bluuebunny/crossref_metadata_2025 huggingface dataset:
Multi-scale cross-correlation characteristics of void fraction wave propagation for gas-liquid two-phase flows in small diameter pipe
Authors: Zhai Lu-Sheng  , Jin Ning-De  ,   School of Electrical Engineering and Automation, Tianjin University, Tianjin 300072, China
Published: December 2020

The void fraction wave is a special physical phenomenon in a gas-liquid two-phase flow system. Understanding the propagation of the void fraction wave is of great significance for uncovering the physical mechanisms in both flow pattern transition and the fluid velocity measurement. In this study, detrended cross-correlation analysis (DCCA) is used to investigate the multi-scale cross-correlation characteristics of the coupled ARFIMA processes. It is found that the DCCA can effectively reveal the multi-scale cross-correlation dynamical behaviors of complex system. Then, we carry out the experimental test in a vertical gas-liquid two-phase flow pipe with small inner diameter. The DCCA is used to detect the cross-correlation characteristics of the void fraction wave on multiple time scales, and the growth rate of the cross-correlation level for the void fraction wave is observed on low time scales. Additionally, the spatial attenuation factor (SAF) of the void fraction wave is calculated to investigate the instability of the wave propagation. The SAF is close to zero under the transitional flow patterns, which means that the void fraction wave is in a stable propagating state. For bubble flows, the void fraction wave presents the attenuation characteristics, whilst the void fraction wave shows the amplification characteristics under the slug and churn flow patterns. Interestingly, the instability behaviors of the void fraction wave are always associated with its multi-scale cross-correlation characteristics. Specifically, the increasing rate of the wave cross-correlation level on low scales is much higher for transitional flow patterns, which is corresponding to the stable propagating characteristic of the void fraction wave. However, when the void fraction wave exhibits attenuation or amplification characteristics under other flow patterns, the increasing rate of the wave cross-correlation level on low scales is much lower.
InSb based subwavelength array for terahertz wave focusing
Authors: Gu Wen-Hao  , Chang Sheng-Jiang  , Fan Fei  , Zhang Xuan-Zhou  ,   Institute of Modern Optics, Nankai University, Tianjin 300071, China
Published: December 2020

With the continuous development of terahertz (THz) technology in recent years, many kinds of THz functional devices including switchers, filters, modulators, isolator and polarizers have been demonstrated. However, researches of the focusing devices in the terahertz frequency range are rarely reported. In this paper, we propose a subwavelength metal-air-InSb-metal periodic array structure to perform terahertz wave focusing. The dependence of permittivity of InSb in the THz regime on external magnetic field and temperature is calculated theoretically. Based on the magneto-optical effect of the semiconductor material InSb and asymmetrical waveguide structure, the influences of external magnetic field and temperature on the focusing and transmittance characteristics of the device are studied in detail. Numerically simulated results show that the structure proposed above can not only improve the transmittance greatly but also perform focusing perfectly. Calculations on the transmission properties show that in a certain range of temperature, the power flow transmittance at the focus point increases with the increase of temperature. In the meantime, for a certain temperature, with increasing the external magnetic field, the power flow continuously increases as well and reaches a maximum value at a certain magnetic field. For example, for a temperature of 172 K and a magnetic field of 0.6 T, the maximum power flow transmitted at the focus point is 10200 W/m2 at 0.8 THz, which is about 28 times larger than that without magnetic field at the same temperature. In addition, the simulation results also show that when the temperature and external magnetic field are fixed at 172 K and 0.5 T, respectively, the power flow transmittances for the incident waves at different frequencies are different. There is a peak value of the transmittance appearing at a specific frequency of 0.8 THz. Moreover, when the incident wave frequency is far from 0.8 THz, the transmittance decreases dramatically. It is worth noting that by choosing different temperatures and external magnetic fields, the structure proposed can not only enhance the transmittance over 20 times at the focus point, but also manipulate effectively the THz wave in a broad operating bandwidth of 400 GHz from 0.4 THz to 0.8 THz. These properties indicate that the proposed structure can act as an ideal tunable, broadband, and high transmittance focusing device in the terahertz regime.
Spectral Stokes singularities of partially coherent edge dislocation beams
Authors: Zheng Shang-Bin  , Tang Bi-Hua  , Jiang Yun-Hai  , Luo Ya-Mei  , Gao Zeng-Hui  ,   Department of Biomedical Engineering, Sichuan Medical University, Luzhou 646000, China;,   Key Laboratory of Computational Physics, Yibin University, Yibin 644000, China
Published: December 2020

