bluuebunny/crossref_metadata_2025 huggingface dataset:
Design and fabrication of broadband radar metamaterial absorber based on the resistor FSS
Authors: Hui Yi-Cong  , Wang Chun-Qi  , Huang Xiao-Zhong  ,   School of Physics and Electronics, Changsha 410073, China;,   School of Aeronautics and Astronautics, Central South University, Changsha 410073, China
Published: December 2020

In this paper, a kind of composite radar absorption materials, consisting of polygonal and seamed resistor with frequency selective surface (FSS) and traditional magnetic radar absorption materials (RAM), is presented. After analyzing such a material and its topological structure, we obtain the equivalent circuit model of this structure, and acquire the reflectivity and input impedance of such models on the basis of transmission line theory. By the application of CST (computer simulation technology), we have made a comparison between structures with nonresistor and resistor FSS. The structure with resistor FSS has a dual-band whose bandwidths are 0.8 GHz from 8.4 to 9.2 GHz and 0.22 GHz from 11.5 GHz to 11.72 GHz with the reflectivity below-10 dB, respectively. The simulated reflection coefficient for the resistor FSS shows two resonant frequencies at 8.7 and 11.5 GHz which respectively make contribute to a higher absorbing peak reaching-24 dB and-23 dB. However, the nonresistor FSS does not have the absorption peak at 8.7 GHz, and the absorption peak at 11.5 GHz reaches-20 dB, confirming the importance of resistors in improving absorption performance. We have observed which part of such a structure influences amost the bsorption by ascertaining power loss density in the absorbing structure. Based on the current distribution of the FSS, two different schemes of LC equivalent circuits can be modeled, at 8.7 GHz and 11.5 GHz, which can explain the anti-resonance and higher absorbing peak of resistor FSS. Moreover, due to the fact that the induced current increases significantly after adding resistors, we could see that the losses happen when the induced current flows through the resistors, Finally the usage of resistor could improve the absorptive performance of FSS at around 8.7 GHz, this result is coincident with that of simulation. In addition, the combination of resistor FSS and RAM can lead to a frequency-doubling effect, meaning that it has remarkable absorptive performance in the range of 8–15 GHz.
Terminating the spiral wave and spatiotemporal chaos in cardiac tissue using the low-pass filtering scheme
Authors: Pan Fei  , Li Wei-Xin  , Wang Xiao-Yan  , Tang Guo-Ning  ,   College of Physical Science and Technology, Guangxi Normal University, Guilin 541004, China
Published: December 2020

To cause the sodium ion activation gate of cardiomyocyte delay to open, the ability of excitation delay should be given to the medium. The time of excitation delay of the medium increases as the control voltage and frequency of stimulation increase. When the control voltage exceeds a threshold value, the medium with excitation delay has the property of low-pass filtering: low-frequency waves can continuously pass through the medium, whereas the high-frequency wave does not pass consecutively. In this paper, the effect of excitation delay of the medium on spiral waves and spatiotemporal chaos is investigated by using Luo-Rudy phase I model. Numerical simulation results show that when the control voltage exceeds the threshold value, the excitation delay of the medium can effectively eliminate the spiral wave and spatiotemporal chaos. When the control voltage gradually increases from a small value, at a small maximal conductance of calcium channel, the excitation delay could reduce the excitability of the medium, making the amplitude of the spiral wave meander increase until conduction failure results in the disappearance of the spiral wave. Under a large maximal conductance of calcium channel, the excitation delay can reduce the unstability of the spiral wave so that spatiotemporal chaos evolve into meandering spiral waves when the control voltage is large enough. The phenomenon that the spiral wave with a large meandering motion of its tip moves out of the system is observed when the control voltage is properly chosen. Further increase of the control voltage leads to the disappearance of spatiotemporal chaos.
A new simulation method of X-ray pulsar signals
Authors: Xue Meng-Fan  , Li Xiao-Ping  , Sun Hai-Feng  , Liu Bing  , Fang Hai-Yan  , Shen Li-Rong  ,   School of Aerospace Science and Technology, Xidian University, Xi'an 710126, China
Published: December 2020

