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## COVER ARTICLE

1,3,5-trinitro-1,3,5-triazacyclohexane (RDX) or hexogen, a high-insensitivity explosive, the accurately description of its energy and properties is of fundamental significance in the sense of security and application. Based on the machine learning method, high-dimensional neural network is used to construct potential function of RDX crystal. In order to acquire enough data in neural network learning, based on the four known crystal phases of RDX, the structural global search is performed under different spatial groups to obtain 15199 structure databases. Here in this study, we use nearby atomic environment to build 72 different basis functions as input neurons, in which the 72 different basis functions represent the interaction with nearby atoms for each type of element. Among them, 90% data are randomly set as training set, and the remaining 10% data are taken as test set. To obtain the better training effect, 9 different neural network structures carry out 2000 step iterations at most, thereby the 30-30-10 hidden layer structure has the lower root mean square error (RMSE) after the 1847 iterations compared with the energies from first-principles calculations. Thus, the potential function fitted by 30-30-10 hidden layer network is chosen in subsequent calculations. This constructed potential function can reproduce the first-principles results of test set well, with the RMSE of 59.2 meV/atom for binding energy and 7.17 eV/Å for atomic force. Especially, the RMSE of the four known RDX crystal phases from 1 atm to 6 GPa are 10.0 meV/atom and 1.11 eV/Å for binding energy and atomic force, respectively, indicating that the potential function has a better description of the known structures. Furthermore, we also propose four additional RDX crystal phases with lower enthalpy, which may be alternative crystal phases undetermined in experiment. In addition, based on molecular dynamics simulation with this potential function, the α-phase RDX crystal can stay stable for a few ps, further proving the applicability of our constructed potential function. Zhao Ji-Jun Acta Physica Sinica.2020, 69(23): 238702.
##### Topics
###### SPECIAL TOPIC—Flexible electronics
2020, 69 (23): 238501. doi: 10.7498/aps.69.20200928
Abstract +
Recently, neuromorphic systems capable of parallel information processing have attracted increasing attention. A neuromorphic system is desired to emulate a human brain, which consists of hundreds of billions of neurons connected with even more synapses. Synapses are important connection parts between neurons to transmit information through release and reception of neurotransmitters. A neuromorphic system could replicate brain learning, cognition and computation of a human brain to process huge data with 1016 floating point numbers per second. The high computing efficiency has attracted many researchers to study artificial synapses for application in future artificial intelligence. The synaptic weight could be adjusted by the received information. This provides a basis for the learning and computing capability of artificial synapses. So far, a number of semiconductor materials have been used in artificial synaptic devices, like some organic materials, e.g. Poly(3-hexylthiophene-2,5-diyl)(P3HT), [1]Benzothieno[3,2-b][1]benzothiophene, 2,7-dioctyl-(C8-BTBT) etc, some inorganic oxides such as zinc oxide, indium zinc oxide(IZO), indium gallium zinc oxide(IGZO), transition metal oxides, etc, and two-dimensional materials, e.g. graphene, black phosphorus, and organic-inorganic hybrid perovskite materials. Among them, transition metal oxides are attractive due to their unique layered structure and inherent properties, which are important in photohydrolysis, lithium ion batteries, and field-effect transistors. MoO3, as a typical transition-metal oxide, has been used in artificial synaptic devices, with different preparation methods, such as mechanical exfoliation, chemical vapor deposition (CVD) and chemical vapor transportation (CVT), pulse-laser deposition (PLD). Here, we report the preparation of a semiconductor layer of MoO3 nanosheets by hydrothermal method, and the use of a TiO2 nanoparticle seed layer to improve the adhesion of MoO3 nanosheets. This is a cost-effective and controllable process. The high surface-to-volume ratio of the material provides large contact area at the interface to allow easy ion diffusion. The device emulates important synaptic functions, such as excitatory post-synaptic current (EPSC), paired-pulse facilitation (PPF), spike-duration dependent plasticity (SDDP), spike-voltage dependent plasticity (SVDP) and spike-rate dependent plasticity (SRDP). This work could be an important addition to the neuromorphic research field.

## EDITOR'S SUGGESTION

2020, 69 (23): 231201. doi: 10.7498/aps.69.20200984
Abstract +
It has been found that many magnetic materials possess the properties arising from skyrmions at room temperature. In addition to the common interaction energy, chiral interaction is also needed to form the skyrmion in magnetic material. There are four chiral magnetic interactions, namely: 1) Dzyaloshinskii-Moriya (DM) interaction; 2) long-ranged magnetic dipolar interaction; 3) four-spin exchange interaction; 4) frustrated exchanged interaction. Through the competition between exchange interaction and chiral interaction, magnetic skyrmion can be realized in magnetic material subject to a certain magnetic field and temperature. The skyrmion generated by the DM interaction features small size (5–100 nm), which is easy to adjust. The skyrmion can be driven by magnetic field or ultralow current density. The magnetic materials with skyrmion can exhibit the properties related to the skyrmion Hall effect, the topological Hall effect and the emergent electrodynamics, which are closely related to the skyrmion number. The existence of skyrmion in the magnetic material can be indirectly measured by topological Hall effect. The movement of skyrmion can be driven by spin polarized current in the direction either parallel or perpendicular to the current direction. The movement of the skyrmion driven by spin polarized currents will continue when the current is present, and will disappear when the current disappears. In previous studies, magnetic skyrmions were realized in a variety of materials. However magnetic skyrmions were found only in very limited types of single crystal materials at room temperature or near room temperature. In recent years, scientists have discovered a variety of magnetic skyrmion materials at room temperature, including film materials (such as multilayer materials, artificial skyrmion materials) and crystal materialssuch as β-Mn-type Co10–x/2Zn10–x/2Mnx, Fe3Sn2. Among all kinds of room temperature magnetic skyrmion materials, the most valuable one is the multilayer film material. The Skyrmion multilayer film has the advantages of small size, adjustable material type, simple preparation, good temperature stability, good device integration,etc. At the same time, skyrmion multilayer film is very easy to optimize by adjusting and constructing a special structure that has the wanted types of materials each with a certain thickness. Artificial skyrmion material obtains artificial skyrmion by constructing a micro-nano structure, therefore the artificial skyrmion with high-temperature stability can be realized by choosing high Curie temperature materials. There are a variety of materials which can realize the skyrmion above room temperature, such as Co9Zn9Mn2 (300–390 K) and Fe3Sn2 (100–400 K). These room temperature materials further widen the temperature application range of skyrmion. The room temperature materials can be prepared or characterized by a variety of techniquesincluding sputtering for fabrication and X-ray magnetic circular dichroism-photoemission electron microscopy (XMCD-PEEM) for characterization. The discovery of the magnetic skyrmion materials at room temperature not only enriches the research content of materials science, but also makes the skyrmion widely applicable in novel electronic devices (such as racetrack memory, microwave detector, oscillators). Because the skyrmion has the advantages of small size, ultra-low driving current density, and topological stability, it is expected to produce racetrack memory based on the skyrmion with low energy consumption, non-volatile and high density. The MTJ microwave detector based on skyrmion can be achieved with no external magnetic field nor bias current but with low power input (< 1.0 μW); the sensitivity of the microwave detector can reach 2000 V·W–1. The frequency of the oscillator based on skyrmion can be tuned by magnetic field or current, and moreover, the oscillato is very easy to integrate with IC. In this paper, first, the basic characteristic of magnetic skyrmion is introduced; and then room temperature magnetic skyrmion is reviewed; finally the advances of the racetrack memory, microwave detectors and oscillators are introduced, highlighting the development trend of room temperature magnetic skyrmion.