The polarization singularities in vector wavefields have been extensively studied analytically and experimentally. The polarization singularities can be analyzed by using electromagnetic theory or Stokes parameters, or be described in terms of complex Stokes scalar fields. In some practical applications, partially coherent beams have more advantages than fully coherent beams. Recently, the concept of the polarization singularities has been extended from fully coherent beams to partially coherent beams. In this paper, using the representation of cross-spectral density matrix propagation, the explicit propagation expressions for the partially coherent edge dislocation beams are derived in free space, and based on the spectral Stokes parameters the spectral singularities are studied in detail. It is shown that there exist spectral s12, s23 and s31 singularities of partially coherent edge dislocation beams in free-space propagation. s12 singularities correspond to circular polarization (C-points) of the partially coherent edge dislocation beams, and s30 (s30) means right-(left-) handedness, where the orientations of the major and minor axes of the polarization ellipse become undefined. s23 and s31 singularities must be located on L-lines, where the handedness of the polarization ellipse is undetermined (linear polarization). The motion, creation and annihilation of spectral Stokes singularities may appear in the variation of a controlling parameter, such as off-axis distance, slope of edge dislocation, spatial correlation length, or in the variation of the propagation distance. By suitably varying the spatial correlation length or propagation distance the V-point, the handedness reversal of C-point, creation and annihilation for a pair of oppositely charged spectral singularities take place. The creation and annihilation occur for a pair of s12 singularities with opposite topological charge but same handedness. The critical points of the controlling parameters and propagation distance, at which pairs of different spectral singularities annihilate, are not the same. The collision of the C-point and L-line results in a V-point (vector singularity), which is unstable. A small perturbation leads to the handedness reversal. At such a point the state of polarization is undetermined and the degree of polarization P=0. The results obtained in this paper would be useful for a deep understanding of polarization singularities of stochastic electromagnetic beams.
Experimental study on three-dimensional ptychography for thick sample
Authors: Pan An  , Zhang Xiao-Fei  , Wang Bin  , Zhao Qing  , Shi Yi-Shi  ,   Xi'an Institute of Optics and Precision Mechanics, Chinese Academy of Sciences, Xi'an 710119, China;,   Changchun Institute of Optics, Fine Mechanics and Physics, Chinese Academy of Sciences, Changchun 130033, China;,   University of Chinese Academy of Sciences, Beijing 100049, China;,   Academy of Opto-electronics, Chinese Academy of Sciences, Beijing 100094, China
Published: December 2020

Ptychography is a new kind of lens-less imaging technology. What restricts the technique is the assumption of a multiplicative interaction between the illuminating coherent beam and the specimen, i.e., and the ptychography cannot be applied to samples no thicker than a few tens of micrometers in the case of visible-light imaging at micron-scale resolution. In the present work, we split a sample into axial sections, thereby realize three-dimensional ptychographic imaging of thick samples at the millimeter level in a series of computer simulations and optical experiments. Our simulation results reveal that by using single wavelength we cannot achieve good-quality images of thick samples. Thus it is necessary to introduce more wavelengths for illumination. With increasing the number of wavelengths, the imaging quality of three-dimensional thick samples can be enhanced continually. Then we make further study on the relationship between the imaging quality and the magnitude of wavelength in optical experiments by using two groups of samples with different thickness values. The results demonstrate that our experimental results are highly consistent with simulations. For our concrete configuration in this paper, the best results of imaging and separation may be obtained for the case of tri-wavelength. At the same time we make a reasonable explanation for the phenomenon of fold-over in the experiment. Our results are important and meaningful for the practical utilizing of three-dimensional ptychography of thick samples.
Adaptive optics correction technique based onfuzzy control
Authors: Liu Zhang-Wen  , Li Zheng-Dong  , Zhou Zhi-Qiang  , Yuan Xue-Wen  ,   Key Laboratory of Science and Technology on High Energy Laser, Institute of Applied Electronic, China Academy of Engineering Physics, Mianyang 621900, China
Published: December 2020

In an adaptive optics system, proportion-integration-differentiation (PID) controller is widely used for correcting wave front, but the controller is strictly dependent on the response model of deformable mirror. In this paper, a novel wave front correction method is proposed. The method, combining fuzzy control and PID control, does not depend on the response model of the deformable mirror. Based on rapid wave front reconstruction, the wave front evaluation indexes, extracted from the reconstructed wave front, are employed for the input of fuzzy controller and PID controller. Thus, the model response matrix of deformable mirror is not required. Each actuator of deformable mirror corresponds to an independent fuzzy PID controller. By designing the fuzzy controller, including fuzzy rule base selection and fuzzy reasoning, the three parameters of PID controller, the proportional kp, the integral ki and the differential kd, are adjusted automatically. A high rapid DSP hardware platform is constructed to verify the method. Test results show that the method can be used to correct the diffraction limit multiplication factor of the light spot from 10-12 to 3-4, which is basically the same as the traditional PID control, but its stability is better. Because the model does not need to calibrate the deformable mirror, the installation of the deformable mirror is easier.
Theoretical and experimental investigation on the narrow-linewidth photonic microwave generation based on parallel polarized optically injected 1550 nm vertical-cavity surface-emitting laser
Authors: Sun Bo  , Wu Jia-Gui  , Wang Shun-Tian  , Wu Zheng-Mao  , Xia Guang-Qiong  ,   School of Physical Science and Technology, Southwest University, Chongqing 400715, China
Published: December 2020

