bluuebunny/crossref_metadata_2025 huggingface dataset:
Time jitter characteristics of GaAs photoconductive semiconductor switch in linear mode
Authors: Gui Huai-Meng  , Shi Wei  ,   College Information Engineering, Shaanxi Polytechnic Institute, Xianyang 712000, China;,   School of Science, Xi'an University of Technology, Xi'an 710048, China
Published: December 2020

Time precision switching is crucial to a high-precision synchronization control system with several synchronized sources. Compared with the other high-power switches, a GaAs photoconductive semiconductor switch (PCSS) with a litter time jitter has been widely used in a precision synchronization control system. There is little work on the time jitter of a GaAs PCSS. In this paper, a formula of GaAs PCSS time jitter is derived by the qualitative theoretical derivation through using the probability distribution of the output electrical pulse and the corresponding relation between the time and electrical waveform of GaAs PCSS, and combining the carrier transport process. In experiment, a neodymium-doped yttrium aluminum garnet nanosecond laser beam is split by a semipermeable half mirror into two optical beams, and then these two beams simultaneously trigger two identical GaAs PCSSs in two parallel circuits. As the energy of a triggering laser pulse is fixed at 0.35 mJ, four different laser pulse widths, namely 30 ns, 22 ns, 16 ns and 11 ns, respectively, are used to trigger the GaAs PCSSs. The bias voltage changes from 0.1 kV to 1 kV in steps of 0.1 kV, and it is used in the above-mentioned experiment. The PCSSs are triggered 20 times at each of the bias voltage values. The time jitter of the GaAs PCSS with a 3-mm gap can be measured. By analyzing the experimental data, we conclude that the time jitter of the GaAs PCSS decreases with the triggering laser pulse width decreasing under the condition of different bias voltage. In the linear mode, the GaAs PCSS illuminated by a photon with a proper wavelength creates an electron-hole pair. The characteristic of the triggering laser pulse determines that of the output electrical pulse. With the energy of triggering laser pulse fixed, the fluctuation of electrical pulse increases fast with its pulse width decreasing. Moreover, according to the derived formula for a time jitter, the GaAs PCSS time jitter decreases with triggering laser pulse width narrowing, under the different externally applied bias voltages. It is demonstrated that the theoretical and experimental results of the relationship between the triggering laser pulse width and the GaAs PCSS time jitter are consistent. The obtained results provide a basis for further reducing the GaAs PCSS time jitter, which is important for a next-generation fusion research facility and laser trigger antenna array of generating short pulse sequence.
Skyrmions in magnetic thin film heterostructures
Authors: Li Wen-Jing  , Guang Yao  , Yu Guo-Qiang  , Wan Cai-Hua  , Feng Jia-Feng  , Han Xiu-Feng  ,   Institute of Physics, Chinese Academy of Sciences, Beijing 100190, China
Published: December 2020

Magnetic skyrmion is expected to function as an ideal information carrier for ultra-high density magnetic storage and logic functional device in the future due to its superior properties, such as topological protection, small size, and low driving current density for motion. In order to meet the basic requirements for writing and reading information in devices, one needs to be able to accurately generate, manipulate, and probe skyrmion at room temperature. Given that the history and latest developments of the skyrmion research will be reviewed comprehensively in other articles, in order to avoid repetition, in this article we briefly review a series of recent research advances we have made in magnetic multilayer materials in recent years, and discuss the advantages of relevant device applications and problems that need to be solved. They are included in three aspects as follows. 1) The room temperature skyrmion was observed in a wedge film Ta (5 nm)/Co20Fe60B20 (CoFeB) (1 nm)/Ta (t)/MgO (2 nm)/Ta (2 nm) by a polar magneto-optical Kerr microscope. Results showed that skyrmion can be created at room temperature by controlling the perpendicular magnetic anisotropy of magnetic thin film. In the following, we designed a thin film heterojunction containing an antiferromagnetic layer IrMn. The introduction of antiferromagnetic material can produce an exchange bias field in the magnetic layer, which can play the same role as an external magnetic field, making it possible to realize zero-field skyrmion. In this study, we have successfully observed a stable skyrmion at room temperature and zero magnetic field. 2) The spin-orbit torque generated by the current proved to be able to be used to manipulate the created skyrmion. In the fourth part of this review, we discuss the dynamic process of skyrmion driven by spin-orbit torque in IrMn/CoFeB heterojunctions, and the chirality of skyrmion can be deduced by the direction of its longitudinal motion driven by an applied current. Finally, a principle device based on the skyrmion is further fabricated. In this device, a set of binary data was recorded in the track in the presence and absence of skyrmion. Generating and manipulating numbers of skyrmions were realized by using a series of pulse currents with different amplitudes and widths. The detection of a skyrmion can be achieved by using a magnetic tunnel junction at the right end of the device. 3) The advantages of skyrmion as a storage device and the problems that need to be solved for practical applications were discussed.
Theoretical and experimental studies on terahertz radiation from laser-driven air plasma
Authors: Wang Wei-Min  , Zhang Liang-Liang  , Li Yu-Tong  , Sheng Zheng-Ming  , Zhang Jie  ,   Beijing National Laboratory for Condensed Matter Physics, Institute of Physics, Chinese Academy of Sciences, Beijing 100190, China;,   Department of Physics, Capital Normal University, Beijing 100048, China;,   School of Physical Sciences, University of Chinese Academy of Sciences, Beijing 100049, China;,   School of Physics and Astronomy, Shanghai Jiao Tong University, Shanghai 200240, China
Published: December 2020

