motion-induced X-ray and terahertz radiation of electrons captured in laser standing wave
Authors: Zhu Wei-Wei  , Zhang Qiu-Ju  , Zhang Yan-Hui  , Jiao Yang  ,   College of Physics and Electronics, Shandong Normal University, Jinan 250014, China
Published: December 2020

The motions of charged particles in electromagnetic fields composed of two or more laser beams show a variety of forms due to the adjustable properties of electromagnetic fields. In this paper, we consider the periodic laser standing wave field composed of two laser beams with opposite propagating directions. The movement of electrons in the standing wave field shows a periodic behavior, accompanied with the obvious radiation, especially when electrons are captured by the laser standing wave field. This phenomenon has aroused much interest of us. Under the existing experimental conditions, the free electron beam with low energy from an electron gun or the relativistic electron beam generated from laser acceleration can be easily obtained and injected into the periodic standing wave field. In this paper, using the single-electron model and the classical radiation theory of charged particles, we study the motion and radiation processes of low and high energy electrons in the polarized laser standing wave field. The results show that when the direction of incident electrons with low-speed is perpendicular to the direction of the laser standing wave electric field, the one-dimensional nearly periodic motion of electrons evolves into a two-dimensional folded movement by gradually increasing the light intensity of the laser standing wave field, and the strong terahertz radiation at micrometer wavelength is produced. High energy electrons generate the high-frequency radiation with the wavelength at several nanometers when the incident direction of high energy electrons is perpendicular or parallel to the direction of the laser standing wave electric field. In the case of low-energy electron, the motion of electron, frequency and intensity of radiation are affected by the laser intensity. In the case of incident high-energy electrons, the laser intensity affects the intensity of electronic radiation, and the initial electron energy influences radiation frequency. The bigger the incident electrons energy, the higher the frequency of radiation is. #br#We can obtain electron beams with different energies by laser acceleration, and they can be promising small radiation sources for terahertz and X-ray by using the electron beam radiation in a laser standing wave field. These studies also provide a basis for experimental researches and the applications of electron radiation in a laser standing wave field.
All-phase target reconstruction method for coherent field imaging
Authors: Cao Bei  , Luo Xiu-Juan  , Chen Ming-Lai  , Zhang Yu  ,   Xi’an Institute of Optics and Precision Mechanics, Chinese Academy of Sciences, Xi’an 710119, China
Published: December 2020

Received signal processing and target reconstruction technique plays a key role in coherent field imaging, and directly influences the quality of the reconstructed image of a target. Based on all-phase fast Fourier transformation (FFT) spectrum analysis theory, a new processing and reconstruction method is proposed. By directly extracting the phase and amplitude information from all-phase FFT spectrum of the return signal, the proposed all-phase target reconstruction method is capable of inhibiting the frequency error caused by many factors, thus reconstructing the target image more precisely. The validity of the proposed method is proved by a coherent field imaging system in outdoor environments, and it has a better reconstruction performance than a traditional method. The resolution of the reconstructed target is close to a theoretical diffraction limit.
Design research and performance analysis of compound parabolic concentrators as optical antennas in visible light communication
Authors: Wang Yun  , Lan Tian  , Li Xiang  , Shen Zhen-Min  , Ni Guo-Qiang  ,   Key Laboratory of Photoelectric Imaging Technology and System, Ministry of Education of China, Beijing Key Laboratory for Precision Optoelectronic Measurement Instrument and Technology, School of Optoelectronics, Beijing Institute of Technology, Beijing 100081, China
Published: December 2020

In order to satisfy the need of visible light communication, compound parabolic concentrators are selected as the optical antennas because of their wide fields of view and high gains in small field of view. Their geometries and optical properties are introduced in order to design compound parabolic concentrators with different fields of view by TracePro. These compound parabolic concentrators are tested under different light source conditions. The distribution of the received power of the receiver which has been coupled with the compound parabolic concentrator, is obtained by a simulation. The obtained gain of compound parabolic concentrator proves that the compound parabolic concentrator works better when the light source has a Lambert radiation pattern than the case under a parallel light condition. The results illustrate that compound parabolic concentrator is suitable to serving as an optical antenna for visible light communication. And it also shows that the smaller the field of view, the greater the gain is. Under the condition of simulation in this paper, a compound parabolic concentrator with 10° field of view could realize a gain of 22.88, which is 31% lower than the theoretical gain because of the effect of its position relative to the light source. On this basis, the model of a visible light communication system is established in a room with a size of 5 m×5 m×3 m. By using a compound parabolic concentrator with a field of view of 60° as an optical antenna, the simulation results show that the average received power is increased by 4.29 dBm for the directed light from light emitting diodes, and by 4.77 dBm with the reflected light being included. And the average received power is increased by 11.2% when the reflected light is considered.
Holographic display based on liquid crystal spatial light modulator
Authors: Xia Jun  , Chang Chen-Liang  , Lei Wei  ,   School of Electronic Science and Engineering, Southeast University, Nanjing 210096, China
Published: December 2020

In conventional phase-only holographic display, the phase-only computer generated hologram is usually calculated based on the fast Fourier transform (FFT) algorithm, in which the Nyquist theory should be satisfied. However, due to the pixel structure of the liquid crystal spatial light modulator and a fixed spatial sampling rate, the size of the reconstructed image is limited by the space-bandwidth product of the liquid crystal phase modulator. The traditional solution is to use convolution algorithm or double-step Fresnel diffraction algorithm to calculate the Fresnel hologram, but FFT has to be calculated many times in both of the methods, thereby increasing the burden of hologram computation. Therefore, in this paper we propose a method to calculate the phase-only hologram based on setting a virtual hologram plane. This virtual hologram plane is set based on the principle of lens imaging. So the calculation of the hologram can be divided into two steps: the first step is to calculate the Fresnel diffraction from the object plane to the virtual hologram plane, and the second step is to calculate the hologram from the virtual hologram plane by being multiplied with a quadratic phase term. In this way, the hologram can be calculated from the original object with any sampling rate we need by adjusting the corresponding parameters of distance. By this method one can calculate the Fresnel diffraction between hologram plane and object plane with variable sampling rates, without considering the space-bandwidth product of the liquid crystal phase modulator, and this algorithm uses only one FFT calculation, which can speed up the calculation of hologram compared with the convolution based method (using three FFTs in calculation) and the double-step Fresnel method (using two FFTs in calculation). Both the computer simulation and the optical experiments demonstrate that the object can be reconstructed with different sizes in the holographic display system. In the optical experiment, the zero-order diffraction can be removed by placing a filter on the back focal plane of the imaging lens and the speckle noise can also be eliminated in order to improve the reconstruction quality by displaying multiple phase-only holograms at a high speed. The proposed method in this paper shows a potential application in zoom-able liquid crystal spatial light modulator based holographic display system.