Photonic microwave generation has attracted much attention in recent years due to its potential applications in various fields such as radio-over-fiber communication, signal processing and radar systems. So far, different photonic microwave generation schemes have been proposed and investigated, such as the optical heterodyne method based on the beat of two independent lasers with a certain wavelength difference, the external modulation method based on electro-optical modulator, the dual-mode beat method based on the monolithic dual-mode semiconductor lasers, and the optoelectronic microwave oscillator method based on optoelectronic feedback loops. These schemes have their own advantages and deficiencies. Unlike the above schemes, in this paper we propose an all optical scheme for generating high-quality microwave based on a 1550 nm vertical-cavity surface-emitting laser (1550 nm-VCSEL). For such a scheme, high frequency microwave can be obtained based on a 1550 nm-VCSEL subjected to external optical injection, where the polarization of the injected light is the same as that of the dominant mode of the free-running 1550 nm-VCSEL (named parallel-polarized optical injection) and its wavelength is adjusted to being close to the wavelength of the suppressed polarization mode of the free-running 1550 nm-VCSEL. With the aid of double optical feedback, the linewidth of the obtained microwave can be narrowed. In this work, firstly, the feasibility of microwave generation based on parallel-polarized optically injected 1550 nm-VCSEL is analyzed theoretically by using the spin-flip model. Next, a corresponding experimental system is constructed, and the performance of microwave generation is preliminarily investigated experimentally. The experimental results show that 30 GHz microwave signals could be obtained based on a parallel-polarized, optically injected 1550 nm-VCSEL under suitable injection parameters, but the linewidth of microwave signal is relatively wide (hundreds of MHz). Finally, after introducing double optical feedback, the linewidth of microwave signal can be reduced by more than two orders of magnitude and narrowed to less than 1 MHz, meanwhile the signal-noise ratio is larger than 40 dB. This work is helpful to develop relevant techniques to acquire high-performance narrow linewidth photonic microwave.
Tunable near-zero index of self-assembled photonic crystal using magnetic fluid
Authors: Geng Tao  , Wu Na  , Dong Xiang-Mei  , Gao Xiu-Min  ,   Shanghai Key Lab of Modern Optical System, Engineering Research Center of Optical Instrument and System, Ministry of Education, School of Optical-Electrical and Computer Engineering, University of Shanghai for Science and Technology, Shanghai 200093, China;,   Electronics and Information College, Hangzhou Dianzi University, Hangzhou 310018, China
Published: December 2020

In a zero index material, the phase velocity of light is much greater than the speed of light in vacuum and can even approach to infinity. Thus, the phase of light throughout a piece of zero-index material is essentially a constant. The zero index material has recently been used in many areas due to its extraordinary optical properties, including beam collimation, cloaking and phase matching in nonlinear optics. However, most of zero index materials usually have narrow operating bandwidths and the operating frequencies are not tunable. In this work, the model of tunable near-zero index photonic crystal is established by using colloidal magnetic fluid. Magnetic fluid, as a kind of easy-made mature nanoscale magnetic material, has proved to be an excellent candidate for fabricating self-assembled photonic crystal, especially the band-tunable photonic crystal with fast and reversible response to external magnetic field. The band structure can be calculated using the plane wave expansion method. For TE mode, it can be seen that a triply-degenerate point (normalized frequency f=0.734) at point under external magnetic field H=147 Oe, forms a Dirac-like point in the band structure, which is called an accidental-degeneracy-induced Dirac-like point. The effective permittivity eff and permeability eff are calculated using an expanded effective medium theory based on the Mie scattering theory. The calculated results show that both eff and eff are equal to zero at Dirac-like point, which means that the effective index neff is zero and the effective impedance Zeff is 1. The lattice structure of such a self-assembled photonic crystal will change with the external magnetic field, leading to the disappearance of Dirac-like point. However, when 143.6 OeH 152.4 Oe (1 Oe=79.5775 A/m), |neff | can keep less than 0.05 under the condition of Zeff = 1. Correspondingly, the operating frequency will change from 0.75 to 0.716. The model is verified by the numerical simulations (COMSOL Multiphysics) and the theoretical results agree well with the numerical ones.
Tunable array beam splitter with different domain-etching depth based on MgO-doped lithium niobate crystal
Authors: Tong Man  , Fan Tian-Wei  , Chen Yun-Lin  ,   Institute of Applied Micro-Nano Materials, School of Science, Beijing Jiaotong University, Beijing 100044, China
Published: December 2020

