bluuebunny/crossref_metadata_2025 huggingface dataset:
Experimental study of the effects of two-photon detuning on slow light and light memory
Authors: Yan Yan  , Li Shu-Jing  , Tian Long  , Wang Hai  ,   State Key Laboratory of Quantum Optics and Quantum Optics Devices, Institute of Opto-Electronics, Shanxi University, Taiyuan 030006, China
Published: December 2020

Electromagnetically induced transparency (EIT) effect is an effective means to store light field into the atom ensemble. The extra noise introduced in the stored procedure can be suppressed greatly under the condition of large one-photon detuning and proper two-photon detuning. In this paper, we experimentally investigate the slow light and light storage in 87Rb vapor by using EIT effect, and study the effects of the two-photon detuning on light pulse delay and light memory at 650 MHz one-photon red detuning. In order to avoid some unwanted effects under the high optical depth condition, such as four-wave mixing, etc., the temperature of the atomic cell is controlled at 65 degrees Celsius. The experimental results show that the delay and the retrieval signals are significant in a two-photon detuning range from 0 to 0.5 MHz. The pulse delay decreases with the increase of two-photon detuning. The delay is 0.36 ups at two-photon resonance, and it is 0.07 ups at 1 MHz two-photon detuning. We simulate the delayed light pulse by using a three-level -type EIT model. The shapes of the measured slow light are in agreement with the theoretical results. The retrieval signals are observed at different two-photon detunings. The shapes of the retrieval pulses change with the two-photon detuning. The shape variations of the retrieval pulses cannot be explained by the three-level EIT theoretical model. By considering the atomic Zeeman sublevels interacting with the left-circular and right-circular polarized components of probe and coupling fields, multiple -type EIT systems will be formed. The interference between the retrieval signals from multiple EIT subsystems causes the shape distortions of retrieval pulses. The retrieval efficiency is measured as a function of two-photon detuning. The retrieval efficiency oscillates, and multiple peaks appear with the increase of two-photon detuning. The first peak appears at two-photon resonance, and the second peak appears at 0.48 MHz two-photon detuning. Finally, we measure the retrieval efficiency as a function of the coupling power at 0.48 MHz two-photon detuning. The optimal retrieval efficiency reaches 25% when the coupling power is 100 mW. These results provide experimental reference for the quantum memory of continuous variables in the hot atom ensemble.
Investigation of detecting biological aerosol by passive Fourier transform infrared spectroscopy technology based on MODTRAN model
Authors: Feng Ming-Chun  , Xu Liang  , Liu Wen-Qing  , Liu Jian-Guo  , Gao Min-Guang  , Wei Xiu-Li  ,   Key Lab of Anhui Province for Environmental Optical Monitoring Technology, Anhui Institute of Optics and Fine Mechanics, Chinese Academy of Sciences, Hefei 230031, China
Published: December 2020

The problem of detecting the biological aerosol at low elevation angle is analyzed and discussed by using MODTRAN model. First of all, the atmospheric model and profile of MODTRAN model are introduced for the biological aerosol detection by Fourier transform infrared (FTIR) spectroscopy. According to the passive detection of biological aerosol by FTIR spectroscopy, the radiation brightness difference L between the background and the target biological aerosol is calculated by using the radiative transfer theory and the simplest three-layer model. The signal value 2Lt under the actual circumstance is derived from the derivative of L combined with the noise equivalent spectral radiance value of the spectrometer. Finally, the detection limit of biological aerosol for each atmospheric mode is predicted with the passive remote sensing method. The limit concentration of detection for each atmospheric mode is different due to the differences in boundary layer temperature, transmittance, and background radiation brightness of atmospheric model, and it is also related to the absorption coefficient of biological aerosol. It is shown that the passive remote sensing of FTIR technology can detect the presence of the biological aerosol. Therefore, the detection of the biological aerosol is feasible. It presents a method of detecting the biological aerosol cloud under the actual circumstance.
An improved Mach-Zehnder acousto-optic modulator on a silicon-on-insulator platform
Authors: Qin Chen  , Yu Hui  , Ye Qiao-Bo  , Wei Huan  , Jiang Xiao-Qing  ,   Integrated Optoelectronics Laboratory, College of Information Science and Electronic Engineering, Zhejiang University, Hangzhou 310027, China
Published: December 2020

