, , Received Date: 2020-03-22
This paper investigates an X band high efficiency klystron-like relativistic backward wave oscillator (RBWO) in detail. The klystron-like RBWO consists of a pre-modulation cavity, a resonant reflector with a ridge, a sectional slow wave structure, and an extraction cavity. First, this paper gives some theoretical studies about beam modulation and energy extraction. For beam modulation, the optimized distance between the pre-modulation cavity and the resonant reflector is studied theoretically, and theoretical results agree well with simulation results. For energy extraction, an ellipse extraction cavity with high power capacity is come up with, and the electric field on the inner surface of the ellipse extraction cavity decreases by 25% in PIC simulation. Also, the paper analyzes the effect of the position of dumped electron on conversion efficiency. Interestingly, it’s found that the efficiency dramatically decreases with the increase of the distance between the extraction cavity and the position of dumped electron, which is caused by the increase of potential energy of electron and the decrease of electric field. Fortunately, we find that the use of guiding magnet with special magnetic field distribution almost eliminate this unfavorable effect. Besides, effects of the distance between the cathode and anode Lak are investigated. It’s shown that the optimized diode voltage decrease with the increase of the distance Lak, and the conversion efficiency is higher at larger Lak. The experimental studies are also given. The power capacity of ellipse extraction cavity is verified, also we find that the efficiency is enhanced by 10% and the width of microwave pulse increases by 7 ns when the roughness of RF structure surface is improved from Ra 0.4 μm to Ra 0.05 μm. Typically, the klystron-like RBWO outputs X band high power microwave with power of 2.15 GW, with pulse duration of 25 ns, and with conversion efficiency of 50%(± 5%). Experimental results agree well with theoretical and PIC simulation results.
Preparation and characteristics of ultra-wide Ga2O3 nanoribbons up to millimeter-long level without catalyst
, , Received Date: 2020-04-01
, , Received Date: 2020-03-25
]. The ratio of source materials- and time-depented growth mechanism was also studied. It was suggested that the transformation of the cross section from triangle to square structure should be derived from the growth mechanism change from vapor-liquid-solid(VLS)process to vapor-solid(VS)process. The doped Mg increased the growth rate of the nanowires sidewalls, which led to a symmetrically growth of GaN nanowires along the twin boundaries. GaN nanowires gradually transformed to square structure by auto-catalytic growth. Moreover, the property of field emission were further investigated. The results showed that the turn-on electric field of square-shaped GaN nanowires was 5.2 V/m and a stable field emission property at high electric field. This research provides a new method for the preparation of square-shaped GaN nanowires and a prospective way for the design and fabrication of novel nano-scale devices.">GaN nanomaterials, as one of the most important third-generation semiconductor materials, have attracted wide attention. In this study, GaN nanowires with square cross section were successfully prepared by microwave plasma chemical vapor deposition system. The diameters of nanowires are from 300 to 500 nm and the lengths from 15 to 20 μm. The results show that the cross section of nanowires could be transformed from triangle into square by adjusting the ratio of Mg to Ga in source materials. X-ray diffraction(XRD)result indicate that the structure of GaN nanowires are agree with the hexagonal wurtzite. X-ray photoelectron spectroscopy (XPS) rusult show that a certain amount of Mg and O impurities incoporated in the square-shaped GaN nanowires. Transmission electron microscopy (TEM) result suggested that square-shaped GaN nanowires had high crystallinity with a growth direction of [ $0\bar 110$]. The ratio of source materials- and time-depented growth mechanism was also studied. It was suggested that the transformation of the cross section from triangle to square structure should be derived from the growth mechanism change from vapor-liquid-solid(VLS)process to vapor-solid(VS)process. The doped Mg increased the growth rate of the nanowires sidewalls, which led to a symmetrically growth of GaN nanowires along the twin boundaries. GaN nanowires gradually transformed to square structure by auto-catalytic growth. Moreover, the property of field emission were further investigated. The results showed that the turn-on electric field of square-shaped GaN nanowires was 5.2 V/m and a stable field emission property at high electric field. This research provides a new method for the preparation of square-shaped GaN nanowires and a prospective way for the design and fabrication of novel nano-scale devices.