Since X-ray pulsar signals cannot be directly detected on the ground, and the space flight detection is both time-consuming and costly, simulation of X-ray pulsar signals with true physical characteristics is of great importance to the validation of various X-ray pulsar signal processing algorithms and X-ray pulsar-based navigation strategies. In this paper, a new simulation method of X-ray pulsar signals is proposed, in which according to the pulsar signal model at the solar system Barycenter (SSB) and the trajectory information of the spacecraft, the real-time photon arrival rate function at the spacecraft is established, then based on this, a scale transforming method is employed to directly generate the photon event time stamps at the spacecraft which follow a non-homogeneous Poisson process. The proposed simulation method takes into account the pulsar spin down law and the influences of the largescale time-space effects introduced in the process of dynamic detection, and thus avoids the complicated iteration procedure involved in the state of the art simulation methods. Finally, a series of simulations are designed to evaluate the performance of the proposed simulation method. The main results can be concluded as follows: 1) The simulated photon event timestamps have a slowly changing period, which are consistent with the pulsar spin down law. 2) The observed pulsar profile accurately reflects how the radiation intensity of pulsars changes over time within a phase cycle, and it has a Pearson correlation coefficient of up to 0.99 with a standard profile. 3) The simulated average fluxes of the pulsars are very close to the true values, and thereby verifies the correctness of the proposed simulation method from an overall point of view. 4) The simulated photon series are very similar to the real data detected by the RXTE explorer, and when the simulation time is longer than 50 s, the relevancy between the simulated profile and the profile obtained from the real data is higher than 0.9. 5) The computational cost of the scale transforming method is much less than that for the commonly used Poisson sifting method and the inverse mapping method. The above results show the validity and high efficiency of the proposed method in terms of the period property, the profile and flux accuracy, the similarity to the RXTE real data and the computational cost.
Chaotic transport of fractional over-damped ratchet with fluctuation and periodic drive
Authors: Liu De-Hao  , Ren Rui-Bin  , Yang Bo  , Luo Mao-Kang  ,   Department of Mathematics, Sichuan University, Chengdu 610064, China
Published: December 2020

The fractional over-damped ratchet model with thermal fluctuation and periodic drive is introduced by using the damping kernel function of general Langevin equation in the form of power law based on the assumption that cytosol in biological cells has characteristics of power-law memory. On basis of the Grunwald-Letnikov definition of fractional derivative, the numerical solution of this ratchet model is obtained. And furthermore, according to the numerical solution, the transport behaviors of stochastic ratchet and corresponding deterministic ratchet (especially when the deterministic ratchet has chaotic trajectory) are investigated, based on which we try to analyze how chaotic properties of the deterministic ratchet and the actions of noise influence the transport properties of molecular motors and moreover find the possible mechanism of current reversal of fractional molecular motor. Numerical results show that, as barrier height, barrier asymmetry and memorability of model change, the current reversal in deterministic ratchet is not necessarily required to appear when happening indeed in corresponding stochastic ratchet; moreover, with the decrease of order p, there exists a chaotic regime in deterministic ratchet model before current reversal, but with the disturbance of noise, current reversal will happen more earlier, namely, chaotic current direction in deterministic ratchet model can be reversed when disturbance of noise exists. This also demonstrates that noise can essentially change the transport behavior of a ratchet; current can change from chaotic state in a ratchet with no noise to directed transport with noise. This is a possible mechanism of current reversal of a fractional stochastic ratchet, and also a reflection that noise plays an active role in directed transport.
Synchronous dynamics of small-world neuronal network system with spatially correlated white noise
Authors: Li Na  , Yang Xiao-Li  ,   College of Mathematics and Information Science, Shaanxi Normal University, Xi'an 710062, China
Published: December 2020

In this paper, by using the Terman-Wang small-world neuronal network with electrical synapse coupling, we investigate the synchronous dynamics of neuronal network system subjected to spatially correlated white noise. First, the dynamical mean-field approximation theory is extended to the small-world network system under spatially correlated white noise, through which the original 2N-dimensional stochastic differential equations of the network system are transformed to 11-dimensional deterministic moment differential equations. Then, based on this set of moment differential equations, the key effects of spatially correlated noise and network structure on the synchronous firing property are discussed in the Terman-Wang neuronal network system. The results show that the synchronization ratio of this considered neuronal network system becomes higher not only as the noise correlation coefficient is increased but also as the coupling strength and the average vertex degree are added. Those results imply that the noise spatial correlation coefficient, the coupling strength, and the average vertex degree can play a positive role in inducing synchronous neuronal behaviors. Furthermore, the synchronous dynamics of the original neuronal network system, obtained by direct numerical simulations, is compared with those obtained by the dynamical mean-field approximation theory, and good consistence between them is revealed.
Modeling and simulation of pedestrian evacuation from a single-exit classroom based on experimental features
Authors: Dong Li-Yun  , Chen Li  , Duan Xiao-Yin  ,   Shanghai Institute of Applied Mathematics and Mechanics, Shanghai University, Shanghai 200072, China;,   Shanghai Key Laboratory of Mechanics in Energy Engineering, Shanghai 200072, China
Published: December 2020