The Talbot effect is a self-imaging phenomenon of near-field diffraction. When a plane wave is incident on a periodic diffraction grating, the image of the grating is repeated at regular distances away from the grating plane. A Talbot array illuminator is a device that splits singular light beam into an array of beams with periodical optical intensity based on Talbot effect. LiNbO3 (LN) crystal is a kind of practicable material for a Talbot array illuminator due to its perfect optical characteristics. MgO-doped LiNbO3 (MgLN) crystal shows shorter absorption edge wavelength and higher resistance to photorefractive damage than LN. Up to now, the usefulness and simplicity of Talbot effect have still aroused the interest of many scholars.In the conventional method, a Talbot array illuminator is fabricated by using high external electric field to modulate the phase difference. However, essentially, high external electric field restricts the Talbot array illuminator to applications in optical integration and optical micro structure devices. Now we are looking forward to a new way which avoids using high external electric field.In this paper, we systematically study the two-dimensional (2D) hexagonal tunable array beam splitter, which is fabricated by domain-etching in MgLN crystal, and its fractional Talbot effect. The self-imaging phenomenon caused by Talbot effect in the Fresnel field for this phase array coherently illuminated is theoretically analyzed according to Fresnel diffraction theory. We numerically simulate the light intensity distributions of Talbot diffraction image under different values of Talbot coefficient and different values of domain-etching depth. The simulation results show that can change the array period and the structure distribution of the fractional Talbot diffraction image, and the domain-etching depth can modulate the light intensity distribution of diffraction image. Based on the numerical simulation results, the 2D hexagonal array beam splitters are fabricated with different values of domain-etching depth. The fractional Talbot diffraction images of array splitters are obtained at different values of through the optical experiments. The results show that domain-etching depth can effectively modulate the intensity distribution of diffraction image, becoming a tunable array beam splitter successfully. The experimental results agree well with the simulation results. The theoretical and experimental results show that the optimal self-image visibility can be obtained at a Talbot coefficient of 0.5 and a domain-etching depth of 0.39 m, while the duty cycle is 52%. Moreover, a good self-image pattern is also observed under thinner domain-etching depth, which is beneficial to optical integration and micro optical devices.
Volume pulsation and scattering of bubbles under the second Bjerknes force
Authors: Ma Yan  , Lin Shu-Yu  , Xian Xiao-Jun  ,   Shaanxi Key Laboratory of Ultrasonics, Shaanxi Normal University, Xi'an 710062, China;,   School of Physics and Information Technology, Ningxia Normal University, Guyuan 756000, China
Published: December 2020

The interaction of bubbles must be taken into consideration in the investigation of sound wave in the liquid containing gas bubbles, particularly in the case where the gas content is high. The force between two air bubbles due to the secondary sound fields radiated by the bubbles is called the secondary Bjerknes force, which makes the dynamics and scattering of bubbles different from a single bubble's. In order to investigate the influence of secondary Bjerknes force on bubbles' pulsation and scattering, we obtain the universal expression of bubbles' pulsation under the secondary Bjerknes force by Lagrange's equation. The influences on volume amplitude and initial phase of different parameter under the second Bjerknes force are discussed, and the scattering of bubbles with phase differences of and 0 is studied. The results show that the radius of neighbouring bubble, distance between two bubbles, polytropic coefficient and the phase can change the volume amplitude of pulsation under the secondary Bjerknes force. The mean radius of bubbles, distance and the frequency of sound have a significant effect on initial phase; the scattering of two bubbles of small distance and phase difference of is directional and decreases with distance r, which is related to the volume amplitude, initial phase and distance between two bubbles. The mean scattering power of bubble pairs of phase difference is 1/6(kd12)2 of single bubble's. The scattering of two bubbles with small distance and same phase also decreases with the distance r and relates to the volume amplitude, initial phase and distance between two bubbles. The mean scattering power of bubble pairs of same phase is 4 times as bigger as the mean scattering power of single bubble. It is expected that the mean radiuses, driving frequency and distance between bubbles can be used to change the scattering of bubbles.
Synchronous behavior of a rotor-pendulum system
Authors: Fang Pan  , Hou Yong-Jun  , Zhang Li-Ping  , Du Ming-Jun  , Zhang Meng-Yuan  ,   School of Mechanical Engineering, Southwest Petroleum University, Chengdu 610500, China;,   College of Geosciences, China University of Petroleum, Beijing Campus, Beijing 102249, China
Published: December 2020

Rotor-pendulum systems are widely applied to aero-power plants, mining screening machineries, parallel robots, and other high-speed rotating equipment. However, the investigation for synchronous behavior (the computation for stable phase difference between the rotors) of a rotor-pendulum system has been reported very little. The synchronous behavior usually affects the performance precision and quality of a mechanical system. Based on the special background, a simplified physical model for a rotor-pendulum system is introduced. The system consists of a rigid vibrating body, a rigid pendulum rod, a horizontal spring, a torsion spring, and two unbalanced rotors. The vibrating body is elastically supported via the horizontal spring. One of unbalanced rotors in the system is directly mounted in the vibrating body, and the other is fixed at the end of the pendulum rod connected with the vibrating body by the torsion spring. In addition, the rotors are actuated with the identical induction motors. In this paper, we investigate the synchronous state of the system based on Poincar method, which further reveals the essential mechanism of synchronization phenomenon of this system. To determine the synchronous state of the system, the following computation technologies are implemented. Firstly, the dynamic equation of the system is derived based on the Lagrange equation with considering the homonymous and reversed rotation of the two rotors, then the equation is converted into a dimensionless equation. Further, the dimensionless equation is decoupled by the Laplace method, and the approximated steady solution and coupling coefficient of each degree of freedom are deduced. Afterwards, the balanced equation and the stability criterion of the system are acquired. Only should the values of physical parameters of the system satisfy the balanced equation and the stability criterion, the rotor-pendulum system can implement the synchronous operation. According to the theoretical computation, we can find that the spring stiffness, the installation title angle of the pendulum rod, and the rotation direction of the rotors have large influences on the existence and stability of the synchronous state in the coupling system. Meanwhile, the critical point of synchronization of the system can lead to no solution of the phase difference between the two rotors, which results in the dynamic characteristics of the system being chaotic. Finally, computer simulations are preformed to verify the correctness of the theoretical computations, and the results of theoretical computation are in accordance with the computer simulations.
Experimental study on improving hohlraum wall reemission ratio by low density gold foam
Authors: Zhang Lu  , Dong Yun-Song  , Jing Long-Fei  , Lin Zhi-Wei  , Tan Xiu-Lan  , Kuang Long-Yu  , Li Hang  , Shang Wan-Li  , Zhang Wen-Hai  , Li Zhi-Chao  , Zhan Xia-Yu  , Yuan Guang-Hui  , Li Hai  , Jiang Shao-En  , Yang Jia-Min  , Ding Yong-Kun  ,   Research Center of Laser Fusion, China Academy of Engineering Physics, Mianyang 621900, China
Published: December 2020