Homemade Confined-doped Fiber with 3kW All-Fiber Laser Oscillating Output Fabricated by MCVD Process
, , Received Date: 2020-04-27
Ytterbium doped fiber lasers (YDFLs) with small volume, good beam quality, good heat dissipation performance and high conversion efficiency is widely used in industrial processing, military, medical and other fields. In past decades, with the development of high performance double cladding gain fiber and fiber devices, the output power of YDFLs has increased rapidly. However, nonlinear effects (NLEs), such as stimulated Brillouin scattering (SBS), stimulated Raman scattering (SRS), are produced which limits the output power of fiber laser further enhanced. Large mode area ytterbium-doped fiber (LMAYDF) can effectively increase the nonlinear effect threshold. However, increasing the core diameter will support more higher order mode (HOM), which may lead to beam quality deterioration and mode instability (MI) effect in fiber lasers. Thus, MI and NLEs have become the main limiting factors for the further improvement of output power and beam quality in fiber lasers. The confined-doped ytterbium-doped double-clad fiber (CDYDF), by reducing the doping diameter of gain ions in the fiber core, makes the fundamental mode (FM) dominate in mode competition and HOM suppressed to achieve LMAYDF gain control for different modes, thus improving the output power of the fiber laser and maintaining good beam quality. The 33/400 μm confined-doped ytterbium-doped double-clad fiber (CDYDF) was fabricated by modified chemical vapor deposition (MCVD) process with solution doping technology (SDT). The Yb3+ doping diameter ratio was 70 % and refractive index profile was close to step-index. Utilizing the master oscillator power amplifier (MOPA) system to verify the beam quality optimization effect of confined-doped fiber，it was optimized to 1.43 as the power increased while the M2 of seed laser was 1.53. An all-fiber structure counter-pumped fiber oscillator was constructed to test the laser performance of homemade confined-doped fiber. When the pump power was ~ 4.99 kW, laser power of 3.14 kW with central wavelength of 1081 nm and linewidth of 3.2 nm at 3 dB was obtained. Moreover, there was no MI and SRS in the whole experiment. We demonstrated that it was the highest output power based on homemade confined-doped fiber. The above results indicate that confined-doped fiber have the potential to achieve high-power and high-beam-quality fiber laser output.
Research on rotational dynamics characteristics of planar superimposed vortexes of exciton polariton condensates
, , Received Date: 2020-05-11
The gyroscope established on quantization vortexes formed from exciton-polariton Bose-Einstein condensate has important potential value on the field of quantum guidance. Thus, we assumed a concept of quantum gyroscope based on Sagnac Effect of the superposition states of quantum vortexes exist in exciton-polariton condensates. To research the gyroscopic effect of superimposed vortexes, which is the core issue of the project, it is essential to study the dynamic characteristics in the case of system rotating. Therefore, in this paper, the stability and dynamics of positive-negative vortex superposed states of two-dimensional exciton-polariton condensates in the disordered potential has been studied under the rotation of the semiconductor microcavity, laying a foundation for the study concerning the gyroscopic effect of the superposed state of exciton-polariton condensates in the semiconductor microcavity. On the basis of reconstructing the mono-component Gross-Pitaevskii equation under the rotational situation, a numerical model with Coriolis items has been constructed using the Runge-Kutta method and the finite difference time domain method, that can be capable to depict the rotation of the system. Moreover, the real-time evolution process of positive-negative vortex superposed states with different topological charges and the relationship between the number of steady-state local particles and the angular speed of the rotation of semiconductor microcavity have been researched using the real-time evolution method when the semiconductor microcavity is rotated. In the meantime, the relationship between the rotation speed in the excitation of vortex superposed states and the rotation speed of the semiconductor microcavity has been also studied with the presence of the influence of the rotation speed of the semiconductor microcavity on the phase stability of vortex superposed states. According to the study, the rotation speed of the semiconductor microcavity has a significant influence on the evolution process and dynamic characteristics of vortex superposed states of exciton-polariton condensates. The rotation of the exciton-polariton system will accelerate the evolution of superimposed vortexes, and overly rapid rotary rate will signalize the fluctuate of the local particle number thus system unstability occurs. Moreover, along with the system rotation, the exciton-polariton superimposed vortexes begin to rotate when the evolution approaches saturation. Noticeable, the angular acceleration of superimposed vortexes is positively associated with the system rotary rate. Further, the topological charge has a significant influence on the rotation of superposition states of vortexes that it rotates more slowly when the topological charge increases but lower evolution stability simultaneously. These conclusions