It is of fundamental importance to investigate the evacuation process from a room with obstacles. The typical case is the evacuation of students from a classroom. Based on evacuation experiments from a classroom, the essential features of evacuee are concluded. In the original floor field model, the dynamic floor field is introduced in order to reflect the interaction among pedestrians. A pedestrian may follow the virtual trace of another one in front. The static floor field does not consider the influence of pedestrians. In this paper, the original dynamic floor field is ignored. These desks and chairs are treated as impassable and passable static obstacles, respectively. The static and passible obstacles, such as chairs, lead to the delay of movement of pedestrians. Furthermore, pedestrians are regarded as movable obstacles. The effect of static obstacles on floor field does not change with time. However, the effect of movable obstacles on floor field is dynamic. Therefore, the whole floor field is updated dynamically according to the movement of crowd. Pedestrians may try to find another uncongested path or exit when they find the crowd in front. It provides a better description of the influence of downstream congestions on upstream crowd. The cellular automaton model based on the dynamic floor field is used to investigate the evacuation process in the case of four layouts and three exit widths. The spatial distributions of evacuation time in different conditions and also the average and maximum evacuation times are obtained. Numerical simulations reproduce the evacuation process observed in the experiment quite well. The evacuation time depends on arrangement of these desks and the exit width. For a given layout, the smaller exit leads to longer evacuation time. It is found that the evacuation time does not decrease monotonically with increasing the number of aisles, which depends on the width of aisle as well. When the aisle is not wide enough, the conflict of pedestrians from both sides reduces the efficiency of evacuation. It is helpful for coping with crowd evacuation with an aisle close to the exit side of the wall. The reasons of the differences between experimental and simulation results are also discussed in more detail.
Raman scattering and X-ray photoelectron spectra of GexSb20Se80-x Glasses
Authors: Xu Si-Wei  , Wang Li  , Shen Xiang  ,   College of Applied Sciences, Beijing University of Technology, Beijing 100124, China;,   College of Information Science and Engineering, Ningbo University, Ningbo 315211, China
Published: December 2020

In this paper, we prepare several GexSb20Se80-x glasses (x=5 mol%, 10 mol%, 15 mol%, 17.5 mol%, 20 mol%, and 25 mol%), and measure their Raman and X-ray photoelectron spectra (Ge 3d, Sb 4d, and Se 3d) in order to understand the evolution of the glass structure with chemical composition. We further decompose the spectra into different structural units according to the assignments of these structural units in the previous literature. It is found that the structural units of SeSeSe trimers exist in the Se-rich glasses, but the number of the structural units of trimers decreases rapidly with the increase of Ge concentration and finally becomes zero in Ge15Sb20Se65 glass. With the increase of Ge concentration, the quantity of GeSe4/2 tetrahedral structures increases, but the number of SbSe3/2 pyramidal structures remains almost unchanged in the Se-rich glasses. On the other hand, the numbers of GeGe and SbSb homopolar bonds increase with the increase of Ge concentration, but those of the GeSe4/2 tetrahedral and SbSe3/2 pyramidal structures decrease in the Se-poor glasses. Moreover, the SeSe homopolar bonds exist in all the glasses, and they cannot be completely suppressed. When the composition is close to stochiometric value, the glass is dominated by heteropolar GeSe and SbSe bonds, but has negligible quantities of GeGe, SbSb and SeSe homopolar bonds. The transition threshold, rather than the transition predicted by the topological constraint model, occurs at the chemically stoichiometric glasses. This suggests that chemical order, rather than topological order, is a main factor in determining structures and physical properties of GeSbSe glasses.
A narrow bandpass filter based on two-dimensional photonic crystals with two reflectors
Authors: Zhuang Yu-Yang  , Zhou Wen  , Ji Ke  , Chen He-Ming  ,   School of Optoelectronic Engineering, Nanjing University of Posts and Telecommunications, Nanjing 210023, China;,   Bell Honors School, Nanjing University of Posts and Telecommunications, Nanjing 210023, China
Published: December 2020

With the rapid development of wavelength division multiplexing technology, narrow bandpass filters have drawn widespread public attention. In this paper, a compact narrow bandpass filter based on two-dimensional photonic crystals is proposed. The transfer characteristics of the filter with a single mode resonator and two reflectors are analyzed by using coupled mode theory. Research results show that the bandwidth of the filter can be controlled by adjusting the distance between the resonator and the two reflectors, which can be applied to the realization of narrow bandpass filters, and even ultra-narrow bandpass filters. Based on the theoretical model mentioned above, we design a narrow bandpass filter based on two-dimensional photonic crystals, which is composed of silicon rods with square lattice in air. Two single mode waveguides are formed by removing two rows of rods. Meanwhile, a point cavity is formed by removing a dielectric column. In order to precisely control the phase change between the resonator and the two reflectors, a phase adjustment region is introduced. We study the transmission spectrum of the structure by the finite-difference time-domain (FDTD) method. We find that the bandwidth of the filter can be narrowed when the phase change between the resonator and the two reflectors satisfies the specific conditions, and the transmission ratio is still high as well. These are consistent with the theoretical analyses. But it is worth noting that there is a difference between the simulation result and theoretical result. This is because in the theoretical analysis, we consider that the propagation constants of the frequencies close to the central frequency are the same. In fact, the propagation constant increases with the increase of frequency, however, this does not affect the central frequency nor its transmission. The performance of the designed filter is analyzed by FDTD, showing that the working frequency is close to 193.40 THz, the bandwidth is smaller than 5.9 GHz, the peak transmittance is up to 94%, and the length of the working area is only 9 m. Compared with the conventional photonic crystal filters, the designed narrow bandpass filer is very compact, and the performance is suitable for dense wavelength division multiplexed communication systems.
Analyses of the time-delay signature and bandwidth of the chaotic output from a master-slave vertical-cavity surface-emitting laser dynamical system
Authors: Yang Xian-Jie  , Chen Jian-Jun  , Xia Guang-Qiong  , Wu Jia-Gui  , Wu Zheng-Mao  ,   School of Physical Science and Technology, Southwest University, Chongqing 400715, China
Published: December 2020

