bluuebunny/crossref_metadata_2025 huggingface dataset:
Numerical study of Prandtl number effects in turbulent thermal convection
Authors: Bao Yun  , Gao Zhen-Yuan  , Ye Meng-Xiang  ,   Department of Mechanics, Sun Yat-Sen University, Guangzhou 510275, China
Published: December 2020

The high-resolution numerical simulations of two-dimensional (2D) turbulent Rayleigh-Bnard convection are conducted by using the Parallel direct method of DNS (PDM-DNS) with Ra=1010 and Pr in a range from 0.05 to 20. Using the flow visualization technique, the effects of Pr on the structure of plumes and large scale circulation (LSC) are investigated. With Pr decreasing, plumes become more active and the flow turns more turbulent. When Pr4.3, pronounced LSC and corner vortex exist. The thickness of thermal boundary layer varies slightly with the value of Pr changing, which obeys a scaling law. Nusselt number (Nu) increases with Pr value increasing when Pr value is low and becomes independent when Pr value is high. Furthermore, two definitions of Reynolds number (Re) are given. The Re〈u〉 angle is calculated from the fluctuation of horizontal velocity near the center of bottom plate, and the ReUmax is calculated from maximal horizontal velocity in the mean field. Both of them follow the same scaling Re~Pr0.81.
Surface structure for manipulating the near-field spectral radiative transfer of thermophotovoltaics
Authors: Yu Hai-Tong  , Liu Dong  , Yang Zhen  , Duan Yuan-Yuan  ,   Key Laboratory of Thermal Science and Power Engineering, Ministry of Education, Beijing Key Laboratory for CO2 Utilization and Reduction Technology, Tsinghua University, Beijing 100084, China;,   School of Energy and Power Engineering, Nanjing University of Science and Technology, Nanjing 210094, China
Published: December 2020

To improve the efficiency and output power of the nano-gap thermophotovoltaic (TPV) power generation system, surface rectangular grating structures are added to the top surface of the group Ⅲ-V semiconductor cell to control the spectrum of near-field radiative transfer. Doped zinc oxide that supports surface waves at near-infrared wavelengths is selected as the TPV emitter. When paired with GaSb grating structures, the surface plasmon polariton excited by the emitter and the light trapping effect by the grating tunnels will be coupled, which results in a significantly and selectively enhanced near-field radiative heat flux within a narrow spectral region above the cell bandgap, thereby fulfilling the design purpose. This physical mechanism is explained by a direct finite-difference time-domain (FDTD) simulation based on the Langevin approach. The material volume meshgrids filled with random dipole sources can act as the thermal emission source and the radiative heat flux is calculated by solving the Maxwell equations numerically. The spectral results show that adding rectangular grating structures to GaSb not only increases radiative transfer in the expected wavelength region over the unstructured case, resulting in a heat flux surpassing that of a far-field blackbody source at the same temperature, but also suppresses the unwanted long-wavelength heat flux that causes radiative loss and cell heating. With a vacuum gap of 200 nm between the emitter and the cell, using a bulk GaSb cell with rectangular gratings can double the spectral flux of the blackbody emitter case, and using an ultrathin GaSb cell with surface structures and back reflectors further increases this ratio to 2.84 due to the total internal reflection controlled by the cell thickness. The amplitude and wavelength of the spectral peak are controlled by the grating size parameters. Low filling ratio gratings with lower-aspect-ratio grating channels generally have sharper enhancement peaks but lower total radiative heat flux, while high filling ratio structures with higher-aspect-ratio channels have better heat flux improvement but might also result in lower conversion efficiency due to the broader spectrum. The rigorous approach reveals the detailed physical mechanism that is otherwise unseen with effective medium approaches for inhomogeneous structures or the Derjaguin proximity approximation. Overall the results of this study enable an enhancement of near-field radiative heat flux limited within a narrow wavelength range shorter than the cell bandgap, offering practical benefit to the application of TPV power generation with higher feasible power and conversion efficiency.
Effect of external field on the I-V characteristics through the molecular nano-junction
Authors: Niu Lu  , Wang Lu-Xia  ,   School of Mathematics and Physics, University of Science and Technology Beijing, Beijing 100083, China
Published: December 2020

As a basic functional unit of molecular electronics, the structure of single molecule sandwiched between nano-electrodes has attracted a lot of interest in molecular science, in particular, its current-voltage (I-V) characteristic induced by an external field. Aiming at the molecular nano-junction which is composed of lead/molecule/lead, we use the method of extended master equation to compute the steady and transient current in the molecular nano-junction under the action of an externally applied electric field. The current can be adjusted by the external field, the relaxation in the molecule, the intra-molecular vibrational energy redistribution, etc. Owing to the strong electronic-vibrational coupling, the I-V curve has an inelastic characteristic in the molecular nano-junction and the stable current increases stepwise with the applied bias voltage increasing. The Franck-Condon blockage can be effectively removed by the external field. The molecular nano-junction being excited by different-width Gaussian pulses, the currents in the molecular nano-junction take different times to reach their steady state. The pulse width has a strong effect on the transient current enhancement. The transient current appears obviously for the 1 ps width pulse excitation. In this case the molecule is at a non-equilibrium state and the currents at both ends of the molecule are different. With the pulse width and the applied voltage increasing, the current through the molecular nano-junction tends to be balanced.
Experimental study on submerged supersonic gaseous jet induced tail cavity
Authors: Xu Hao  , Wang Cong  , Lu Hong-Zhi  , Huang Wen-Hu  ,   School of Astronautics, Harbin Institute of Technology, Harbin 150001, China;,   R & D Center, Chinese Academy of Launch Vehicle Technology, Beijing 100076, China
Published: December 2020