It is important to improve the hohlraum radiation temperature for the research of high energy density physics, especially for study of inertial confinement fusion. Increasing the wall reemission ratio is an effective way to improve the temperature. It is found in theory that low density foam could reduce hohlraum wall energy loss, and then increase hohlraum temperature. In previous studies, experiments have shown that laser-to-X-ray conversion is enhanced by Au foam. However, improving reemission ratio is more important to increase hohlraum radiation temperature, because most of energy is lost in the wall.In this paper, we report our experiments carried out on SGⅢ prototype to compare the X-ray flux reemitted by Au foam and that by Au. For the experimental design, Au solid and Au foam are irradiated symmetrically along the axis by hohlraum radiation source Tr(t), which is assessed by broadband X-ray spectrometer flat-response X-ray diodes. The measured peak temperature is about 190 eV. Reemission flux from sample is measured by transmission grating spectrometer (TGS). The space-resolved image for pure Au sample shows that the hohlraum radiation is asymmetrical along the axis in the experimental conditions, temperature of top is higher than that at the bottom, which is consistent with simulation results obtained by using IRAD3D code. In order to compare the reemission flux from Au solid sample and that from Au foam sample in same conditions, we need to correct the symmetry of hohlraum radiation. By multiplying the ratio of top flux to bottom flux in pure Au target by the bottom flux in Au-Au foam target, where Au foam is on, we make sure that they are ablated by the same radiation source. The calculated results show that X-ray flux is increased by 20% by Au foam of 0.4 g/cc density when the hohlraum temperature is 190 eV. The typical observed time-integrated X-ray reemission spectra for Au solid and Au foam by TGS are also shown. We see that N-band and O-band reemission are clearly enhanced by Au foam, and the O-band reemission is almost the same as M-band reemission. The increased flux concentrates below 1 keV of the soft X-ray emission.The self-similar solution results and MULTI 1D simulation results show that the wall loss energy fraction is saved by Au foam, whose relation to reemission flux can be described by a simple expression. The theoretical solution shows that the emission flux increases about 10%, and the MULTI simulation indicates that the emission flux increases about 6.8%. They are in qualitative agreement with the experiments results. These results show an alluring prospect for Au foam to be used as hohlraum wall.
Growths of Fe-doped GaN high-resistivity buffer layers for AlGaN/GaN high electron mobility transistor devices
Authors: Wang Kai  , Xing Yan-Hui  , Han Jun  , Zhao Kang-Kang  , Guo Li-Jian  , Yu Bao-Ning  , Deng Xu-Guang  , Fan Ya-Ming  , Zhang Bao-Shun  ,   Key Laboratory of Opto-electronics Technology, Ministry of Education, College of Electronic Information and Control Engineering, Beijing University of Technology, Beijing 100124, China;,   Key Laboratory of Nano Devices and Applications, Suzhou Institute of Nano-Technology and Nano-Bionics, Chinese Academy of Sciences, Suzhou 215123, China
Published: December 2020