Strong terahertz (THz) radiation of MV/cm can be generated from air via two-color laser scheme. In this paper, we introduce three recent theoretical and experimental researches conducted by Wang et al., in which they explored the long-standing problem of THz generation mechanism and extended the scheme with uncommon frequency ratio. In the widely-studied two-color laser scheme, the frequency ratio of the two lasers is usually fixed at 2/1=1:2. In 2013 they predicted according to the plasma current model, for the first time, that the two-color scheme can be extended to a new frequency ratio 1:2n, where n is an positive integer. In 2017 they found that the frequency ratio can be further extended to much broader values. In that year, their experiments showed, for the first time, efficient THz generation with new ratios of 2/1=1:4 and 2:3. They observed that the THz polarization can be adjusted by rotating the longer-wavelength laser polarization, but the polarization adjustment becomes inefficient by rotating the other laser polarization, which is inconsistent with the symmetric nature in the susceptibility tensor required by the multi-wave mixing theory; the THz energy shows similar scaling laws with different frequency ratios, which is inconsistent with the scaling predicted according to the multi-wave mixing theory. These experimental results are in agreement with the plasma current model and particle-in-cell simulations. Therefore, their studies not only push the development of the two-color scheme, but also show that the THz generation mechanism should be mainly attributed to the plasma current model.
Temperature dependence of characteristic parameters of 1550 nm vertical-cavity surface-emitting laser
Authors: Ma Ling-Hua  , Xia Guang-Qiong  , Chen Jian-Jun  , Wu Zheng-Mao  ,   School of Physical Science and Technology, Southwest University, Chongqing 400715, China;,   School of Medical Engineering Technology, Xinjiang Medical University, Urumqi 830011, China
Published: December 2020

Compared with conventional edge-emitting semiconductor lasers, vertical-cavity surface-emitting lasers (VCSELs) exhibit many advantages such as low power consumption, low threshold current, single longitudinal-mode operation, circular output beam with narrow divergence, on-wafer testing capability, high bandwidth modulation, low cost and easy large-scale integration into two-dimensional arrays, etc. VCSELs have been widely adopted in various applications such as optical communication, optical storage, parallel optical links, etc. At the same time, the rich dynamic characteristics of VCSELs have always been one of the frontier topics in the field of laser research, and many theoretically and experimentally investigated results have been reported. For theoretically investigating the dynamical characteristics of VCSELs, the spin-flip model (SFM) is one of most commonly and effectively used methods. In order to accurately predict the nonlinear dynamical performance of a 1550 nm-VCSEL, six characteristic parameters included in the rate equations of the SFM need to be given accurately. The six characteristic parameters are the decay rate of field k, the decay rate of total carrier population N, the linear anisotropies representing dichroism a, the linear anisotropies representing birefringence p, the spin-flip rate s, and the linewidth enhancement factor . In this work, through experimentally analyzing the output performances of a 1550 nm-VCSEL under free-running and parallel polarized optical injection, such six characteristic parameters included in the SFM are extracted first in the case that the temperature of the VCSEL is set to be 20.00℃. Furthermore, through gradually increasing the temperature of the 1550 nm-VCSEL from 10.00℃ to 30.00℃, the dependence of the six characteristic parameters on the temperature of the 1550 nm-VCSEL is investigated emphatically. The results show that with the increase of temperature of the 1550 nm-VCSEL, the linear anisotropy representing birefringence p behaves as an increasing trend, and the linewidth enhancement factor shows a decreasing trend. However, the other four characteristic parameters present complex varying trends with the increase of the temperature of the 1550 nm-VCSEL. The research in this paper is helpful in accurately understanding and controlling the dynamical characteristics of the VCSEL, and we hope that it can give a guidance for practical applications.
Effect of leading-edge curvature variation on flat-plate boundary-layer receptivity
Authors: Shen Lu-Yu  , Lu Chang-Gen  ,   School of Marine Sciences, Nanjing University of Information Science and Technology, Nanjing 210044, China
Published: December 2020

