bluuebunny/crossref_metadata_2025 huggingface dataset:
Ligand with strong electronegativity induced blue emitting of CsPbBr3 nanocrystals
Authors: Liu Xiao-Bing , Guo Ruo-Tong , Zhong Yu-Xuan , Zhao Li-Xin , Shi Hao-Nan , Liu Li-Juan ,   College of Physcis and Engineering, Qufu Normal University, Qufu 273165, China
Published: December 2020

All-inorganic cesium lead halide (CsPbX3, X = Cl, Br, I) perovskite nanocrystals (NCs) are promising candidates for the next-generation luminescent materials due to their fascinating physic-optical properties, such as size-tunable optical band gaps, high luminescent quantum yields, and narrow emissive bandwidths. At present, the prepared CsPbX3 NCs are concentrated in the range of green and red. The research of blue CsPbX3 NCs is lacking and these CsPbX3 NCs still suffer problems of low quantum efficiency and poor stability, which limit their application areas. In this paper, 2-acrylamide-2-methyl-propionic sulfonic acid (AMPS) with strong electronegativity is used to prepare CsPbX3 NCs by the thermal injection method. All CsPbBr3 NCs each have a uniform size, good crystallization, and nanoplate morphology. The CsPbBr3 NCs each exhibit an optical absorption at 450 nm and a photoluminescence (PL) emission at 462 nm with a full width of half maximum of 20 nm. To further investigate the physical mechanism of the PL shift and explore the effect of AMPS on the transient dynamics of the photocarriers in CsPbBr3 NCs, we measure the time-resolved PL spectrum and transient absorption spectrum. It can be found that the CsPbBr3 NCs have only one lifetime of 222 ns, which is one order of magnitude longer than that of the CsPbBr3 NCs without AMPS. Meanwhile, there is no obvious transient absorption signal. Based on the above experimental results, this blue shift is caused by three reasons. Firstly, AMPS has a strong attraction to the excited electrons, which causes the electrons in the excited state to stay for a long time before returning to the ground state. Because of the relaxation behavior before the radiation transition, the energy released by the radiation transition is larger and the fluorescence wavelength is shorter. Secondly, the prepared CsPbBr3 NCs have stronger quantum confinement than CsPbBr3 NCs with cubic block morphology. Finally, AMPS can passivate the surface defects of CsPbBr3 NCs more effectively. The prepared CsPbBr3 NCs have less defects, which also causes the PL to be blue-shifted. This study provides not only a method of synthsizing the CsPbBr3 NCs with blue emitting but also an insight into the surface engineering or physical functionalization of inorganic perovskite NCs.
Design and research of two-dimensional magneto-optical trap of sodium atom using permanent magnets
Authors: Li Zi-Liang , Shi Zhen-Lian , Wang Peng-Jun ,   State Key Laboratory of Quantum Optics and Quantum Optics Devices, Institute of Opto-Electronics, Shanxi University, Taiyuan 030006, China,   Collaborative Innovation Center of Extreme Optics, Shanxi University, Taiyuan 030006, China
Published: December 2020

It is helpful to make full use of the laboratory space by simplifying the cold atom experimental system, especially in the area of aerospace and precision measurement. We present a two-dimensional magneto-optical trap (2DMOT) for sodium atoms, whose magnetic field is produced by four sets of permanent magnets, and the residual field in the vertical direction is used for a Zeeman slower. The atoms are cooled and trapped in a 2DMOT which provides a highly efficient atomic flux for three-dimensional magneto-optical trap (3DMOT) in a high-vacuum chamber. The maximum 3DMOT loading rate is measured to be 2.3 × 109/s by optimizing the parameters of the Zeeman slower and the 2DMOT. The atom number trapped in 3DMOT is 6.2 × 109. The 2DMOT designed by using permanent magnets has the property of compact structure and simple size, which can be used to cool and trap other neutral atoms.
Dynamic behaviors of RDX single crystal under ramp compression
Authors: Chong Tao , Mo Jian-Jun , Zheng Xian-Xu , Fu Hua , Zhao Jian-Heng , Cai Jin-Tao ,   Institute of Fluid Physics, China Academy of Engineering Physics, Mianyang 621900, China,   Institute of Applied Electronics, China Academy of Engineering Physics, Mianyang 621900, China
Published: December 2020

The dynamics of RDX single crystal under ramp wave loading is studied experimentally and numerically. The ramp wave loading experiments on RDX single crystal in the orientation of (210) and (100) within 15 GPa are carried out with the magnetic driven device CQ-4, which can provide a loading pressure waveform with a rising time of 450–600 ns. The particle velocity curves of the interface between RDX single crystal and LiF window are obtained with the photonic Doppler velocimetry (PDV). The velocity profiles show an obvious three-wave structure, indicating that the RDX undergoes physical processes such as elastic-plastic transition and α-to-γ phase transition in the loading section. The stress yield limits of different crystallographic orientations of RDX single crystal show obvious difference. The onset phase transition pressures in two crystallographic orientations are the same, which is between 3.5 GPa and 4.0 GPa. The pressure range of phase transition is between initial phase transition pressure and 5 GPa. The γ phase is stable from 5 GPa to 15 GPa. The Hayes multi-phase equation of state and non-equilibrium phase transition kinetic model are employed to simulate the experimental process, and the numerical results can well describe the experimental physical processes such as elastoplastic transformation and phase transformation in the loading section. The calculated results reveal that the correction of the bulk modulus with pressure is necessary under ramp wave compression.
State-to-state dynamics of D + DBr reaction
Authors: Yuan Fang-Yuan , Zhu Zi-Liang ,   College of General Education, Weifang University of Science and Technology, Shouguang 262700, China
Published: December 2020