Multiphase jets and cavitation problems are inevitable for high-speed underwater vehicles propelled by jet engines. Unlike being injected into stagnant water, the gaseous jet behind a underwater vehicle is usually conjugated with a tail cavity. The pulsation and collapse of such cavities can seriously affect the vehicle performance. In this study, the shape character, forming mechanism and control conditions for the supersonic gaseous jet induced tail cavity at the wake of a revolution body are experimentally investigated in a water tunnel. The induced cavity is ventilated only by a convergent-divergent nozzle with a designed Mach number of 2.45. The form of the cavity is recorded through two high-speed cameras both horizontally and vertically under different Froude numbers and ventilation rates. The time averaged form is thus obtained through digital image processing to eliminate the transient characteristics of the cavity. The experiment is conducted with the Froude number ranging from 3.2 to 16.2, and the ventilation rate 0 to 0.5. Due to the high density and velocity ratio between water and gas, the structure of such flow is usually very complicated. Many novel phenomena of the jet-cavity interaction are observed. With increasing stagnation pressure of the central jet, the induced cavities evolves form foamy, intact, partially break, to pulsating foamy closure type. The foamy and intact tail cavities share the same profile and characteristics that of a supercavity. And the pulsating foamy closure type was never observed before in a traditional supercavitating flow. The outline of the pulsating foamy cavity is the same as the foamy cavity's, indicating that they have the similar forming mechanism. A comparison with the jet-cavity interaction model is made and the following conclusions are obtained:the real ventilation rate, which corresponds to the re-entrant jet gas blocked by the cavity boundary, is the key factor in controlling the cavity form. When the gaseous jet is completely blocked by the water-gas interface, an intact or foamy cavity will be formed. A partially break cavity appears only when some fraction of the jet is blocked and this is when some of the strongest interactions between jet and cavity occurs. When little gas was blocked by the interface, a pulsating foamy cavity forms. With the structure of gaseous jet considered, the transition of the induced cavity closure between different types is in favour of the prediction from Paryshev's model of cavity closure to a central jet. The variation of the cavity form, thus the interaction strength between jet and cavity, coincides with the real ventilation rate estimated through the theoretical model.
Thermal entanglement in a five-qubit XXZ Heisenberg spin chain with the next nearest neighboring interaction
Authors: Liu Gui-Yan  , Mao Zhu  , Zhou Bin  ,   Faculty of Physics and Electronic Science, Hubei University, Wuhan 430062, China
Published: December 2020

In the study of thermal entanglement of the Heisenberg spin chain model, one usually considers only the spin interaction between the nearest neighboring qubits. Actually, a generalized Heisenberg model, so-called J1-J2 Heisenberg model, which is constructed by considering the fact that not only the nearest neighboring but also the next nearest neighboring spin interaction also plays an important role. In J1-J2 Heisenberg model, due to the next nearest neighboring spin interaction, the frustration effect can occur and has an important influence on the magnetic properties of the model. In this paper we investigate the thermal entanglement of a five-qubit XXZ Heisenberg spin chain with the next nearest neighboring interaction in a magnetic field. Using the numerical method, we calculate the pairwise concurrences of the nearest neighbouring qubits and the next nearest neighboring qubits, abbreviated as C12 and C13 respectively. The numerical results show that the frustration parameter α has an important effect on the pairwise thermal entanglement. Moreover, C12 and C13 have different variations with the change of the frustration parameter α. Meanwhile, it is found that the temperature, magnetic field, Dzyaloshinkii-Moriya (DM) interaction and anisotropic parameter also have great effects on the thermal entanglement. The increasing of temperature can reduce the thermal entanglement. The magnetic field can enhance the thermal entanglement between both two nearest and next nearest neighboring qubits, but when the magnetic field becomes strong enough, only the thermal entanglement between the two nearest neighboring qubits is suppressed. A certain extent of DM interaction can enhance the thermal entanglement between the two nearest neighboring qubits. But for the next nearest neighboring qubits, without the magnetic field, the increasing of DM interaction mainly enlarge the entanglement vanishing area of frustration parameter α. When the system changes from anisotropic to isotropic state, the entanglement vanishing area also changes obviously for C12 and C13. Thus, we can choose appropriate magnetic field strength, temperature, frustration parameter, DM interaction parameter and anisotropic parameter to effectively control and enhance the thermal entanglement of the system.
New transmission mode of electromagnetic wave in high-density rod cavity structure
Authors: Jiao Jiao  , Tong Ji-Sheng  , Ma Chun-Guang  , Guo Ji-Yu  , Bo Yong  , Zhao Qing  ,   School of Physical Electronics, University of Electronic Science and Technology of China, Chengdu 610054, China
Published: December 2020