Boundary-layer receptivity is the initial stage of the laminar-turbulent transition, which is the key step to implement the prediction and control of laminar-turbulent transition in the boundary layer. Current studies show that under the action of acoustic wave or vortical disturbance, the variation of leading-edge curvature significantly affects the boundary-layer receptivity. Additionally, the free-stream turbulence is universal in nature. Therefore, direct numerical simulation is performed in this paper to study the receptivity to free-stream turbulence in the flat-plate boundary layer with an elliptic leading edge. To discretize the Navier-Stokes equation, a modified fourth-order Runge-Kutta scheme is introduced for the temporal discretization; the high-order compact finite difference scheme is utilized for the x- and y-direction spatial discretization; the Fourier transform is conducted in the z-direction. The pressure Helmholtz equation is solved by iterating a fourth-order finite difference scheme. In addition, the Jaccobi transform is introduced to convert the curvilinear coordinate system into Cartesian coordinate system. And elliptic equation technique is adopted to generate the body-fitted mesh. Then the effect of elliptic leading-edge curvature on the receptivity mechanism and the propagation speed of the excited Tollmien-Schlichting (T-S) wave packet in the flat-plate boundary layer are revealed. Subsequently, a group of multi-frequency T-S waves is extracted from the T-S wave packets by temporal fast Fourier transform. The influences of different leading-edge curvatures on the amplitudes, dispersion relations, growth rates, phases and shape functions of the excited T-S waves are analyzed in detail. Finally, the position occupied by leading-edge curvature in the boundary-layer receptivity process for the excitation of T-S wave is also confirmed. The numerical results show that the more intensive receptivity is triggered in the smaller leading-edge curvature; on the contrary, the less intensive receptivity is triggered in the greater leading-edge curvature. But in different leading-edge curvatures, the structures of the excited T-S wave packets are almost identical, and the group velocity is close to constant, which is approximate to one-third of the free-stream velocity. Similarly, the greater amplitude of the excited T-S wave can be induced with the smaller leading-edge curvature; whereas the smaller amplitude of the excited T-S wave can be induced with the greater leading-edge curvature. Moreover, the dispersion relations, growth rates, phases and shape function of the excited T-S waves in the boundary layer are found to be nearly invariable in different leading-edge curvatures. Through the above study, a further step can be made to understand the boundary-layer leading-edge receptivity and also improve the theory of the hydrodynamic stability.
Physics and devices of quanutm light emission from semicoductor self-assembled quantum Dots
Authors: Shang Xiang-Jun  , Ma Ben  , Chen Ze-Sheng  , Yu Ying  , Zha Guo-Wei  , Ni Hai-Qiao  , Niu Zhi-Chuan  ,   State Key Laborotory of Superlattices and Microsturctures, Institute of Semiconductors, Chinese Academy of Sciences, Beijing 100083, China;,   College of Materials Science and Opto-Electronic Technology, University of Chinese Academy of Sciences, Beijing 100049, China
Published: December 2020

Self-assembled semiconductor single quantum dots (QDs), as a good candidate of solid-state real single photon (SP) emitters in high purity and counting rate, have attracted great attention in recent two decades, promising for quantum information, optical quantum computation, quantum storage, and quantum coherent manipulation. To isolate single QD from the other QDs surrounding, 1) dilute QD density is well controlled during epitaxy; 2) micro-pillars or nanowires individually in space as hosts are fabricated. To enhance their uni-directional emission, GaAs/AlAs distributed Bragg reflector (DBR) planar cavity is integrated. To improve the system (i.e. confocal microscope, traditionally) stability and its optical collection efficiency, a near-field fiber coupling by adhering a micro-pillar chip to fiber facets directly is used. To enhance the coherence of QD spontaneous emission, resonant excitation technique is applied. In this article, we review our research progress in self-assembled QD SP emission, including SP emission from InAs or GaAs QDs on Ga droplet-self-catalyzed GaAs nanowires (with g2(0) of 0.031 or 0.18, respectively), SP emission from InAs/GaAs QDs coupled with high-Q (1000-5000) DBR micro-pillar cavities and their fiber-coupled device fabrication with SP fiber output rate ~1.8 MHz, single QD resonant fluorescence with inter-dot coherent visibility of 40%, strain-coupled bilayer InAs QDs to extend their emission wavelength to 1320 nm and parametric down conversion of 775 nm SP emission from single QD in nanowire to realize entangled photon pairs at 1550 nm (entanglement fidelity of 91.8%) for telecomm application, and definite quantum storage of InAs QD SPs at 879 nm in ion-doped solid (at most 100 time-bins). In future, there will be still several urgent things to do, including 1) puring the environment of a single QD (e.g. growing GaAs QDs to avoid the wetting layer, and optimizing QD growth to avoid smaller QDs) to reduce its spectral diffusion and developing a high-symmetric QD (e.g. GaAs QD) to reduce the fine structure splitting of its emission; 2) positioning single QD precisely for a good alignment of single QD to a micro-cavity or fiber cone (single mode with high numerical aperture) to increase optical excitation efficiency and SP collection efficiency; 3) developing optical quantum integrated chip, including hybrid structures of active micro-cavity and passive waveguide, and high-transmission waveguide beamsplitter or Mach-Zender interferometer to improve SP extraction (micro-cavity), collection (optical setup) and counting rate (at avalanched photon detectors and coincidence counting module).
Expansion characteristics of atom and ion component in laser-induced aluminum plasma
Authors: Lin Zhi-Yi  , Jian Jun-Tao  , Wang Xiao-Hua  , Hang Wei  ,   Department of Chemistry and Key Laboratory of Spectrochemical Analysis and Instruments, Ministry of Education, College of Chemistry and Chemical Engineering, Xiamen University, Xiamen 361005, China
Published: December 2020