The state-to-state quantum dynamics studies of the abstraction channel D + DBr → Br + D2 and exchange channel \begin{document}$\rm D' + DBr \to D + D'Br$\end{document} of the D +DBr reaction are carried out by using the time-dependent wave packet method with second-order split operator in a collision energy range from 0 to 2.0 eV. The potential energy surface reported by Li et al. (Li W T, He D, Sun Z G 2019 J. Chem. Phys. 151 185102) is adopted in this work. The dynamics properties such as reaction probability, integral cross section (ICS), differential cross section (DCS), the distribution of product ro-vibrational states, specific-state rate constant, etc. are reported and compared with available theoretical and experimental values. The ICSs are compared with the values reported by Zhang et al. and good agreement is achieved between each other, except a little difference at high collision energy. The specific-state rate constants of the title reaction are studied in a temperature range from 200 to 1000 K and present values are in good agreement with experimental data and the Zhang et al.’s results. For abstraction reaction, the backward DCSs reflect the head on “rebound” mechanism dominates in the low collision energy region and abstract mechanism plays a dominant role for the abstraction reaction at high collision energy. In addition, sideward DCSs are observed which stem from the crossing of the two electronic states on the potential energy surface and these values are not reliable. For exchange reaction, the head on “rebound” mechanism dominates the reaction in the collision energy range studied. However, the forward and sideward DCSs are more and more apparent as the collision energy increases.
Objective assessment of image quality based on image content contrast perception
Authors: Yao Jun-Cai , Shen Jing ,   School of Computer Engineering, Nanjing Institute of Technology, Nanjing 211167, China,   School of Physics and Telecommunication Engineering, Shaanxi University of Technology, Hanzhong 723000, China,   School of Information and Communications Engineering, Xi’an Jiaotong University, Xi’an 710049, China
Published: December 2020

Image quality assessment (IQA) plays a very important role in acquiring, storing, transmitting and processing image and video. Using the characteristics of human visual perception and the features of the gray, gradient, local contrast, and blurring of image, an IQA method based on the image content contrast perception is proposed in the paper, which is called MPCC. In the proposed method, firstly, combining with the characteristics of human visual perception, based on the definition of the contrast in physics, a novel definition for image quality and its calculation method are proposed. Then, based on the gray gradient co-occurrence matrix, a novel concept, namely the gray-gradient entropy of image, and its calculation method, are proposed. And based on the gray-gradient entropy, local contrast and blurring of image, a method of describing the image content and their visual perception are proposed. Finally, based on the image content features and the image quality definition, an IQA method and its mathematical model are proposed by comprehensive analysis. Further, the proposed IQA model MPCC is tested by using 119 reference images and 6395 distorted images from the five open image databases (LIVE, CSIQ, TID2008, TID2013 and IVC). Moreover, the influences of the 52 distortion types on IQA are analyzed. In addition, in order to illustrate the advantages of the MPCC model, it is compared with the seven existing typical IQA models in terms of the accuracy, complexity and generalization performance of model. The experimental results show that the accuracy PLCC of the MPCC model can achieve 0.8616 at lowest and 0.9622 at most in the five databases; among the 52 distortion types, the two distortion types, namely the change of color saturation and the local block-wise distortions of different intensity, have the greatest influence on IQA, and the accuracy PLCC values of the seven existing IQA models are almost all below 0.6, but the PLCC of the MPCC model can reach more than 0.68; and the comprehensive benefit of the performance of the MPCC model is better than those of the seven existing IQA models. These results of test and comparison above show that the proposed IQA method is effective and feasible, and the corresponding model has an excellent performance.
Wide-angle method for vortex electromagnetic wave generation using field transformation
Authors: Feng Jia-Lin , Shi Hong-Yu , Wang Yuan , Zhang An-Xue , Xu Zhuo ,   Key Laboratory for Multifunctional Materials and Structures, Ministry of Education, Xi’an Jiaotong University, Xi’an 710049, China,   Faculty of Electronics and Information, Xi’an Jiaotong University, Xi’an 710049, China,   Key Laboratory for Electronic Materials Research Laboratory, Ministry of Education, Xi’an Jiaotong University, Xi’an 710049, China
Published: December 2020

The Field transformation (FT) is a novel theory for controlling the polarization and impedance of electromagnetic waves, which is independent on the angle of incidence. Thus, the FT method is superior for wide-angle devices design. In this paper, we propose a wide-angle method for generating vortex beam based on the FT theory. According to this method, an artificial media for vortex beam generation is designed and simulated, which demonstrates the proposed method. The designed artificial media is a multi-layered structure, which can generate vortex beam of order 2 with an incident angle stability up to 60°.
Synchronization in coupled oscillators with multiplex interactions
Authors: Wang Xue-Bin , Xu Can , Zheng Zhi-Gang ,   Institute of Systems Science, Huaqiao University, Xiamen 361021, China,   College of Information Science and Engineering, Huaqiao University, Xiamen 361021, China
Published: December 2020