The time-delay signature (TDS) and the bandwidth (BW) are two important performance indexes to assess the chaos signal from a delayed laser system. Based on the spin flip model of vertical-cavity surface-emitting laser (VCSEL), we numerically investigate the characteristics of chaos dynamics in a master-slave VCSEL system, where a chaotic signal generated by a master VCSEL (M-VCSEL) under external optical feedback is unidirectionally injected into a slave VCSEL (S-VCSEL). The influences of injection strength, frequency detuning between M-VCSEL and S-VCSEL, and feedback strength of M-VCSEL on chaos TDS (including intensity TDS (I-TDS) and phase TDS (P-TDS)) and BW are investigated. The results show that by adjusting the injection strength and the frequency detuning, both I-TDS and P-TDS of two polarization components (referred to as X-PC and Y-PC respectively) of the chaotic output from the system can be suppressed simultaneously. Through further analyzing the influences of the injection strength and frequency detuning on the BW of chaotic signal, we find that the BWs of both X-PC and Y-PC of chaotic outputs can simultaneously exceed 30 GHz within a large negative frequency detuning range. Furthermore, by combining the evolution characteristics of the TDS and BW of chaotic outputs in the parameter space of injection strength and frequency detuning, the parameter region for generating the chaotic signals with wide BW and low TDS can be determined. In addition, by reasonably adjusting feedback strength, the quality of chaotic signal from the system can be further optimized.
Continuous-wave 1.57 m/3.84 m intra-cavity multiple optical parametric oscillator based on MgO:APLN
Authors: Yu Yong-Ji  , Chen Xin-Yu  , Cheng Li-Bo  , Wang Chao  , Wu Chun-Ting  , Dong Yuan  , Li Shu-Tao  , Jin Guang-Yong  ,   Jilin Key Laboratory of Solid Laser Technology and Application, School of Science, Changchun University of Science and Technology, Changchun 130022, China;,   Department of Engineering Mathematics Basic Teaching, School of Science, Changchun University of Science and Technology, Changchun 130022, China
Published: December 2020

Continuous-wave (CW) coherent sources emitting two wavelengths of 1.57 m and 3.84 m have aroused much interest of scientists due to their many applications such as military multiband composite guidance, remote monitoring of the special environment, etc. Quasi-phase matching (QPM) optical parametric oscillator (OPO) device with periodically inverted structure of nonlinear coefficient can implement an efficient and wavelength conversion at arbitrary wavelength in the transparent range of the QPM material. Nowadays, using MgO:PPLN for QPM, various MgO:PPLN-OPOs pumped by conventional 1.06 m laser source can produce 1.57 m and 3.84 m laser and also achieve good results. But as a result of the limitation of momentum conservation condition and periodically poled structure, 1.57 m and 3.84 m laser can only meet a single band. To obtain the two-wavelength laser output at the same time, the MgO:PPLN-OPO could not be applied. In this paper, a CW 1.57 m and 3.84 m intra-cavity multiple optical parametric oscillator based on MgO:APLN is reported. The cross period parameter light is obtained by using a folded type doubly cavity which consists of 1064 nm resonator and multiple optical parametric oscillator. Considering both its thermal stability under high power pump and the light spot mode matching of multiple optical parametric oscillation process, through numerical simulation and theoretical analysis of two sub cavities, the optimum parameters of the cavity structure are determined. On this basis, the influences of output coupler transmittance on oscillation threshold, the down-conversion efficiency, output power stability are investigated in experiment. With T=10% at 1.47 m and 3.3 m output coupler used, the maximum output powers of 3.13 W at 1.57 m and 0.85 W at 3.84 m are obtained, corresponding to slope efficiencies of 6.8% and 1.9%, respectively. The power stabilities are better than 1.8% and 3% at the maximum output power in half an hour. The experimental results show that the intra-cavity multiple optical parametric oscillator based on a single poled crystal MgO:APLN is an effective method of obtaining a 1.57 m and 3.84 m CW laser.
Fast simulation of atmospheric turbulence phase screen based on non-uniform sampling
Authors: Cai Dong-Mei  , Ti Pei-Pei  , Jia Peng  , Wang Dong  , Liu Jian-Xia  ,   College of Physics and Optoelectronics, Taiyuan University of Technology, Taiyuan 030024, China;,   College of Information Engineering, Taiyuan University of Technology, Taiyuan 030024, China
Published: December 2020