A series of experiments is designed in order to investigate the expansion and movement characteristics of atoms and ions of the plasma in the presence of ambient gas. To obtain two-dimensional spectral images of different components in the plasma, a nanosecond laser with a wavelength of 532 nm is used to ablate an aluminum sample, forming the plasma. A C-T type of tri-grating monochromator with an emICCD detector is used for diagnosing the plasma chronologically. At the same time, a 2400 gmm-1 grating is used to replace the narrowband filter for imaging diagnosis of different components in vacuum. The spectrally resolved images of Al I (396.1 nm), Al Ⅱ (466.3 nm), and Al Ⅲ (447.9 nm) in aluminum plasma are obtained. Besides, the spectral images of plasma components under different ambient pressures are collected to explore the influence of background gas on plasma evolution. The results show that in the plasma formation process, the ion component is distributed in the anterior segment of the plume relative to the atom component, and its angular distribution is smaller. The vacuum expansion rates of atoms and ions are all on the order of 104 ms-1. The movement speed of the ion component in the plasma is higher than that of atom component, and its movement speed increases with the valence of the ion increasing. In the energy density range used in this experiment, the velocity varies slightly with the laser energy. For the neutral atom, the velocity increases obviously as the energy increases. With the expansion process progressing, each component of the plume advances along the direction normal to the sample surface, and the emission intensity gradually decreases, the corresponding plume density and its temperature also decrease. With the ambient pressure increasing, the movement characteristics of each component are obviously different from those under high vacuum. At a pressure higher than 1 Pa, the plasma and the ambient gas are infiltrated with each other, vignetting appears in the front of the plume, disturbance occurs, causing the expansion speed to decrease. In addition, the plasma plume shrinks due to the increase of pressure, and the probability of collision with the background gas increases, so that the plume emission intensity is strengthened and the plasma lifetime is prolonged. The results of the new diagnosis method and the experimental results demonstrated in this study can provide a reference for the study of plasma component dynamic process.
Test of weak equivalence principle of microscopic particles based on atom interferometers
Authors: Wang Jin  , Zhan Ming-Sheng  ,   State Key Laboratory of Magnetic Resonance and Atomic and Molecular Physics, Wuhan Institute of Physics and Mathematics, Chinese Academy of Sciences, Wuhan 430071, China;,   Center for Cold Atom Physics, Chinese Academy of Sciences, Wuhan 430071, China
Published: December 2020

The equivalence principle is one of the two basic assumptions of general relativity. It is the extension of weak equivalence principle. At present, a lot of experiments have tested the weak equivalence principle within a certain accuracy. However, the new theories that unify the gravity and the standard model require the weak equivalence principle to be broken, so the highly accurate test of the weak equivalence principle has important scientific significance. The test of the weak equivalence principle using microscopic particles complements and extends that using macroscopic objects. In this paper, the principle of the atomic interferometer is introduced, and the history and status quo of experimental study on weak equivalence principle of microscopic particles using atomic interferometer are reviewed. The precision of experiments using different-mass atoms is improved from 10-7 to 10-8, the precision of experiment using different spin-orientation atoms reaches the level of 10-7, and the precision of quantum test using superposition-state atoms reaches the level of 10-9. The key problems in the weak equivalence principle test using dual-species atom interferometers are summarized. Advances have been made in vibration noise suppression, frequency shift and phase noise suppression of Raman laser, four-wave double-diffraction Raman-transition atom interference, signal detection and data processing. The development of long-baseline atom interferometers for improving the free evolution time of atoms has progressed. The precision of demonstration experiment of weak equivalence principle test using atom interferometers in weightlessness reaches 10-4 level. The space plan for atom interferometer based weak equivalence principle test is also gradually implemented. The test precision of microparticles' weak equivalence principle using long-baseline atom interferometers or space atom interferometers is expected to reach the level of 10-15-10-17 in the future.
First-principles study of H, Cl and F passivation for Cu2ZnSnS4(112) surface states
Authors: Wang Xiao-Ka  , Tang Fu-Ling  , Xue Hong-Tao  , Si Feng-Juan  , Qi Rong-Fei  , Liu Jing-Bo  ,   State Key Laboratory of Advanced Processing and Recycling of Non-ferrous Metals, Department of Materials Science and Engineering, Lanzhou University of Technology, Lanzhou 730050, China;,   Department of Chemistry, Texas A & M University, 700 University Blvd, Kingsville, TX 78363, USA
Published: December 2020