The study of synchronizations in coupled oscillators is very important for understanding the occurrence of self-organized behaviors in complex systems. In the traditional Kuramoto model that has been extensively applied to the study of synchronous dynamics of coupled oscillators, the interaction function among oscillators is pairwise. The multiplex interaction mechanism that describes triple or multiple coupling functions has been a research focus in recent years. When the multiplex coupling dominates the interactions among oscillators, the phase oscillator systems can exhibit the typical abrupt desynchronization transitions. In this paper, we extensively investigate the synchronous dynamics of the Kuramoto model with mean-field triple couplings. We find that the abrupt desynchronization transition is irreversible, i.e. the system may experience a discontinuous transition from coherent state to incoherent state as the coupling strength deceases adiabatically, while the reversed transition cannot occur by adiabatically increasing the coupling. Moreover, the coherent state strongly depends on initial conditions. The dynamical mechanism of this irreversibility is theoretically studied by using the self-consistency approach. The neutral stability of ordered state is also explained through analyzing the linear-stability of the incoherent state. Further studies indicate that the system may experience a cascade of desynchronized standing-wave transitions when the width of the distribution function of natural frequencies of oscillators is changed. At the critical coupling, the motion of coupled oscillators in high-dimensional phase space becomes unstable through the saddle-node bifurcation and collapses into a stable low-dimensional invariant torus, which corresponds to the standing-wave state. The above conclusions and analyses are further extended to the case of multi-peak natural-frequency distributions. The results in this work reveal various collective synchronous states and the mechanism of the transitions among these macroscopic states brought by multiplex coupling. This also conduces to the in-depth understanding of transitions among collective states in other complex systems.
Temperature dependence of nitrogen-vacancy optical center in diamond
Authors: Wang Kai-Yue , Guo Rui-Ang , Wang Hong-Xing ,   Key Basic Materials Collaborative Innovation Center of Shanxi Province, School of Materials Science and Engineering, Taiyuan University of Science and Technology, Taiyuan 030024, China,   Key Laboratory for Physical Electronics and Devices, Ministry of Education, Faculty of Electronic and Information Engineering, Xi’an Jiaotong University, Xi’an 710049, China
Published: December 2020

Diamond, a wide band gap semiconductor material, has been attracting interest in several fields from electrics and optics to biomedicine and quantum computing due to its outstanding properties. These properties of diamond are related to its unique lattice and optically active defect centers. In this paper, the dependence of nitrogen-vacancy (NV) center on measurement temperature is studied by using the low-temperature photoluminescence (PL) spectroscopy in a temperature range of 80–200 K. The results show that with the increase of the measurement temperature, the zero phonon lines of NV defects are red-shifted, its intensity decreases and its full width at half maximum increases. These results are attributed to the synergetic process of the lattice expansion and quadratic electron-phonon coupling. The NV— and NV0 centers have similar values in the quenching activation energy and the thermal softening coefficient, resulting from their similar structures. The small differences may be associated with the electron-phonon coupling. The broadening mechanism of the NV centers is carefully distinguished by \begin{document}$T^3,\; T^5,\; T^7$\end{document} Voigt function fitting with the relation. These results show that the full width at half maximum of the Gaussian component of NV— and NV0 centers are randomly distributed near 0.1 meV and 2.1 meV, respectively, while the full width at half maximum of the Lorentz component of NV— and NV0 centers increase with measurement temperature increasing. The full width at half maximum of Lorentz of NV— and NV0 centers conform to the \begin{document}$ T^3 $\end{document} relationship. It can be proved that under the action of the fluctuating field, the zero phonon lines of the NV defects exhibit an obvious homogeneous widening mechanism.
Research on acousto-optic switch based on optical tamm state
Authors: Zhang Ruo-Yu , Li Pei-Li , Gao Hui ,   Department of Opto-Electronic Engineering, Nanjing University of Posts and Telecommunications, Nanjing 210023, China
Published: December 2020