During reentry of hypersonic spacecraft into the atmosphere, a break in the radio communication due to the presence of a plasma sheath on the spacecraft can occur. The break is commonly known as reentry communication blackout. Normally, for high density plasma, it is difficult for the electromagnetic waves of L and S bands to penetrate through. They may be decayed rapidly or reflected. That is why reentry communication blackout happens. In recent years, initiative methods are used to reduce the effects of reentry communication blackout such as by designing ideal shape for reentry vehicle, sprinkling special substances on the surface of the vehicle to improve efficiency of electromagnetic wave, adding magnetic field within the blackout area, etc. However, these methods not only fail to fully resolve the problems caused by blackout but also bring some new ones. Therefore, to resolve the problems, transmission mechanism of electromagnetic waves in plasmons should be analyzed.
In this paper, we use the finite difference time domain, consider the mechanism of electromagnetic waves in a structure consisting of high-density plasma rods, and refer to the two-dimensional (2D) photonic crystal and surface wave local coupling theory. A new type of high-density plasma micro-rod cavity structure is designed. The special structure, consisting of metal cavity, high-density plasma rod, and dielectric medium filled within the cavity, is quite different from traditional 2D sub-wavelength plasma rod arrays. This kind of design takes advantage of cavity structure to couple electromagnetic wave within the plasma rod so that the surface wave diffraction transmission mode can be changed into a local coupling enhancement penetrating mode. In this paper, we investigate the plasma micro-rod cavity structures with two shapes:cylinder and square, respectively. It is found that electromagnetic waves of L and S bands can have unusual transmission properties in certain frequency ranges, such that electromagnetic waves can pass through the interior of the high-density plasma rod.
Passively Q-switched mode-locked low threshold Tm, Ho: LLF laser with an single walled carbon nanotubes saturable absorber
Authors: Ling Wei-Jun  , Xia Tao  , Dong Zhong  , Zuo Yin-Yan  , Li Ke  , Liu Qing  , Lu Fei-Ping  , Zhao Xiao-Long  , Wang Yong-Gang  ,   Institute of Laser Technology, Tianshui Normal University, Tianshui 741001, China;,   School of Physics and Information Technology, Shaanxi Normal University, Xi'an 710062, China
Published: December 2020

Employing single walled carbon nanotubes (SWCNT) grown by the vertical growth method as a saturable absorber for the initiation of the pulse generation, and designing a low threshold resonant cavity, we demonstrate a stable passively Q-switched mode-locked (QML) Tm, Ho:LiLuF4 solid-state laser with low threshold for the first time. With wavelength tunable Ti:sapphire solid laser operating at 785 nm as a pumping source, continuous-wave (CW) absorbed pump thresholds of 52, 59 and 62 mW are obtained by using 1.5%, 3% and 5% output coupled mirrors respectively. In this case, the maximum output powers are 645, 828 and 940 mW respectively, whose corresponding slope deficiencies are 31.02%, 39.16% and 43.78%, respectively. When the SWCNT-SAs is inserted in the cavity, the cavity loss is further increased, so the laser threshold is improved. Employing the 1.5% output mirror, a laser threshold is obtained to be as low as 85 mW, but the maximum laser output power is only 70 mW, corresponding slope efficiency is 3.42%; employing the 3% output coupling mirror, the laser threshold is obtained to be as low as 99 mW, the maximum output power is 154 mW, and the corresponding slope efficiency is 8.47%; employing the 5% output mirror, owing to the loss in the cavity being too large, the QML operation cannot be achieved. The output power of the 3% output mirror is twice higher than that of the 1.5% output mirror, but the laser threshold difference is only 14 mW. With a comprehensive analysis, we use the 3% output mirror. In this case, a stable QML operation with a threshold of 250 mW is obtained. When the absorption pump power is 1.85 W, the maximum output power is 154 mW with a typical Q-switched pulse envelope width of 300 s, which is corresponding to a 178.6 MHz of the mode-locked frequency. The modulation depth in Q-switching envelope is close to 100%. According to the definition of the rise time and considering the symmetric shape of the mode locked pulse, we could assume the duration of the pulse to be approximately 1.25 times more than the rise time of the pulse. So the width of the mode locked pulse is estimated to be about 663 ps. The results show that the SWCNT is a promising SA for QML solid-state laser with the 2 m wavelength. In the later stage, we increase the pump power, optimize the quality of the SWCNT material, and compensate for the dispersion in the cavity. It is expected to achieve a stable continuous mode-locking operation, and obtain a femtosecond mode-locked ultrashort pulse output. The mode-locked mid-infrared pulses have a lot of potential applications such as ultrafast molecule spectroscopy, the generation of mid-IR pulse, laser radar, atmospheric environment monitoring, etc.
Millimeter-wave half-waveplate based on field transformation
Authors: Wang Cheng  , Zhao Jun-Ming  , Jiang Tian  , Feng Yi-Jun  ,   Department of Electronic Engineering, School of Electronic Science and Engineering Nanjing University, Nanjing 210093, China
Published: December 2020