2020, 69 (23): 238101. doi: 10.7498/aps.69.20200692
Abstract +
Diamond has a series of extreme characteristics superior to other materials, and also very wide application scope. The large diamond single crystal can play a role in its functional characteristics, which has become a research hotspot. In this paper, we introduce the principle and process of synthesizing large diamond single crystal by temperature gradient method (TGM) under high pressure and high temperature (HPHT), and summarizes the research status and research directions of different types of and additives-doped large diamond single crystals respectively. The principle of the temperature gradient method is that the carbon source, driven by the temperature gradient, diffuses from the high concentration region at the high temperature end to the low concentration region at the low temperature end, and diamonds are grown on the seed crystal. The growth rate of diamond crystal is controlled by adjusting the axial temperature gradient at synthesis cell, and the shape growth of Ib-type large diamond is controlled by the distribution in the V-shaped growth area. We introduce different kinds of diamond large single crystals from five aspects. Firstly, the Ia-type diamond large single crystal can be obtained by the annealing treatment of Ib-type diamond under HPHT. The conversion rate of C centre to A centre for nitrogen in diamond is improved by optimizing the conditions of HPHT. Secondly, the Ib-type larger diamond is studied very much in the following areas: the analysis of its surface characteristic, the control of inclusions and cracks, the precipitation mechanism and the elimination measures of regrown graphite and the mass production technology of multiseed method. Thirdly, IIa-type large diamond single crystal is introduced in which the nitrogen getter is selected due to the ability Al and Ti (Cu) to getter nitrogen, the catalyst is selected because of its effect on the nitrogen content in the diamond synthesized with Fe or Ni based catalyst, and the elimination method of microcrystalline graphite precipitation is presented by analyzing its mechanism. Fourthly, the boron elements exist in IIb-type diamond and have influence on the growth characteristics of synthetic diamond. Fifthly, introduced is the research status of diamond synthesized with B, N, S, P doping elements, in which its individual substance or their compound is used as a doping source or boron and other elements with small radius are used as co-doping agent. Then S or P with B co-doping is more conducive to the improvement of the performance of n-type diamond large single crystal semiconductor. Therefore, it is proposed that the large diamond single crystal need strengthening in mass production of IIa-type large diamond single crystal, superconducting characteristics of IIb-type large diamond single crystal, and doping of n-type semiconductors.
###### GENERAL
2020, 69 (23): 230201. doi: 10.7498/aps.69.20201052
Abstract +
Triboelectric nanogenerator (TENG) and its self-powered sensor based on the principles of contact electricity generation and electrostatic induction have important application prospects in the fields of new energy and internet of things (IoT). In the contact separation process of polymer materials with different electronegativity values, due to the transfer of electrons, a changing electrostatic field will be generated in the space around the polymer. In the existing TENG research, the field strength perpendicular to the plane of the friction layer and the electrode layer is mainly used to generate electrostatic induction, and the electric field effect around the polymer is ignored. According to the principle of electrostatic induction, the internal charge of the conductor in the electric field will be redistributed, which provides a way for the conductor to generate an induced electrical signal on the surface of the conductor without contacting the friction material. In this paper, we design a non-contact triboelectric nanogenerator (NC-TENG) based on changing electrostatic field. The influence of the distance between the conductor and the friction material, the induction area of the conductor and the position of the conductor relative to the friction material on the induced electrical output performance are studied when silicone rubber and nitrile rubber are used as a friction material. The results show that the NC-TENG can produce a stable electrical signal output when the conductor is completely separated from the friction material. The induced voltage of NC-TENG decreases with the increase of the distance between the conductor and the friction material, and gradually increases with the increase of the conductor's induction area. For the friction material with a size of 30 mm × 30 mm, the electrical output of NC-TENG tends to be stable when its conductor area is 60 mm × 45 mm. In addition, the different orientation of the conductor relative to the friction material also has a significant effect on the induced electrical output. The NC-TENG designed in this paper provides a novel electrical output generation mode, which provides a higher possibility for the subsequent research on TENG and the application of self-powered sensors.
2020, 69 (23): 230301. doi: 10.7498/aps.69.20200802
Abstract +
$C$ at the last moment $\tau$ is studied in detail. The results show that on the whole, $C$ first increases from zero to a maximal value with $\tau$ increasing, then decreases, and finally it approaches to zero. At first, the system is in a pure state and $C=0$. Finally, the system is in a maximally mixed state due to the interaction with the environment and $C=0$ again. When the number of measurements $N$ is relatively small, $C$ fluctuates with $\tau$ increasing, but when $N$ is relatively large, the fluctuations disappear. Due to the quantum Zeno effect, as $N$ is larger, the variation of $C$ with $\tau$ is similar to that for the case of no intermediated measurement. Because of the quantum superposition principle, uncertainty principle, and quantum collapse, quantum measurement can disturb the system, so quantum statistical complexity $C$ exhibits a complex behavior. In the quantum realm, the complexity of a system can be transformed into a resource. The quantum state needs creating, operating, or measuring. Therefore, all our results provide a theoretical reference for the optimal controlling of quantum information process and condensed matter physics. At the same time, the number of the degrees of freedom is two for the damped and linearly driven two-level system, so this system is simple and easy to study. The complexity of such a system can be tailored by properly tuning the driving strength. Therefore, the model can be used as a typical example to study the quantum statistical complexity."> Recently, quantum statistical complexity based quantum information theory has received much attraction. Quantum measurements can extract the information from a system and may change its state. At the same time, the method of measuring multiple quantum is an important quantum control technique in quantum information science and condensed matter physics. The main goal of this work is to investigate the influence of multiple quantum measurements on quantum statistical complexity. It is a fundamental problem to understand, characterize, and measure the complexity of a system. To address the issue, a damped and linearly driven two-level system (qubit) is taken for example. The driving amplitude and dephasing intensity are considered. By using the Lindblad equation and the Born-Markov approximation, the time evolution of the system can be obtained. Under multiple intermediated measurements, the system has a complex dynamic behavior. Quantum statistical complexity $C$ at the last moment $\tau$ is studied in detail. The results show that on the whole, $C$ first increases from zero to a maximal value with $\tau$ increasing, then decreases, and finally it approaches to zero. At first, the system is in a pure state and $C=0$. Finally, the system is in a maximally mixed state due to the interaction with the environment and $C=0$ again. When the number of measurements $N$ is relatively small, $C$ fluctuates with $\tau$ increasing, but when $N$ is relatively large, the fluctuations disappear. Due to the quantum Zeno effect, as $N$ is larger, the variation of $C$ with $\tau$ is similar to that for the case of no intermediated measurement. Because of the quantum superposition principle, uncertainty principle, and quantum collapse, quantum measurement can disturb the system, so quantum statistical complexity $C$ exhibits a complex behavior. In the quantum realm, the complexity of a system can be transformed into a resource. The quantum state needs creating, operating, or measuring. Therefore, all our results provide a theoretical reference for the optimal controlling of quantum information process and condensed matter physics. At the same time, the number of the degrees of freedom is two for the damped and linearly driven two-level system, so this system is simple and easy to study. The complexity of such a system can be tailored by properly tuning the driving strength. Therefore, the model can be used as a typical example to study the quantum statistical complexity.