The generation of atmosphere turbulence wave-front is important for studying the light propagation and imaging through the atmosphere, and correcting the atmosphere turbulence, such as the adaptive optics system. The power spectral density method generates phase screens quickly for using the fast Fourier transform (FFT). The main drawback to this approach is that lower order aberrations such as tilt are often under represented. The reason is that the low frequency is sampled inadequately. Since the low order aberrations include a major percentage of the atmospheric energy spectrum, the error of simulated phase screens makes this method less desirable to use. To overcome this shortcoming, a non-uniform sampling method is proposed to generate phase screens accurately. Unfortunately, when the sampling is nonuniform, the FFT does not apply directly. Generating such a phase screen is computation intensive which greatly reduces simulation speed. In this paper, we develop a fast, more accurate method to generate atmospheric turbulence phase screens, according to non-uniforming sampling.
The nonequispaced fast Fourier transform (NUFFT) arises in a variety of application areas, ranging from medical imaging to radio astronomy to the numerical solution of partial differential equations. Speeding up the simulation of atmospheric turbulence phase screens is possible by using the non-uniform fast Fourier transform. In this paper, the atmospheric turbulence phase screen is decomposed into a series of harmonics. Then the non-uniform distributed harmonics are projected onto over-sampled uniform grid by using the Gaussian kernel function. Atmospheric turbulence phase screen will be generated using the standard fast Fourier transform on the over-sampled uniform grid. The atmospheric turbulence phase screens can be generated quickly. Using Kolmogorov spectrum model in this paper, the phase screens can be generated quickly. The performances of generated phase screens are analyzed through their phase structure functions. The statistical results are in very good agreement with the theoretical values. The relative error curve of simulation phase screens is calculated and analyzed. The more the oversampling grid, the more the relative error is. Compared with the result from the direct harmonics summation method, the error here mainly concentrates in high-frequency region where the sampling frequency points are sparse. However, the atmosphere turbulence phase screen is simulated in high accuracy on the whole. Compared with the time cost of the harmonics summation, the time using NUFFT is decreased to about 800 times. The simulated phase screens indicate that non-uniform fast Fourier transform is able to generate atmospheric turbulence phase screen with high accuracy and fast speed.
Effect of intense laser irradiation on the electronic properties of 2H-SiC
Authors: Deng Fa-Ming  ,   Mathematics Department, Sichuan University for Nationalities, Kangding 626001, China;,   Institute of Atomic and Molecular Physics, Sichuan University, Chengdu 610065, China
Published: December 2020

By using first-principles with pseudopotentials method based on the density functional perturbation theory, in this paper we calculate the electronic properties of wurtzite 2H-SiC crystal under the strong laser irradiation and analyze the band structure and the density of state. Calculations are performed by using the ABINIT code in the generalized gradient approximation for the exchange-correlation energy. And the input variable tphysel, which is a variable in the ABINIT code and relates to the laser intensity, is used to define a physical temperature of electrons Te. The size of Te is set to simulate the corresponding electron temperature of the crystal when intensive laser irradiates it in an ultrafast time. The high symmetry points selected in the Brillouin zone are along -A-H-K--M-L-H in the energy band calculations. After testing, we can always obtain a good convergence of the total energy when choosing a 20 Hartree cut-off energy and a 442 k-points grid. Then, optimizing the structure, and the structural parameters and the corresponding electronic properties of 2H-SiC in the different electron-temperature conditions are studied using the optimized equilibrium lattice constant. The calculation results indicate that the equilibrium lattice parameters a and c of 2H-SiC gradually increase as the electronic temperature Te goes up. With the electronic temperature going up, the top of valence band is still at , while the bottom of conduction band shifts from the K point with increasing electronic temperature, resulting in the fact that 2H-SiC is still an indirect band-gap semiconductor in a range of 0-2.25 eV and when the electronic temperature reaches 2.25 eV and even more than 2.5 eV, the crystal turns into a direct band-gap semiconductor. With Te rising constantly, the bottom of the conduction band and the top of valence band both move in the direction of high energy or low energy. When Te exceeds 3.5 eV, the top of valence band crosses the Fermi level. When Te varies in a range of 0-2.0 eV, the forbidden bandwidth increases with temperature rising, and when Te varies in a range of 2-3.5 eV, the forbidden bandwidth quickly decreases. This variation shows that the metallic character of 2H-SiC crystals increases with electronic temperature Te rising. The total density of states (DOS) and partial density of states are calculated at Te=0 eV and 5 eV. The DOS figures indicate that 2H-SiC is a semiconductors and its energy gap equals 2.3 eV. At Te =5 eV, the gap disappears, exhibiting metallic properties. This result shows that the crystal covalent bonds weaken and metallic bonds strengthen with temperature rising and the crystal experiences the process of melting, shifting to metallic state.
Interfacial barrier effects in blocked impurity band infrared detectors
Authors: Liao Kai-Sheng  , Li Zhi-Feng  , Li Liang  , Wang Chao  , Zhou Xiao-Hao  , Dai Ning  , Li Ning  ,   National Laboratory of Infrared Physics, Shanghai Institute of Technical Physics, Chinese Academy of Sciences, Shanghai 200083, China
Published: December 2020