An acousto-optic switch scheme based on optical tamm state (OTS) is proposed. The acousto-optic switch’s structure is one-dimensional photonic crystal heterostructure, which is composed of three materials: silicon dioxide, gallium arsenide and tellurium dioxide. All three materials are acousto-optic materials, which can ensure the acousto-optic effect when the ultrasonic wave and the light wave are incident at the same time. Due to the acousto-optic effect, the refractive index and thickness of one-dimensional photonic crystal heterostructures are changed by ultrasonic. The acousto-optic switch changes the ultrasonic amplitude to shift the intrinsic wavelength of OTS to the shorter wave direction. With the increase of ultrasonic amplitude, the intrinsic wavelength of OTS hardly changes after the amplitude exceeds 0.4 nm. This means that the ultrasonic wave with an amplitude of 0.4 nm can shift the intrinsic wavelength to 1538 nm. The acousto-optic switch realizes the on-off function within the permitted range. In this paper, the theoretical model of the acousto-optic switch is established. The propagation of ultrasonic wave in one-dimensional photonic crystal heterostructure is analyzed by theoretical model. The propagation of light in the medium after acousto-optic effect is analyzed by transmission matrix method. The simulation is carried out through COMSOL Multiphysics software. The results show OTS exists and localization can be seen in the electric field diagram. The acousto-optic switch of 1548.8–1551.7 nm can be realized by applying certain amplitude of ultrasonic or not. In this wavelength range, the extinction ratio is not lower than 12 dB and the insertion loss is not higher than 0.97 dB. The maximum extinction ratio is 13.17 dB, and the minimum insertion loss is only 0.65 dB. The acousto-optic switch of 1536.6–1543.3 nm can be realized by applying ultrasonic wave with amplitude corresponding to the length of incident light. In this wavelength range, the extinction ratio is not lower than 12 dB, and the insertion loss is not higher than 0.99 dB. The maximum extinction ratio is 13.15 dB, and the minimum insertion loss is only 0.65 dB. The response time of the acousto-optic switch is less than 13 ns. The acousto-optic switch has the characteristics of high extinction ratio and low insertion loss. It has a good application prospect and can be effectively applied in future optical communication.
Study on two-photon induced ultrafast carrier dynamcis in Ge-doped GaN by transient absorption spectroscopy
Authors: Fang Yu , Wu Xing-Zhi , Chen Yong-Qiang , Yang Jun-Yi , Song Ying-Lin ,   Jiangsu Key Laboratory of Micro and Nano Heat Fluid Flow Technology and Energy Application, School of  Physical Science and Technology, Suzhou University of Science and Technology, Suzhou 215009, China,   School of Physical Science and Technology, Soochow University, Suzhou 215006, China
Published: December 2020

Gallium nitride (GaN) is a key material in blue light-emitting devices and is recognized as one of the most important semiconductors after Si. Its outstanding thermal conductivity, high saturation velocity, and high breakdown electric field have enabled the use of GaN for high-power and high-frequency devices. Although lots of researches have been done on the optical and optoelectrical properties of GaN, the defect-related ultrafast dynamics of the photo-excitation and the relaxation mechanism are still completely unclear at present, especially when the photo-generated carrier concentration is close to the defect density in n-type GaN. The transient absorption spectroscopy has become a powerful spectroscopic method, and the advantages of this method are contact-free, highly sensitive to free carriers, and femtosecond time resolved. In this article, by employing optical pump and infrared probe spectroscopy, we investigate the ultrafast photo-generated carriers dynamics in representative high-purity n-type and Ge-doped GaN (GaN:Ge) crystal. The transient absorption response increased as probe wavelengths increased, and hole-related absorption was superior to electron-related absorption, especially at 1050 nm. The transient absorption kinetics in GaN:Ge appeared to be double exponential decay under two-photon excitation. By modelling the carrier population dynamics in energy levels, which contained both radiative and non-radiative defect states, the carrier dynamics and carrier capture coefficients in GaN: Ge can be interpreted and determined unambiguously. The faster component (30–60 ps) of absorption decay kinetics corresponded to the capturing process of holes by negatively charged acceptor CN. However, the capturing process was limited by the recombination of electron and trapped holes under higher excitation after the saturation of deep acceptors. As a result, the slower component decayed slower as the excitation fluence increased. Moreover, the experimental and theoretical results found that, the carrier lifetime in n-GaN can be modulated by controlling the defect density and carrier concentration under a moderate carrier injection, making GaN applicable in different fields such as LED and optical communication.
High performance temperature and refractive index dual-purpose sensor based on the ethanol-sealed metal-dielectric-metal waveguide
Authors: Qi Yun-Ping , Zhang Ting , Guo Jia , Zhang Bao-He , Wang Xiang-Xian ,   College of Physics and Electronic Engineering, Northwest Normal University, Lanzhou 730070, China,   School of Science, Lanzhou University of Technology, Lanzhou 730050, China
Published: December 2020

In order to enhance the working performance of existing temperature sensor and refractive index sensor of sub-wavelength waveguide, the design of ring regular octagon surface plasmon resonance sensor with sharp transmission peak, high sensitivity and high integration was proposed in this paper based on surface plasmon polaritons. The feasibility of using ethanol as a thermosensitive filler to establish a linear conversion relationship between temperature and effective refractive index was analyzed theoretically. The reason why the real part of effective refractive index changes abruptly with the change of waveguide width is also explained. The multimode interference coupled mode theory (MICMT) was used to fit and analyze the transmission peak of the sensor, and then the finite element methods (FEM) is used for simulation analysis. Results obtained by the theory of the MICMT are consistent very well with those from simulation. In order to obtain the optimal parameter setting of the ring regular octagon surface plasmon resonance sensor, various parameters of the sensor are simulated by FEM. It is found that increasing L and decreasing H will improve the sensitivity of the sensor, while decreasing parameter w can not only improve the amplitude of transmission peak, but also keep the sensitivity unchanged. This characteristic of parameter w greatly improves the robustness of the sensor. All kinds of physical phenomena in this paper are analyzed in detail. Firstly, the phenomenon of transmission peak displacement caused by parameter changes is explained through the analysis of magnetic field distribution, and then the phenomenon of inconsistent sensitivity of different transmission peaks is explained through photon energy formula. Compared with the previous structural design, the dual-purpose sensor has many advantages such as wide operating wavelength range, narrow full width at half maximum and easy to integrate. As a temperature sensor and refractive index sensor, its sensitivity was as high as 0.9 nm/℃ and 2400 nm/RIU. The study of this structure broke through the limitations of some traditional cavities, in order to provide a high- performance cavity selection for the micro-nano photon temperature and refractive index dual-purpose sensor based on the design of surface plasmon polaritons in the future.
Enhanced third-order nonlinear processes based on Raman resonance
Authors: Pei Li-Ya ,   College of Mathematics and Physics, Beijing University of Chemical Technology, Beijing 100029, China
Published: December 2020