Over the last decades, manipulating polarization has received much attention due to its wide applications in science and technology. In this paper, a half-waveplate based on a field transformation (FT) method is proposed and investigated in order to convert polarization, which works at millimeter-wave band with a wide incident angle and broad working bandwidth.Inspired by the FT method, we confine our attention to a two-dimensional (2D) case of in-plane wave propagation on the x-y plane, with both material properties and fields unchanged in the z direction. The fields are denoted with a subscript “(0)” in the virtual space. Then a series of theoretical calculations is analyzed in detail. Under the guidance of theoretical analysis, it is shown that the main job for realizing this half-wavepalate is to obtain a material with specific permittivity and permeability. The proposed waveplate is composed of periodically arranged two dielectric layers each with sub-wavelength in height. By using the effective medium theory, the effective electromagnetic (EM) parameters of the waveplate can be tuned by manipulating the heights of the two dielectric layers. Among them one layer is a material with a permittivity of 10 and height of 0.68 mm, and another layer material has a permittivity of 1, and height of 5 mm. We alternately arrange the two materials along one direction periodically to obtain a waveplate for realizing the TE-to-TM and LCP-to-RCP conversion. The thickness of whole waveplate is 5.5 mm. A broadband EM half-waveplate is achieved in millimeter-wave region, which possesses a nearly 90% conversion efficiency across the frequency band from 24 GHz to 37 GHz. At the same time, we also find that when the incident angle gradually increases from 0° to 60°, the performances of polarization conversion efficiency and working bandwidth are still good. For the incident angle of 60°, a 3-dB bandwidth over 26-33 GHz is still achieved. The performance of the waveplate is verified through both full-wave simulation and experimental measurement, which are in good agreement with each other. Meanwhile, three-dimensional (3D) printing technology makes the sample fabricated more easily. Another advantage of our design is that the 3D printing technology can be used to carry out the experimental fabrication, which may pave a new way to manufacturing more microwave devices.
Propagation characteristics of speckle field in plasma
Authors: Yang Chun-Lin  ,   Chengdu Fine Optical Engineering Reserch Center, Chengdu 610041, China
Published: December 2020

The interaction between light and plasma is one of the key problems in an inertial confinement fusion system. Some instability processes will occur when the energy of laser is absorbed by plasma. Because reducing the coherence of laser can significantly restrain the instability of the plasma, in practice, a continuous phase plate (CPP) is often used to generate the speckle and thereby to restrain the nonlinear effect in plasma. To clarify the working mechanism of CPP, the propagation characteristics of speckle field are studied in this paper. Since there are two different kinds of media in the light path, the statistical optics theory and the matrix optics method are combined to analyze the propagation characteristics of the speckle field in plasma. The ABCD matrix of the plasma is deduced. And then intensity distribution properties of the speckle filed in the plasma are calculated. Meanwhile, the autocorrelation length of the speckle field is calculated and the mechanism of the nonlinear restraint is explained. The results show that the speckle field is a paseudorandom field. It will bring a random phase disturbance to the wavefront in the propagation direction. It is very different form the ordinary Gaussian beam, the speckle filed has a limited longitudinal autocorrelation length. Though the propagation rule of the speckle field in plasma is similar to that in air, when the laser transmits into plasma, the coherence of the laser speckle weakens rapidly. The autocorrelation length of the speckle field in the plasma is shorter than that in air. Therefore, many kinds of nonlinear effects can be restrained when the speckle transmits into plasma. Specially, the autocorrelation length of the speckle is much shorter in the high density plasma. So the result of suppressing the nonlinear effect is better in plasma with high density than that with low density. This characteristic is very helpful in restraining the different nonlinear effects in plasma.
Theoretical and experimental study of average reflection optical path length of diffuse cubic cavity
Authors: Zhang Yun-Gang  , Liu Ru-Hui  , Wang Mei-Ting  , Wang Yun-Xuan  , Li Zhan-Xun  , Tong Kai  ,   College of Electrical Engineering, Yanshan University, Qinhuangdao 066004, China
Published: December 2020

The most direct and efficient method to improve the sensitivity of gas sensor is to increase the effective optical path length (Leff) of gas cell according to the Beer-Lambert law. Moreover through experimental research and analysis, the diffuse cubic cavity, as a kind of gas cell, can effectively increase the value of Leff, which is crucial to the study of the reflection law of light in the diffuse cubic cavity. Based on the analysis of the reflection law of light in the diffuse cubic cavity, the theoretical value of the single reflection average optical path length (Lave) is obtained, the theoretical approximation model of the light reflection in the diffuse cubic cavity is established, and the simulation values are obtained by the finite element method. The tunable diode laser absorption spectroscopy (TDLAS) is a perferred gas dection technique with high selectivity, fast response and non-contact measuring. We develop diffuse cubic cavities of different sizes and study the reflection law and characteristics of the light in the cavities. We obtain the Leff values of the cubic cavities using TDLAS, with that and the theoretical formula between Leff and Lave, which in relation to the side length a, the diffuse reflectivity of coating and port fraction f, the experimental values of the Lave are obtained. The accuracies and stabilities of the theoretical approximation model and the simulation results by the finite element method are verified. According to the relationship between the Lave and the number of reflections established by the finite element method, the relative errors between the simulation values and the theoretical values of Lave are less than 3.6%, when each inner surface of the diffuse cubic cavity is divided into 10001000 or more small patches. It shows that the finite element method has a satisfactory effect on the cubic cavities with different sizes, and the error range is less than 0.1%. The TDLAS is used to measure the Leff values of three different cubic cavities with side lengths of 5 cm, 8 cm, and 12 cm, and the corresponding experimental values of the Lave are calculated indirectly. A comparison among the theoretical values, simulation values and experimentical values of the Lave, shows that these three values are well consistent with each other, which indicates that the simulation of the reflection law of light in the diffuse reflection cubic cavity has a significant reference value for the experimental study. Also, the present study of the diffuse cubic cavity will provide a technical support for studying the diffuse cavity of arbitrary shape in the future.
Heterojunction polarization beam splitter based on self-collimation in photonic crystal
Authors: Zuo Yi-Fan  , Li Pei-Li  , Luan Kai-Zhi  , Wang Lei  ,   Department of Opto-Electronic Engineering, Nanjing University of Posts and Telecommunications, Nanjing 210023, China
Published: December 2020

