bluuebunny/crossref_metadata_2025 huggingface dataset:
Motion characteristics of coaxial gun discharge plasma current sheet
Authors: Qi Liang-Wen , Zhao Chong-Xiao , Yan Hui-Jie , Wang Ting-Ting , Ren Chun-Sheng ,   Key Laboratory of Materials Modification by Laser, Ion and Electron Beams of the Ministry of Education, School of Physics, Dalian University of Technology, Dalian 116024, China
Published: December 2020

The coaxial gun discharge, used as plasma jet with high density and velocity, has a wide variety of applications such as plasma space propulsion, simulation experiment of thermal transient events in the International Thermonuclear Experimental Reactor, plasma refueling for fusion reactors and a laboratory scale platform for studying astrophysical phenomena. The plasma produced in the coaxial gun can be accelerated by self-induced Lorentz force, and the ionization in the transport process can be based on " snow-plow model” in which a coaxial current sheet moves forward and sweeps a large amount of the gas between two electrodes to cause the plasma dump. Based on the measurements of discharge current, voltage, photocurrent and magnetic signal, the experimental investigation on the characteristics of plasma motion and current sheet channel distribution in the gun operated under different discharge conditions and various pressures is carried out. In this paper, it is emphasized to explore the electrical and dynamic properties about plasma in the first half-cycle of current. The results show that the plasma velocity increases with the increase of the current amplitude, and that the transport distance is proportional to the axial kinetic energy of ions when the pressure is fixed at 10 Pa and discharge current is adjusted from 35.7 kA to 69.8 kA. Moreover, in the case of high current, the plasma jet from the nozzle tends to form a new current path at the bottom of the gun. However, when the discharge current is fixed at 49.8 kA and the gas pressures range from 5 Pa to 40 Pa, the plasma motion velocity and transport distance are continuously reduced. Meanwhile, it is not found that new current paths are generated at the bottom of the coaxial gun under high pressure. The generation of the new current path is relevant to the channel impedance formed by more charged particles left at the bottom of the gun and neutral particles leaking from current sheet during discharge. Besides, a multiple discharge phenomenon is presented in experiment and the secondary discharge breakdown position occurs at the head of the electrode when the current is reversed to a positive value. Therefore, this study provides a reasonable choice of electrical parameters to obtain optimal plasma characteristics during the discharge of the coaxial gun.
Application and evaluation of Chinese spallation neutron source in single-event effects testing
Authors: Wang Xun , Zhang Feng-Qi , Chen Wei , Guo Xiao-Qiang , Ding Li-Li , Luo Yin-Hong ,   State Key Laboratory of Intense Pulsed Radiation Simulation and Effect, Northwest Institute of Nuclear Technology, Xi’an 710024, China
Published: December 2020

Due to the lack of available spallation neutron source, the atmospheric neutron single event effect (SEE) in China were studied mainly by means of simulation and single energy neutron test. Since the Chinese spallation neutron source (CSNS) passed the national acceptance, it has become possible to carry out the research on atmospheric neutron SEE by using the CSNS. In this paper, the neutron SEE experiments of 3 kinds of SRAMs with different feature sizes are carried out for the first time by using the CSNS back-n. The application of CSNS back-n in the study of atmospheric neutron SEE is evaluated by comparing with the results of the earlier plateau experiment. The results show that the cross section of the single event upset is smaller than that of the plateau test, and the cross sections of different devices have no obvious monotonic relationship with feature size. The reason for the former result is that the energy spectrum of CSNS back-n is slightly softer than that of the atmospheric neutron. The reason for the second result is that small feature size means small critical charge and small sensitive volume, and these two factors compete with each other when they make the contribution to the cross section. According to the difference in energy spectrum and cross section among the SRAM devices, a correction factor is proposed to correct the test results based on CSNS back-n. For the difference in energy spectrum, different energy thresholds will produce different ratios between the cross sections by using CSNS back-n and atmospheric neutron. The neutrons of CSNS back-n are mainly concentrated around 1 MeV, which is close to the energy threshold of general SRAM devices. Thus, inaccurate energy threshold estimation will introduce a large error into the cross section of SEU. Thus, the relation between the correction factor and the energy threshold is analyzed. If 12 MeV is selected as the energy threshold to calculate the cross section, more consistent results could be obtained for our DUT in CSNS back-n and atmospheric neutron environment. In a word, the results show that the CSNS back-n can be used to speed up the atmospheric neutron SEE test, but the result should be corrected to evaluate the threat from atmospheric neutron. Fortunately, with the continuous increase of CSNS operating power, the neutron flux and the accelerated factor of CSNS will increase synchronously. Besides, other 3 white light neutron beams are planned in the CSNS project, the planned energy spectra are closer to those of atmospheric neutron. It is expected that the CSNS will be better applied to the study of atmospheric neutron SEE.
Refractive index and thermo-optic coefficient of Ge-Sb-Se chalcogenide glass
Authors: Yang An-Ping  , Wang Yu-Wei  , Zhang Shao-Wei  , Li Xing-Long  , Yang Zhi-Jie  , Li Yao-Cheng  , Yang Zhi-Yong  ,   1. Jiangsu Key Laboratory of Advanced Laser Materials and Devices, School of Physics and Electronic Engineering, Jiangsu Normal University, Xuzhou 221116, China;,   2. Shanghai Aerospace Control Technology Institute, Shanghai 201109, China
Published: December 2020