We observe experimentally huge enhanced four-wave mixing based on Raman resonance in an 85Rb atomic vapor system. With the decrease of coupling field power or the increase of experimental temperature, the signal tends to be narrowed down in linewidth, and be basically the same spectrum as the stimulated Raman spectroscopy. It is found that the macroscopic polarization interference effect plays a crucial role in determining the nonlinear spectra. Further more, in the Doppler-broadened Λ-type energy level system, there is a strong relationship among electromagnetically induced transparency, four-wave mixing and stimulated Raman spectroscopy. The sharp transparent window in electromagnetically induced transparency can be interpreted as the suppression of Raman gain on the linear absorption of the probe field. However, the four-wave mixing signal is a new field generated by the atomic vapor system, and it comes from the Raman gain which is affected and modified by the absorption and dispersion of the nonlinear optical medium. It shows that in a Λ-type Doppler-broadened system, in essence, both the electromagnetically induced transparency and enhanced four-wave mixing stem from stimulated Raman scattering based on the third-order nonlinear processes, just the spectra of which are from different ways and objects of detection.
Investigation of an X band high efficiency klystron-like relativistic backward wave oscillator
Authors: Yang De-Wen , Chen Chang-Hua , Shi Yan-Chao , Xiao Ren-Zhen , Teng Yan , Fan Zhi-Qiang , Liu Wen-Yuan , Song Zhi-Min , Sun Jun ,   Science and Technology on High Power Microwave Laboratory, Northwest Institution of Nuclear Technology, Xi’an 710024, China,   Tsinghua University, Beijing 100084, China
Published: December 2020

This paper investigates an X band high efficiency klystron-like relativistic backward wave oscillator (RBWO) in detail. The klystron-like RBWO consists of a pre-modulation cavity, a resonant reflector with a ridge, a sectional slow wave structure, and an extraction cavity. First, this paper gives some theoretical studies about beam modulation and energy extraction. For beam modulation, the optimized distance between the pre-modulation cavity and the resonant reflector is studied theoretically, and theoretical results agree well with simulation results. For energy extraction, an ellipse extraction cavity with high power capacity is come up with, and the electric field on the inner surface of the ellipse extraction cavity decreases by 25% in PIC simulation. Also, the paper analyzes the effect of the position of dumped electron on conversion efficiency. Interestingly, it’s found that the efficiency dramatically decreases with the increase of the distance between the extraction cavity and the position of dumped electron, which is caused by the increase of potential energy of electron and the decrease of electric field. Fortunately, we find that the use of guiding magnet with special magnetic field distribution almost eliminate this unfavorable effect. Besides, effects of the distance between the cathode and anode Lak are investigated. It’s shown that the optimized diode voltage decrease with the increase of the distance Lak, and the conversion efficiency is higher at larger Lak. The experimental studies are also given. The power capacity of ellipse extraction cavity is verified, also we find that the efficiency is enhanced by 10% and the width of microwave pulse increases by 7 ns when the roughness of RF structure surface is improved from Ra 0.4 μm to Ra 0.05 μm. Typically, the klystron-like RBWO outputs X band high power microwave with power of 2.15 GW, with pulse duration of 25 ns, and with conversion efficiency of 50%(± 5%). Experimental results agree well with theoretical and PIC simulation results.
Micromagnetic simulations of reversal magnetization in core ((Nd0.7, Ce0.3)2Fe14B)-shell (Nd2Fe14B) type
Authors: Li Dong , Dong Sheng-Zhi , Li Lei , Xu Ji-Yuan , Chen Hong-Sheng , Li Wei ,   Division of Functional Materials Research, Central Iron and Steel Research Institute, Beijing 100081, China
Published: December 2020