Fe-doped high-resistivity GaN films and AlGaN/GaN high electron mobility transistor (HEMT) structures have been grown on sapphire substrates by metal organic chemical vapor deposition. The lattice quality, surfaces, sheet resistances and luminescent characteristics of Fe-doped high-resistivity GaN with different Cp2Fe flow rates are studied. It is found that high resistivity can be obtained by Fe impurity introduced Fe3+/2+ deep acceptor level in GaN, which compensates for the background carrier concentration. Meanwhile, Fe impurity can introduce more edge dislocations acting as acceptors, which also compensate for the background carrier concentration to some extent. In a certain range, the sheet resistance of GaN material increases with increasing Cp2Fe flow rate. When the Cp2Fe flow rate is 100 sccm, the compensation efficiency decreases due to the self-compensation effect, which leads to the fact that the increase of the sheet resistance of GaN material is not obvious. In addition, the compensation for Fe atom at the vacancy of Ga atom can be explained as the result of suppressing yellow luminescence. Although the lattice quality is marginally affected while the Cp2Fe flow rate is 50 sccm, the increase of Cp2Fe flow rate will lead to a deterioration in quality due to the damage to the lattice, which is because more Ga atoms are substituted by Fe atoms. Meanwhile, Fe on the GaN surface reduces the surface mobilities of Ga atoms and promotes a transition from two-dimensional to three-dimensional (3D) GaN growth, which is confirmed by atomic force microscope measurements of RMS roughness with increasing Cp2Fe flow rate. The island generated by the 3D GaN growth will produce additional edge dislocations during the coalescence, resulting in the increase of the full width at half maximum of the X-ray diffraction rocking curve at the GaN (102) plane faster than that at the GaN (002) plane with increasing Cp2Fe flow rate. Therefore, the Cp2Fe flow rate of 75 sccm, which makes the sheet resistance of GaN as high as 1 1010 /\Box, is used to grow AlGaN/GaN HEMT structures with various values of Fe-doped layer thickness, which are processed into devices. All the HEMT devices possess satisfactory turn-off and gate-controlled characteristics. Besides, the increase of Fe-doped layer thickness can improve the breakdown voltage of the HEMT device by 39.3%, without the degradation of the transfer characteristic.
Recent progress of two-dimensional layered molybdenum disulfide
Authors: Gu Pin-Chao  , Zhang Kai-Liang  , Feng Yu-Lin  , Wang Fang  , Miao Yin-Ping  , Han Ye-Mei  , Zhang Han-Xia  ,   Tianjin Key Laboratory of Film Electronic and Communication Devices, School of Electronics Information Engineering, Tianjin University of Technology, Tianjin 300384, China
Published: December 2020

Recently, two-dimensional (2D) layered molybdenum disulfide (MoS2) has attracted great attention because of its graphene-like structure and unique physical and chemical properties. In this paper, physical structure, band gap structure, and optical properties of MoS2 are summarized. MoS2 is semiconducting and composed of covalently bonded sheets held together by weak van der Waals force. In each MoS2 layer, a layer of molybdenum (Mo) atoms is sandwiched between two layers of sulfur (S) atoms. There are three types of MoS2 compounds, including 1T MoS2, 2H MoS2, and 3R MoS2. As the number of layers decreases, the bad gap becomes larger. The bad gap transforms from indirect to direct as MoS2 is thinned to a monolayer. Changes of band gap show a great potential in photoelectron. Preparation methods of 2D MoS2 are reviewed, including growth methods and exfoliation methods. Ammonium thiomolybdate (NH4)2MoS4, elemental molybdenum Mo and molybdenum trioxide MoO3 are used to synthesize 2D MoS2 by growth methods. (NH4)2MoS4 is dissolved in a solution and then coated on a substrate. (NH4)2MoS4 is decomposed into MoS2 after annealing at a high temperature. Mo is evaporated onto a substrate, and then sulfurized into MoS2. MoO3 is most used to synthesize MoS2 on different substrates by a chemical vapor deposition or plasma-enhanced chemical vapor deposition. Other precursors like Mo(CO)6, MoS2 and MoCl5 are also used for MoS2 growth. For the graphene-like structure, monolayer MoS2 can be exfoliated from bulk MoS2. Exfoliation methods include micromechanical exfoliation, liquid exfoliation, lithium-based intercalation and electrochemistry lithium-based intercalation. For micromechanical exfoliation, the efficiency is low and the sizes of MoS2 flakes are small. For liquid exfoliation, it is convenient for operation to obtain mass production, but the concentration of monolayer MoS2 is low. For lithium-based intercalation, the yield of monolayer MoS2 is high while it takes a long time and makes 2H MoS2 transform to 1T MoS2 in this process. For electrochemistry lithium-based intercalation, this method saves more time and achieves higher monolayer MoS2 yield, and annealing makes 1T MoS2 back to 2H MoS2. The applications of 2D MoS2 in field-effect transistors, sensors and memory are discussed. On-off ratio field effect transistor based on MoS2 has field-effect mobility of several hundred cm2V-1-1 and on/off ratio of 108 theoretically.
Density functional study on the adsorption characteristics of O, O2, OH, and OOH of B-, P-doped, and B, P codoped graphenes
Authors: Sun Jian-Ping  , Zhou Ke-Liang  , Liang Xiao-Dong  ,   School of Electrical and Electronic Engineering, North China Electric Power University, Beijing 102206, China
Published: December 2020