Ge-Sb-Se chalcogenide glass is environmentally friendly, and has wide infrared transmitting window, high optical nonlinearity, as well as good mechanical property. These make it a good material for infrared transmission and nonlinear optics. In optical designs, the refractive index (n) and thermo-optic coefficient (ζ) of the glass are key technical parameters. In order to predict and tailor the n and ζ of Ge-Sb-Se glass, compositions with different chemical and topological features are prepared, their n, ζ, density (d) and volume expansion coefficient (β) are measured, and the composition dependence of the parameters is systematically investigated. The chemical feature of the glass is quantified by the percentage deviation of the composition from the stoichiometric ratio and denoted as dSe. The topological feature is represented by the mean coordination number <r> of each atom in the composition. It is shown that the n of Ge-Sb-Se glass increases with d increasing; the ζ decreases almost linearly with β increasing; and the β decreases with dSe decreasing or <r> increasing. When the Ge content is fixed, the d increases with dSe decreasing or <r> increasing; when the Sb concentration is fixed, the d has a minimum value at dSe=0. Based on the measured d and n, the molar refractivity (Ri) of Ge, Sb and Se elements in a spectral range of 2-12 μm are calculated. The obtained value of RGe is in a range of 10.16-10.50 cm3/mol, RSd in a range of 16.71-17.08 cm3/mol, and RSe in a range of 11.15-11.21 cm3/mol. When the Ri and d are used to compute n of any composition, the discrepancy between the calculated value and the measured one is less than 1%. According to the measured ζ and β, the thermal coefficients of the molar refractivity (φi) of Ge, Sb, and Se elements in a wavelength region of 2-12 μm are computed. The optimal value of φGe is in a range of 21.1-22.6 ppm/K, φSb in a range of 7.2-8.4 ppm/K, and φSe in a range of 90.2-94.2 ppm/K. When the φi and β are used to compute ζ of any composition, the discrepancy between the calculated value and the measured value is less than 6 ppm/K.
Impact of initial states on the quantum speed limits metric in linear molecules
Authors: Feng Hai-Ran , Li Peng , Yue Xian-Fang ,   Department of Physics and Information Engineering, Jining University, Qufu 273155, China
Published: December 2020

Quantum speed limit (QSL) in a quantum system originates from the essential principle of the quantum mechanics. It gives a maximum speed of evolution or a minimum evolution time of the quantum system, which has potential applications in the fields of quantum information, quantum communication and quantum control and so on. In the last decades, the QSL bounds have been explored from the isolated quantum systems to the open quantum systems, several different geometric measures have been adopted to investigate the distinguishability between the initial and the evolved state. The QSL bounds in many systems have been discussed, indicating that the tightness of the QSL metric is related to the researched dynamical system. However, the QSL problem for the molecular system has rarely been reported. The study of the quantum speed limits in the different molecular systems is helpful for realizing the quantum information technology based on the molecules. In this paper, the generalized quantum speed limit metric for linear molecular dynamics is studied by the algebraic approach. The quantum Fisher information metric and the Wigner-Yanase information metric are both used to study the QSLs in the dynamical evolution of the two linear molecules. Here the dynamical evolutions begin with the two kinds of vibrational states, Fock initial state and coherent initial state. The results show that the quantum Fisher information metric is more appropriate than the Wigner-Yanase information metric for HCN and DCN molecules. The relative differences between the generalized geometric QSL and the two geodesic QSL metrics become bigger gradually with the increase of the initial vibrational quantum number. However, the relative difference for the DCN molecule is smaller than for the HCN molecule. The relative difference between the strong coherent states is smaller, which indicates that the generalized geometric quantum speed limit is suitable to describing high-coherent case. In conclusion, the different QSL metrics reveal the discrepancy in the evolution of the molecular system, and the relative difference is related to the initial state of the molecules and molecular parameters. More molecular systems need to be investigated in order to obtain the criteria between the QSL metrics and the molecular parameters.
Magnetic properties of one-dimensional Fermi gases in an optical cavity
Authors: Feng Yan-Lin , Fan Jing-Tao , Chen Gang , Jia Suo-Tang ,   State Key Laboratory of Quantum Optics and Quantum Optics Devices, Institute of Laser Spectroscopy, Shanxi University, Taiyuan 030006, China,   Collaborative Innovation Center of Extreme Optics, Shanxi University, Taiyuan 030006, China
Published: December 2020