2020, 69 (23): 230302. doi: 10.7498/aps.69.20200796
Abstract +
$\left| {\rm{0}} \right\rangle$, qubit in the superposition state of $\left| {\rm{0}} \right\rangle$ and $\left| {\rm{1}} \right\rangle$, and qubit in the superposition state of $\left| {\rm{0}} \right\rangle$, $\left| {\rm{1}} \right\rangle$ and $\left| {\rm{2}} \right\rangle$. Then, the Aulter-Townes splitting (ATS) experiment can be fulfilled in this system. Unlike the method given by Novikov et al. [Novikov S, Robinson J E, Keane Z K, et al. 2013 Phys. Rev. B 88 060503], our method only needs to apply continuous microwave excitation signal to the qubit, and does not need to carry out precise timing test on the qubit, thus reducing the test complexity of observing ATS effect. The ATS effect in resonance and non-resonance regime are observed. In the resonance ATS experiment, in order to obtain the peak value and frequency of resonance peak, Lorentz curve can be used for fitting peaks, and the ATS curve of double peak can be fitted by adding two Lorentz curves together. In the non-resonance ATS experiment, the detection signal is scanned, and the ATS double peak will shift with the different coupling signal detuning, forming an anti-crossing structure. The two curves formed by crossing free structure give two eigenvalues of Hamiltonian. By solving the equation, the experimental results can also be found to be consistent with the theoretical results.">The superconducting quantum bit(qubit) based on Josephson junction is a macroscopic artificial atom. The basic parameters of the artificial atom can be changed by micro and nano machining. The three-dimensional (3D) Transmon qubit is a kind of qubit with the longer decoherence time. It is coupled with a 3D superconducting cavity by means of capacitance. It is a man-made coupling system between atom and cavity field, which can verify the effects of atomic physics, quantum mechanics, quantum optics and cavity quantum electrodynamics. In this paper, transmon qubits are prepared by the double angle evaporation method, and coupled with aluminum based 3D superconducting resonator to form 3D transmon qubits. The basic parameters of 3D transmon are characterized at an ultra-low temperature of 10 mK. The 3D transmon parameters are EC = 348.74 MHz and EJ = 11.556 GHz. The coupling coefficient g2/Δ between qubit and the 3D cavity is 43 MHz, which is located in the dispersive regime. The first transition frequency of qubit is f01 = 9.2709 GHz, and the second transition frequency is f12 = 9.0100 GHz. The 3D resonator is made of the material 6061T6 aluminum, the loaded quality factor is 4.8 × 105, and the bare frequency of the resonator is 8.108 GHz. The Jaynes-Cummings readout method is used to find the optimal readout power to distinguish among the qubit in the ground state $\left| {\rm{0}} \right\rangle$, qubit in the superposition state of $\left| {\rm{0}} \right\rangle$ and $\left| {\rm{1}} \right\rangle$, and qubit in the superposition state of $\left| {\rm{0}} \right\rangle$, $\left| {\rm{1}} \right\rangle$ and $\left| {\rm{2}} \right\rangle$. Then, the Aulter-Townes splitting (ATS) experiment can be fulfilled in this system. Unlike the method given by Novikov et al. [Novikov S, Robinson J E, Keane Z K, et al. 2013 Phys. Rev. B 88 060503], our method only needs to apply continuous microwave excitation signal to the qubit, and does not need to carry out precise timing test on the qubit, thus reducing the test complexity of observing ATS effect. The ATS effect in resonance and non-resonance regime are observed. In the resonance ATS experiment, in order to obtain the peak value and frequency of resonance peak, Lorentz curve can be used for fitting peaks, and the ATS curve of double peak can be fitted by adding two Lorentz curves together. In the non-resonance ATS experiment, the detection signal is scanned, and the ATS double peak will shift with the different coupling signal detuning, forming an anti-crossing structure. The two curves formed by crossing free structure give two eigenvalues of Hamiltonian. By solving the equation, the experimental results can also be found to be consistent with the theoretical results.
2020, 69 (23): 230303. doi: 10.7498/aps.69.20200697
Abstract +
The gyroscope established on quantization vortices formed from exciton-polariton Bose-Einstein condensate has important potential applications in the field of quantum guidance. Thus, we assume a concept of quantum gyroscope based on Sagnac effect of the superposition states of quantum vortices existing in exciton-polariton condensates. To study the gyroscopic effect of superimposed vortices, which is the core issue of the project, it is essential to study the dynamic characteristics in the case of system rotating. Therefore, in this paper, the stability and dynamics of positive-negative vortex superposed states of two-dimensional exciton-polariton condensate in the disordered potential are studied under the rotation of the semiconductor microcavity, thereby laying a foundation for studying the gyroscopic effect of the superposed state of exciton-polariton condensates in the semiconductor microcavity. On the basis of reconstructing the mono-component Gross-Pitaevskii equation under the rotational situation, a numerical model with Coriolis items is constructed by the Runge-Kutta method and the finite difference time domain method, which is capable of depicting the rotation of the system. Moreover, the real-time evolution process of positive-negative vortex superposed states with different topological charges and the relationship between the number of steady-state local particles and the angular speed of the rotation of semiconductor microcavity are investigated by the real-time evolution method when the semiconductor microcavity is rotated. In the meantime, the relationship between the rotation speed in the excitation of vortex superposed states and the rotation speed of the semiconductor microcavity is also studied in the presence of the influence of the rotation speed of the semiconductor microcavity on the phase stability of vortex superposed states. According to the study, the rotation speed of the semiconductor microcavity has a significant influence on the evolution process and dynamic characteristics of vortex superposed states of exciton-polariton condensates. The rotation of the exciton-polariton system will accelerate the evolution of superimposed vortices, and overly rapid rotary rate will signalize the fluctuation of the local particle number thus the system unstability occurs. Moreover, along with the system rotation, the exciton-polariton superimposed vortices begin to rotate when the evolution approaches to saturation. It is noticeable that the angular acceleration of superimposed vortices is positively associated with the system rotary rate. Further, the topological charge has a significant influence on the rotation rate of exciation region of superposition state of vortices that it rotates more slowly when the topological charge increases but lower evolution stability simultaneously. These findings possess important guiding significance for establishing the quantum gyroscope in the future.
2020, 69 (23): 230501. doi: 10.7498/aps.69.20200856
Abstract +
The autaptic structure of neurons has the function of self-feedback, which is easily disturbed due to the quantum characteristics of neurotransmitter release. This paper focuses on the effect of conductance disturbance of chemical autapse on the electrophysiological activities of FHN neuron. First, the frequency encoding of FHN neuron to periodic excitation signals exhibits a nonlinear change characteristic, and the FHN neuron without autapse has chaotic discharge behavior according to the maximum Lyapunov exponent and the sampled time series. Secondly, the chemical autaptic function can change the dynamics of FHN neuronal system, and appropriate autaptic parameters can cause the dynamic bifurcation, which corresponds to the transition between different periodic spiking modes. In particular, the self-feedback function of chemical autapse can induce a transition from a chaotic discharge state to a periodic spiking or a quasi-periodic bursting discharge state. Finally, based on the quantum characteristics of neurotransmitter release, the effect of random disturbance from autaptic conductance on the firing activities is quantitatively studied with the help of the discharge frequency and the coefficient of variation of inter-spike interval series. The numerical results show that the disturbance of autaptic conductance can change the activity of ion channels under the action of self-feedback, which not only improves the encoding efficiency of FHN neuron to external excitation signals, but also changes the regularity of neuronal firing activities and induces significant coherent or stochastic bi-resonance. The coherent or stochastic bi-resonance phenomenon is closely related to the dynamic bifurcation of FitzHugh-Nagumo(FHN) neuronal system, and its underlying mechanism is that the disturbance of autaptic conductance leads to the unstable dynamic behavior of neuronal system, and the corresponding neuronal firing activity may transit between the resting state, the single-cycle and the multicycle spike states, thereby providing the occurring possibility for coherent or stochastic bi-resonance. This study further reveals the self-regulatory effect of the autaptic structure on neuronal firing activities, and could provide theoretical guidance for physiological manipulation of autapses. In addition, according to the pronounced self-feedback function of autaptic structure, a recurrent spiking neural network with local self-feedback can be constructed to improve the performance of machine learning by applying a synaptic plasticity rule.