The interdigital transducer (IDT) of the traditional Mach-Zehnder (MZ) acousto-optic modulator on a silicon-on-insulator (SOI) platform is located outside its two arms. The crest and trough of the surface acoustic wave (SAW) are used to modulate the two arms of the MZ interferometer so as to achieve a high modulation efficiency. Therefore, the distance between the two arms must be odd multiples of half acoustic wavelength. However, since the substrate is usually not uniform, the wavelength of the SAW changes as it transmits through the surface of the device. As a result, the exact distance between the two arms is difficult to choose. On the other hand, the SAW losses a portion of energy after passing through the first arm of the MZ interferometer, so the modulation of the second arm becomes much weaker. To solve these problems, we propose a new structure where its IDT is situated in the middle of the two arms of the MZ interferometer. With this scheme, the two arms of the MZ interferometer are located exactly at the crest and the trough of the SAW, while they are modulated with equal strength. In this paper, we first use the finite element method to simulate the acoustic frequency and the surface displacement of the excited SAW. Then we deduce the refractive index variations of all layers according to their acousto-optic effects. After that, we analyze the influences of different factors on the acousto-optic modulation efficiency, including the type and size of waveguide, the thickness of zinc oxide (ZnO) layer, and the area it covers, the number of electrodes, etc. These parameters are accordingly optimized to enhance the modulation efficiency. Modeling result based on COMSOL Multiphysics indicates that when the width of the strip waveguide is 6 m, the ZnO layer covers only the area under the IDT and has a thickness of 2.2 m, and the number of the electrodes is 50, the effective refractive index variation of the waveguide reaches 4.0810-4 provided that the amplitude of the driving voltage is 1 V. This value is 12% higher than that of the traditional structure.
A coupled lattice Boltzmann model for fluid flow and diffusion in a porous medium
Authors: Liu Gao-Jie  , Guo Zhao-Li  , Shi Bao-Chang  ,   Key Laboratory of Coal Gasification and Energy Chemical Engineering of Ministry of Education, East China University of Science and Technology, Shanghai 200237, China;,   State Key Laboratory of Coal Combustion, Huazhong University of Science and Technology, Wuhan 430074, China
Published: December 2020

The flow and diffusion of miscible fluid in a porous medium with a high Plcet number (Pe) and large viscosity ratio widely exist in industrial processes, such as oil recovery, geological sequestration of carbon dioxide, and chemical engineering process. When these problems are studied by numerical methods, the key point is to accurately describe the flow dynamics and diffusion process in a porous medium at the same time. As an alternative to conventional numerical methods, the lattice Boltzmann method based on kinetic theory is well suited to pore-scale simulations of miscible fluid flows and molecular diffusion. However, most of the existing lattice Boltzmann models have many difficulties (e.g. robustness and numerical stability) in simulating such systems at high Pe and large viscosity ratio. In this paper, in order to overcome the above difficulties, we propose a coupled lattice Boltzmann model based on the multiple-relaxation-time model and the lattice kinetic scheme for the fluid flow and diffusion, respectively. It can be shown that the incompressible Navier-Stokes equations and the convection-diffusion equation can be derived from the presented coupled model through the Chapman-Enskog procedure. The proposed model is validated by simulating a concentration gradient driven flow in a porous channel. Numerical results demonstrate that the model is of second-order accuracy in space. We further simulate a flow through two types of artificial porous media. The robustness of the presented model is investigated by measuring the permeability and diffusivity under different relaxation times. It is found that the model is insensitive to relaxation parameters. In addition, the miscible viscous displacement in two parallel plates is simulated to test the numerical stability of the model. It is observed that the results accord well with those reported in previous work, and the model is very stable at high Pe and large viscosity ratio in comparison with the standard lattice Bhatnagar-Gross-Krook model. Overall, the coupled lattice Boltzmann model can serve as an effective tool for directly simulating the fluid flow and diffusion at high Pe and large viscosity ratio in the pores of a porous medium.
An experimental study of water-entry cavitating flows of an end-closed cylindrical shell based on the high-speed imaging technology
Authors: Lu Zhong-Lei  , Wei Ying-Jie  , Wang Cong  , Sun Zhao  ,   School of Astronautics, Harbin Institute of Technology, Harbin 150001, China
Published: December 2020

The objective of this present study is to address the cavitating flow patterns and regimes in the water-entry cavity. For this purpose, an experimental study of vertical water-entry cavity of an end-closed cylindrical shell is investigated by using high-speed video cameras and visualization technique. According to the cavitating flows as observed in the experiments, two flow pattern forms of fluctuation cavitation and cloud cavitation are found around the body. A further insight into the characteristics of the cavity shape and the variation in the cavity fluctuations parameters is gained by analyzing the image data. Furthermore, the experiments at different impact velocities are conducted to analyze the effects of impact velocity on the flow patterns and parameters. Finally, the formation mechanisms of cavitation fluctuations and cavitation clouds are studied based on the basic theory of fluid mechanics. The obtained results show that the cavitation flow pattern form of fluctuation cavitation occurs under the impact velocity condition of low speed, and the cloud cavitation occurs under the velocity condition of high speed. As fluctuation cavitation, the maximal extension diameters of cavitation fluctuate periodically along the water depth, and the speeds of extension and shrinkage are both proportional to the extension diameter. The collapses are different for the two flow pattern cavitations, i.e., the fluctuation cavitation, which is of deep closure and closed at the trough of wave cavitation more than once, and the cloud cavitation, which falls off and forms the leading edge of the cylindrical shell. The frequency fluctuation is independent of the impact velocity, the corresponding pinch-off time decreases with increasing the impact velocity, and the pinch-off time decreases in a nearly linear relation with Froude number. The water poured to the cylindrical shell causes the internal air to compress and expand, and as a consequence of these effects, periodic disturbances of pressure distribution and velocity field occur around the leading edge of the cylindrical shell, then the extended intensity of the cross section of the cavity shows variation in this process, which can be defined as fluctuation cavitation pattern. It appears that the re-entrant flow after the pinch-off at the trailing edge of cavity, then the laminar-turbulent transition is waken as a consequence of the re-entrant flow moving upstream, which flow pattern involved in this structure occurs as cloud cavitation.
A novel method based on the fuzzy C-means clustering to calculate the maximal Lyapunov exponent from small data
Authors: Zhou Shuang  , Feng Yong  , Wu Wen-Yuan  , Wang Wei-Hua  ,   Chongqing Key Laboratory of Automated Reasoning and Cognition, Chongqing Institute of Green and Intelligent Technology, Chinese Academy of Sciences, Chongqing 400714, China;,   University of Chinese Academy of Sciences, Beijing 100049, China
Published: December 2020