In this work we show that the superradiance of the cavity photons can give rise to a magnetic transformation for the atomic system when the quasi one-dimensional Fermi gases are coupled to an optical cavity. This magnetic transformation has a close relationship with the atomic detuning and the filling number. When the interaction between the atoms is neglected, the mean-field approximation may be used in the superradiant phase. In this approximation, we analyze the static spin structure factors of the system with different filling numbers and atomic detuning. Then we characterize the cavity photons-assisted magnetic transformation and obtain the phase diagrams which are dependent on the cavity parameters. Finally, the feasible experimental parameters of our results are also discussed.
Critical breakdown path under low-pressure and slightly uneven electric field gap
Authors: Yu Bo , Liang Wei , Jiao Jiao , Kang Xiao-Lu , Zhao Qing ,   Center for Information Geoscience, University of Electronic Science and Technology of China, Chengdu 611731, China,   Shanghai Institute of Space Propulsion, Shanghai 201112, China
Published: December 2020

The determination of the critical breakdown path in slightly uneven electric field has played a significant role in gas discharge starting process and electrode surface erosion. In order to study the law of the critical path position in the low-pressure breakdown case, a new algorithm based on the Monte-Carlo collision model and the postulation of " forward-back trajectory for electrons” is established, namely the determination of the critical path(DCP) model. In the DCP model, some electric field lines among the electrodes are regarded as the potential breakdown paths, and the probability of the excitation and ionization collisions between the electrons and the neutrals can be obtained by the Monte-Carlo model. The most probable path to trigger the breakdown will be selected from among all the potential paths, namely the critical breakdown path, and the corresponding breakdown voltage of the critical path will be calculated. A breakdown test with two different electrode devices is performed to examine the accuracy of the DCP model: the critical path and breakdown voltage obtained by the DCP could be examined respectively by capturing the surface traces of negative electrode and measuring the breakdown voltage. According to the test results, the critical breakdown path can transit at different gap pressures or flow rates, and this observation is qualitatively consistent with the calculation results. Meanwhile, the relative error maximum of the breakdown voltage obtained by DCP is less than 7.9%. The accuracy of the DCP model partly depends on the background pressure, and the background pressure in the application case should be less than 103 Pa. Based on the DCP model, the numerical analyses of another four electrode devices are conducted to acquire the common law about the critical breakdown path. According to the calculation results, the transition zone has both a high frequency of critical path transition and a " fluctuant and similarly straight” breakdown voltage curve with the gap pressure or flow rate increasing, and the critical path transition direction follows the rule of " from longer paths to shorter paths”. Lastly, the inherent laws of those regulations about the critical path are revealed by the DCP model.
Synthesis and long afterglow characteristics of Sr2MgSi2O7:Eu2+, Dy3+ by experimental optimization design
Authors: Liu Sheng-Yi , Zhang Jin-Su , Sun Jia-Shi , Chen Bao-Jiu , Li Xiang-Ping , Xu Sai , Cheng Li-Hong ,   College of Science, Dalian Maritime University, Dalian 116026, China
Published: December 2020

An optimization method is used to obtain the longest effective afterglow time in the rare earth ions doped long lasting phosphors. The effective afterglow time is defined as the time for the intensity to decays to 10% of the initial intensity. In this paper, we choose the Eu2+ and Dy3+ coped Sr2MgSi2O7 as the experimental objects. In order to obtain the longest effective afterglow time of Sr2MgSi2O7:Eu2+, Dy3+ phosphor, the experiment is optimized by quadratic general rotation combination design. The Sr2MgSi2O7:Eu2+, Dy3+ phosphor are synthesized via a solid-state reaction. The effective afterglow time is obtained by the afterglow decay curve. A binary quadratic regression equation model relating the rare earth ions Eu2+/Dy3+ doping concentrations to the effective afterglow time is established. The genetic algorithm is used to solve the equation. The optimal doping concentration of Eu2+ and Dy3+ are 0.5 mol% and 1.0 mol%, respectively. The theoretical maximum value of effective afterglow time is calculated to be 321 s. The phosphor with the optimal doping concentration Sr2MgSi2O7:0.5 mol% Eu2+, 1.0 mol% Dy3+ are synthesized by the same method as that of synthesizing the frontal samples. The X-ray diffraction shows that the optimal sample prepared is of pure phase, and the doping ions have no effect on the lattice structure of the matrix. A characteristic emission at 465 nm due to the 4f65d1−4f7 transition of Eu2+is observed under the 370 nm excitation. The afterglow curve of the optimal sample is measured and the effective afterglow time is 333 s which has a good match with the theoretically calculated value of 321 s. The thermoluminescence spectrum of the optimal phosphor is measured, and the trap depth is calculated to be 0.688 eV according to the Chen’s model. Moreover, the long-lasting luminescence process of Eu2+ as the luminescence center of Sr2MgSi2O7 matrix is discussed in the energy level diagram.
Atom-subjected optical dipole force exerted by femtosecond laser field
Authors: Liu Ji-Cai , Cheng Fei , Zhao Ya-Nan , Guo Fen-Fen ,   College of Mathematics and Physics, North China Electric Power University, Beijing 102206, China
Published: December 2020

 In 2011, Kumar et al. (2011 Phys. Rev. A 84 043402) studied the light force acting on a beam of neutral two-level atoms superimposed on a few-cycle-pulse Gaussian laser field under both resonant and off-resonant conditions by solving the optical Bloch equation beyond the rotating-wave approximation, and they found that under resonant condition the transverse component of the light force shows oscillatory behavior but vanishes when a time average is taken, and the time averaged longitudinal force is nonzero only when the Rabi frequency is smaller than the resonant frequency and vanishes when the Rabi frequency is equal to or larger than the resonant frequency.