The first-principles calculation method is used to systematically investigate the lattice structure, energy band, density of states of the bulk Cu2ZnSnS4, surface reconstruction, and mechanism of adsorption and passivation of F, Cl and H atoms on Cu2ZnSnS4 (112) surface. We find that the surface reconstruction occurs on the Cu-Zn-Sn-terminated Cu2ZnSnS4 (112) surface and this reconstruction introduces surface self-passivation. By analyzing the partial density of states of the atoms on the S-terminated Cu2ZnSnS4 (112) surface, it can be seen that surface states near the Fermi level are mainly contributed by 3d orbitals of Cu atoms and 3p orbits of S atoms at the top of the valence band. When a single F, Cl or H atom is adsorbed on the S-terminated Cu2ZnSnS4 (112) surface, all three kinds of atoms exhibit an optimal stability at a specific top adsorption site in comparison with at the bridge, hcp and fcc sites. And this top position is also the position of the S atom that has the greatest influence on the surface states. When two atoms of the same kind are adsorbed on the surface, H, Cl or F atoms occupy the top sites of two S atoms that cause surface states on the Cu2ZnSnS4 (112) surface, which have the lowest adsorption energy. And the surface states near the Fermi level are partially reduced. Therefore, two S atoms that cause the surface states are the main targets of S-terminated Cu2ZnSnS4 (112) surface passivation. It has also been found that the passivation effect of H atom for surface states is the most significant and the effect of Cl atom is better than that of F atom. Comparing the partial density of states, the Bader charge and the differential charge of the atoms before and after adsorption, we find that the main reason for the decrease of the surface states is that the adsorption atoms obtain electrons from the S atoms, and the state density peaks of the Cu and S atoms at the Fermi level almost disappear completely. In the surface model, the F atom obtains the same number of electrons from the two S atoms, while the two S atoms have different effects on the surface states. And the H and Cl atoms obtain fewer electrons from the S atoms, that have less influence on the surface states. It may be the reason why the passivation effect of F atom is slightly less than that of H and Cl atoms.
Nematic fluctuations in iron-based superconductors studied by resistivity change under uniaxial pressure
Authors: Li Shi-Liang  , Liu Zhao-Yu  , Gu Yan-Hong  ,   Beijing National Laboratory for Condensed Matter Physics, Institute of Physics, Chinese Academy of Sciences, Beijing 100190, China;,   School of Physical Sciences, University of Chinese Academy of Sciences, Beijing 100049, China
Published: December 2020

Antiferromagnetic, nematic and superconducting phases have been widely found in iron-based superconductors. The study on their relationships is thus crucial for understanding the low-energy physics and high-temperature superconductivity. The so-called nematic phase represents a spontaneous in-plane rotational symmetry breaking of the electronic states, which results in strong in-plane anisotropic properties. We have developed a uniaxial pressure device, which enables us to obtain nematic susceptibility by studying the resistivity change under uniaxial pressure at low temperature. In this paper, we brief two of our recent researches on nematic fluctuations in iron-based superconductors. The first research shows the presence of a nematic quantum critical point in BaFe2-xNixAs2, which exhibits several characteristics, including the zero mean-field nematic transition temperature x=0.11, broad hump feature in the nematic susceptibility in overdoped samples, strongest nematic susceptibility along the (100) direction at x=0.11, and the divergence of zero-temperature nematic susceptibility at x=0.11 for uniaxial pressure along both the (110) and (100) directions. We further study the nematic susceptibility in many other iron-based superconductors and find that the ordered moment at zero temperature linearly scales with nematic Curie constant, which is obtained from the Curie-Weiss-like temperature dependence of nematic susceptibility in these materials. Accordingly, we propose a universal phase diagram for iron-based superconductors, where superconductivity is achieved by suppressing the long-range antiferromagnetic order in a hypothetical parent compound though the enhancement of nematic fluctuations by doping, including both carrier doping and isovalent doping. Our results suggest that nematic fluctuations play a very important role in both the antiferromagnetism and superconductivity in iron-based superconductors.
Buckling behavior of boron nitride nanotubes under combined axial compression and torsion via molecular dynamics simulations
Authors: Zeng Qiang  , Zhang Chen-Li  ,   Department of Engineering Mechanics, Shanghai Jiao Tong University, Shanghai 200240, China
Published: December 2020

Buckling behavior of boron nitride nanotubes under combined axial compression and torsion is presented by using molecular dynamics simulation. In order to study the effect of helicity and nanotube size, three groups of nanotubes are considered. The first group is a pair of boron nitride nanotubes with a similar geometry but different helicities, the second group includes three armchair naotubes having equal length but different radii, and three armchair (8, 8)-nanotubes with different lengths form the third group. The simulation is conducted by applying Nose-Hoover thermostat in a temperature range from 50 K to 1200 K. Based on the interatomic interactions given by Tersoff-type potentials, the molecular dynamics method is used to study variations of atomic interaction in initial linear deformation and postbuckling stages with various load-proportional parameters, and to determine the interactive buckling loads relationship. By comparing typical buckling modes under different loads, it is found that the boron nitride nanotube experiences complex micro-deformation processes, resulting in different variations of atomic interaction and strain energies. When the axial compressive load is relatively large, the change of atomic interaction for boron nitride nanotubes under combined loads is similar to that found under the pure axial compression. The onset of buckling leads to the abrupt releasing of strain energy. As the torsional load is relatively large, the nanotube shows torsion-like buckling behavior, no obvious reduction of strain energy is observed after the critical point. The present simulation results show that both the armchair and zigzag nanotubes exhibit nonlinear interactive buckling load relationships. Rise in temperature results in the decrease of interactive buckling load, and the effect of temperature varies with the value of load-proportional parameter. That is, the axial compressive load is relatively large, and the effect of temperature is more significant. It is found that the buckling behavior in the case of combined loading is strongly size dependent. The interactive critical axial and shear stress decrease as nanotube radius or length increases. The studies also reveal that under both simple loading and combined load condition, carbon nanotubes possess higher buckling loads than those of boron nitride nanotubes with a similar geometry, which provides valuable guidance for forming carbon and boron nitride hybrid nanotubes as well as coaxial nanotubes with superior mechanical properties.
Transmission characteristics of terahertz wave in high temperature plasma
Authors: Meng Ling-Hui  , Ren Hong-Bo  , Liu Jian-Xiao  ,   School of Electronics and Information Engineering, Hengshui University, Hengshui 053000, China
Published: December 2020