In order to reduce errors caused by human factors to identify the linear region, we propose a new method based on the fuzzy C-means clustering for calculating the maximum Lyapunov exponent from small data. The method based on the changing characteristic of divergence index curve is used to identify the linear region. Firstly, the divergence index data are calculated from the small data algorithm for the given chaotic time series. Secondly, the fuzzy C-means clustering method is used for dividing the data into two classes (unsaturated and saturated data), and the unsaturated data are retained. Thirdly, the retained data are divided by the same clustering method into three classes (positive fluctuation data, zero fluctuation data and negative fluctuation data), and the zero fluctuation data are retained. Fourthly, the 3$ criterion is used for excluding gross errors to retain the valid from the selected data. Finally, the regression analysis and statistical test are used to identify the linear region from the valid data. The effectiveness of the proposed method can be demonstrated by the famous chaotic systems of Logistic and Henon. The calculated results are closr to the theoretical values than the subjective method. Experimental results show that the proposed new approach is easier to operate, more efficient and more accurate as compared with the subjective recognition. But this method has its own shortcomings. (1) As the new method is verified by the simulation experiment, there exists no strict mathematical proof. (2) Since the difference algorithm is used in this new method, it will miss some detailed information in some cases. (3) The calculation accuracy still needs to be improved, so this method only serves as a reference to detect the linear region, it can not be applied to high precision engineering field. Considering the deficiencies of the new method, we will make further research to improve the calculation method for maximum Lyapunovexponent, so as to make it solve the real-time problem of the signal detection, and find the accurate location of abrupt climate change in the field of meteorology, to provide accurate satellite launch safety period in the field of space weather and other aspects. In short, studying the largest Lyapunov exponent from chaotic time series has a wide application prospect and practical significance.
Absolute distance ranging by means of chirped pulse interferometry
Authors: Liu Ting-Yang  , Zhang Fu-Min  , Wu Han-Zhong  , Li Jian-Shuang  , Shi Yong-Qiang  , Qu Xing-Hua  ,   State Key Laboratory of Precision Measurement Technology and Instruments, Tianjin University, Tianjin 300072, China;,   National Institute of Metrology, Beijing 100013, China;,   Beijing Institute of Control and Engineering, Beijing 100190, China
Published: December 2020

Large-range and high-accuracy absolute distance measurement plays an important role in many practical applications, such as industrial production, aerospace and scientific research, etc. In this paper, a method is proposed for absolute distance measurement by chirped pulse interferometry based on the femtosecond optical frequency comb. The interference spectra obtained in experiments are analyzed by means of the principle of the dispersive interference, and the distance can be determined by the shift of the widest fringe in the interference spectra. An absolute distance measurement system can be set up based on the modified Michelson interferometer with a pair of gratings to chirp the reference pulses in the reference arm. Experimental results are in agreement well within 33 m in a range up to 65 m, i.e. a relative precision of 5.110-7. In addition, the optimization of the measurement uncertainty is theoretically and experimentally performed.
Research on the temperature sensing characteristics of triple cladding quartz specialty fiber based on cladding mode resonance
Authors: Fu Xing-Hu  , Xie Hai-Yang  , Yang Chuan-Qing  , Zhang Shun-Yang  , Fu Guang-Wei  , Bi Wei-Hong  ,   Key Laboratory for Special Fiber and Fiber Sensor of Hebei Province, School of Information Science and Engineering, Yanshan University, Qinhuangdao 066004, China
Published: December 2020