 In this paper, we investigate further the strong nonlinear optical interaction between a two-level atomic system and a femtosecond Gaussian laser pulse by solving numerically the full-wave optical Bloch equations through using the predictor-corrector method. It is found that the light forces and the light potentials are sensitive to the value of the Rabi frequency and the detuning of the laser field. Under the resonant condition, the instant light forces induced by the femtosecond laser pulse change their signs as a function of time. The instant longitudinal light force changes its sign at twice the Rabi frequency, while the instant transverse light force changes its sign at twice the light carrier-wave frequency. However, none of the time-averaged light forces is zero, showing periodical oscillation characters as a function of Rabi frequency. Both of the time-averaged longitudinal and transverse light forces oscillate at the Rabi frequency corresponding to the pulse area of 2\begin{document}${\text{π}}$\end{document}. The time-averaged transverse light force shows also a trend of enhancement with Rabi frequency increasing, and the time-averaged longitudinal light force shows also a saturation trend with the increase of the Rabi frequency. The optical potential depends strongly on the detuning. It changes gradually from repulsive potential to attractive potential when the detuning defined here changes from negative to positive detuning. When the field is nearly resonant, the optical potential then oscillates between repulsive and attractive potentials. Therefore, neutral atoms can be focused, defocused, trapped, splitted or steered by the femtosecond laser field with appropriate detuning and Rabi frequency.
Role of optical angular momentum in enhanced transmission process of plasmonic coaxial nanoring aperture
Authors: Wang Shuai , Deng Zi-Lan , Wang Fa-Qiang , Wang Xiao-Lei , Li Xiang-Ping ,   Guangdong Provincial Key Laboratory of Optical Fiber Sensing and Communications, Institute of Photonics Technology, Jinan University, Guangzhou 510632, China,   Guangdong Provincial Key Laboratory of Nanophotonic Functional Materials and Devices, School of Information and Optoelectronic Science and Engineering, Guangzhou 510632, China,   Institute of Modern Optics, Nankai University, Tianjin 300350, China
Published: December 2020

The nano groove can highly improve the transmittance of coaxial nanoring aperture due to the excitation of surface plasmon polariton (SPP). The total angular momentum carried by incident beam is reserved in the whole process and transferred to the SPP, thus the vortex SPP carrying orbital angular momentum is generated. The enhanced transmittance of nano aperture by vortex SPP has a wide range of applications, but its physical mechanism has been ignored for a long time. Here we study the process of the enhanced transmittance of the coaxial nanoring aperture and provide a model to describe the process of photon transmission. When the incident light irradiates on the coaxial nanoring aperture and nano groove, the vortex SPP induced by the groove propagates to coaxial nanoring aperture. Some of the photons in the SPP are coupled into the coaxial nanoring aperture and some are reflected back. The reflected photons travel back and forth multiple times between the coaxial nanoring aperture and nano groove. The vortex SPP interacts with the incident beam at the round of coaxial nanoring aperture, which determines the intensity at the round of the coaxial nano aperture, and thus affecting the transmittance. We systematically study the influence of optical angular momentum and the radius of the nano groove on the transmittance of coaxial nanoring aperture by using theoretical analysis and numerical simulations. The results show that the optical angular momentum and radius of the nano groove both affect the radial propagation phase of vortex SPP from nano groove to coaxial nanoring aperture, hence affecting the intensity of the electric field at the round of coaxial nanoring aperture and consequently determine the transmittance. The transmittance peaks of incident beams with different optical angular momenta will appear at different radii of the nano grooves, which provides a potential way to modulate the transmittance by adjusting the radius of the nano groove. This study is instructive for designing the enhanced optical transmission nano device based on vortex SPP.
Quantum broadcasting multiple blind signature protocol based on three-particle partial entanglement
Authors: Zhang Wei , Han Zheng-Fu ,   Key Laboratory of Complex Systems and Intelligent Computing, School of Mathematics and Statistics, Qiannan Normal University for Nationalities, Duyun 558000, China,   CAS Key Laboratory of Quantum Information, University of Science and Technology, Hefei 230026, China
Published: December 2020

Recently, a quantum broadcasting multiple blind signature scheme based on GHZ state has been proposed, which could be used to settle the problem that a message is so important that it needs to be signed by multiple signatories, in order to guarantee the message privacy: none of signatories can acquire the content of the message they have signed. Maybe it can be applied to an E-bank system. For example, a large amount of money has to be transferred through E-bank system on the internet. The E-bank system operator submits the request to the bank after filling the application form including payment amount, bank transfer account and some other information. When the request arrives, the bank clerk signs to approve. However, it is not enough, it has to ask the manager for authority, and then it needs to be signed by the manager. In the whole process, all the signatories cannot learn what they have signed, but the application form has been recorded in the E-bank system. So, once disagreement takes place, the bank can track the message sender.