###### ATOMIC AND MOLECULAR PHYSICS
2020, 69 (23): 233201. doi: 10.7498/aps.69.20201103
Abstract +
${I^2}$. Owing to homogeneous broadening, the full width at half maximum of transmission spectrum of the same cell is broadened to $\Delta {\nu _t} = {\rm{6}}.{\rm{9}}\;{\rm{GH}}{\rm{z}}$. Center frequency of transmission spectrum is set to be $\varDelta = {\rm{0}}$. The probe beam’s frequency detuning is larger than the half width at half maximum of the transmission spectrum $\left| \varDelta \right| > {{\Delta {\nu _t}}}/{{\rm{2}}}$, so the integrated power of Rubidium atomic spin noise spectra is scaled as $\varDelta^{-1}$. And there is a dip for $|\varDelta| < {{\Delta {\nu _t}}}/{{\rm{2}}}$ as a result of collisions between the buffer gas and Rubidium atoms. The integrated power of Rubidium atomic spin noise spectra is scaled as $\sqrt n$. The Rubidium atomic spin's transverse relaxation time becomes shorter while the temperature increases. Only at the condition of non-perturbative probe, including far-off-resonant laser, weak laser intensity and uniform transverse magnetic field, the measured full width at half maximum will be close to the intrinsic linewidth of spin noise spectrum. In this way, we can obtain the Rubidium atomic spin's transverse relaxation time. This work can be applied to the field of physical constants precision measurement, like Lande g factor and isotopic abundance ratio. In addition, it provides an important reference for developing the high signal-to-noise ratio and compact spin noise spectrometer.">Spin noise spectroscopy is a very sensitive undisturbed spectroscopic technique for measuring atomic spin fluctuations by using a far-detuned probe laser beam. In this paper, we describe an experimental setup for measuring the spin noise spectroscopy. The spin noise spectra of Rubidium atomic vapor cell filled with 10 Torr of Neon gas and 20 Torr of Helium gas as buffer gas are investigated in a magnetically shielded environment. The dependence of the spin noise power spectral density, separately, on the probe beam’s intensity (I ), the probe beam’s frequency detuning (Δ) and Rubidium atomic number density (n) are measured. The integrated power of Rubidium atomic spin noise spectra is scaled as${I^2}$. Owing to homogeneous broadening, the full width at half maximum of transmission spectrum of the same cell is broadened to $\Delta {\nu _t} = {\rm{6}}.{\rm{9}}\;{\rm{GH}}{\rm{z}}$. Center frequency of transmission spectrum is set to be $\varDelta = {\rm{0}}$. The probe beam’s frequency detuning is larger than the half width at half maximum of the transmission spectrum $\left| \varDelta \right| > {{\Delta {\nu _t}}}/{{\rm{2}}}$, so the integrated power of Rubidium atomic spin noise spectra is scaled as $\varDelta^{-1}$. And there is a dip for $|\varDelta| < {{\Delta {\nu _t}}}/{{\rm{2}}}$ as a result of collisions between the buffer gas and Rubidium atoms. The integrated power of Rubidium atomic spin noise spectra is scaled as $\sqrt n$. The Rubidium atomic spin's transverse relaxation time becomes shorter while the temperature increases. Only at the condition of non-perturbative probe, including far-off-resonant laser, weak laser intensity and uniform transverse magnetic field, the measured full width at half maximum will be close to the intrinsic linewidth of spin noise spectrum. In this way, we can obtain the Rubidium atomic spin's transverse relaxation time. This work can be applied to the field of physical constants precision measurement, like Lande g factor and isotopic abundance ratio. In addition, it provides an important reference for developing the high signal-to-noise ratio and compact spin noise spectrometer.
###### ELECTROMAGNETISM, OPTICS, ACOUSTICS, HEAT TRANSFER, CLASSICAL MECHANICS, AND FLUID DYNAMICS
2020, 69 (23): 234101. doi: 10.7498/aps.69.20200514
Abstract +
Mirrors can be seen everywhere in daily life and play an important role in modern optical systems. A traditional mirror, which is made of noble metals, usually has a zero electric field strength and maximal magnetic field strength at its surface induced by the out-of-phase of electric field and in-of-phase of magnetic field between the reflected field and incident field due to the boundary condition of perfect electric conductor. As the magnitude of local electric field determines the strength of the light-matter interaction, it is clear that this interaction is suppressed near the mirror surface. Magnetic mirror, which can enhance electric field on the surface, has been widely applied to strong light-matter interaction for biological sensing, material analysis, and imaging. However, the conventional smooth magnetic mirror with a plane surface is difficult to induce sufficient light-matter interaction when the matter has a complex geometrical shape. Here in this work, we propose a concept of magnetic mirror with a rough interface designed by an array of artificial surface plasmonic structures. The artificial surface plasmonic structure on a subwavelength scale is designed by periodically inserting spiral metallic strips into a dielectric cylinder to support the strong magnetic dipolar resonant mode. The magnetic dipolar resonance of the excited structure is induced by the displacement current circle. Therefore, the resonant frequency is related to the geometrical parameters of the helical structure closely. When we reduce the outer radius of the structure, the magnitude of the displacement current circle will change, resulting in blue-shift of the resonant frequency. At the same time, we also find that increasing the spiral degree of the structure will cause the magnetic dipolar resonance frequency to become red-shifted. Particularly, the same magnetic dipolar mode can be supported in a spiral structure of different size by tuning the spiral degree accordingly. In this context, we design a rough magnetic mirror constructed by the artificial surface plasmonic structures with various sizes, and demonstrate that the efficiency of rough magnetic mirror is in agreement with that of smooth magnetic mirror. The proposed rough magnetic mirror can provide the unique ability to enhance the interaction between light and complicated matter for the application of biological sensing and imaging in microwave and terahertz band.
2020, 69 (23): 234102. doi: 10.7498/aps.69.20200797
Abstract +
The transmission polarization conversion metasurface has been widely concerned, because it has the advantage of being easy to be conformal with the antenna. Based on the reasonable arrangement of transmission polarization conversion units, various and complex electromagnetic functions can be realized. As the electromagnetic open window on the flight platform, the antenna is the bottleneck that restricts the decrease of radar cross section (RCS) of the whole flight platform. It is difficult to simultaneously realize the normal and efficient radiation of the antenna and the decrease of the RCS of the antenna. When the designed transmission metasurface is used in the antenna design, the radiation and scattering of the antenna can be regulated comprehensively. In this paper, a composite polarization conversion metasurface is proposed and verified. The unit cell of composite polarization conversion metasurface consists of two mirror symmetrical anisotropic metal patches in the upper layer, a dielectric layer and a polarization gate in the lower layer. When the polarization direction of the incident electromagnetic wave is perpendicular to the extension direction of the polarization gate and arrives at the composite polarization conversion surface, the conversion surface can realize the conversion from transmission linear polarization to right-hand circular polarization in a frequency range from 9.3 GHz to 10.9 GHz. When the polarization direction of the incident electromagnetic wave is parallel to the extension direction of the polarization gate, co-polarized total reflection can be realized. The chessboard arrangement metasurface is composed of composite polarization conversion unit and its mirror unit. A novel linearly polarized chessboard arrangement metasurface antenna is composed of the linearly polarized source microstrip antenna with a bandwidth of 9.4–10.7 GHz and the chessboard arrangement metasurface. By using the counter rotating cancellation characteristic of circular polarization, the chessboard arrangement metasurface antenna maintains linearly polarized radiation. Comparing with the source microstrip antenna, the linear polarization purity of chessboard arrangement metasurface antenna is improved from 9.5 GHz to 10.5 GHz. At the same time, the forward gain of the chessboard arrangement antenna increases and the radar cross section decreases. The maximum reduction is 39.2 dB. To further verify the practicability of the design and analysis, the chessboard arrangement metasurface antenna sample is fabricated and measured in microwave anechoic chamber with an Agilent 5230C network analyzer. The experimental results are in good agreement with the simulation results. This study has important reference value in the design of high gain, low RCS antenna and integrated regulation radiation and scattering of antenna.