A triple-cladding quartz specialty fiber (TCQSF) temperature sensor based on cladding mode resonance is made. The sensor is fabricated by just splicing a short, few-centimeter-long segment of TCQSF between two standard single-mode fibers (SMFs), so the sensor structure is simple. In order to explain its sensing principle in detail, we assume that the TCQSF is equivalent to three coaxial waveguides based on coupling mode theory. Utilizing the scalar method and the relationship between Bessel function and mode field distribution of step-index circular symmetry waveguide, the mode field distribution of these waveguides and their characteristic equation can be easily obtained. Then the dispersion curves of each mode which is transmitted in the three waveguides can be calculated. The intersection between the fundamental core mode LP01(rod) in the rod waveguide and the cladding mode LP01(tube) in the tube waveguide I indicates that the two modes have the same propagation constant, and satisfy the phase-matching condition when the wavelength is 1563.7 nm which is known the resonant wavelength. And only when the sensor length is equal to the beatlength, can the light be coupled completely from the core to the fluorine-doped silica cladding. Thus, the cladding mode resonance phenomenon occurs and a band-stop filter spectrum will be obtained. Then the sensor is applied to the simulation calculation of the temperature sensing characteristics. With increasing temperature, both the refractive index of each layer and the sizes of the axial and radial fibers will change, which will finally lead to a big difference on the dispersion curves of LP01(rod) and LP01(tube). Therefore, the resonant wavelength shift of the sensor can be obtained by just calculating the dispersion curves of these two modes at different temperatures, and the scope of curvature sensitivity is 70.76-97.36 pm/℃. Finally, a straight forward experiment is performed to prove the temperature sensing properties. Experimental results show that the sensor has a sensitivity in temperature of 73.74 pm/℃ at 35 ℃-95 ℃, which is completely consistent with the theoreticaly calculatied results. Thus, the proposed sensor has the advantages of simple structure, easy fabrication, highly sensitivity, controlled cladding mode excitation, and so on. It can be used in industrial production, biomedical and other temperature sensing areas.
Vanadium K-shell X-ray emission induced by xenon ions at near the Bohr velocity
Authors: Zhou Xian-Ming  , Zhao Yong-Tao  , Cheng Rui  , Lei Yu  , Wang Yu-Yu  , Ren Jie-Ru  , Liu Shi-Dong  , Mei Ce-Xiang  , Chen Xi-Meng  , Xiao Guo-Qing  ,   Institute of Modern Physics, Chinese Academy of Sciences, Lanzhou 730000, China;,   School of Physical Science and Technology, Lanzhou University, Lanzhou 730000, China;,   University of Chinese Academy of Sciences, Beijing 100049, China;,   School of Science, Xi'an Jiaotong University, Xi'an 710049, China
Published: December 2020

The X-ray emission spectra produced by 2.4-6.0 MeV Xe20+ ions impacting on vanadium surface were measured. The V K-shell X-ray production cross sections were extracted from the experimental yield data and compared with the theoretical predictions of the binary encounter approximation (BEA), the plane wave born approximation (PWBA), and the energy-loss coulomb-repulsion perturbed-stationary-state relativist (ECPSSR). In order to predict reasonably the inner-shell ionization induced by highly charged heavy ions during the asymmetric collisions at near the Bohr velocity, the corrections of BEA model are discussed. It is found that the X-ray production cross section induced by highly charged heavy ions moving at near the Bohr velocity is on the magnitude of 1 barn, which is almost four orders of magnitude larger than that induced by proton. The ECPSSR, which is regarded as the best model to simulate the inner-shell ionization by light ions, may underestimate the experimental data at least three orders of magnitude. The PWBA model presents a prediction to the results on an order of magnitude better than the ECPSSR simulation, but gives a worse tendency than the BEA model. The BEA calculations, corrected both by Coulomb repulsion and effective nuclear charge, present the best agreement with the experimental results. It is proposed, that in the energy region near the Bohr velocity, during the asymmetric collisions of Xe20+ ions with V atoms, the K-shell electron of V is ionized by direct ionization, and that it can be described by the binary encounter process between the xenon ions and the bound electrons. The X-ray production cross section can be simulated by BEA model, but the corrections of Coulomb repulsion and effective nuclear charge must be considered.
Vibrational density of states and boson peak in two-dimensional frictional granular assemblies
Authors: Niu Xiao-Na  , Zhang Guo-Hua  , Sun Qi-Cheng  , Zhao Xue-Dan  , Dong Yuan-Xiang  ,   Department of Physics, University of Science and Technology Beijing, Beijing 100083, China;,   State Key Laboratory for Hydroscience and Engineering, Tsinghua University, Beijing 100084, China
Published: December 2020

In this paper, the two-dimensional granular assemblies composed of 2048 mono-dispersed frictional disks are simulated by the discrete element method. A set of eigenvalues and corresponding eigenvectors is obtained by diagonalizing the Hessian matrix for each stable configuration. The effects of the friction coefficient of disk on mechanical and geometrical properties of these systems under isotropic confining are studied. Results show that at a fixed pressure, with increasing from 0.001 to 1.0, the crossover frequency *, which separates the Debye scale region from the platform of vibrational density of states, and the boson peak BP gradually shift towards lower frequency, and the intensity of the boson peak D(BP) / BP increases. These results are mainly attributed to the fact that the system becomes more and more disordered with the increase of (i.e., the decrease of the average coordination number), resulting in more excess modes at *. For a better understanding of the different vibration modes of the two-dimensional frictional granular systems, we plot the polarization vector diagrams for different frequencies ( 1 = 0.15, 2 = 1.5 and 3 = 6.0) for configurations with = 0.001 and = 1.0, respectively. Mode analysis results show that the mode at low ( 1.0) has a mixed translational-rotational but translational-dominated character; the mode at intermediate frequency (1.0 4.0) is localized and has a mixed translational-rotational but translational-dominated character; and the mode at high frequency ( 4.0) have a strongly rotational in character. It is worth noting that the low-frequency modes become more localized and the rotational participation fraction also increases as increases, implying that the rotational modes play more important role in the system with higher friction coefficient.
Method for detecting high-speed rail surface defects by photoacoustic signal
Authors: Sun Ming-Jian  , Cheng Xing-Zhen  , Wang Yan  , Zhang Xin  , Shen Yi  , Feng Nai-Zhang  ,   Harbin Institute of Technology, School of Astronautics, Harbin 150001, China;,   Harbin Institute of Technology at Weihai, School of Information and Electrical Engineering, Weihai 264200, China
Published: December 2020