Numerical solution of three-dimensional time-dependent Schrodinger equation based on graphic processing unit acceleration
, , Received Date: 2020-05-11
In the field of quantum mechanics, the theoretical study of the interaction between intense laser field and atoms and molecules depends very much on the numerical solution of the time-dependent Schrodinger equation. however, solving the three-dimensional time-dependent Schrodinger equation is not a simple task, and the analytical solution can not be obtained, so it can only be solved numerically with the help of computer. In order to shorten the computing time and get the results faster, it is necessary to use parallel methods to accelerate computing. In this paper, under the background of strong field ionization, the three-dimensional time-dependent Schrodinger equation of hydrogen atom is solved in parallel, and the suprathreshold ionization of hydrogen atom under the action of linearly polarized infrared laser electric field is taken as an example. Based on the spherical polar coordinate system, the time-dependent Schrodinger equation is discretized by the splitting operator-Fourier transform method, and the photoelectron continuous state wave function under the length gauge can be obtained. In Graphics processing unit (GPU) accelerated applications, the sequential portion of the workload runs on central processing unit (CPU) (which is optimized for single-threaded performance), while the compute-intensive part of the application runs in parallel on thousands of GPU cores. GPU can make full use of the advantage of fine-grained parallelism based on multi-thread structure to realize parallel acceleration of the whole algorithm. Two accelerated computing modes of CPU parallel and GPU parallel are adopted, and their parallel acceleration performance is discussed. compared with the existing physical laws, the calculation error is also within an acceptable range, and the result is also consistent with the existing physical laws of suprathreshold ionization, which also verifies the correctness of the program. In order to obtain a relatively accurate acceleration ratio, many different experiments have been carried out in order to find the optimal acceleration ratio. Computational experiments show that under the condition of ensuring accuracy, GPU parallel computing has a maximum speedup of about 60 times based on the computational performance of CPU. It can be seen that the accelerated numerical solution of three-dimensional time-dependent Schrodinger equation based on GPU can significantly shorten the computational time. This work has important guiding significance for the rapid solution of three-dimensional time-dependent Schrodinger equation by using GPU.
Improve the sensitivity of Silicon Vacancy Spin-based sensors based on the Double Spiral Coil RF Resonance Structure
, , Received Date: 2020-05-20
Aiming at the problem of non-uniform broadening of the RF field of silicon vacancy spin magnetic resonance signals, this paper proposes and designs a double-spiral coil RF resonance structure, which uses the parallel symmetry of the double-spiral coil to construct a uniform region of the RF field. Compared with a single linear structure, the uniformity is improved by 56.889 times. At the same time, the use of RF signal close-distance mutual inductance coupling resonance characteristics, to achieve the enhancement of the RF field, compared to the single coil structure enhanced by 1.587 times, the equivalent spin sensor sensitivity increased by 4.833 times. In the experiment, a light detection magnetic resonance sensor measurement system based on the SiC silicon color center spin magnetic resonance effect was built. By comparing different types of RF antennas, the silicon vacancy color center spin magnetic resonance based on the double spiral coil RF resonance antenna structure was tested. The signal contrast is increased by a maximum of 6 times, the sensitivity of the sensor obtained by the modulation and demodulation information calculation method is increased by 4.833 times, the sensor noise is reduced by 8 times, and the measurement sensitivity of the silicon vacancy spin sensor is improved, combined with the SiC wafer chip manufacturing Technology provides technical support for the manufacture of high-precision, chip-scale spin quantum sensor
Research on selection of whispering-gallery modes and Fano resonance of prolate microbottle resonators
, , Received Date: 2020-05-31
Optical microresonators supporting whispering-gallery modes have been intensively studied in past decades due to their practical applications ranging from fundamental science to engineering physics. Among such microresonators, microsphere resonators have been demonstrated ultra-high quality (Q) factor, however, their shapes usually becomes non-standard spherical body, leading to irregular resonant spectra. Microring resonators have unique potential in integraibility on chip, but the fabrication imperfections limit their Q-factor only to 106. In addition, the free spectral range (FSR) just deponds on their radius. Due to the advantages of high Q-factor, standard shape, slender mode field distribution, microbottle resonators have been demonstrated excellent performance in cavity quantum dynamics, nonlinear optics, high-sensitivity sensing, and micro-laser. In this paper, we theoretically and experimentally investigate a systematic