2020, 69 (23): 234103. doi: 10.7498/aps.69.20200511
Abstract +
In recent years, due to their features nonexistent in natural matirials, the perfect absorbers based on metasurfaces have become a hot research point. Although great progress has been made, the absorbers with spin-selection are rarely reported. However, the absorbers with spin-selection have more widespread applications in chiral sensors and satellite communication. Therefore, a spin-selection absorber based on the metasurface with modified square split-ring structure is proposed. Firstly, the theoretical conditions for generating the spin-selection absorption are analyzed theoretically, and then the qualified metasurface unit cell is designed under the guidance of the theory. We design an asymmetric modified square split-ring resonator to break both the n-fold (n>2) rotational symmetry and mirror symmetry. The unit cell is composed of three layers, i.e. the top layer, which is a modified square split-ring, the middle layer, which is an FR4 dielectric plate with a thickness of 4 mm, and the bottom layer, which is an all-metal plate acting as the reflecting incident wave. In order to obtain the optimal performance, the designed meta-atom is optimized by CST Microwave Studio, a well-known commercial full wave simulation software.The numerical simulation results show that the unit cell can selectively absorb the right-handed circularly polarized waves and reflect left-handed circularly polarized waves at 7.2 GHz. A maximum absorption rate for the absorption of right-handed circularly polarized (RCP) waves reaches a value higher than 90%, while the absorption rate of the other spin state is kept lower than 19%. In addition, to meet the need of practical applications, the absorption performance is also further investigated under different oblique incident angles, with the wave vectors confined in the x-z plane and y-z plane, respectively. Finally, to further understand the mechanism of spin-selection absorber, the surface current distributions are also simulated for LCP and RCP wave, respectively. The different surface current distributions are obtained for incident LCP and RCP wave, which is a solid evidence for spin-selection absorption. This paper offers a reference for the generation of spin-selection absorber. The proposed method not only is suitable for microwave region, but also can be extended to higher frequencies, and hopefully it can be widely used in the field of communication.
2020, 69 (23): 234201. doi: 10.7498/aps.69.20201064
Abstract +
The reflection light field of surface of the Non-Lambertian body in nature has both specular reflection and diffuse reflection components. In the process of three-dimensional(3D) reconstruction, image matching and pattern recognition are based on the ideal Lambert body. The imaging effect is limited due to the presence of specular reflection components, and the accuracy of feature recognition is low. In order to obtain the diffuse reflection component accurately, a large number of studies have been conducted for a long time, which can be mainly divided into two parts: intensity- and polarization-based separation techniques. The intensity-based separation algorithm is limited in many aspects due to the prior knowledge, such as light source chromaticity, direction and image color information. With the maturity of detection technology, the acquisition and interpretation of multi-dimensional physical properties of light-field have made great progress of the utilization of polarization characteristics of light wave. Compared with traditional intensity imaging technology, the polarization imaging technology has strongr and many advantages in highlighting targets. However, in traditional polarization-based separation techniques, it is often necessary to assume that the diffuse light is completely unpolarized, which is used in some specific cases but not universally.In this work, we report a method to obtain the diffuse reflection components of the target surface based on the polarization characteristics of the light-field. According to Fresnel's law and Lawrence B. Wolff's reflection model, the reflected light-field on the target surface can be divided into diffuse and specular components with partial polarization. The partial polarization characteristics of diffuse and specular components are explored in depth and the Stokes vector is used to calculate the minimum light intensity of each pixel modulated by polarizer, which is completely unpolarized light. By subtracting completely unpolarized light from the obtained polarized sub-images, the diffuse and specular components in the polarized part satisfy the linear constraint model. Based on the independent component analysis (ICA) model, the diffuse and specular components in the polarized part are regarded as independent and non-interfering additive vectors. The singular value decomposition method and optical relevancy of mutual information are used to determine the optimal mix coefficients matrix of the subcomponents in the linear constrained model. Thus, the diffuse components are accurately acquired and explained from the complex reflected light-field. Simulation and experimental results show that the algorithm mentioned above can accurately obtain the optimal mix coefficients’ matrix without the prior knowledge of illuminant chromaticity, or direction or image chromatic information, or others. This technique can accurately obtain and remove the specular reflection part, at the same time, restore the diffuse light intensity which is covered by the specular reflection and conforms to the change trend of the surface shape. Meanwhile, the pretty good results also demonstrate that the proposed separation method has the strong stability and wide applicability. This technology does not have to make the assumption that 3D imaging technology and computer vision algorithms such as pattern recognition rely on natural objects as ideal Lambert bodies, and it can eleminate the influence of complex reflected light-field on target results, which makes passive remote 3D imaging technology more applicable and more robust.
2020, 69 (23): 234202. doi: 10.7498/aps.69.20200700
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In the field of quantum mechanics, the theoretical study of the interaction between intense laser field and atoms and molecules depends very much on the numerical solution of the time-dependent Schrödinger equation. However, solving the three-dimensional time-dependent Schrödinger equation is not a simple task, and the analytical solution cannot be obtained, so it can only be solved numerically with the help of computer. In order to shorten the computing time and obtain the results quickly, it is necessary to use parallel methods to speed up computing. In this paper, under the background of strong field ionization, the three-dimensional time-dependent Schrödinger equation of hydrogen atom is solved in parallel, and the suprathreshold ionization of hydrogen atom under the action of linearly polarized infrared laser electric field is taken for example. Based on the spherical polar coordinate system, the time-dependent Schrödinger equation is discretized by the splitting operator-Fourier transform method, and the photoelectron continuous state wave function under the length gauge can be obtained. In Graphics processing unit (GPU) accelerated applications, the sequential portion of the workload runs on central processing unit (CPU) (which is optimized for single-threaded performance), while the compute-intensive part of the application runs in parallel on thousands of GPU cores. The GPU can make full use of the advantage of fine-grained parallelism based on multi-thread structure to realize parallel acceleration of the whole algorithm. Two accelerated computing modes of CPU parallel and GPU parallel are adopted, and their parallel acceleration performance is discussed. Compared with the results from the existing physical laws, the calculation error is also within an acceptable range, and the result is also consistent with the result from the existing physical laws of suprathreshold ionization, which also verifies the correctness of the program. In order to obtain a relatively accurate acceleration ratio, many different experiments are carried out. Computational experiments show that under the condition of ensuring accuracy, the GPU parallel computing speeds by up to about 60 times maximally based on the computational performance of CPU. It can be seen that the accelerated numerical solution of three-dimensional time-dependent Schrödinger equation based on GPU can significantly shorten the computational time. This work has important guiding significance for rapidly solving the three-dimensional time-dependent Schrödinger equation by using GPU.