Railway plays a major role in our daily life and national economy. In recent years, people payed much more attention to the safety operation of the high-speed train. In fact, the rail cracks originate from surface micro cracks will directly affect the safety of high-speed train. Therefore, it is vital to detect the rail surface micro cracks.
Numerous nondestructive testing methods have been developed and applied in the detection of high speed rail cracks, such as magnetic particle testing, eddy current testing, and ultrasonic testing, etc. However, all the above conventional methods could only achieve crack information from the point of one-dimensional signal but not effective for the detection of surface micro cracks. A surface defect detection method based on photoacoustic (PA) signal from high speed rail is proposed soas to detect the surface crack more exactly and visually.
Simulation and experiments are designed to validate the proposed method. Firstly, three models of high-speed rail with transverse crack, oblique crack, and scale stripping are established respectively. Meanwhile, the PA effect is simulated by finite element analysis and K-wave. Then, PA image of the rail surface is reconstructed by time inversion reconstruction algorithm, and some parameters, such as the center frequency of ultrasonic sensor and the laser power are also confirmed in further simulation. Subsequently, an experimental platform is established to collect the actual PA signal from a rail surface and to reconstruct PA images of the rail surface and shallow layer. The crack appearing in PA images are clear enough to show the receive crack information, such as sizes, propagating directions, and locations, which can be used to evaluate the rail states and decide processing scheme.
It is proved that clear images of rail surface and shallow layer can be received by the detecting method of high-speed rail surface defects based on photoacoustic signal, and the surface cracks can be detected effectively.
Coherence length and magnetic penetration depth of the s-wave holographic superconductor model in Lifshitz spacetime
Authors: Yang Zhuo-Qun  , Wu Ya-Bo  , Lu Jun-Wang  , Zhang Cheng-Yuan  , Zhang Xue  ,   School of Physics and Electronic Technology, Liaoning Normal University, Dalian 116029, China;,   Department of Physics, Qiannan Normal College for Nationalities, Duyun 558000, China
Published: December 2020

The AdS/CFT duality provides us a powerful guidance to study the strong-coupled conformal field theory by using its dual weak-coupled gravity. One of the interesting applications of the duality is to study high temperature superconductors, which are supposed to be a strongly coupled system. According to Ginzburg-Landau theory, a superconductor can be characterized by only two parameters, coherence length and the magnetic penetration length ; therefore, it is important to determine the two parameters. In this paper in the D=d+2-dimensional Lifshitz black hole, we analytically study the static fluctuation of the scalar field with nonzero spatial momentum along one spatial coordinate of the boundary, and investigate the perturbation of the gravitational system near the critical temperature Tc. Working in the probe limit (the gauge field and scalar field do not backreact on the original metric), we obtain the superconducting coherence length via AdS/CFT (anti-de Sitter/conformal field theory) correspondence, which is (1/Tc)(1-(T/Tc)-1/2. Moreover, in the probe limit (the magnetic field does not backreact to the background spacetime), we have calculated the diamagnetic current induced by a homogeneous external magnetic field perpendicular to the surface of the superconductor. Then, we obtain the magnetic penetration depth (Tc-T)-1/2, which agrees with the result in Ginzburg-Landau theory. And these results strongly support the idea that a superconductor can be described by a charged scalar field on the Lifshitz black hole via AdS/CFT (anti-de Sitter/conformal field theory) duality.
Different time regularization of the Breit quark potential and the mass splittings of c-J/ and other mesons
Authors: Jirimutu  , Aodeng  , Bao tmurbagan  ,   College of Computer and Information, Inner Mongolia Medical University, Hohehot 010110, China;,   College of Physics and Electronic Information, Inner Mongolia University for the Nationalities, Tongliao 028043, China
Published: December 2020

The study on the mass splittings of the mesons with the same structure but different spin-and orbit-quantum numbers is one of the important methods for checking the efficiency of potential models. In previous calculations for quark potential models, the splitting between - is easily obtained while that of the c-J/ is however too small to meet the experimental results. In this paper, the third term of the complete Breit quark potential in the momentum space is regularized twice by applying the form factor 2/(q2+2), and the other terms except the first term of the Coulombic potential and the seventh term of the constant potential are regularized once. The mass splittings are calculated by using these values. Our results indicate that the mass splittings of light mesons -, heavy mesons c-J/, b-(1s), and c0-c1-c2 can meet the experimental results with high accuracy only when the screen mass is expanded to the third-order polynomial with respect to the meson reduced mass r=mr mj/(mr+mj), while the masses of other mesons are improved greatly. An efficient quark potential model is thus described in this paper.
One-dimensional far-detuned optical lattice realized with a multimode 1064 nm laser
Authors: Yuan Yuan  , Lu Xiao-Gang  , Bai Jin-Hai  , Li Jian-Jun  , Wu Ling-An  , Fu Pan-Ming  , Wang Ru-Quan  , Zuo Zhan-Chun  ,   Beijing National Laboratory for Condensed Matter Physics, Institute of Physics, Chinese Academy of Science, Beijing 100190, China;,   College of Physics and Information Engineering, Hebei Normal University, Shijiazhuang 050000, China
Published: December 2020