Over past years, the excessive use of fossil fuel has posed serious problems such as greenhouse effect and environmental pollution, which threaten human life. Regarded as an ideal substitution for traditional internal combustion engine, low temperature proton exchange membrane fuel cell (PEMFC) converts chemical energy through electrode reaction directly into electrical energy with high efficiency and low pollution. However, the main problem behind the industrialization of PEMFC, is that oxygen reduction reaction (ORR) occurring on the cathode needs precious metal platinum (Pt) as catalyst, which has a limited reserve and is costly. Owing to high activity and stability, the graphenes doped with non-metal B and P, have proven to be excellent alternatives to Pt experimentally. However, the relevant theoretical work is scarce.Adsorptions of the ORR intermediates, i.e., O, O2, OH, and OOH, of doped graphenes are essential for the cathode reaction, which also bring some difficulties to the next step reaction. Therefore, in this paper, based on density functional theory, the adsorption characteristics of O, O2, OH, and OOH of B-doped, P-doped and B, P-codoped graphenes are studied using first-principles calculation code VASP first. By analyzing the adsorption energies, bond lengths, densities of states and charge transfers, the influences of the different dopants on the intermediates are evaluated. Then, the ORR steps are discussed, and the free energy change of each step is further given. The results show that for B-doped and P-doped graphenes, the adsorption energies of various intermediates exhibit similar linear relationships. The adsorption energy of OOH of P-doped graphene (3.26 eV) is much larger than that in B-doped grapheme (0.73 eV). The large adsorption energy of P-doped graphene is beneficial to the fracture reaction of OO bond in OOH, while the small adsorption energy of B-doped graphene can promote the reaction of OH converting into water. Owing to the synergistic effect, the graphene codoped with B and P possesses better catalyzing ability than single B-and P-doped ones. The results are helpful for understanding the excellent performances of codoped graphenes.
Phase-Field Modeling of Facet Hexagonal Spirals with Anisotropy, Deposition, and Kinetic Effects
Authors: Dong Xiang-Lei  , Xing Hui  , Chen Chang-Le  , Sha Sha  , Wang Jian-Yuan  , Jin Ke-Xin  ,   Shaanxi Key Laboratory for Condensed Matter Structure and Properties, School of Science, Northwestern Polytechnical University, Xi'an 710129, China
Published: December 2020

In this paper, we perform the quantitative phase-field simulations based on the surface morphology and growth regime of the hexagonal GaN spiral structure. We investigate the highly anisotropic energy, the deposition rate and the kinetic attachment and detachment effects. A regularized equation including the modified gradient coefficient is employed to study the anisotropic effect. Results show that the highly anisotropic energy modulates the equilibrium state by changing the local curvature of the tip step and thus leading to the changed spiral spacing. Under the weak anisotropy, the spiral spacing and morphology keep stable with the increase of the anisotropic strength. In the case of facet anisotropy, however, the larger anisotropic strength facilitates the spiral growth due to the local interfacial instability caused by increasing the supersaturation for the tip step. As to the effect of deposition, the deposition rate imposes the reaction on the curvature of interface due to the variations of supersaturation and step velocity. The larger rate of deposition enables the shorter spacing for both anisotropic and isotropic spirals. We carry out a convergence study of spiral spacing with respect to the step width to estimate the precision of the phase-field simulation. Results show that the larger deposition rate and the higher anisotropy give rise to the lower convergence of the spiral model. Moreover, we find that the kinetic attachment affects the instinct regime of spiral growth by changing the step spacing and the scaling exponents of spiral spacing versus deposition rate. The anisotropic spiral exhibits the more significant hexagonal structure and the lower value of step velocity by reducing the value of kinetic coefficient. The scaling exponent decreases with anisotropy increasing, but it increases with kinetic effect strengthening. The highly anisotropic energy contributes to weakening the sensitivity of the spiral spacing to the kinetic effect.
Strain field tuning the electronic and magnetic properties of semihydrogenated two-bilayer GaN nanosheets
Authors: Xiao Mei-Xia  , Liang You-Ping  , Chen Yu-Qin  , Liu-Meng  ,   College of Materials Science and Engineering, Xi'an Shiyou University, Xi'an 710065, China
Published: December 2020

In this paper, first-principles calculations based on the density functional theory, are performed to investigate the effects of strain field on the electronic and magnetic properties of two-bilayer gallium nitride (GaN) nanosheets. The two-bilayer GaN nanosheet without surface modification forms a planar graphitic structure, whereas that with full hydrogenation for the surface Ga and N atoms adopts the energetically more favorable wurtzite structure. Surface hydrogenation is proven to be an effective way to induce a transition from indirect to direct band gap. The bare and fully-hydrogenated GaN nanosheets are nonmagnetic semiconductors. When only one-side Ga or N atoms on the surface are hydrogenated, the semihydrogenated two-bilayer GaN nanosheets will preserve their initial wurtzite structures. The two-bilayer GaN nanosheet with one-side N atoms hydrogenated transforms into a nonmagnetic metal, while that with one-side Ga atoms hydrogenated (H-GaN) is a ferromagnetic semiconductor with band gaps of 3.99 and 0.06 eV in the spin-up and spin-down states, respectively. We find that the two-bilayer H-GaN nanosheets will maintain ferromagnetic states under a strain field and the band gaps Eg in spin-up and spin-down states are a function of strain . As the tensile strain is +6%, the band gap in spin-up state reduces to 2.71 eV, and that in spin-down state increases to 0.41 eV for the two-bilayer H-GaN nanosheets. Under the compressive strain field, the two-bilayer H-GaN nanosheets will show a transition from semiconducting to half-metallici state under compression of -1%, where the spin-up state remains as a band gap insulator with band gap of 4.16 eV and the spin-down state is metallic. Then the two-bilayer H-GaN nanosheets will turn into fully-metallic properties with bands crossing the Fermi level in the spin-up and spin-down states under a compressive strain of -6%. Moreover, the value of binding energy Eb for the two-bilayer H-GaN nanosheet decreases (increases) monotonically with increasing compressive (tensile) strain. It is found that although hydrogenation on one-side Ga atoms of the two-bilayer H-GaN nanosheets is preferred to be under compressive strain, the two-bilayer H-GaN nanosheets are still the energetically favorable structures. The physical mechanisms of strain field tuning band gaps in the spin-up and spin-down states for the two-bilayer H-GaN nanosheets are mainly induced by the combined effects of through-bond and p-p direct interactions. Our results demonstrate that the predicted diverse and tunable electronic and magnetic properties may lead to the potential application of GaN nanosheets in novel electronic and spintronic nanodevices.
A circular polarization antenna designed based on the polarization conversion metasurface
Authors: Li Wen-Hui  , Zhang Jie-Qiu  , Qu Shao-Bo  , Shen Yang  , Yu Ji-Bao  , Fan Ya  , Zhang An-Xue  ,   College of Science, Air Force Engineering University, Xi'an 710051, China;,   School of Information and Communication Engineering, Xi'an Jiaotong University, Xi'an 710049, China
Published: December 2020