Polarization beam splitter (PBS) is an important device in optical system, in which the optical signal can be separated into two mutually orthogonal polarized light and transmit along different paths. It is difficult for the traditional PBS to meet the needs of the modern optical integrated systems because of its low transmission efficiency and high dependence on the incident angle. Therefore, it is necessary to design more efficient and compact PBSs. In recent years, photonic crystals have attracted more attention due to their ability to manipulate photon motion.
In this paper, a photonic crystal PBS with a non-orthogonal heterojunction structure is proposed, which is based on the self-collimation effect and bandgap properties of photonic crystal. The proposed PBS structure is composed of two square lattice photonic crystals with the same lattice constant and different air hole radii in silicon (Si), in which the beam can be self-collimated and propagate without diffraction, and the polarization separation of and transverse electric (TE) mode from transverse magnetic (TM) mode is realized at the interface. The self-collimation effect can be used to control the transmission of light in order to realize the general light guiding of the waveguide, and it can greatly reduce the difficulty in manufacturing process because of no additional defects introduced. The splitting properties, transmission properties and polarization extinction ratio of the PBS are numerically simulated and analyzed by using Rsoft software combined with the plane wave expansion method and the two finite-difference time-domain method. It is shown that a high efficiency and a large separating angle for TE and TM modes in a wide frequency range 0.275-0.285(a/ ) can be achieved. The transmission efficiency is above 88% for both TE and TM modes, and the extinction ratios are more than 26.57 dB for TE mode and 17.50 dB for TM mode, respectively. This structure can be applied to the transmission system of terahertz band: a=26 m, the size is 572 m546 m, and the separation of TE mode from TM mode can be achieved in a wavelength range of 91-95 m. A PBS for optical communication system can be also designed by using the same structure: n=3.48, a=426.25 nm, and the proposed PBS is only 9.38 m8.95 m in size, which can separate these two polarization beams in a wavelength range of 1511-1579 nm. What is more, the proposed PBS based on photonic crystal is simple and easy to integrate, which has important application value in optical communication technology.
Variational study of the 2DEG wave function in InAlN/GaN heterostructures
Authors: Li Qun  , Chen Qian  , Chong Jing  ,   School of Automation and Information Engineering, Xi'an University of Technology, Xi'an 710048, China;,   China Satellite Maritime Tracking and Control Department, Jiangyin 214431, China
Published: December 2020

The variational method has been widely used to study the electronic structures of heterostructure materials in spite of this method being less accurate than the numerical method, because analytical formulas for some electrical parameters can be derived using this method. However, effects of surface states on the two-dimensional electron gas (2DEG) have not been taken into account in the variational studies of GaN-based heterostructures. In the present study, analytical formulas for the electron wave function and ground state energy level of the 2DEG in InAlN/GaN heterostructures are derived using the variational method, and the influences of structural parameters of InAlN/GaN heterostructures on the electrical properties are discussed. In the theoretical model, evenly distributed surface states below the conduction band are assumed to be the origin of the 2DEG, and the polarization charges at the InAlN surface and the InAlN/GaN interface due to spontaneous and piezoelectric polarization effects in InAlN/GaN heterostructures are taken into account. A trial envelope wave function with two variational parameters is used to derive the expectation value of the total energy per electron. The variational parameters are determined by minimizing the expectation value. The model predicts a linear conduction band profile in InAlN barrier layer and an approximately triangular-shaped potential well on the GaN side of the InAlN/GaN interface. Electrons released from the surface states are confined in the potential well, forming the 2DEG. The 2DEG sheet density for the lattice-matched InAlN/GaN heterostructure with a 15 nm InAlN layer is 1.961013 cm-2, and the average distance from the InAlN/GaN interface of electrons is 2.23 nm. The 2DEG sheet density increases rapidly with InAlN thickness increasing when the InAlN layer exceeds the critical thickness, and starts to be saturated above 15 nm. The dependence of the calculated 2DEG sheet density on the InAlN thickness quantitatively agrees with recently reported experimental data. The increasing 2DEG sheet density results in increasing the ground state energy level and Fermi energy, and the energy spacing between the two also increases for containing more electrons. The polarization discontinuity at the InAlN/GaN interface decreases with increasing In mole fraction, causing the 2DEG sheet density to decrease, and thus the ground state energy level and the Fermi energy to decrease. This model is conducive to understanding the electrical behaviors of InAlN/GaN heterostructures and providing readily applicable formulas for studying the electron transport and optical transitions.
Characteristic parameters of adaptive optical imaging system in oceanic turbulence
Authors: Wu Tong  , Ji Xiao-Ling  , Luo Yu-Juan  ,   College of Physics and Electronic Engineering, Sichuan Normal University, Chengdu 610068, China
Published: December 2020