Blocked impurity band (BIB) detectors, developed from extrinsic detectors, have long been employed for ground-based and airborne astronomical imaging and photon detections. They are the state-of-the-art choice for highly sensitive detection from mid-infrared to far-infrared radiation. In this work, we demonstrate the existence of an interfacial barrier in blocked impurity band structures by evidence of temperature-dependent dark currents, bias-dependent photocurrent spectra and corresponding theoretical calculations. The origin of the build-in field is studied. The temperature-dependent characteristics of space charge effects are also investigated in detail. It is found that at higher temperature (T 14 K), the space charge influence is negligible, and the interfacial barrier is mainly caused by bandgap narrowing effects. Based on interfacial barrier effects, a dual-excitation model is proposed to clarify the band structure of BIB detectors. The photocurrent spectra related to the two excitation processes, i.e., the direct excitation over the interfacial barrier and excitation to the band edge with subquent tunneling into blocking layer, are successfully extracted and agree reasonably well with the calculated band structure results. The effects of interfacial barrier on the photocurrent spectrum, peak responsivity and internal quantum efficiency of the devices are investigated. With the consideration of interfacial barrier effects, the calculated peak responsivity shows good agreement with the experimental result. It is suggested that interfacial barrier effects should be considered for successfully designing the BIB detectors. Additionally, the build-in field is found to equivalently lower the critical field for impact ionization. This study provides a better understanding of the working mechanism in BIB detectors and also a better device optimization.
First-principles study of effects of quantum confinement and strain on the electronic properties of GaSb nanowires
Authors: Li Li-Ming  , Ning Feng  , Tang Li-Ming  ,   School of Physics and Microelectronics Science, Hunan University, Changsha 410082, China
Published: December 2020

Using first-principles calculations based on density functional theory and projector augmented wave method, we investigate the electronic structures of one-dimensional wurtzite (WZ) and zinc-blende (ZB) GaSb nanowires with different diameters along the [0001] and [111] directions, respectively. The results show that the band gap of the GaSb nanowire increases as the size of the nanowire decreases due to the quantum confinement, and the band structures of the GaSb nanowires display an indirect band structures feature when the diameter of the nanowire is smaller than 3.0 nm, whereas bulk GaSb has a direct gap. Owing to the different responses of the valence band maximum/conduction band minimum energies to strain, the band structures of GaSb nanowires experiences a noticeable indirect-to-direct transition when the nanowires are under the uniaxial strain. For example, an indirect-to-direct band gap transition in the band structure of [111] ZB GaSb nanowires can be realized by applying a uniaxial tensile strain, and this transition in the band structure of [0001] WZ GaSb nanowires can take place by applying both uniaxial tensile and compression strain when the diameter of the nanowire is about 2.0 nm. In addition, it is found that carrier effective mass is dependent on the diameter of the GaSb nanowire, therefore both the electron and hole effective mass values decrease as diameter increases. It is also found that the hole effective mass is smaller than the electron effective mass for GaSb nanowires with the same directions and sizes, indicating that the hole transportation is more prominent than the electron transportation.
Effects of nitrogen and hydrogen co-doped on {100}-oriented single diamond under high temperature and high pressure
Authors: Fang Chao  , Jia Xiao-Peng  , Yan Bing-Min  , Chen Ning  , Li Ya-Dong  , Chen Liang-Chao  , Guo Long-Suo  , Ma Hong-An  ,   State Key Laboratory of Superhard Materials, Jilin University, Changchun 130012, China;,   Center for High Pressure Science and Technology Advanced Research, Changchun 130012, China
Published: December 2020

As is well known, most natural diamonds usually contain not only aggregated nitrogen up to thousands of ppm but also hydrogen. Therefore, the studies of nitrogen and hydrogen impurities in a diamond are of interest for improving the physical properties of a diamond and solving the problems about natural diamond genesis. From this point of view, in this paper, we choose C3N6H6 powders as a nitrogen and hydrogen source and select high-quality seed crystals with {100} facets as the growth facets. The effects of nitrogen and hydrogen co-doped on {100}-oriented single diamond in the NiMnCo-C system at pressures ranging from 5.5 GPa to 6.2 GPa and temperatures of 1280-1450 ℃ are investigated. Experimental results show that both pressure and temperature, which are the synthesis conditions, increase with the increases of nitrogen and hydrogen content in diamond-growth environment, and the V-shape region of diamond-forming moves up. From the obtained Fourier transform infrared spectra, we notice that there is a significant change of the nitrogen concentration in the synthesized diamond with increasing the nitrogen and hydrogen content in the diamond-growth environment. We calculate the nitrogen concentrations in those diamonds and the results indicate that the highest concentration of nitrogen is up to 2000 ppm. Meanwhile, we notice that the hydrogen associated infrared peaks of 2850 and 2920 cm-1 are gradually enhanced, which shows that both nitrogen and hydrogen are successfully co-doped into the diamond. Scanning electron microscope micrographs show that the {111} face is elongated and has triangulated textures appearing on the surface with nitrogen and hydrogen co-doped into the diamond. This result indicates that the synergistic doping of nitrogen and hydrogen has a great influence on the morphology of {100}-oriented single diamond. From the obtained Raman spectra, we find a shift towards higher frequency of the Raman peak from 1330.23 cm-1 to 1330.40 cm-1 and the full width at half maximum increases from 3.12 cm-1 to 4.66 cm-1 with increasing the concentrations of nitrogen and hydrogen in diamond-growth environment. This is the first report about nitrogen and hydrogen co-doped on 100-oriented single diamond by far. This work can provide a new method to study the influences of nitrogen and hydrogen impurities on diamond synthesis and it will help us to further understand the genesis of natural diamond in the future.
Terahertz high-sensitivity superconducting detectors
Authors: Shi Sheng-Cai  , Li Jing  , Zhang Wen  , Miao Wei  ,   Millimeter and Sub-Millimeter Wave Laboratory, Purple Monntain Observatory, Chinese Academy of Sciences, Nanjing 210008, China;,   Key Laboratory of Radio Astronomy, Chinese Academy of Sciences, Nanjing 210008, China
Published: December 2020