In the hypersonic flight, the air surrounding an aircraft under the effect of high temperature will be ionized. The ionized gas is called plasma. Because of the influence of interaction between electromagnetic wave, in some cases the communication will be interrupted. High temperature effect is an important characteristic of the plasma. Therefore, the study of terahertz wave propagation in high temperature plasma is of great significance. In this paper, the transmission of terahertz wave in a high temperature plasma slab is studied. Generally, high temperature plasma is an anisotropic medium. The electromagnetic wave propagates in anisotropic high-temperature plasma and forms left-hand circular polarization mode or right-hand circular polarization (RCP) mode. It is found that the RCP wave can exhibit some novel characteristics, such as the forbidden band transmission characteristics, which is discovered in this paper. The transmission characteristics of terahertz wave in high temperature plasma are studied analytically. The results show that when the frequency of terahertz wave is lower than plasma frequency, the wave cannot be propagated in high temperature plasma, and it shows a stopband characteristic. When the frequency is higher, it can be transmitted through the plasma, and it presents a passband characteristic. These are consistent with the propagation characteristics of electromagnetic waves in cold plasma. However, some characteristics in high temperature plasma are different from those in the cold plasma. In high temperature plasma, the transmission characteristics are influenced by the electron temperature and external magnetic field. When the two parameters are chosen appropriately, a sharp transmission peak will be produced in the stopband. This phenomenon has never been found in cold plasma models before. And the paper will discuss this problem by the two influencing factors. It is also found that the frequency of the transmission peak is affected by magnetic field, and the peak amplitude is influenced by electron temperature. The electron temperatures at high transmittance (transmittance is about 1) under different applied magnetic fields are calculated. In order to study the law embodied in the data, the method of data fitting is adopted. And the formula of transmission peak frequency is obtained by curve fitting. The fitting results show that the transmission peak frequency is proportional to the external magnetic field. The relationship between peak electron temperature and external magnetic field is exponential. Finally, the fitting formula is verified by the finite-difference time-domain method. The numerical results are in good agreement with the analytical solution results, which proves the correctness of the work.
Orientation determination and manipulation of single ice crystal via unidirectional solidification
Authors: Zhang Tong-Xin  , Wang Zhi-Jun  , Wang Li-Lin  , Li Jun-Jie  , Lin Xin  , Wang Jin-Cheng  ,   State Key Laboratory of Solidification Processing, Northwestern Polytechnical University, Xi'an 710072, China;,   School of Materials Science and Engineering, Xi'an University of Technology, Xi'an 710048, China
Published: December 2020

The growth of ice crystal has been widely investigated by researchers from various fields, but efficient method that can meet the experimental requirements for identifying and reproducing the ice crystal with specific orientation is still lacking. In this paper, an ice crystal can be characterized with unique orientation information, where tilt angle of optical axis α, extinction angle β and the angle γ relative to preferred orientation 〈1120〉 in the basal plane (0001) and the direction of temperature gradient G are determined based on the properties of optic polarization of hexagonal ice in the directional solidification. An integrated criterion for determining the orientation of hexagonal ice is proposed by combining the crystal optics and solidification interface morphology. Precise manipulation of the orientation of single ice crystal is achieved by using a step-by-step method via a unidirectional platform combined with a polarized optical microscope. Three coordinate systems are established to achieve the manipulation of ice. They are the microscope coordinate system termed as “A-P-L”, where A, P and L refer to the directions of analyzer, polarizer and incident beam of the optical microscope, respectively, the specimen box coordinate system named “xyz”, and the crystallographic coordinate system described by the optical axis and 〈1120〉 in the basal plane (0001). Ice crystals are all confined in a series of glass specimen boxes filled with KCl solution (0.2 mol/L) and the growth sequence of the single ice crystal from one specimen box to another is specially designed to ensure the specific orientation relations among specimen boxes, and the orientation relations among the specimen boxes are adjusted according to the integrated criterion. Single ice crystals with three typical orientations (α3=90°, β3 a=0°; α3=90°, β3b=90°; α4=90°, β4 dose not exist, γ ≈ 33°) relative to the microscope coordinate A-P-L are obtained, and their morphological characteristics of S/L interface are observed in situ under different pulling velocities (10.3 μm/s, 13.4 μm/s and 100 μm/s, respectively). In this paper we successfully solve the problem of orientation determination and manipulation of ice orientation in the study of directional solidification of ice crystal, which may provide an effective experimental approach for investigating the theoretical problems concerning ice crystal growth.
Multi-wavelength light-emitting diode light source radar system and near-ground atmospheric aerosol detection
Authors: Zhong Wen-Ting  , Liu Jun  , Hua Deng-Xin  , Hou Hai-Yan  , Yan Ke-Jun  ,   School of Mechanical and Precision Instrument Engineering, Xi'an University of Technology, Xi'an 710048, China
Published: December 2020