Since recently one is interested in underwater communications, imaging, sensing and lidar appeared, it is important to study characteristic parameters of the adaptive optical imaging system in oceanic turbulence. Until now, the characteristic parameters of the adaptive optical imaging system in atmospheric turbulence have investigated widely and in depth, but those in oceanic turbulence have been examined seldom. It is known that the atmospheric turbulence is induced by the temperature fluctuation. However, the oceanic turbulence is induced by both the temperature fluctuation and the salinity fluctuation. The temperature and salinity spectra have similar ''bumped'' profiles, with bumps occurring at different wave numbers. Thus, the behavior of light propagation in oceanic turbulence is very different from that in atmospheric turbulence. In this paper, the influence of oceanic turbulence on characteristic parameters (i.e., strehl ratio, Greenwood time constant, and isoplanatic>) of the adaptive optical imaging system is studied. The approximate analytical expression of the Strehl ratio for the short-exposure imaging case is derived. It is demonstrated by the numerical calculation method that this Strehl ratio approximate expression is accurate enough except the near field when DG/r0=1 (where DG is the pupil diameter of the optical system, r0 is the seeing parameter in oceanic turbulence), and the relative error maximum of this Strehl ratio approximate expression in the far field is much smaller than that in the near field. In addition, the analytical expressions of the Greenwood time constant and the isoplanatic> in oceanic turbulence are also obtained in this paper. It is shown that the values of the three characteristic parameters (i.e., Strehl ratio, the Greenwood time constant and the isoplanatic>) decrease when salinity-induced optical turbulence dominates gradually. The Strehl ratio, the Greenwood time constant and the isoplanatic> also decrease as the rate of dissipation of kinetic energy per unit mass of seawater decreases or the rate of dissipation of mean-squared temperature increases. It is known that the isoplanatic> at wavelength λ=0.5 μm are roughly 7-10 μrad for a nearly vertical path from Earth to space in atmospheric turbulence. However, it is shown in this paper that the isoplanatic> may be on the order of μrad after 100 m propagation distance in oceanic turbulence. Therefore, the influence of oceanic turbulence on the isoplanatic> is very large. The results obtained in this paper will be useful in the applications of adaptive optics imaging systems involving oceanic turbulence channels.
Flexible solid-state supercapacitors based on shrunk high-density aligned carbon nanotube arrays
Authors: Zhu Qi  , Yuan Xie-Tao  , Zhu Yi-Hao  , Zhang Xiao-Hua  , Yang Zhao-Hui  ,   Center for Soft Condensed Matter Physics and Interdisciplinary Research, Soochow University, Suzhou 215006, China
Published: December 2020

Nowadays flexible solid-state supercapacitors (FSCs) have received more and more attention than conventional capacitors due to the good operability and flexible fabrication process as well as high specific/volumetric energy density. In general, carbon based materials including amorphous carbon, carbon nanotube, grapheme, etc. can be used to fabricate electrolytic double-layer capacitance (EDLC)-type FSCs due to its extraordinary cyclic stability at high current density. Aligned carbon nanotube (ACNT) arrays are one of the ideal electrode candidates for energy storage due to their good capacity, highly efficient charge transfer rate, excellent rate performance and long cycle life compared with those of other carbon-based materials carbon nanotubes. However, the low density and the weak interaction between the carbon tubes cause the CNT arrays to tend to easily collapse during processing and transferring. Thus pure carbon nanotube arrays are unable to be directly used to assemble flexible electronic devices. In this paper, we use ethyl alcohol to shrink the CNT array to increase the density and mechanical strength. At the same time we embed the conductive polyvingle alcohol (PVA) gel into the carbon nanotube array to fabricate a flexible solid supercapacitor. Hydrogel-based solid electrolytes have been long considered to be used to prepare FSCs, because this method possesses obvious advantages including low cost, good environmental compatibility and simple manufacturing process. The ACNT/PVA complex can maintain good mechanical stability and flexibility during its folding and bending, and can also keep the high orientation of carbon nanotubes. The maximum capacitance of the hybrid flexible device can reach 458 mFcm-3 at a current density of 10 mAcm-3, which is much higher than the capacitance reported in the literature. After 5000 charging-discharging cycles, a capacity still keeps nearly 100%. The maximum energy density of CNTs/gel composite device can reach 0.04 mWhcm-3 with an average power density of 3.7 mWcm-3. The capacitance can be further increased to 618 mFcm-3 by a simple in-situ electrochemical oxidation treatment. The energy density can be further increased to 0.07 mWhcm-3 by the electro-oxidation treatment. The electrochemical performance of the device is far superior to that of EDLC-typed FSC reported in the literature. Additionally the equivalent series resistance (RESR) of the devices decreases from 120 to 30 and also the charge transfer resistance declines from 90 to 10 . This is mainly due to the effect of pseudo capacitance and electro-wetting effect caused by electro-oxidation. This easy-to-assemble hybrid devices thus potentially pave the way for manufacturing wearable devices and implantable medical devices.
Ultra-low frequency active vibration control for cold atom gravimeter based on sliding-mode robust algorithm
Authors: Luo Dong-Yun  , Cheng Bing  , Zhou Yin  , Wu Bin  , Wang Xiao-Long  , Lin Qiang  ,   Center for Optics and Optoelectronics Research, Collaborative Innovation Center for Bio-Med Physics Information Technology, College of Science, Zhejiang University of Technology, Hangzhou 310023, China
Published: December 2020