study on the spectral characteristics of prolate microbottle resonators. Firstly, in theory, field distribution theory of the microbottle resonator was studied in detail based on Helmholtz equation. In experiment, prolate microbottle resonators were fabriated via arc discharge technology. Secondly, the radial modes and axial modes of the microbottles were efficiently excited with the help of a coupled tapered fiber waveguide. By adjusting the coupling gap between the microbottle and the waveguide, three cupling states concluding under-coupling, critically-coupling and over-coupling were transfomed. In our experiment, the whispering-gallery modes excited were identifiable and recognized. The resonant modes with an ultra-high Q-factor up to 1.78×10^8 was achieved. The characteristic of ultra-high Q-factor make the microbottle hold great potential in biochemical sensing, nonlinear optics, and micro-laser. The tuning stability was enhanced by keeping the waveguide in touch with the microbottle. We investigated selective excitation of whispering-gallery modes by adjusting different coupling points. As a result, clean spectra with robust coupling were observed. The stable device is suitable to improve sensing performance. Finally, Fano resonance effect was obtained by choosing the diameter of the tapered fiber waveguide. The results showed would has a great significance for an enhanced approach in sensing, nonlinear optics and cavity quantum dynamics.
, , Received Date: 2020-06-14
On the basis of laser scanning confocal microscopy, line-scanning fluorescence microscopy (LSFM) uses linear scanning instead of point scanning to improve the speed of image acquisition. It has the advantages of simple system structure, fast imaging speed, weak phototoxicity, and is more suitable for high-resolution and fast imaging of living thick samples. It is of great significance for the research in the fields of life science, biomedicine and other fields. However, the current LSFM technologies still faces many urgent problems in terms of system flexibility, imaging speed, resolution and optical sectioning capabilities. Therefore, in this paper a digital line-scanning fluorescence microscopy (DLSFM) based on digital micromirror device (DMD) is presented. In the illumination path, a high-speed spatial light modulator DMD is introduced to realize multi-line parallel scanning excitation, which simplify the optical system and improve the flexibility and scanning speed of the system; A DLSFM image reconstruction algorithm based on the standard deviation of fluorescence signal is proposed, which is combined with three-dimensional (3D) Landweber deconvolution algorithm to achieve 3D high-resolution optical slice image reconstruction. On this basis, the imaging experiments of fluorescent beads and standard samples of mouse kidney section were carried out by using DLSFM. The experimental results show that DLSFM has the ability of fast 3D high-resolution optical sectioning imaging.
, , Received Date: 2020-03-16
An acousto-optic switch scheme based on optical tamm state (OTS) is proposed. The acousto-optic switch’s structure is one-dimensional photonic crystal heterostructure, which is composed of three materials: silicon dioxide, gallium arsenide and tellurium dioxide. All three materials are acousto-optic materials, which can ensure the acousto-optic effect when the ultrasonic wave and the light wave are incident at the same time. Due to the acousto-optic effect, the refractive index and thickness of one-dimensional photonic crystal heterostructures are changed by ultrasonic. The acousto-optic switch changes the ultrasonic amplitude to shift the intrinsic wavelength of OTS to the shorter wave direction. With the increase of ultrasonic amplitude, the intrinsic wavelength of OTS hardly changes after the amplitude exceeds 0.4 nm. This means that the ultrasonic wave with an amplitude of 0.4 nm can shift the intrinsic wavelength to 1538 nm. The acousto-optic switch realizes the on-off function within the permitted range. In this paper, the theoretical model of the acousto-optic switch is established. The propagation of ultrasonic wave in one-dimensional photonic crystal heterostructure is analyzed by theoretical model. The propagation of light in the medium after acousto-optic effect is analyzed by transmission matrix method. The simulation is carried out through COMSOL Multiphysics software. The results show OTS exists and localization can be seen in the electric field diagram. The acousto-optic switch of 1548.8–1551.7 nm can be realized by applying certain amplitude of ultrasonic or not. In this wavelength range, the extinction ratio is not lower than 12 dB and the insertion loss is not higher than 0.97 dB. The maximum extinction ratio is 13.17 dB, and the minimum insertion loss is only 0.65 dB. The acousto-optic switch of 1536.6–1543.3 nm can be realized by applying ultrasonic wave with amplitude corresponding to the length of incident light. In this wavelength range, the extinction ratio is not lower than 12 dB, and the insertion loss is not higher than 0.99 dB. The maximum extinction ratio is 13.15 dB, and the minimum insertion loss is only 0.65 dB. The response time of the acousto-optic switch is less than 13 ns. The acousto-optic switch has the characteristics of high extinction ratio and low insertion loss. It has a good application prospect and can be effectively applied in future optical communication.