In this paper, we present a new quantum broadcasting multiple blind signature scheme which is based on a three-particle partial entanglement state. Comparing with the original scheme, the partial entanglement state is utilized in our new scheme in place of the GHZ state, and this does not bring down the security of the scheme. Particularly, using the partial entanglement state can not only save the entanglement resource to some extent, but also make the scheme much easier to be realized. As is well known, It is not easy to keep the maximum entanglement state shared among the participants in the whole quantum communication process. By using the partial entanglement in place of the maximum entanglement can improve the new scheme applicability to make it more practical. It is also indicated that multi-qubit entangled systems which are partially entangled can be efficiently used as a resource in quantum information processing with perfect performance.
Numerical calculation and optimization of photoacoustic cell for photoacoustic spectrometer
Authors: Cheng Gang , Cao Yuan , Liu Kun , Cao Ya-Nan , Chen Jia-Jin , Gao Xiao-Ming ,   Anhui University of Science and Technology, State Key Laboratory of Mining Response and Disaster Prevention and Control in Deep Coal Mines, Huainan 232001, China,   Anhui Institute of Optics and Fine Mechanics, Chinese Academy of Sciences, Hefei 230031, China,   University of Science and Technology of China, Hefei 230031, China
Published: December 2020

Photoacoustic spectroscopy (PAS) offers intrinsic attractive features in the detection of trace gases, including ultra-compact size and background-free absolute absorption measurement. The photoacoustic (PA) cell is a key component in the PAS system, which determines the performance of the PAS sensor. In this paper, a cylindrical resonant photoacoustic cell is taken as a research target. Based on the fundamental theory of acoustics and absorption spectrum, a mathematical model of acoustic field excitation in the PA cell is established. The acoustic resonance frequency, quality factor and cell constant of the PA cell are used as three key parameters to describe its performance. By employing advanced computer numerical calculation and finite element simulation technology, we establish a simulation model and impose the excitation load and boundary conditions on the model according to the actual working conditions. Then we calculate and simulate the acoustic modal of the PA cell, and the first eight acoustic modal values of the cavity and the visual vibration model of the acoustic pressure are obtained. With considering the acoustic loss, the thermo-acoustic coupling multi-physical field simulation of photoacoustic cell is carried out. Comparing with analytical calculation and experiment results, the reliability and feasibility of using numerical simulation method to calculate the relevant parameters of photoacoustic cell are demonstrated. In order to obtain a better structure of photoacoustic cell, an optimization algorithm combining response surface proxy model with multi-objective genetic algorithm is proposed. We try to change the shapes of both ends of the resonator in the original photoacoustic cell into the shape of the bell mouth. Take into account the case that the longitudinal acoustic normalization frequency of the PA cell is larger than 1000 Hz, Pareto optimal solution set with the maximum quality factor Q and cell constant Ccell of the PA cell is obtained. The results show that the maximum error between the predicted and simulated values of the proxy model of the PA cell Q and Ccell is only 1.3%. Comparing with the original PA cell, the Q factor and the Ccell of the optimized PA cell are increased by 48.9% and 34.4%, respectively. The performance of the optimized photoacoustic cell is obviously improved. The proposed algorithm of photoacoustic numerical simulation combined with multi-objective optimization design can provide helpful reference for designing the PA cell in PAS sensor development.
A model of femtosecond laser ablation of metal based on dual-phase-lag model
Authors: Tan Sheng , Wu Jian-Jun , Huang Qiang , Zhang Yu , Du Xin-Ru ,   College of Aerospace Science and Engineering, National University of Defense Technology, Changsha 410073, China
Published: December 2020