For a quantum memory to be useful as a quantum repeater, a long coherence time is a crucial requirement. In recent years, the most commonly explored medium for quantum storage has been atomic gases. We report an experiment to realize a quantum memory based on an Rb atomic ensemble in a one-dimensional far-detuned optical lattice. A multimode 30 W continuous wave fiber laser was used to construct a travelling wave lattice with a period of 25 m. The Rb atoms were loaded into a magneto-optical-trap, which was then adjusted to optimize the polarization gradient cooling. To trap the cooled atoms, we turned on a laser which has a wavelength of 1064 nm and therefore is red-detuned from the resonance frequencies of D1 and D2 transitions of 87Rb atoms. By taking the short-distance time-of-flight image the temperature of the atoms was found to be about 20 K. This system will provide a foundation for future quantum information storage studies.
An analytic approach for density gradient injection in laser wake field acceleration
Authors: Wang Tong  , Wang Xiao-Fang  ,   Department of Modern Physics, University of Science and Technology of China, Hefei 230026, China
Published: December 2020

In laser wake field acceleration, it is relatively easy to achieve and control electron injection by adopting a plasma density gradient. This scheme of plasma density gradient injection has been studied in recent years both theoretically and experimentally, but thus far theoretical studies have been done mostly by particle-in-cell simulations. In this paper the density gradient injection and acceleration of electrons are studied with a newly developed analytic approach. The energy threshold for electron injection versus plasma density gradient scale length is given. It is shown that in the plasma density gradient region, the energy threshold of electron injection becomes lower at later times after the driving laser pusle or when the gradient becomes sharper. Evolution of plasma wave's phase velocity and motion of the background electrons in the plasma density gradient are worked out in the linear plasma wave regime, i.e. the normalized laser intensity is a0～1. The energy, the location, and the timing of the injected electrons are obtained. Separatrices of test electrons in the gradient region are obtained by Hamiltonian theory. The influence of injection timing in the density gradient region on the succeeding acceleration in the homogeneous plasma density region is also discussed. It is indicated that whether the injected electrons may be accelerated efficiently or not in the homogeneous region depends on both the energy of the electrons and the phase of the plasma wave at the gradient-to-homogeneous turning point. The analytic results are confirmed by particle-in-cell simulations.
Theoretical study on generating mid-infrared ultrashort pulse in mode-locked Er3+: ZBLAN fiber laser
Authors: Wang Shao-Qi  , Deng Ying  , Zhang Yong-Liang  , Li Chao  , Wang Fang  , Kang Min-Qiang  , Luo Yun  , Xue Hai-Tao  , Hu Dong-Xia  , Su Jing-Qin  , Zheng Kui-Xing  , Zhu Qi-Hua  ,   Research Center of Laser Fusion, China Academy of Engineering Physics, Mianyang 621900, China;,   Graduate School of China Academy of Engineering Physics, Beijing 100088, China;,   Science and Technology on Plasma Physics Laboratory, Mianyang 621900, China;,   Collaborative Innovation Center of IFSA (CICIFSA), Shanghai JiaoTong University, Shanghai 200240, China
Published: December 2020

Fiber lasers show several advantages over other types of lasers. They are efficient, compact, and rugged since they require few bulk components and are virtually unaffected by the surrounding environment. Mode-locked mid-infrared (mid-IR) lasers are essential for a wide variety of applications. The promising applications of mode-locked fiber lasers at wavelengths near 3 m include combs generation (metrology), spectroscopic sensors, infrared countermeasures, laser surgery, high-efficient pump sources for longer-wavelength oscillators and mid-IR supercontinuum source pumping. Based on the nonlinear Schrdinger equation (NLSE), a theoretical model of passively mode-locked Er3+-doped fluoride fiber laser using a saturable absorber is set up. Some mechanisms for generating mid-IR ultrashort pulse in fiber lasers are investigated. When the dispersion of the cavity is managed properly, the numerical simulation mainly focuses on the evolution process of mid-IR ultrashort pulse in fluoride fiber oscillators. Influences of the intracavity net dispersion and the small-signal gain on the generation of mode-locked pulses are analyzed in detail. And the reasonable parameter windows are given. Just as the simulated results showed, for a case of 4 m Er3+-doped fluoride fiber, small-signal gain g0= 0.6 m-1 and unsaturated loss l0 = 0.7, the stable mode-locked pulses are achieved by tuning the net intracavity dispersion within a certain range from 0.72 ps2 to 0.83 ps2. As the net intracavity dispersion increases, the output pulse duration increases gradually, while the spectrum width (FWHM) and peak power decrease accordingly. In addition, for the case of 4 m Er3+-doped fluoride fiber, unsaturated loss l0 = 0.7 and net intracavity dispersion of 0.8 ps2, the stable mode-locked pulses can also be obtained by tuning the small-signal gain within a certain range from 0.55 to 0.70 m-1. As the small-signal gain increases, the output pulse duration, spectral width, and peak power increase gradually. This work may be beneficial to the design of experiments for achieving more narrow pulse duration, wide spectral width, and high peak power mid-infrared ultrashort pulse.
Semi-Dirac cone and singular features of two-dimensional three-component phononic crystals
Authors: Gao Han-Feng  , Zhang Xin  , Wu Fu-Gen  , Yao Yuan-Wei  ,   Department of Physics and Optoelectronic Engineering, Guangdong University of Technology, Guangzhou 510006, China;,   Department of Experiment Education, Guangdong University of Technology, Guangzhou 510006, China
Published: December 2020