, , Received Date: 2020-03-20
We observe experimentally huge enhanced four-wave mixing based on Raman resonance in an 85Rb atomic vapor system. With the decrease of coupling field power or the increase of experimental temperature, the signal tends to be narrowed down in linewidth, and be basically the same spectrum as the stimulated Raman spectroscopy. It is found that the macroscopic polarization interference effect plays a crucial role in determining the nonlinear spectra. Further more, in the Doppler-broadened Λ-type energy level system, there is a strong relationship among electromagnetically induced transparency, four-wave mixing and stimulated Raman spectroscopy. The sharp transparent window in electromagnetically induced transparency can be interpreted as the suppression of Raman gain on the linear absorption of the probe field. However, the four-wave mixing signal is a new field generated by the atomic vapor system, and it comes from the Raman gain which is affected and modified by the absorption and dispersion of the nonlinear optical medium. It shows that in a Λ-type Doppler-broadened system, in essence, both the electromagnetically induced transparency and enhanced four-wave mixing stem from stimulated Raman scattering based on the third-order nonlinear processes, just the spectra of which are from different ways and objects of detection.
Study on two-photon induced ultrafast carrier dynamcis in Ge-doped GaN by transient absorption spectroscopy
, , Received Date: 2020-03-16
Gallium nitride (GaN) is a key material in blue light-emitting devices and is recognized as one of the most important semiconductors after Si. Its outstanding thermal conductivity, high saturation velocity, and high breakdown electric field have enabled the use of GaN for high-power and high-frequency devices. Although lots of researches have been done on the optical and optoelectrical properties of GaN, the defect-related ultrafast dynamics of the photo-excitation and the relaxation mechanism are still completely unclear at present, especially when the photo-generated carrier concentration is close to the defect density in n-type GaN. The transient absorption spectroscopy has become a powerful spectroscopic method, and the advantages of this method are contact-free, highly sensitive to free carriers, and femtosecond time resolved. In this article, by employing optical pump and infrared probe spectroscopy, we investigate the ultrafast photo-generated carriers dynamics in representative high-purity n-type and Ge-doped GaN (GaN: Ge) crystal. The transient absorption response increased as probe wavelengths increased, and hole-related absorption was superior to electron-related absorption, especially at 1050 nm. The transient absorption kinetics in GaN: Ge appeared to be double exponential decay under two-photon excitation. By modelling the carrier population dynamics in energy levels, which contained both radiative and non-radiative defect states, the carrier dynamics and carrier capture coefficients in GaN: Ge can be interpreted and determined unambiguously. The faster component (30–60 ps) of absorption decay kinetics corresponded to the capturing process of holes by negatively charged acceptor CN. However, the capturing process was limited by the recombination of electron and trapped holes under higher excitation after the saturation of deep acceptors. As a result, the slower component decayed slower as the excitation fluence increased. Moreover, the experimental and theoretical results found that, the carrier lifetime in n-GaN can be modulated by controlling the defect density and carrier concentration under a moderate carrier injection, making GaN applicable in different fields such as LED and optical communication.