The terahertz regime, as a last radio window, remains to be fully explored, and astronomical and atmospheric observations in this regime are scientifically important. Like other frequency regimes, developing high-sensitivity detectors (coherent and incoherent) is of particular significance for both ground-based and space-borne facilities. As the coherent detector of choice below 1.4 THz, superconductor-insulator-superconductor (SIS) heterodyne mixers have achieved as high a sensitivity as five times the quantum limit around 1.4 THz. It is, however, still a challenge to developing SIS mixers at frequencies beyond 1.4 THz with considerable transmission loss in superconducting circuits due to the Cooper-pair breaking by energetic photons and increased many difficulties in designing and fabricating.
So far, superconducting hot electron bolometer (HEB) mixers have been the most sensitive heterodyne detectors at frequencies above 1.5 THz, and successfully used to detect molecular spectral lines up to 2.5 THz from ground-based and space telescopes. Although spiral-antenna coupled NbN HEB mixers show a good sensitivity in the whole THz frequency range, the directly measured spectral response with Fourier transform spectrometer falls quickly as frequency increases, especially above 3 THz.
The terahertz band is also of particular importance to observe astronomical objects such as cosmic microwave background, early distant objects, cold objects and dusty objects. Aiming at such objects, we develop a terahertz imaging array system by combining advanced superconducting detectors such as transition edge sensor (TES) and microwave kinetic inductance detectors (MKIDs), thus the system has a frequency band centred at 350 m, an operational temperature of 0.3 K, and a sensitivity reaching background limit performance for ground-based applications. In addition, it is expected to have some breakthroughs in ultra-sensitive superconducting TES and MKID, low noise multi-channel readout and multiplexing, efficient terahertz-wave coupling technology, and large-scale array system integration. The developed terahertz imaging array system will serve as the next-generation instrument of Dome A 5 m terahertz telescope, conducting a 350 m-band legacy survey for studying the planets, stars, galaxies and cosmology. Besides the application in astronomy, the developed terahertz imaging array system can also be applied to some areas requiring rapid detection such as security, deep space exploration, and biomedical imaging.
In this paper, we mainly introduce the superconducting detectors developed at Purple Mountain Observatory and those for international collaborative projects.
Analysis of improved characteristics of pentacene thin-film transistor with an embedded copper oxide layer
Authors: Nie Guo-Zheng  , Zou Dai-Feng  , Zhong Chun-Liang  , Xu Ying  ,   School of Physics and Electronic Science, Hunan University of Science and Technology, Xiangtan 411201, China;,   State Key Laboratory of Luminescent Materials and Devices, Institute of Polymer Optoelectronic Materials and Devices, South China University of Technology, Guangzhou 510640, China;,   College of Science, Hunan University of Technology, Zhuzhou 412007, China
Published: December 2020

Organic thin-film transistor (OTFT) based on pentacene semiconductor with an embedded copper oxide (CuO) thin layer is investigated. With the 3 nm-thick CuO layer embedded in the pentacene semiconductor, the drain current of the OTFT increases more than 3 times compared with that of pentacene organic field-effect transistor without CuO layer, and the absolute threshold voltage reduces from -21 V to -7.9 V. The hole mobility and current on/off ratio are much improved. It is interpreted by the mechanism based on the analysis of the interface charge transfer between pentacene layer and CuO layer. Results of X-ray photoelectron reveal electron transfer from pentacene to high work function CuO and the formation of charge transfer (CT) complexes based on electron transfer near the contact of CuO and pentacene. The CT complexes between pentacene layer and CuO layer could reduce the exponential density of state near the band edge of pentacene and the pentacene bulk hole trap density, and enhance the pentacene bulk hole carriers injection, which leads to the improvement of the field-effect mobility of OTFT with CuO layer. Electrons are transfered from the highest occupied molecular orbital of pentacene to the thin CuO layer which can generate holes in pentacene. The generated hole has the same effect as that with applying negative gate voltage which influences the threshold voltage. The drain current of the device increases and the threshold voltage shifts from -21 V to -7.9 V. Therefore, the thin CuO layer that is directly embedded in the organic semiconductor layer, serves as the hole-injection layer, which is responsible for reducing the contact barrier of OTFT with CuO layer.
Progress of terahertz devices based on graphene
Authors: Feng Wei  , Zhang Rong  , Cao Jun-Cheng  ,   Department of Physics, Faculty of Science, Jiangsu University, Zhenjiang 212013, China;,   Key Laboratory of Terahertz Solid-State Technology, Shanghai Institute of Microsystem and Information Technology, Chinese Academy of Science, Shanghai 200050, China
Published: December 2020