The effects of core size, shell thickness and shell distribution on the coercivity of single-grain core ((Nd0.7,Ce0.3)2Fe14B)-shell (Nd2Fe14B) magnets are studied by programming and modeling them through using the C++ language. All the micromagnetic simulations are carried out via object oriented micro magnetic framework (OOMMF). The results show that the coercivity decreases with the increase of core size when the shell thickness is constant. It is considered that for the grain, the increase in the size of the core leads the average magnetocrystalline anisotropy field to increase and the total demagnetization energy to increase, thereby contributing to the magnetization reversal occurring under a smaller external field. When the core size is unchanged, as the shell thickness increases gradually, the coercivity first increases and then decreases. The analysis of the position of the nucleation point shows that the reason why the coercivity increases in the early period is mainly that the nucleation point is located at the core-shell junction and belongs to the core. As the thickness of the shell increases, the exchange interaction effect between the magnetic moment of the shell and the one of the nucleation point is strengthened, so a larger external field is needed in the nucleation process. As for the decrease of the coercivity in the later period, the main reason is that the nucleation points are exactly the vertices of the shell (also the vertices of the grain), and the increase of the shell thickness conduces to increasing the total demagnetization energy, so the nucleation points can be formed under a smaller external magnetic field. With core size and shell volume kept unchanged, when the shell is distributed on the two easy-axis planes (i.e. the planes perpendicular to the easy axis) of the core, the coercivity of the magnet reaches a largest value. It is because that the nucleation points are located at the vertices of the shell (also the vertices of the grain), of which the magnetocrystalline anisotropy field is larger, and the demagnetization field is smaller. Via magnetocrystalline anisotropy field, the demagnetization energy, nucleation point, etc, the changes of coercivity in above cases can be explained.
Polarization-dependent nonlinear optical response in GeSe2
Authors: Ouyang Hao , Hu Si-Yang , Shen Man-Ling , Zhang Chen-Xi , Cheng Xiang-Ai , Jiang Tian ,   College of Advanced Interdisciplinary Studies, National University of Defense Technology, Changsha 410073, China,   State Key Laboratory of Pulsed Power Laser Technology, Changsha 410073, China,   Hunan Provincial Key Laboratory of High Energy Laser Technology, Changsha 410073, China
Published: December 2020

Germanium diselenide (GeSe2), a layered IV-VI semiconductor, has an in-plane anisotropic structure and a wide band gap, exhibiting unique optical, electrical, and thermal properties. In this paper, polarization axis Raman spectrum and linear absorption spectrum are used to characterize the crystal axis orientation and energy band characteristics of GeSe2 flake, respectively. Based on the results, a micro-domain I scan system is used to study the optical nonlinear absorption mechanism of GeSe2 near the resonance band. The results show that the nonlinear absorption mechanism in GeSe2 is a superposition of saturation absorption and excited state absorption, and is strongly dependent on the polarization and wavelength of incident light. Under near-resonance excitation (450 nm), the excited state absorption is more greatly dependent on polarization. With different polarizations of incident light, the modulation depth can be changed from 4.6% to 9.9%; for non-resonant excitation (400 nm), the modulation depth only changes from 7.0% to 9.7%. At the same time, compared with saturation absorption, the polarization-dependent excited state absorption is greatly affected by the distance away from the resonance excitation wavelength.
Tunable metasurfaces based on phase-change materials
Authors: Yan Wei , Wang Ji-Yong , Qu Yu-Rui , Li Qiang , Qiu Min ,   Key Laboratory of 3D Micro/Nano Fabrication and Characterization of Zhejiang Province, School of Engineering, Westlake University, Hangzhou 310024, China,   Institute of Advanced Technology, Westlake Institute for Advanced Study, Hangzhou 310024, China,   State Key Laboratory of Modern Optical Instrumentation, College of Optical Science and Engineering, Zhejiang University, Hangzhou 310007, China
Published: December 2020

Due to their superior ability to control light, metasurfaces, ultrathin two-dimensional metamaterials composed of subwavelength nanostructures, have attracted great attention in recent years. Exploring geometric and material freedom in designing elementary nanostructures and their ambient environment of metasurfaces enables versatile optical devices, such as planar metalenses, holographic imaging and thermal radiators. With phase-change materials (PCMs) such as GeSbTe and VO2 integrated into metasurfaces, the optical functionalities of metasurfaces can be flexibly tuned by exploiting the phase transitions of PCMs induced by external stimuli, thereby opening up new directions and perspectives for dynamic tunable metamasurfaces. In this article, we review the recent progress of tunable metasurfaces based on PCMs, analyze their underlying working mechanisms and highlight their important applications. We conclude this review by bringing our perspectives on challenges and future directions in this field.
Ultrafast optical switches and pulse lasers based on strong nonlinear optical response of plasmon nanostructures
Authors: Zhang Duo-Duo , Liu Xiao-Feng , Qiu Jian-Rong ,   Institute of Materials Science and Engineering, Zhejiang University, Hangzhou 310027, China,   State Key Laboratory of Modern Optical Instrumentation, College of Optical Science and Engineering, Zhejiang University, Hangzhou 310027, China
Published: December 2020