, , Received Date: 2020-01-14
Based on the principle of micro-scale discharge, the micro-nano ionization gas sensor has the characteristics of fast response, high precision and easy integration. It is expected to achieve rapid and accurate detection of gas. At present, there is a lack of systematic analysis of the inter-polar discharge process of the new sensor. This paper uses the fluid-chemical dynamics methodology to create a 2D space discharge model of the N2-O2 mixed gas at the micron gap and the nano-tip field in ambient atmosphere at normal temperature and pressure. Meanwhile, by analyzing the mutual coupling between the space electron transport process, the discharge current density, and the space electric field strength, the paper clarifies the dynamics of space discharge in the field, improves how internal discharges work in such micro-nano structured ionization gas sensors, and analyzes the pattern of influence of different polar distances on space discharges. The results show that the electric field in the space remains constant as the production and consumption of positive and negative ions reaches a dynamic equilibrium in the field. It is reflected in the field strengthening effect of positive ion groups to the cathode plate and of negative ion groups to the anode plate, as well as in the field weakening effect between positive and negative ion groups. The resulting stable and strong electric field of the cathode makes sure that space discharge is maintained, and the discharge current density stabilizes. Initially, as the polar distance decreases gradually, the electric field strength between the poles and plates increases. It plays a leading role in the accumulation of electron energy and in the increase in the number density of electrons, thus leading to the increase of the output current density up to the peak value when the polar distance D = 50 μm. As the polar distance decreases, the field strength between the poles and plates increases. Despite that, when electrons accumulate energy up to such a level that gas molecules can be ionized, the necessary movement distance and the distance required to increase the number density of electrons decreases. As a result, the degree of ionization weakens, and the field strengthening effect of positive ions decreases. In other words, the increment of the field strength caused by positive ions at the tip decreases, and in turn, the discharge current density decreases. This pattern serves as a theoretical support in the optimization of the micro-nano structured ionization gas sensors.
, , Received Date: 2020-01-07
When the satellite is on orbit, the surrounding plasma environment will interact with the spacecraft surface, accumulate charges on the spacecraft surface and cause surface charging effect, which could lead to electrostatic discharge and affect the running of the spacecraft. SMILE is a satellite operating in a solar synchronous and high inclination large elliptical orbit. The on-orbit motion will encounter ionospheric plasma, magnetospheric plasma and solar wind plasma, pass through the region of the outer radiation belt enriched by high-energy electrons. These environmental factors can cause the surface charging effect on satellite and affect on-orbit security of the satellite and the acquisition of scientific data. Utilizing the software simulation of spacecraft plasma interaction system, the charging effects of SMILE satellite surface in solar wind plasma, magnetic tail plasma and extremely harsh plasma environment have been simulated, and the charging potential distribution on its surface have been obtained. The results show that the surface charging potential varies in different environments, but all comfort with the design requirements. The analysis of surface current shows that the secondary electron emission has great influence on surface charging in various plasma environments. Under sun illumination, photoelectron emission dominates surface charging. By analyzing the charge current on the surface on the eclipse, the calculated results can supply the experimental curve of the secondary electron emission coefficient of indium tin oxide materials.
Information security is the cornerstone and lifeblood of national security in the information society, and anonymous quantum communication is one of the important ways to protect information security. Using quantum walk randomness to effectively solve sensitive problems such as leakage of identity information. In this paper, an anonymous communication scheme based on quantum walks on the Cayley graph is proposed. First, both parties in the communication hide their identity information, and the sender Alice anonymously selects the receiver Bob through logic or operation. Secondly, the trusted third party and the communicating parties use the BB84 protocol to generate and distribute the security key. Alice encrypts the information sequence according to the security key to obtain the blind information; Bob uses the joint Bell state measurement and security key to sign and the trusted third party verifies the signature information. Third, the trusted third party calculates the position probability distribution function of Bob’s quantum walk via the Fourier transform, converts the position information corresponding to the maximum probability into a confirmation frame and sends it to Alice; Alice uses the quantum compression algorithm by decreasing dimensions to reduce the number of transmitted information bits(the length of the information bit can be reduced by up to 37.5%) and uses the security key to complete the information encryption and then transmit the information to the location indicated by the confirmation frame. Bob uses quantum walks to search the location node to obtain the transmission information and complete the anonymous quantum communication. Finally, the security analysis of the scheme is carried out, and the numerical simulation results of the Cayley graph of 200 nodes are given. At the 10-step walk, the maximal probability of the 6th node is 45.31%. According to the simulation results, the probability that Bob is eavesdropped on the specific location at his 10-step walk during the communication of this scheme is approximately 6 × 10-7%, so the receiver can avoid the identity information from the eavesdropping with a high probability, and the quantum network anonymity protocol is not broken.