Graphene has unique electronic properties stemming from a linear gapless carrier energy spectrum, and has dominant advantages in the research of devices such as lasers, detectors and modulators in terahertz region due to its tunable energy gap and extremely high carrier mobility. In this review, we summarize its latest progress in applications of terahertz devices such as lasers, detectors and modulators. Terahertz lasers based on graphene can reach a gain as high as 104 cm-1, and terahertz detectors with different structures such as a bilayer graphene field-effect transistor with top gate and buried gate can achieve NEP (noise equivalent power) ～ m nW/Hz. Graphene terahertz modulators, which are equipped with transmission configuration and reflection configuration, can have a very high modulation depth. These results may be helpful for developing the high-efficiency graphene terahertz devices.
Water permeability in carbon nanotube arrays
Authors: Han Dian-Rong  , Zhu Xing-Feng  , Dai Ya-Fei  , Cheng Cheng-Ping  , Luo Cheng-Lin  ,   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

The membrane composed of carbon nanotube arrays may be widely used in biological molecular devices, image display area and optoelectronic devices. In this paper, the water permeability of the (11, 11) carbon nanotube arrays is simulated by using the SPC/E water model and the molecular dynamics program LAMMPS at 300 K. It is found that the distance between carbon nanotubes has a significant impact on water density distribution and the electric dipole moment orientation. Regardless of the distance between the neighboring tubes, water molecules will get into the nanotubes and form a double-layer cylindrical ring structure inside the nanotubes. However, water molecules can fill into the interstitial space of the nanotube array only when the nearest distance between the neighbor the tubes is greater than 3.4 Å, or the interstitial cross area becomes greater than 57.91 Å2. As the interstitial space increases, the structure of water molecules in the interstitial space will evolve from disconnected single-file chains to boundary-shared close-packing-like columnar circles. Meanwhile, the radius of the water ring inside the nanotube will increase and its boundary becomes more sharp due to the attractions from those water molecules filled in the interstitial space. Relative to the tube axis, the distributions of the water molecular electric dipole moments in the interstitial space depend upon water structures. Under the condition of single-file chain, the distribution exhibits a bimodal characteristic, which is very similar to the distribution of water dipole moments inside the nanotube. Whereas, for the boundary-shared close-packing-like water columnar circle, the distribution of dipole moments shows a unimodal characteristic and the peak corresponds to the angle 90°. This indicates that the preferred orientation of the water dipoles points to the direction perpendicular to the tube axis. These conclusions are helpful in the understanding of the water transport properties in carbon nanotube arrays.
Watermarking algorithm based on ptychographical imaging
Authors: Liu Xiang-Lei  , Pan Ze  , Wang Ya-Li  , Shi Yi-Shi  ,   University of Chinese Academy of Sciences, Beijing 100049, China;,   Academy of Opto-Electronics, Chinese Academy of Sciences, Beijing 100094, China;,   State Key Laboratory of Information Security, Institute of Information Engineering, Chinese Academy of Sciences, Beijing 100093, China
Published: December 2020

We propose a method for watermarking algorithm using ptychographical imaging. The proposed method uses four probes and one fake probe for illumination, orderly. And five weighted double phase-encoded hidden images are added to a host image that is referred to as the transmitted image. Specifically, the amplitude of the transmitted image contains encoded and attenuated watermark image. And the phase of the transmitted image is the host image due to the application of the fake probe. We develop an analytical presentation for the experimental principle using mathematical derivation of the fake probe and ptychographical imaging. We test analytically the distortion of the transmitted image that is due to noise jamming and the effect of the occlusion of the pixels of the transmitted image. Moreover, we discuss the impact of the probe number and probe overlap rate on system robustness, respectively. Results of computer simulations are presented. First, the amplitude and phase of the watermark image are extracted well. Second, the contradiction between watermark imperceptible and watermark extraction quality can be resolved. Third, the simulations illustrate the system ability to extract the watermark image under distortions and the robustness of the transmitted image against removal trials. Furthermore, the robustness of the system is improved, as the number or overlap rate of the probe increases. In the end, the proposed method is applied to GIF images.