An ultra-low frequency vibrational noise isolation apparatus from external vibration can be a critical factor in many fields such as precision measurement, high-technology manufacturing, scientific instruments, and gravitational wave detection. To increase the accuracies of these experiments, well performed vibration isolation technology is required. Until recently the cold atom gravimeter has played a crucial role in measuring the acceleration due to gravity and earth gravity gradient. The vibration isolation is one of the key techniques in the cold atom gravimeter. To reduce the vibrational noise caused by the reflecting mirror of Raman beams in the cold atom gravimeter, a compact active low-frequency vibration isolation system based on sliding-mode robust control is designed and demonstrated. The sliding-mode robust control active vibration isolation method is used to solve the vibration problem of Raman mirror in the cold atomic gravimeter. The purpose of vibration control is that the controller enables the system to be at zero state as the system states are away from the equilibrium due to vibration disturbance. In this system, the mechanical setup is based on a commercial passive isolation platform which only plays a role at higher frequency. A sliding-mode robust control subsystem is used to process and feed back the vibration measured by a seismometer which can measure the velocity of the ground vibration. A voice coil actuator is used to control and cancel the motion of a passive vibration isolation platform. The simulation and experiment results of vibration isolation platform show, on the one hand, that the vibration noise power spectral density decreases by up to 99.9%, and that the phase noise in cold atom interferometry produced by vibration decreases by up to nearly 85.3% compared with the results of the passive vibration isolation platform. On the other hand, compared with the lead-lag control method, the vibration noise power spectral density decreases by up to 83.3% and the phase noise in cold atom interferometry produced by vibration decreases by nearly 40.2%. Therefore, the sliding-mode robust control has the advantages of less tuning parameters, strong anti-interference ability, and more obvious vibration isolating effect.
Chaotic time series prediction based on robust extreme learning machine
Authors: Shen Li-Hua  , Chen Ji-Hong  , Zeng Zhi-Gang  , Jin Jian  ,   School of Mechanical Science and Engineering, Huazhong University of Science and Technology, Wuhan 430074, China;,   School of Automation, Huazhong University of Science and Technology, Wuhan 430074, China
Published: December 2020

Chaos is seemingly irregular and analogous to random movement happening in a determinative system in nature,and more and more types and numbers of time series with chaotic characteristics are obtained from the actual systems,such as atmospheric circulation,temperature,rainfall,sunspots,and the Yellow River flow.The chaotic time series prediction has become a research hotspot in recent years.Because neural network can be strongly approximated nonlinearly,it has better prediction performance in the chaotic time series modeling.Extreme learning machine is a kind of neural network, and it is widely used due to its simple structure,high learning efficiency and having global optimal solution.Extreme learning machine initializes the input weight randomly and just adjusts the output weight in the training process,in order to be able to obtain the global optimal solution,so it has faster convergence speed and can overcome the disadvantage of gradient vanishing.Due to the above advantages,in recent years,the improved algorithms of the extreme learning machine have been developed rapidly.However,the traditional training methods of extreme learning machine have very poor robustness and can be affected easily by noise and outliers.And in practical applications,the time series are often contaminated by noise and outliers,so it is important to improve the forecasting model robustness and reduce the influence of noise and abnormal points to obtain better prediction accuracy.In this paper,a robust extreme learning machine is proposed in a Bayesian framework to solve the problem that outliers exist in the training data set.Firstly,the input samples are mapped onto the high-dimensional space,and the output weight of the extreme learning machine is used as the parameter to be estimated,then the proposed model utilizes the more robust Gaussian mixture distribution as the likelihood function of the model output.The marginal likelihood of the model output is analytically intractable for the Gaussian mixture distribution,so a variational procedure is introduced to realize the parameter estimation.In the cases of different noise levels and the different numbers of outliers,the proposed model is compared with the other prediction models.The experimental results of Lorenz,Rossler and Sunspot-Runoff in the Yellow River time series with outliers and noise demonstrate that the proposed robust extreme learning machine model could obtain a better prediction accuracy.The proposed robust extreme learning machine not only has the strong capability of the nonlinear approximation but also can learn the model parameters automatically and has strong robustness.At the same time,the time complexities of different models are compared and the convergence of the proposed model is analyzed at the end of the paper.
Multi-channel physical random number generation based on two orthogonally mutually coupled 1550 nm vertical-cavity surface-emitting lasers
Authors: Yao Xiao-Jie  , Tang Xi  , Wu Zheng-Mao  , Xia Guang-Qiong  ,   School of Physical Science and Technology, Southwest University, Chongqing 400715, China
Published: December 2020