A novel circular porlarization (CP) antenna is designed and fabricated by loading a kind of polarization conversion metasurface. The structure of polarization conversion metasurface is composed of metal slash and copper ground sheet, which is separated by an FR4 dielectric substrates with a thickness h=3 mm. When a normal electromagnetic wave incident on the whole surface vertically, the electric field of the wave can be decomposed into two components Evi and Evi. Under the excitation of the two components, resonance is induced between the metal slash and the copper ground sheet respectively, making their own reflection phase changed to = u - v = 180, eventually making the reflection wave appear with 90 polarization rotation. By taking the peculiarity of 90polarization rotation of the polarization conversion metasurface, we can modulate the linear polarization of microstrip slot antenna into the circular polarization radiation. Through adjusting the distance between the slot antenna and the polarization rotation metasurface, we can regulate the working frequency of the circular polarization. Simulated and experimental results show that the polarization conversion metasurface makes a high-efficiency polarization rotation at 8-12 GHz. And the center working frequency of the CP antenna is 9.1 GHz, the impedance bandwidth is 8.3-10 GHz. When the distance H = 4.5 mm between the microstrip slot antenna and the polarization conversion metasurface, the 3 dB axial ratio bandwidth is 8.3-8.8 GHz, the microstrip slot antenna may realize circular polarized radiation. When H = 20 mm, the 3 dB axial ratio bandwidth is 8.8-9.3 GHz, and the slot antenna can realize circular polarized radiation. When H = 8 mm, the 3 dB axial ratio bandwidth is 9.3-10 GHz, the slot antenna can realize circular polarized radiation. Experimental results are in agreement with the simulation results, showing the validity of this design method which may become a new approach for the CP antennain designing
Tapered structure based all-fiber probe for endoscopic optical coherence tomography
Authors: Yan Xue-Guo  , Shen Yi  , Pan Cong  , Li Peng  , Ding Zhi-Hua  ,   State Key Laboratory of Modern Optical Instrumentation, College of Optical Science and Engineering, Institute of Laser Biomedicine, Zhejiang University, Hangzhou 310027, China
Published: December 2020

A tapered structure based all-fiber endoscopic probe for endoscopic optical coherence tomography (OCT) is presented in this paper. The designation and fabrication of a miniaturized high-performance probe are critical in endoscopic OCT. Compared to the conventional fiber-lens structure based endoscopic probe, the all-fiber probe has a prominent edge in size and flexibility. Due to its lower beam-divergence, the large core multi-mode fiber makes a better fit than a general single mode fiber does when utilized to replace the micro lens in a conventional endoscopic probe as the imaging component. Furthermore, a tapered fiber is introduced as a transition section between the single mode fiber and the large core multi-mode fiber in order to enhance the light transmission efficiency and reduce the rigid length of the probe simultaneously. First, in order to obtain an optimal performance, optical simulation software(Rsoft) is adopted to determine the probe's proper lengths of the tapered section and the large core multi-mode fiber. Second, the all-fiber structure based endoscopic probe is fabricated by means of large core multi-mode fiber tapering, cutting and fusing processes. The beam characterization and insertion loss of the fabricated probe are measured experimentally The probe itself is 250 m, and after covering with a stainless steel protective tube, its outer diameter becomes 325 m. The rigid length of the probe is about 1 cm, which is more flexible and easier for inserting into curved blood vessels. The insertion loss of the probe is measured to be about 0.3 dB. To the best of our knowledge, it is the lowest among all of the all-fiber endoscopic probes. Finally, the probe is integrated with a custom-built swept-source optical coherence tomography system. Imaging of human fingertip and ex-vivo chicken trachea is conducted to demonstrate the key performance parameters of our probe. The effective imaging range of the probe is up to 800 microns in air without the help of any extra mechanism to expand its depth of focus. The probe offers a compact, efficient and flexible candidate for endoscopic optical coherence tomography, which is promising in cardiovascular investigations.