2020, 69 (23): 234203. doi: 10.7498/aps.69.20200817
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Optical microresonators supporting whispering-gallery modes have been intensively studied in past decades due to their practical applications ranging from fundamental science to engineering physics. Among such microresonators, microsphere resonators have been demonstrated to possess ultra-high quality (Q) factor, however, their shapes usually become non-standard spherical body, leading to irregular resonant spectra. Microring resonators have unique potential in integraibility on chip, but the fabrication imperfection limits their Q-factor only to 106. In addition, the free spectral range (FSR) just depends on their radius. Due to the advantages of high Q-factor, standard shape, slender mode field distribution, the microbottle resonators are demonstrated to possess excellent performance in cavity quantum dynamics, nonlinear optics, high-sensitivity sensing, and micro-laser. In this paper, we carry out a systematic study on the spectral characteristics of prolate microbottle resonator theoretically and experimentally. First, theoretically, the field distribution theory of the microbottle resonator is studied in detail based on Helmholtz equation. Experimentally, prolate microbottle resonators are fabriated via arc discharge technology. Second, the radial modes and axial modes of the microbottles are efficiently excited with the help of a coupled tapered fiber waveguide. By adjusting the coupling gap between the microbottle and the waveguide, The controlling of three cupling states i.e. undercoupling, critical coupling and overcoupling are realized. In our experiment, the whispering-gallery modes excited are identifiable and recognizable. The resonant mode with an ultra-high Q-factor of up to 1.78 × 108 is achieved. The characteristic of ultra-high Q-factor makes the microbottle hold great potential in biochemical sensing, nonlinear optics, and micro-laser. The tuning stability is enhanced by keeping the waveguide in touch with the microbottle. We investigate the selective excitation of whispering-gallery modes by adjusting different coupling points. As a result, clean spectra with robust coupling are observed. The stable device is suitable for improving the sensing performance. Finally, Fano resonance effect is obtained by choosing the diameter of the tapered fiber waveguide. The results presented in this paper will be of great significance for enhancing the sensing, nonlinear optics and cavity quantum dynamics.

## EDITOR'S SUGGESTION

2020, 69 (23): 234204. doi: 10.7498/aps.69.20200890
Abstract +
The field of squeezed state is an important quantum resource in the study of quantum optics. In the application of quantum information, the spectrum bandwidth of the squeezed light field is an important index to limit the information transmission capacity. Currently, the optical parametric oscillator (OPO) is one of the most efficient ways to generate high squeezed non-classical optical fields. In this paper, the degenerate singly-resonant and doubly-resonant OPO structures are introduced. Both OPOs are composed of concave mirrors and periodically poled potassium titanyl phosphate crystals (PPKTP). The length of PPKTP crystal is 10 mm. The curvature radius of the curved surface is 12 mm, and it has high reflectivity at 1550 nm and 775 nm. The plane surface is coated with anti-reflection coating. The air gap length is 21 mm. The concave mirror is an output coupling mirror, and its radius of curvature is 25 mm. In the singly-resonant OPO, only the signal light resonates in the cavity, and the pump light passes through the nonlinear crystal twice and then outputs out of the cavity. The reflectivity of OPO output coupling mirror to the wavelength of 1550 nm is 88%. The linewidth of the corresponding fundamental frequency wave is 77.4 MHz. For doubly-resonant OPO, both the signal light and the pump light resonate simultaneously in the cavity. The reflectivity of OPO output coupling mirror to 1550 nm and 775 nm is 85% and 97.5%, respectively. The linewidth of the corresponding fundamental frequency wave and harmonic is 97.1 MHz and 15.6 MHz, respectively. Then the threshold of OPO is calculated. The threshold pump power of OPO increases with signal light transmittance increasing, but the threshold value of doubly-resonant OPO is obviously smaller than that of singly-resonant OPO. After that, the variation of the squeezing bandwidth of the squeezed light field generated by OPO with the transmittance of the signal is analyzed. Finally, we complete the design of quantum squeezer with low threshold (18 mW), broadband (84.2 MHz) and high stability (the standard deviation of locking baseline is 0.32 MHz) experimentally. The results show that compared with the singly-resonant optical parametric oscillator, the doubly-resonant cavity has the characteristics of low threshold and high stability, which is more suitable for the preparation and practical application of broadband squeezed light field.

## EDITOR'S SUGGESTION

2020, 69 (23): 234205. doi: 10.7498/aps.69.20200620
Abstract +
Ytterbium doped fiber lasers (YDFLs) with small volume, good beam quality, good heat dissipation performance and high conversion efficiency are widely used in industrial processing, military, medical and other fields. In past decades, with the development of high-performance double cladding gain fiber and fiber devices, the output power of YDFLs increases rapidly. However, nonlinear effects (NLEs), such as stimulated Brillouin scattering (SBS), stimulated Raman scattering (SRS), are produced, which limits the further enhancement of the output power of fiber laser. Large mode area ytterbium-doped fiber (LMAYDF) can effectively increase the nonlinear effect threshold. However, increasing the core diameter will support more high-order modes (HOMs), which may lead the beam quality to deteriorate and induce the mode instability (MI) effect to occur in fiber lasers. Thus, MI and NLEs have become the main limiting factors for the further improving of output power and beam quality in fiber lasers. The confined-doped ytterbium-doped double-clad fiber (CDYDF), by reducing the doping diameter of gain ions in the fiber core, makes the fundamental mode (FM) dominate in mode competition and HOM suppressed to achieve LMAYDF gain control for different modes, thus improving the output power of the fiber laser and maintaining good beam quality. The 33/400 μm confined-doped ytterbium-doped double-clad fiber (CDYDF) is fabricated by modifying the chemical vapor deposition (MCVD) process with solution doping technology (SDT). The Yb3+ doping diameter ratio is 70% and refractive index profile is close to step-index. Utilizing the master oscillator power amplifier (MOPA) system the beam quality optimization effect of confined-doped fiber is verified and optimized to 1.43 as the power increases while the M2 of seed laser is 1.53. An all-fiber structure counter-pumped fiber oscillator is constructed to test the laser performance of home-made confined-doped fiber. When the pump power is ~4.99 kW, laser power of 3.14 kW with a central wavelength of 1081 nm and line width of 3.2 nm at 3 dB is obtained. Moreover, there is no MI nor SRS in the whole experiment. We demonstrate that it is the highest output power based on home-made confined-doped fiber. The above results indicate that confined-doped fibers have the potential to achieve high-power and high-beam-quality fiber laser output.
2020, 69 (23): 234206. doi: 10.7498/aps.69.20200765
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Due to the power instability and field non-uniformity of radio frequency (RF), microwave (MW) and laser signals, inhomogeneous broadening of spin magnetic resonance line causes the absorption to decrease in a nuclear resonance system, which can reduce the sensitivity of spin-based sensing and testing technology. In this paper, we propose and design a double solenoid coil RF resonance antenna structure. The nearly uniform RF field density is produced by the two solenoid coil antenna structures that are parallel to the symmetry axis. The size of the uniformity in the center region of double solenoid coil RF resonance antenna structure is about π×375 mm2 × 10 mm. And the non-uniformity is less than 0.9%. Comparing with a single straight wire antenna and the single solenoid coil RF resonance antenna structure, the uniformity is improved by about 56.889 times and 42.889 times, respectively. At the same time, based on the near-field mutual inductance coupled resonance effect, the intensities of RF field in the center region of the two-solenoid coil antenna structure is enhanced. Comparing with the single solenoid coil antenna structures, it is enhanced by about 1.587 times. And the equivalent sensitivity of the silicon vacancy color center spin based sensor is enhanced by about 4.833 times. In the experiment, an optical detection magnetic resonance measurement system based on the spin magnetic resonance effect of silicon vacancy color center in single crystal SiC is built. Comparing with the single straight wire antenna and the single solenoid coil RF resonance antenna structure, the contrast of the silicon vacancy color center spin magnetic resonance signals of the double solenoid coil RF resonance antenna structure increases about 6 times and 2.4 times, respectively. The sensitivity of the spin-based sensor is increased by 4.833 times and 2.071 times through using the modulation and demodulation method, and the noise decreases by 8 times and twice. Hence, based on this double solenoid coil RF resonance antenna structure, the sensitivity of the silicon vacancy spin sensor can be improved. Combined with chip manufacturing technology of SiC wafer, it proves to be a potential approach to developing the high precision, chip scale spin sensor devices and measurement technology.