Femtosecond laser ablation possesses a variety of applications due to its better control, high power density, smaller heat-affected zone, minimal collateral material damage, lower ablation thresholds, and excellent mechanical properties. The non-Fourier effect in heat conduction becomes significant when the heating time becomes extremely small. In order to analyze the femtosecond laser ablation process, a hyperbolic heat conduction model is established based on the dual-phase-lag model. Taken into account in the model are the effect of heat source, laser heating of the target, the evaporation and phase explosion of the target material, the formation and expansion of the plasma plume, and interaction of the plasma plume with the incoming laser. Temperature-dependent optical and thermophysical properties are also considered in the model due to the fact that the properties of the target will change over a wide range in the femtosecond laser ablation process. The effects of the plasma shielding, the ratio of the two delay times, and laser fluence are discussed and the effectiveness of the model is verified by comparing the simulation results with the experimental results. The results show that the plasma shielding has a great influence on the femtosecond laser ablation process, especially when the laser fluence is high. The ratio between the two delay times (the ratio B) has a great influence on the temperature characteristic and ablation characteristic in the femtosecond laser ablation process. The augment of the ratio B will increase the degree of thermal diffusion, which will lower down the surface temperature and accelerate the ablation rate after the ablation has begun. The ablation mechanism of femtosecond laser ablation is dominated by phase explosion. The heat affected zone of femtosecond laser ablation is small, and the heat affected zone is less affected by laser fluence. The comparison between the simulation results and the experimental results in the literature shows that the model based on the dual-phase-lag model can effectively simulate the femtosecond laser ablation process.
Coherent perfect absorption in nonlocal particle composite medium
Authors: Chen Zhi-Peng , Yu Wen-Jing , Gao Lei ,   School of Physical Science and Technology, Soochow University, Collaborative Innovation Center of Suzhou Nano Science and Technology, Soochow University, Suzhou 215006, China,   Jiangsu Key Laboratory of Thin Films, Soochow University, Suzhou 215006, China,   School of Mathematics and Physics, Jiangsu University of Technology, Changzhou 213001, China
Published: December 2020

We explore the coherent perfect absorption of light in a nonlocal metal-dielectric composite film in which metallic nanoparticles (gold) are randomly embedded in the dielectric host medium (silica). The two coherent light beams illuminate the gold-silica composite slab respectively from the left and right sides at the same angle of incidence and the conditions required for coherent perfect absorption are investigated each as a function of different system parameters. Under different system parameters, we study the coherent perfect absorption of a nonlocal particle composite medium. A nonlocal effective medium theory is proposed to approximately describe the metal-dielectric composite film. The effective permittivity and effective permeability of the composite medium are approximated by using the effective medium theory under the model of coated sphere with core and shell. According to the effective dielectric parameters of the composite medium, we can obtain the transmission coefficient and reflection coefficient of the plane wave incident on the slab. By comparing and analyzing the coherent perfect absorptions of the composite medium under nonlocal and local conditions, we find that under the influence of nonlocal effect when the size of particle is very small, the frequency range of incident light that produces the coherent perfect absorption of the composite medium increases and the small size can also cause the coherent perfect absorption to occur in wider frequency range. Especially, we pay attention to the choosing of physical parameters in the design of coherent perfect absorption with macroscopic composite slab when we take the nonlocal effect (or spatial dispersion) into account. In the further calculation, the coherent perfect absorption of the composite medium can be realized by changing the system parameters such as the thickness of composite slab, the wavelength of incident light, the volume fraction of metal particles, etc. We also bring about the coherent perfect absorption at a small volume fraction which satisfies all the conditions. Finally, according to these results, we can realize the control of the coherent perfect absorption with nonlocal effect. Our study may be helpful in designing the optical nanoabsorbers.
Low-coherece laser based lens array beam smoothing techique
Authors: Gao Yan-Qi , Zhao Xiao-Hui , Jia Guo , Li Fu-Jian , Cui Yong , Rao Da-Xing , Ji Lai-Lin , Liu Dong , Feng Wei , Huang Xiu-Guang , Ma Wei-Xin , Sui Zhan ,   Shanghai Institute of Laser Plasma, China Academy of Engineering Physics, Shanghai 201800, China,   Collaborative Innovation Center of IFSA (CICIFSA), Shanghai Jiaotong University, Shanghai 200240, China,   National Laboratory on High Power Laser and Physics, Shanghai Institute of Optics and Fine Mechanics, Chinese Academy of Sciences, Shanghai 201800, China
Published: December 2020

The experimental study of laser-driven material state equation puts forward extremely high requirements for the uniformity and stability of the target spot intensity distribution, and these two characteristics greatly determine the accuracy and repeatability of the experimental results. In this paper, a beam smoothing scheme combining diffraction-weakened lens array (LA) with induced spatial incoherent (ISI) technique based on low-coherence laser is proposed to solve the problems, that is, the uniformity and stability of the target spot intensity distribution in the material state equation experiments driven with narrow-band coherent laser. The super-Gaussian soft aperture used in our scheme can improve the intensity fluctuation caused by the hard-edge diffraction of the lens elements, and the temporal smoothing technique, ISI, can reduce the interference effect between the lens array elements. The speckle patterns of target spot, which are caused by interference between beamlets and determine the high nonuniformity, will randomly reconstruct after each coherent time. The high-frequency components are further smoothed by the time-average effect. In broadband high-power laser devices, ISI can be combined with LA by making the lens elements with different thickness values. This scheme can enhance the focal spot uniformity and improve the tolerance of the system to the wavefront phase distortion. The influence of wavefront phase distortion on target surface uniformity and stability are analyzed. The simulation results show that this smoothing scheme significantly reduces the target spot nonuniformity, improves the tolerance of random wavefront phase distortion, and presents a uniform and stable target spot intensity distribution. The nonuniformity of target spot will be reduced to about 10% after 10 ps, and about 3% after 100 ps. In addition, statistical analysis shows that the peak-to-valley value and the nonuniformity of the target spot intensity distribution are strongly correlated with the gradient of root-mean-square of the wavefront phase distortion. Using this method, the tolerance range of the wavefront phase distortion can be given according to the requirements of the experiments, which has reference value for designing and optimizing the laser driver parameters in the state equation experiment.
Molecular dynamics simulation of characteristic water molecular arrangement on graphene surface and wetting transparency of graphene
Authors: Shi Chao , Lin Chen-Sen , Chen Shuo , Zhu Jun ,   School of Aerospace Engineering and Applied Mechanics, Tongji Univesity, Shanghai 200092, China,   Nantong Blue Island Offshore Co. Ltd., Nantong 226259, China
Published: December 2020