Physical random number, which is non-reproducible and non-periodical, has attracted much attention due to its potential applications in various fields such as secure communication, statistical analysis, and numerical simulation. Recently, fast physical random number generators based on optical chaotic entropy sources have been demonstrated to reach a rate of up to several hundreds of Gbit/s. Although many efforts have been made to optimize the schemeis of chaotic-based random number generation, most of them are based on distributed feedback semiconductor lasers and can only generate single-channel physical random number. After taking into account the costs and technological applications, the multi-channel physical random number generation technique needs developing. On the other hand, vertical-cavity surface-emitting lasers (VCSELs) can simultaneously emit two orthogonally polarized components under appropriate parameter conditions, and then each polarized component can be used as an entropy source for generating random number. As a result, VCSEL-based chaotic entropy sources may be suitable for multi-channel random number generation. In this work, a scheme for achieving multi-channel physical random number is proposed. Also the influence of the coupling parameters on the performance of the randomness of final bit sequences is investigated. For such a scheme, two orthogonally mutually coupled VCSELs are used to supply four-channel chaotic signals with a comparable output power and weak time-delay signature (TDS). The four-channel chaotic signals, which serve as chaotic entropy, are quantized by 8-bit analog-to-digital converters (ADCs) with 20 GHz sampling rate, and then the m least significant bit (m-LSB) post-processing method is adopted for generating final four-channel random bit sequences. Firstly, based on the spin-flip mode of VCSELs, the influences of coupling strength and frequency detuning on the dynamics of two orthogonally mutually coupled 1550 nm VCSELs are analyzed. Next, the optimized parameter regions for generating four-channel chaotic signals with comparable output power and weak TDS are preliminarily determined. For a given optimized value of coupling strength and different frequency detunings within the optimized parameter regions, the generated four-channel chaotic signals are taken as the entropy sources for obtaining final bit sequence by quantizing the 8-bit ADC and m-LSB post-processing. Finally, the randomness of the four final bit sequences is tested by NIST SP 800-22 statistical test suite, and the regions of preferred coupling parameters for simultaneously generating four-channel random numbers are determined.
Investigation on voltaic effect based on one-dimensional TiO2 nanotube array thin film
Authors: Wang Na  , Ma Yang  , Chen Chang-Song  , Chen Jiang  , San Hai-Sheng  , Chen Ji-Ge  , Cheng Zheng-Dong  ,   Pen-Tung Sah Institute of Micro-Nano Science and Technology, Xiamen University, Xiamen 361005, China;,   Shenzhen Research Institute of Xiamen University, Shenzhen 518000, China;,   Shenzhen β Energy Technologies Co., Ltd., Shenzhen 518063, China;,   Texas A & M University, Texas 77843, USA
Published: December 2020

This work is to develop a high-reliability long-life high-conversion-efficiency radio-isotope microbattery in order to meet power requirements of micro-electromechanical systems, micro-sensors, micro-actuators, wireless sensing net, and other electron devices working in harsh circumstances, such as polar, desert, subsea, outer surface, etc. Compared with traditional dry batteries, chemical batteries, fuel cells and solar cells, the radioactive isotope batteries have long service life, higher energy density, strong adaptability to environment, good work stability, no maintenance, and miniaturized size, etc. These advantages make the voltaic battery an attractive alternative. In this paper we present a voltaic battery with enhanced voltaic effect by using a wide-bandgap semiconductor TiO2 nanotube array thin film. An electrochemical anodic oxidation method is used to prepare the vertically oriented and highly ordered TiO2 nanotube array film on Ti plate. Electrolyte solution consists of ammonium fluoride, ethylene glycol, and deionized water. The structure (TiO2 nanotube array with diameter about 80-100 nm, wall thickness about 15-25 nm, and length 9 m) is characterized by field emission scanning electron microscope. The microstructure of the TiO2 nanotube array is characterized using X-ray diffraction. The effects of annealing condition on optical and electrical properties are studied. The electrical property is characterized by Keithley model 2450 source meter semiconductor characterization system in dark at room temperature. The voltaic batteries are assembled as a sandwiched structure (63Ni/TiO2 nanotube arrays film/Ti) using a radioisotope 63Ni plate and TiO2 nanotube array films. The experimental results show that the black TiO2 nanotube array film annealed at 450 ℃ in argon atmosphere could creates high visible-ultraviolet absorption due to a great many of oxygen vacancy defects generated in TiO2 nanotube array film. The oxygen vacancy signals are found by electron spin resonance. Compared with the planar structure, the nano-porous array structure has strong absorption to particles:most of the particles enter into the pores and are reflected or absorbed by the surface of the tube walls. With a 10 mCi 63Ni radiation source, the voltaic battery using black TiO2 nanotube array film can generate an open-circuited voltage of 1.02 V, a short-circuited current of 75.52 nA, and a maximum effective conversion efficiency of 22.48%.