2020, 69 (23): 234207. doi: 10.7498/aps.69.20200939
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Laser-based diagnostic techniques are critical nonintrusive methods of measuring the in-situ temperature in combustion flow fields. Developing temperature measurement techniques with high accuracy and precision is of great significance for studying the combustion. At present, nanosecond (ns) lasers are commonly used in these methods. However, the researches based on femtosecond (fs) lasers are relatively few. Here, we develop a thermometry technique for combustion fields based on fs laser-induced filament. When the fs laser propagates in an optical medium, a long uniformly distributed plasma channel (also named filament) will be generated. The clamped intensity inside the filament is high enough to generate excited atoms/molecules through fs laser-induced photochemical reactions. Subsequently, the excited atoms/molecules release fluorescence signals. The length of the filament can be measured by imaging the fluorescence signal with an ICCD camera, which is evaluated by the full width at half maximum (FWHM) of the spatial distribution of the filament emission signal. Based on theoretical analysis, the experimental data of the filament length are fitted with a power function, and the result is satisfactory compared with the R-squared measure of goodness (R2) of 0.984. This indicates that the filament length is correlated well with the temperature of the combustion field. A monotonic quantitative relationship between the filament length and the temperature can be established by a calibration process, and then the temperature of the combustion field can be measured. When the temperature changes from 1630 to 2007 K, the length of the filament shortens by 38%. This indicates that the filament length is sensitive to the temperature of the flow field. When the temperature is 2007 K, the absolute uncertainty of the measurement is ±25 K, and the relative uncertainly is about 1.2%. The spatial resolution of the measurement system is 50 μm, which was determined by a USAF 1951 Target. Based on the spatial resolution, the measurement precision can arrive at 17 K. Although, at present, this temperature measurement technique based on femtosecond laser-induced filament is used only in laminar premixed flames, it has potential applications in temperature measurements ranging from room temperature to combustion temperatures.
2020, 69 (23): 234208. doi: 10.7498/aps.69.20201035
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Femtosecond extreme ultraviolet (XUV) light pulses play an important role in investigating the ultrafast dynamics of atoms and molecules, and are complementary to the conventional large facilities like synchrotron radiation and free electron laser. We build a table-top femtosecond extreme ultraviolet light source based on the high-order harmonic generation (HHG) process of gaseous medium in a strong laser field. We implement HHG by focusing an intense IR laser into a 5 cm long gas-filled hollow waveguide, instead of the conventional tightly focusing geometry with gas jet. Inside the waveguide, the laser peak intensity is nearly constant and the gas pressure is well-controlled, making it possible to maintain the phase matching condition over an extended distance. And a fully coherent high harmonic beam builds up along the waveguide, leading to a dramatically higher HHG efficiency. Monochromatic XUV light pulses are obtained by spectral selection of the HHG through employing the conical diffraction method of grating. With this geometry used, the pulse broadening caused by wave front tilting during the diffraction can be strongly suppressed, especially for the case of grazing incidence. And the femtosecond temporal character of the light pulse can be preserved while keeping a high reflectivity. The temporal broadening of the XUV light pulse in our setup is estimated to be within 100 femtosecond. By using different noble gases, photons with energy values ranging from 20 eV to 90 eV are produced. For the 27th-order harmonic centered at 41.9 eV, the flux is measured to be 1 × 1010 photons per second, with an energy spread of 0.4 eV. In order to investigate the ultrafast dynamic behaviors of gaseous atoms and molecules with an HHG-based XUV source, we develop a reaction microscope with ultrahigh vacuum of about 10–11 mbar. The combination of HHG-based XUV with the newly developed reaction microscope provides a unique tool for studying the XUV photon and atom/molecule interaction. A series of experiments has been successfully carried out on the platform and the system shows good performance.
2020, 69 (23): 234209. doi: 10.7498/aps.69.20200835
Abstract +
Diffraction gratings have been widely used in waveguides. They can transmit light beams or images from the in-coupling end to the out-coupling end at predetermined positions. However, when they are applied to augmented reality and virtual reality with large field of view and color light sources, there will arise some problems such as mismatch and missing field of view, non-uniform emission, and others. Therefore, starting from these physical problems, the upper limit of the field of view for diffractive waveguide and the complete theoretical boundary formula of the field of view are derived, and on this basis, in-depth research is conducted on monochromatic waves and multicolor waves, respectively. It is concluded that the single-layer diffractive waveguide supports the theoretical upper limit of the monochromatic wave field angle of about 48° under normal high refractive index of n = 1.75, and supports the theoretical upper limit of the multicolor wave field angle of 26.4° for coefficient q = 1.3. Clearly, a larger field of view requires a higher refractive index n and a smaller q value. The boundary conditions of field integrity indicate that reducing the maximum diffraction angle of the long wave and thinning the thickness of the waveguide layer can solve the problem of missing field of view. The practical maximum diffraction angle generally does not exceed 75°, and the thickness of the waveguide layer is about 0.5 to 1.0 mm generally based on the incident field of view. Finally, a method of expanding each total internal reflection field of view into a ray tracing diagram and a distribution function of pupils to receive light energy at various angles are obtained. In this way, the optimal position of the out-coupling grating region can be achieved, and the inverse of the distribution function is used to constrain the angular distribution of the projected light or the grating efficiency, and then receiving uniform exit image at any position becomes possible. The uniformity of the monochromatic waveguide increases from 0.27 to 0.15, and the uniformity of the long wave in the single grating multicolor waveguide rises from 0.4 to 0.28. The results of these studies will undoubtedly help to solve the problem in the diffractive waveguides used in large field of view and multicolor light.
2020, 69 (23): 234301. doi: 10.7498/aps.69.20200764
Abstract +
A theory is developed to model the dynamic of bubble and particle inside a spherical liquid-filled cavity surrounded by an elastic medium. The aim of this work is to study how the outer elastic medium affects the interaction between bubble and particle. Starting from the theory of velocity potential distribution, combined with Lagrangian equations, the motion equations of bubbles and particles in the cavity are obtained. The resonance frequency of the bubbles and influence of the interaction between particle and bubble on the translational behavior under the action of sound waves are analyzed. The results show that the properties of medium elasticity and density can change the resonance frequency of the bubble in the cavity. As the radius of the spherical cavity increases, the resonance frequency of the bubble has a tendency to first decrease and then increase, and gradually tends to the resonance frequency of a single bubble in an unbounded liquid. The translation of bubble and particle in the spherical liquid cavity is affected by factors such as acoustic field parameters, the characteristics of the outer elastic medium, and the characteristics of the bubble and particle themselves. The overall characteristic is that the particle has a tendency to move to the cavity wall, and the translation of bubble is closely related to the interaction characteristics between bubble and particle.