Due to accidental degeneracy, a semi-Dirac point is realized at the center of the Brillouin zone in a two-dimensional phononic crystal (PC) consisting of a square array of core-shell-structure elliptical cylinders in water. In the vicinity of the semi-Dirac point, the dispersion is linear along the X direction, but it is quadratic along the Y direction. The semi-Dirac point is formed by the degeneracy of dipole and quadrupole modes, through accurately adjusting the radius of the cores and shells, the two modes will coincide and the dispersion relation will become linear. It is worth to be emphasised that the frequency of the semi-Dirac point is very low in our designed PC, and this is exactly the special advantage of a three-component system. Since the dispersion relation is different in the vicinity of the semi-Dirac point, some new features may be seen. Firstly, the anisotropic transmission phenomenon is demonstrated. A PC slab is placed in a rectangular waveguide where the sound hard boundary conditions are used on the upper and lower walls; a plane wave impinges on the PC slab along the X direction at the semi-Dirac point frequency, and total transmission can be achieved, so that the sound energy transmissivity is also equal to one. In the meantime, the waves experience no spatial phase changes when they are transmitting through the PC slab; this behavior indicates that the PC can be equivalent to zero index medium along the X direction. However, when the plane wave is incident along the Y direction, the transmitted field is very weak, and the sound energy transmission is nearly zero. Secondly, the properties of the semi-Dirac point can be applied to design acoustic diode. The scatterers of the PC are arranged in triangular prism shapes and placed into a straight waveguide; when the wave is incident along the X direction, it can be transmitted through the PC slab and emerge in the right area, but when the waves is incident from the opposite direction, it will be totally reflected back. Therefore, the semi-Dirac point in PC provides a way to realize the acoustic diode. Finally, the unidirectional wave-front shape effect can also be observed in our considered system. We put a square sample with 16-by-16 coating rods into water medium. When a tightly focused Gaussian beam impinges on the PC sample along the X direction at the semi-Dirac point frequency, the outgoing wave will be modulated to a plan wave. Whereas, when the incident wave along the Y direction, the Gaussian beam will be totally reflected. In conclusion, the singular features of semi-Dirac point in PC will provides an advantageous means to manipulate acoustic waves and exploit new functional materials.
Direct-sequence spread-spectrum underwater acoustic communication based on single vector differential energy detector
Authors: Yin Jing-Wei  , Du Peng-Yu  , Zhang Xiao  , Zhu Guang-Ping  ,   Acoustic Science and Technology Laboratory, Harbin Engineering University, Harbin 150001, China;,   College of Underwater Acoustic Engineering, Harbin Engineering University, Harbin 150001, China
Published: December 2020

By taking advantage of spread processing gain, the direct-sequence spread-spectrum (DSSS) for underwater acoustic (UWA) communication system can be carried out at low signal levels, which is the preferred method for high-quality UWA communication and remote UWA communication. However, phase fluctuation caused by complex marine environment seriously affects the performance of spread spectrum system, leading to the reduction of spread processing gain. Differential energy detector is proposed for DSSS UWA communication system in this paper, which has a good ability of anti-carrier phase fluctuation and multi-path interference by detecting the output energy of two correlators. Differential coding can avoid error propagation when determining the relationship between adjacent symbols. Differential energy detector combined with improved active average sound intensity detector is further proposed in this paper, which can get vector processing gain by updating the estimated azimuth so as to make the system operate stably at a low signal to noise ratio. Improved active average sound intensity detector also has the ability of anti-carrier phase fluctuation, and the feedback code bit information of differential energy detector can ensure that the processing gain of improved active average sound intensity detector is not affected by Doppler's accumulation. Simulation and Dalian sea test have verified the robustness of single vector differential energy detector algorithm. Using the single-vector differential energy detector, good performance is achieved for a signal-to-noise ratio as low as -18 dB based on at-sea data.
Comparative study on intense emission of velvet and cabon nanotube cathode
Authors: Cai Dan  , Liu Lie  , Ju Jin-Chuan  , Wang Hai-Tao  , Zhao Xue-Long  , Wang Xiao  ,   College of Optoelectronic Science and Engineering, National University of Defense Technology, Changsha 410073, China;,   The PLA Unit 78010, Chengdu 610000, China
Published: December 2020