Graphene has received a lot of attention for its excellent physical and chemical properties, and the unique wettability of graphene is still under investigation. Most of previous studies focused on graphene or carbon nanotubes, and less of them on the comparison of wettability between graphene and other materials to reveal the characteristic wettability of graphene. In the present study, the wettability of monolayer graphene, copper and silica are studied by using the molecular dynamics simulation, in which the contact angle and the water molecule arrangement (i.e. density distribution and angle distribution of water molecules) on the substrates are analyzed. The results show that although both copper and graphene are weak hydrophilic materials, there are two neat layers of water molecule structure on the surface of graphene, and water molecules are disordered on the surface of copper. Silica is a kind of strong hydrophilic material and graphene is a kind of weak hydrophilic material, but both of them have two layers of dense water molecule layer, which are in different states, on the surface. On the silica surface, the two layers of water molecules, whose densities are greatly different, are close to the substrate, and hydrogen bonds are randomly arranged, which is very different from the arrangement of water molecules on the graphene surface. By making a comparison of wettability among the three materials, the characteristic water molecule arrangement on graphene surface is obtained: there exist two layers of water molecules on the surface of graphene. Within the dense layer of water molecules near the substrate, the angles between the O−H bonds of water molecule and the vertical direction of substrate focus on 90°, while the hydrogen bonds on the surface are almost perpendicular to the substrate. Furthermore, it is found that adding a layer of graphene on copper (the main force between water molecules and copper is van der Waals force) will have a less influence on copper wettability. However, adding a layer of graphene on silica (the main force between water molecule and silica is from chemical bonds) will have a significant influence on the wettability of silica, i.e. not only the upper contact angle increases significantly, but also the arrangement of water molecules on the surface of the substrate becomes similar to that of graphene. These simulated results are found to be in agreement with the experimental results of Rafiee et al. [Rafiee J, Mi X, Gullapalli H, Thomas A V, Yavari F, Shi Y, Ajayan P M, Koratkar N A 2012 Nature 11 217]. This work can provide a theoretical guidance for further developing the applications of graphene in microstructure design.
Selective reflection spectrum in a quasi-lambda four-level atomic system
Authors: Meng Teng-Fei , Tian Jian-Feng , Zhou Yao-Yao ,   Department of Physics, Taiyuan Normal University, Jinzhong 030619, China
Published: December 2020

Selective reflection (SR) from the interface between transparent medium and dilute vapour is caused by the atomic vapor near the interface. The sub-Doppler structure in SR is due to the deexcitation caused by the collision between atomic vapor and the wall. Beacuse the interaction region between atomic vapor and incident light is on the order of a few hundred nanometers, SR has low optical loss and high spatial resolution. The experimental device of SR is simple. Because of the above characteristics, the SR has been widely studied and applied. The nonlinear SR spectrum of quasi lambda-type four-level system at gas-solid interface is studied theoretically in this paper. By sloving the density matrix equations, the approximate analytic solution of the matrix element associated with the probe field is obtained at normal incidence when the intensity of the probe field is very weak. The effect of the Rabi frequency, the detuning of the signal field and the detuning of the coupling field on the lineshape are analyzed by numerical simulation, respectively. Three peaks and two transparent windows appear in SR spectrum when the detuning of coupling field and signal field are both zero. The middle peak is generated due to the participation of signal field, and the other two peaks are caused by the other two fields. The linewidth and the amplitude of the middle peak can be changed by varying the Rabi frequency of signal field, and the other two peaks have little effect on the Rabi frequency of signal field. The signal generated due to the participation of signal field can be transformed from peak to transparent window when the detuning value of the signal field is equal to the Rabi frequency of coupling field. When the detuning value of the signal field is not equal to the Rabi frequency of coupling field, a dispersion-like signal between reflection peak and transparent window is generated due to the participation of signal field. The position of peak and transparent window can be manipulated by controlling the detuning value of the coupling field. When the detuning value of coupling field decreases from zero, three peaks all shift to red detuning direction. When the detuning value of coupling field is blue-detuned and increases, three peaks all shift to blue detuning direction. The numerical results can be explained by using the various electric transition pathways and dressed state theory. This study is helpful in investigating quantum coherence and dynamic process of atoms at gas-solid interface.
Growth and characterization of Ti:MgAl2O4 laser crystal by Czochralski method
Authors: Sun Gui-Hua , Zhang Qing-Li , Luo Jian-Qiao , Sun Dun-Lu , Gu Chang-Jiang , Zheng Li-Li , Han Song , Li Wei-Min ,   Anhui Provincial Key Laboratory of Photonic Devices and Materials, Anhui Institute of Optics and Fine Mechanics, Chinese Academy of Sciences, Hefei 230031, China
Published: December 2020