Near-ground atmospheric aerosol has a direct effect on the living and production of human, and the research on its detection attracts substantial attention from engineers and scholars in the community of environment. Traditional monitoring instruments can accurately and continuously detect the aerosols above the ground, but it is difficult for them to obtain the information about vertical distribution of near-ground aerosols. As is well known, lidar can act as an efficient method to detect the aerosol's temporal and spatial distribution. However, this technique is restricted in the potential applications of the detection of aerosol at a certain wavelengths or near range. That is because it usually presents fixed wavelengths and a large blind area. In this work, a new multi-wavelength light-emitting diode (LED) light source radar system is designed and established for detecting the characteristics of atmospheric aerosol near ground. The paper is outlined as follows. Firstly, the composition and working principle of the LED light source radar system are introduced. Based on the structure of radar's transmitter and receiver, the geometric overlap factor is analyzed and calculated. The minimum detection height of the LED light source radar system is then determined to be 60 m. Secondly, the inversion method for the echo signal of LED light source radar is studied. In consideration of the suitability of near-range detection of LED light source radar, the Fernald forward integration method is used for inversing the aerosol extinction coefficient. The calibration value of aerosol extinction coefficient is further determined with the ground visibility data. Finally, by using the designed multi-wavelength LED light source radar system (475 nm, 530 nm, and 625 nm), near-ground aerosol observation at night in Xi'an city is carried out and three atmospheric conditions including light, moderate and severe pollution days are considered. The height distribution curves of atmospheric aerosol extinction coefficient of three wavelengths within a height of nearly 300 m are obtained. The characteristics of the distribution and change of near ground aerosols are accordingly discussed. The experimental results show that the multi-wavelength LED light source radar provides an efficient implementation for detecting the vertical distribution of atmospheric aerosol near the ground, and can make up for the inadequacy of lidar in near range detection.
Theoretical and experimental investigation of light guidance in hollow-core anti-resonant fiber
Authors: Ding Wei  , Wang Ying-Ying  , Gao Shou-Fei  , Hong Yi-Feng  , Wang Pu  ,   Institute of Physics, Chinese Academy of Sciences, Beijing 100190, China;,   Institute of Laser Engineering, Beijing University of Technology, Beijing 100124, China
Published: December 2020

The inherent material imperfections of solid core optical fiber, for example, Kerr nonlinearity, chromatic dispersion, Rayleigh scattering and photodarkening, set fundamental limitations for further improving the performances of fiber-based systems. Hollow-core fiber (HCF) allows the light to be guided in an air core with many unprecedented characteristics, overcoming almost all the shortcomings arising from bulk material. The exploitation of HCF could revolutionize the research fields ranging from ultra-intense pulse delivery, single-cycle pulse generation, nonlinear optics, low latency optical communication, UV light sources, mid-IR gas lasers to biochemical sensing, quantum optics and mid-IR to Terahertz waveguides. Therefore, the investigations into the guidance mechanism and the ultimate limit of HCF have become a hot research topic. In the past two decades, scientists and engineers have fabricated two types of high-performance HCFs with loss figures of 1.7 dB/km and 7.7 dB/km for hollow-core photonic bandgap fiber (HC-PBGF) and hollow-core anti-resonant fiber (HC-ARF) respectively. In comparison with the twenty-years-old HC-PBGF technology, the HC-ARF that recently appeared outperforms the former in terms of broadband transmission and high laser damage threshold together with a quickly-improved loss figure, providing an ideal platform for many more challenging applications. While the guidance mechanism and fabrication technique in HC-PBGF have been well recognized, the HC-ARF still has a lot of room for improvement. At the birth of the first generation of broadband HC-ARF, the guidance mechanism was unclear, the fiber design was far from perfect, the fabrication was immature, and the optical properties were not optimized. In the past five years, we have developed an intuitive and semi-analytical model for the confinement loss of HC-ARF and managed to fabricate high-performance nodeless HC-ARF. We further employ our theoretical model and fabrication technique to well control and design other interesting properties, such as polarization maintenance and bending loss in HC-ARF. For a long time, the anti-resonant theory of light guidance has been regarded as being qualitative, and the leaky-mode-based HC-ARF have been considered to have worse performances than the guided-mode-based HC-PBGF. Our investigations in theory and experiment negative these prejudices, thus paving the way for the booming development of HC-ARF technologies in the near future.
First-principle calculation of electronic structures and absorption spectra of lithium niobate crystals doped with Co and Zn ions
Authors: Wu Sheng-Yu  , Zhang Yun  , Bai Hong-Mei  , Liang Jin-Ling  ,   School of Physical Science and Technology, Southwest University, Chongqing 400715, China
Published: December 2020