So far, the investigations of carbon nanotube (CNT) cathode have been focused on the field emission with low current and voltage. However, the properties of the intense pulsed emission of CNT cathode have not been discussed deeply and comprehensively. In this paper, the intense emission properties of velvet and CNT cathode are studied in various aspects, such as emission capability, cathode plasma expansion, cathode initialization, emission uniformity, operation stability, outgassing property, and so on. Three different CNT cathodes are made by using electrophoresis deposition, chemical vapor deposition and also CNT paper (or buckypaper) gluing. Results show that the emission capability of CNT arrays and CNT paper cathode is definitely better than the velvet cathode. At the same diode voltage (～300 kV), the amplitudes of diode current of CNT array and CNT paper are 2.75 and 3.1 kA respectively, which are bigger than that of the velvet cathode (～1 kA). The orientation of CNT should not affect the emission capability of CNT cathodes. And the small radius of the tube wall and the existence of defects are suggested to be the reasons for the emission of electrons from the body of the tubes. The threshold electric field strength of intense emission of CNTs is about two-thirds of velvet cathode. The onset delay time of CNT cathode is shorter than the velvet cathode by about 12-17 ns at the same electric field growth rate. The time-evolution processes of the plasma expansion velocity of CNT and velvet cathodes are similar, which could be divided into three phases (rapid rising, quick decreasing and stable phase). In summary, the plasma expansion velocity of CNT cathode is less than one fourth that of velvet at the end of the first phase. During the stable phase, which sustains until the end of the voltage pulse, both cathodes have the same plasma expansion velocity (7 cm/s). The emission uniformity of the cathode has been studied by analyzing the distributions of cathode plasma spots and Cherenkov radiation light, which are captured by the high speed frame camera. The emission uniformity of CNT cathode is much better than that of the velvet cathode. Especially, the cathode plasma spots on the whole surface of CNT array cathode are very dense and uniform. The peak outgassing pressure of the CNT paper cathode is 0.3 Pa, which is one fifth that of the velvet cathode; while the peak outgassing pressure of the CNT array cathode is 0.042 Pa, which is the lowest, and the outgassing pressure of the CNT cathode is related to the fabrication methods. Volatile such as epoxy should be avoided in the fabrication processes. This CNT cathode appears to be suitable for intense emission source and high-power microwave device applications.
JMCT Monte Carlo analysis of BEAVRS benchmark: hot zero power results
Authors: Li Gang  , Deng Li  , Zhang Bao-Yin  , Li Rui  , Shi Dun-Fu  , Shangguan Dan-Hua  , Hu Ze-Hua  , Fu Yuan-Guang  , Ma Yan  ,   Institute of Applied Physics and Computational Mathematics, Beijing 100094, China;,   CAEP Software Center for High Performance Numerical Simulation, Beijing 100088, China
Published: December 2020

J Monte Carlo transport code (JMCT), a new three-dimentional (3 D) Monte Carlo transport code, is introduced in this paper. The code is developed on the basis of 3 D geometry infrastructure JCOGIN and composed of multilayer modules. JMCT is capable of simulating the collision of particles with multi-group energy or providing energy data libraries. Two forms of parallelism supported in JMCT are domain decomposition and domain replication. The code has very good expansibility. JMCT Monte Carlo results have been compared with hot zero power (HZP) measurements of BEAVRS benchmark model from the MIT Computational Reactor Physics Group. Included in the comparisons are the eigenvalues, control rod bank worths, isothermal temperature coefficients, axially integrated full core detector measurements, axial detector profiles, etc. The eigenvalues for the HZP condition with different control rods positions and boron concentrations are calculated and the error is less than 0.2% compared with the theoretical error 1.000. The results of JMCT for isothermal temperature coefficients are also listed together with MC21 results and measured data. Each calculation for the eigenvalue is run by 1000 cycles in total, discarding 600 cycles, tracking 4 million neutrons each cycle. It takes 5.3-5.7 hours to run on 200 CPU cores. The JMCT results of axially integrated radial detector relative power distribution (RPD) and axial normalized detector signal are compared with the measured data. Power depressions from grid spacers are clearly seen in the JMCT results and accord with the measured data. The JMCT results of axially integrated assembly RPD power distribution are in good agreement with MC21 results, the maximum difference being 3.173% for 193 assemblies. So is the result of pin power RPD relative power at the axial elevation of peak power; the minimum relative power RPD 0.278 of JMCT is comparable to 0.283 of MC21, and the max relative power RPD 2.422 of JMCT is comparable to 2.452 of MC21. The calculation for RPD is run with 3000 inactive cycles and 5000 active cycles, tracking 4 million particles each cycle. It takes about 4.8 days to run on 200 CPU cores. Shannon entropy is used to demonstrate that the fission source distribution is converged after 3000 inactive cycles. With the development of computers and parallel computing, the Monte Carlo method can be used in reactor design instead of benchmarking other calculated results.