The melting point of Ti:MgAl2O4 crystal is as high as 2130 °C, it is a challenge to obtain a large-sized and high-quality laser crystal. By optimizing the crystal growth process, Ti:MgAl2O4 crystal with a size of 30 mm× 70 mm is successfully grown by the Czochralski method under the condition of weak reducing atmosphere. The X-ray diffraction pattern is studied, and the x-ray rocking curve indicates that the grown crystal has a high crystalline quality in terms of the lower full width at half maximum(FWHM) intensity, which provides a material basis for the next laser output experiment. In a range of 100–1000 cm–1, there are four Raman vibration peaks located at 312, 410, 675 cm–1 and 771 cm–1 respectively. The grown crystal has an absorption cutoff range of 250–318 nm and two wide absorption bands of 395–495 nm and 550–1100 nm. Excited by 271 nm, the grown crystal shows a strong broadband emission ina range of 340–650 nm with a peak centered at 480 nm. After annealing in hydrogen atmosphere, shape of the transmittance spectrum and emission spectrum are both unchanged, but the fluorescent emission intensity is significantly reduced. After annealing in air atmosphere, the original two absorption bands disappear while none of the characteristics of fluorescence emission in a 340–650 nm range changes significantly. In addition, a new fluorescence emission peak near 725 nm is observed. Combining with the ESR spectrum, what we canconfirm is that the Ti:MgAl2O4 as-grown crystal contains Ti3+ and Ti4+ ions, and no ESR signal of Ti3+ is observed after annealing in air atmosphere. Moreover, excitationspectrum is also recorded. The fluorescence lifetime is 14 μs at room temperature, which is 4–5 times that of Ti:Al2O3 crystal and Ti:BeAl2O4 crystal. Furthermore, the emission cross section of the grown Ti:MgAl2O4 crystal is calculated from the Füchtbauer-Ladenburg (F-L) formula and its value is 2 × 10–20 cm2, large emission cross section which is beneficial for realizing laser oscillation. All the above results show that the Ti:MgAl2O4 crystal is a potential crystal material for realizing broadband tunable blue laser output.
Mechanical, electronic properties and deformation mechanisms of Ti3B4 under uniaxial compressions: a first-principles calculation
Authors: Li Jun , Liu Li-Sheng , Xu Shuang , Zhang Jin-Yong ,   Hubei Key Laboratory of Theory and Application of Advanced Materials Mechanics, Wuhan University of Technology, Wuhan 430070, China,   State Key Laboratory of Advanced Technology for Materials Synthesis and Processing, Wuhan University of Technology, Wuhan 430070, China,   Institute of Advanced Material Manufacturing Equipment and Technology, Wuhan University of Technology, Wuhan 430070, China
Published: December 2020

As an important Ti-B component, Ti3B4 has been widely used in industry and military applications. However, its deformation behaviors are not clear, which greatly limits its applications. First-principles methods based on density function theory were employed to investigate the mechanical, electronic properties and deformation mechanisms of Ti3B4 under uniaxial compressions along different axis. The results show that the structure underwent a massive change under different axial compressions. Strong anisotropic of deformation behaviors in Ti3B4 was observed. The compressive strength along b-axis is the highest in Ti3B4 structure. Under a-axis compression, the interaction between intralayer Ti—Ti bonds becomes weaker as the compressive strain increases, causing the partly damage of Ti3B4. However, in this process, the structure is not destroyed and can sustain the stress continuously. After that, the interlayer Ti—Ti bonds and the intralyer B—B bonds which are along b-axis, are broken and then it causes the sudden drop in stress, implying that the Ti3B4 structure is fully destroyed. Under b-axis compression, the changes of Ti—B bonds in Ti3B4 structure lead to the decrease of stress. Similarly, the structure can sustain the stress continuously in the process. Then, the B—B bonds which are along b-axis are broken, resulting in the sudden drop in stress. Under c-axis compression, the formation of interlayer Ti—B bonds and the breakage of intralayer Ti—B bonds result in structural instability of Ti3B4. Meanwhile, the deformed Ti3B4 still exhibits a metallic feature in the crystalline state after uniaxial compressions. However, there is no noticeable pseudogap in DOS spectra for a-axis and b-axis compressions. While for c-axis compression, there still exists a pseudogap around the Fermi energy, but it moves to the lower energy. And the pseudogap becomes narrower than that of the initial structure, which means that the covalent properties of Ti3B4 are reduced after deformations. The present work provides necessary insights in understanding the mechanical behaviors and deformation mechanisms of Ti3B4, which is the basis for improving the mechanical performance of Ti3B4 at macroscale.