Nonlinear optical (NLO) effects are ubiquitous in the interaction of light with different materials. However, the NLO responses of most materials are inherently weak due to the small NLO susceptibility and the limited interaction length with the incident light. In plasmonic nanostructures the optical field is confined near the surface of the structures, so that the electromagnetic field is greatly enhanced in a localized fashion by spectral resonance. This effect results in the enhancement of light-matter interaction and NLO response of the material. Ultrafast pulse lasers have been widely used in optical communication, precise measurement, biomedicine, military laser weapons and other important fields due to their excellent performances. Although commercial lasers become very matured, they can achieve ultra-high peak power and ultra-short pulse width and ultra-high repetition rate, but the ultra-fast pulses in the mid-to-far infrared band are seldom studied, so finding a saturable absorber material with excellent performance is of great significance for developing the pulsed lasers. In this paper, we review the recent research progress of the applications of exiton nanostructure in ultrafast optical switches and pulse lasers based on noble metal and non-noble metals. The metallic system mainly refers to gold and silver nanoparticles. For non-noble metals, we mainly introduce our researches of chalcogenide semiconductor, heavily doped oxide and titanium nitride. A variety of wide bandgap semiconductors can exhibit metal-like properties through doping. Since doping can form free carriers, when their size is reduced to a nanometer scale, they will show the characteristics of local surface plasmon resonance, thus realizing ultra-fast nonlinear optical response, and the concentration of doped carriers cannot reach the level of metal carriers, thus being able to effectively reduce the inter-band loss caused by excessively high carriers. Through pump probe detection and Z-scan testing, we found that these plasmonic nanostructures exhibit ultrafast NLO response in tunable resonance bandwidth, which has been utilized as a working material for developing the optical switch to generate the pulsed laser with duration down to a femtosecond range. These results take on their potential applications in ultrafast photonics. Finally, we make a comparison of the pros and cons among different plasmonic materials and present a perspective of the future development.
Migration characteristics of droplet condensation on end surface of single-finger microgripper
Authors: Fan Zeng-Hua , Rong Wei-Bin , Liu Zi-Xiao , Gao Jun , Tian Ye-Bing ,   School of Mechanical Engineering, Shandong University of Technology, Zibo 255049, China,   State Key Laboratory of Robotics and System, Harbin Institute of Technology, Harbin 150080, China
Published: December 2020

Liquid droplet is a prerequisite for micro-robot based on liquid medium. The investigation of the migration characteristics of condensed droplets on the end surface of a single-finger microgripper is of significance for obtaining stable droplets. The principle of flexible operation for micro-components using droplet condensation is analyzed first. The liquid bridge force acting on a microsphere is derived. A growth model of condensed droplet on the tip of a single-finger microgripper is established, including single-droplet growth, droplet coalesce, droplet movement, and pining effect. Condensation process on the tip of single-finger microgripper with a diameter of 130－400 μm is observed experimentally. Small droplets are formed by directly growing with a big growth rate in the initial stage, then the droplet growth is determined by droplet coalesce. The experimental results show that a single droplet is formed on the end surface after direct growth and droplets coalesce. The maximum droplet volume of 5.5 nL appears on the tip of a single-finger actuator with a diameter of 400 μm under the conditions of surface temperature of –5 °C, room temperature of 24 °C and humidity of 37%. The stability of the formed droplets is dominated by temperature gradients and edge effects during growth process. The distribution of condensed droplets is asymmetric while the microgripper is placed on a cooling surface with temperature gradient. A big growth rate is shown in a low temperature range. A single asymmetric droplet with an offset of 13 μm with respect to the axis of the actuator is formed, which is caused by the temperature gradient. A stable contact angle of 112° is obtained on the end surface of a single-finger microgripper with a diameter of 137 μm because of edge effect using the ambient temperature of 24 °C and humidity of 42%. Condensed droplets located on the end surface of hydrophobic microgripper are more stable than the untreated microgripper. Compared with the droplet formation (0.3 nL) on an untreated microgripper with a diameter of 150 μm, a lager stable droplet of 0.4 nL is obtained on the end face of a small microgripper with a diameter of 130 μm because of the hydrophobic action. The validity of theoretical analysis is verified by experimental results. The experimental investigation of the migration characteristics of condensed droplets on the end surface of a single-finger microgripper shows that the droplet shape can be changed by adjusting the temperature gradient and hydrophilic/hydrophobic performance, which plays an important role in achieving a stable droplet on the end surface.
Counter-rotating effect on frequency shift of flux qubit in ultrastrongly coupled circuit-quantum-electrodynamics system
Authors: Chen Zhen , Wang Shuai-Peng , Li Tie-Fu , You Jian-Qiang ,   Quantum Physics and Quantum Information Division, Beijing Computational Science Research Center, Beijing 100193, China,   Department of Physics, Zhejiang University, Hangzhou 310027, China,   Center for Quantum Information Frontier Science, Institute of Microelectronics, Tsinghua University, Beijing 100084, China,   Beijing Academy of Quantum Information, Beijing 100193, China
Published: December 2020

In recent years, quantum Rabi model has aroused considerable interest because of its fundamental importance and potential applications in quantum technologies. For a conventional cavity-quantum-electrodynamic (cavity-QED) system involving the interaction between an atom and photons in a cavity, the atom-photon coupling frequency is much smaller than the transition frequency of the atom and the frequency of the cavity mode. This cavity-QED system is usually described by the Jaynes-Cummings model in which the rotating-wave approximation can be adopted by neglecting the counter-rotating coupling terms in the Hamiltonian of the system. However, by designing the unique structure of the superconducting circuit, the ultrastrong-coupling regime can be achieved in a circuit-QED system in which the counter-rotating coupling terms become as important as the rotating terms. Thus, the rotating-wave approximation cannot be used in the ultrastrongly coupled circuit-QED system. Owing to the ultrastrong coupling, this circuit-QED system is described by the standard quantum Rabi model when a superconducting qubit is coupled only to a single resonator mode. In this work, we experimentally study an ultrastrongly coupled circuit-QED system consisting of a four-junction superconducting flux qubit and a muti-mode coplanar-waveguide resonator. The transmission-spectrum measurement and numerical simulations show that the system is in the ultrastrong-coupling regime. By changing the photon number in the resonator, we observe the frequency shift of the flux qubit via the spectroscopic measurement. This frequency shift contains the contributions from not only the rotating-coupling terms but also the counter-rotating terms, which is in good agreement with the theory. The result indicates that this ultrastrongly-coupled quantum system can be used as a good platform to investigate the quantum Rabi model and has potential applications in various aspects of quantum technology, such as quantum simulation, ultrafast quantum gates, entangled-state preparation and protected qubits.
Photoluminescence spectrum study of defects of potassium dihydrogen phosphate crystals irradiated by different laser fluences
Authors: Li Xiang-Cao , Liu Bao-An , Li Meng , Yan Chun-Yan , Ren Jie , Liu Chang , Ju Xin ,   School of Mathematics and Physics, University of Science and Technology Beijing, Beijing 100083, China
Published: December 2020