In this paper, the electronic structures and absorption spectra of Co doped and Co, Zn co-doped LiNbO3 crystals are studied by the first-principle using the density functional theory, to explore the characteristics of charge transfer in Co, Zn co-doped LiNbO3 crystals, and to build the relationship between these characteristics and the holographic storage quality. The basic model is built as a supercell structure of 211 of near-stoichiometric pure LiNbO3 crystal with 60 atoms, including 12 Li atoms, 12 Nb atoms and 36 O atoms. Four models are established as the near-stoichiometric pure LiNbO3 crystal (LiNbO3), the cobalt doped LiNbO3 crystal (Co:LiNbO3), the zinc and cobalt co-doped LiNbO3 crystal [Co:Zn(L):LiNbO3] with doping ions at Li sites, and the other zinc and cobalt co-doped LiNbO3 crystal [Co:Zn (E):LiNbO3)] with zinc ions at Li sites and Nb sites. The last two models would represent the concentration of Zn ions below the threshold (6 mol%) and near the threshold, respectively. The charge compensation forms are taken as CoLi+-VLi-, CoLi+-ZnLi+-2VLi- and CoLi+-ZnNb3--2ZnLi+ respectively in doped models. The results show that the conduction band and valence band of pure LiNbO3 crystal are mainly composed of O 2p orbit and Nb 4d orbit respectively, and energy gap is 3.48 eV. The band gap of the doped LiNbO3 crystal is narrower than that of pure LiNbO3 crystal, due to the Co 3d and Zn 3d orbit energy levels superposed with that of O 2p orbit energy levels, and thus forming the upside of covalent bond. The band gap of Co:LiNbO3 crystal is 3.32 eV, and that of Co:Zn:LiNbO3 crystals are 2.87 eV and 2.75 eV respectively for Co:Zn(L):LiNbO3 and Co:Zn(E):LiNbO3 model. The Co 3d orbit is split into eg orbit and t2g orbit with different energies. The absorption peak at 2.40 eV appears in the band gap of Co:LiNbO3 crystal, which is attributed to the transfer of the Co 3d splitting orbital t2g electrons to conduction band. The absorption peaks of 1.58 eV and 1.10 eV could be taken as the result of eg electron transfers of both Co2+ and Co3+ in crystal, especially the latter ion. These two absorption peaks are obviously enhanced in Co:Zn (E):LiNbO3 crystal compared with in other samples in this paper. Based on that, it could be proposed that a charge transfer between Zn2+ and Co2+ as Co2++Zn2+Co3++Zn+ exist in the crystal, which results in the decrease of eg orbital electron number, but hardly affect the t2g orbital electron. The Co ion in crystal could act as the deep-level center (2.40 eV) or the shallow-level center (1.58 eV) with the different accompanying doped photorefractive ions in the two-light holographic storage applications. In both cases, the choice of Zn ion concentration near threshold could be helpful for the photo damage resistance and recording light absorption in storage applications.
Progresses of ultrastable optical-cavity-based microwave source
Authors: Jiang Hai-Feng  ,   Key Laboratory of Time and Frequency Primary Standards, National Time Service Center, Xi'an 710600, China;,   School of Astronomy and Space Science, University of Chinese Academy of Sciences, Beijing 100049, China
Published: December 2020

With the progress of science and technology and the continuous improvement of the precision measurement application technology, the technical requirements for the stability and noise level of the ultra-stable microwave source are increasing. Its application range becomes more and more wide, including high performance frequency standard research, network radar development, deep space navigation system, etc. Up to now, the photonic microwave generators based on ultra-stable laser and femtosecond light comb are believed to be the highest microwave frequency source with the highest frequency stability and the relative frequency stability 10-16 in 1 s. This device is also the basis of the application for the next frequency standard (optical frequency standard). Whether the generation of time or most of the precision measurements, the output laser of the optical frequency standard should be transformed into a super stable baseband frequency signal. In this paper, we first introduce the development, current situation and application requirements of ultra-stable photonic microwave source, then we present the principle and structure of the ultra-stable photonic microwave source and the technical development of its components based on the first set of domestic-made ultra-stable microwave frequency sources developed by the National Time Service Center. For the ultra-stable laser, we mainly focus on the research and development of the ultra-stable cavity design, the Pound-Drever-Hall frequency locking technology, and the residual amplitude noise effect rejection. For the optical frequency combs, we mainly focus on the development of laser mode-locking and frequency control technology based on erbium-doped fiber combing system. For the low noise photonic-to-microwave detection and low noise synthesizer techniques, the noise effect rejection of wideband photoelectric detection and the microwave phase noise induced by the amplitude noise of the laser are emphatically introduced. Finally, we summarize and prospect the photonic ultra-stable microwave generation technique.