The laser-induced damage to potassium dihydrogen phosphate (KDP) crystal restricts the development of high power laser systems and attract the attention of researchers. The defects are essential for the understanding of the laser-induced damage to KDP crystals. The defects in KDP crystals are commonly related to \begin{document}$ \rm H_2PO_4^{-} $\end{document} groups. The defects of KDP crystal have been studied extensively, however the changes of defects of KDP crystal with low fluence and high fluence have not been investigated sufficiently. The synchrotron radiation technology is a sensitive method of detecting the defects. The vacuum ultraviolet photoluminescence (PL) emission spectra can provide microscopic structural changes in KDP crystals. In this work, we investigate the defects of KDP crystals irradiated with different fluences by vacuum ultraviolet PL emission spectra. The vacuum ultraviolet spectra are obtained at the 4B8 beam line in Beijing synchrotron radiation facilities. Each KDP crystal spectrum is measured from 200 to 400 nm and 400 to 800 nm. The emission spectra of KDP crystal irradiated with different fluences are fitted for illustration. Each Gaussian curve represents a kind of defect. Comparing the retired components with KDP crystal irradiated by 11.5 J/cm2, the new band at 231.55 nm emerges in the spectra of KDP crystal irradiated by 9.0 J/cm2. The intrinsic luminescence band is assigned to the radiative annihilation of self-trapped excitons. According to our previous work, the short chain structures mainly exist in the crystal irradiated by 9.0 J/cm2, and the long chain structure is mainly in the crystal irradiated by 11.5 J/cm2. The retired components have the short, medium and long chain. The length of P—O bond in the short chain is shorter than that in the long chain structure. The overlap between phosphorus 3s orbitals and oxygen 2p increases, and the radiative annihilation of STEs becomes stronger. So the band at 231.55 nm emerges in the spectrum of KDP crystal irradiated by 9.0 J/cm2. It suggests that the structure of the retired component and the structure of KDP crystal irradiated by 9.0 J/cm2 are different. The results provide an insight into the defects in KDP crystals. It is meaningful to study the mechanism of laser-induced damage to KDP crystal.
A new type of adiabatic invariant for fractional order non-conservative Lagrangian systems
Authors: Xu Xin-Xin , Zhang Yi ,   School of Mathematics and Physics, Suzhou University of Science and Technology, Suzhou 215009, China,   School of Civil Engineering, Suzhou University of Science and Technology, Suzhou 215011, China
Published: December 2020

The Herglotz variational problem is also known as Herglotz generalized variational principle whose action functional is defined by differential equation. Unlike the classical variational principle, the Herglotz variational principle gives a variational description of a holonomic non-conservative system. The Herglotz variational principle can describe not only all physical processes that can be described by the classical variational principlen, but also the problems that the classical variational principle is not applicable for. If the Lagrangian or Hamiltonian does not depend on the action functional, the Herglotz variational principle reduces to the classical integral variational principle. In this work, in order to describe the dynamical behavior of complex non-conservative system more accurately, we extend the Herglotz variational principle to the fractional order model, and study the adiabatic invariant for fractional order non-conservative Lagrangian system. Firstly, based on the Herglotz variational problem, the differential variational principle of Herglotz type and the differential equations of motion of the fractional non-conservative Lagrangian system are derived. Secondly, according to the relationship between the isochronal variation and the nonisochronal variation, the transformation of invariance condition of Herglotz differential variational principle is established and the exact invariants of the system are derived. Thirdly, the effects of small perturbations on fractional non-conservative Lagrangian systems are studied, the conditions for the existence of adiabatic invariants for the Lagrangian systems of Herglotz type based on Caputo derivatives are established, and the adiabatic invariants of Herglotz type are obtained. In addition, the exact invariant and adiabatic invariant of fractional non-conservative Hamiltonian system can be obtained by Legendre transformation. When \begin{document}$ \alpha \to 1$\end{document}, the Herglotz differential variational principle for fractional non-conservative Lagrangian system degrades into classical Herglotz differential variational principle, and the corresponding exact invariants and adiabatic invariants also degenerate into the classical exact invariants and adiabatic invariants of Herglotz type. At the end of the paper, the fractional order damped oscillator of Herglotz type is discussed as an example to demonstrate the results.