## EDITOR'S SUGGESTION

2020, 69 (23): 234302. doi: 10.7498/aps.69.20200958
Abstract +
The Indo-Pacific humpback dolphins (Sousa chinensis) are nearshore odontocetes, distributed in tropical and sub-tropical oceans. This species has been studied to unveil its ability to echolocate. Indo-Pacific humpback dolphin, like its Odontocetes companion, relies on echolocation system to navigate and detect targets, which contains a sound transmitting system in the forehead and a sound reception in the jaw. Their soft tissues present gradient sound speed and density distributions in the forehead. Solid skull, air structures and soft tissues form a natural multi-phase meta-material to modulate sounds into energy focused beams. This multi-phase property is also applied to the hearing system as revealed in current papers. Here in this work, the physical mechanism of sound reception in the Indo-Pacific humpback dolphin is studied by using the computed tomography (CT) scanning, physical measurements and numerical simulation. Hounsfield units (HUs) of the forehead tissues are extracted from CT scanning results. A linear relationship is revealed between HU and sound speed, HU and density, which are combined with HU distribution to reconstruct the sound speed and density distribution of the sound reception system. The CT scanning shows that the sound reception system located at lower head is composed of external mandibular fat, internal mandibular fat, mandible and hearing bones. Model of sound reception system is developed on the basis of CT scanning results and used in subsequent simulations. The physical process of sound reception reveals that the hearing system can guide sounds through variable pathways to reach hearing bones. Sounds can enter into the reception system along the acoustic pathways composed of mandible, external mandibular fat and internal mandibular fat. Mandibular fat and mandible form a unique sound pathway. In addition, another pathway which is composed of external mandibular fat, pan bone and internal mandibular fat can lead the sound to propagate and finally arrive at hearing bones. The diversity of acoustic pathways is applicable to a range of frequencies from 30 to 120 kHz. The variability of acoustic pathways in Indo-Pacific humpback dolphin shows the complexity of its biosonar system. The anatomy and simulation results can deepen our understanding of the mechanism of echolocation of Indo-Pacific humpback dolphin and provide references for designing man-made sound reception devices.
2020, 69 (23): 234701. doi: 10.7498/aps.69.20200971
Abstract +
With advantages in biosafety and efficiency, gene delivery based on mechanical approaches has received more and more attention in academic research. In the present paper, a method based on zero-net-mass-flux jet is proposed to apply fluid shear to the moving cells in the microchannel, which causes cell to deform, and then open its mechano-sensitive channel on the cell membrane. This novel method is verified theoretically by numerical simulation in this study. In this paper, an immersed finite element method is utilized to numerically simulate the deformation of red blood cells subjected to zero-net-mass-flux jet during the movement of red blood cells in microchannel, aiming at investigating how to efficiently introduce small molecules into cells. The important parameters of numerical simulation are pressure gradient Δp along the microchannel, the amplitude Am and frequency f of the zero-net-mass-flux jet. Through the analysis of the characteristic of flow field and the stress on the red blood cells, we find that when cell surface tension T0 is greater than critical surface tension τ c, the gating of cell surface mechano-sensitive channel will occur, and the percentage of gating Popen on the cell membrane can be obtained at each moment. Addtionally, the channel opening integral I is defined to measure the gating degree of the membrane mechano-sensitive channel under different flow parameters, and the influences of pressure gradient, jet vibration frequency and amplitude on the I are further discussed in order to find the optimized process parameters, The method we proposed is simpler and easier to implement, and the applied fluid shear stress can be controlled precisely, so that it is possible for proteins, genes and other substances to be transported into the cell across the membrane, and to implement reprogramming.
2020, 69 (23): 234702. doi: 10.7498/aps.69.20200903
Abstract +
${t_{\rm valid}}$) is defined and used to judge the end point of the liquid column quantitatively. The liquid jet trajectory and cross-section deformation can be calculated using MATLAB software. The instantaneous images of continuous liquid columns in supersonic crossflow are captured using high-spatial-resolution microscopic imaging methods. The microscopic imaging system is composed of a double pulse solid-state laser, computer, CCD camera, synchronous controller, microscope lens and laser diffuser. After passing through the laser diffuser, a plane background light with uniform distribution is formed on the scattering plate. The mean filtering method is used to filter the original image. After filtering, the range of gray distribution in the background area is obviously reduced. The distribution of gray value is more concentrated, and the background of the image is more uniform. Then the image edge detection function is used to obtain the near-field jet trajectory. The parameter variables studied include liquid injection pressure drop (1–2 MPa), liquid nozzle diameter (0.5 mm/1.0 mm), and liquid gas momentum ratio (3.32–7.27). The results show that the continuous liquid column model can better predict the jet trajectory on the center plane and the shape of the liquid column in three-dimensional space. It is indicated that the predictive result matches well with the experimental result. This study is of great significance for establishing the solid-particle coupling model of liquid jet in supersonic crossflows.">The trajectory of the spray is studied theoretically and experimentally when a round liquid jet is injected into a supersonic crossflow vertically. A solid model of continuous liquid column is established in three-dimensional space. The cross-section deformation equation of the continuous liquid column along the injection direction is established using a method of micro-element analysis. The stress analysis of cross section is simplified into a two-dimensional droplet. The shape of the cross section is considered to continuously change from circular to elliptical shape. And the bow shock wave in front of the jet column is simplified into an oblique shock wave with a known shock angle. Based on this, the calculation of aerodynamic force is greatly simplified. A dimensionless parameter named effective deformation time of liquid column (the logogram is ${t_{\rm valid}}$) is defined and used to judge the end point of the liquid column quantitatively. The liquid jet trajectory and cross-section deformation can be calculated using MATLAB software. The instantaneous images of continuous liquid columns in supersonic crossflow are captured using high-spatial-resolution microscopic imaging methods. The microscopic imaging system is composed of a double pulse solid-state laser, computer, CCD camera, synchronous controller, microscope lens and laser diffuser. After passing through the laser diffuser, a plane background light with uniform distribution is formed on the scattering plate. The mean filtering method is used to filter the original image. After filtering, the range of gray distribution in the background area is obviously reduced. The distribution of gray value is more concentrated, and the background of the image is more uniform. Then the image edge detection function is used to obtain the near-field jet trajectory. The parameter variables studied include liquid injection pressure drop (1–2 MPa), liquid nozzle diameter (0.5 mm/1.0 mm), and liquid gas momentum ratio (3.32–7.27). The results show that the continuous liquid column model can better predict the jet trajectory on the center plane and the shape of the liquid column in three-dimensional space. It is indicated that the predictive result matches well with the experimental result. This study is of great significance for establishing the solid-particle coupling model of liquid jet in supersonic crossflows.
###### CONDENSED MATTER: STRUCTURAL, MECHANICAL, AND THERMAL PROPERTIES
2020, 69 (23): 236801. doi: 10.7498/aps.69.20200860
Abstract +
${\gamma }_{\left\{100\right\}/\left\{010\right\}/\left\{001\right\}}$ = 1.76 J/m2. While the (110) and (111) lattices of ITO consist of In/Sn/O, in which the O atom percent on (110) and (111) lattices are 56 at.% and 25 at.% respectively. Besides, the calculated values of surface energy ${\gamma }_{\left\{110\right\}/\left\{101\right\}/\left\{011\right\}}$ and ${\gamma }_{\left\{111\right\}}$ are 1.07 and 0.89 J/m2, respectively. Combining the XPS measurement results and crystal structure of ITO, it can be understood that in the diffusion process of low-energy NHx ion/group into ITO film without preferred orientation, lots of In/Sn—O—N bonds are formed in the ITO lattices, which are rich in O and have lower surface energy $\gamma$. Then, after the low-energy NHx ion/group bombardment, the electrical properties of ITO film without preferred orientation are deteriorated seriously. On the contrary, because of the absence of O and the highest surface energy $\gamma$, it is hard for the low-energy NHx ion/group to diffuse into ITO (100) lattice. Then, after the low-energy NHx ion/group bombardment, the electrical properties of ITO film with (100) preferred orientation have little change. With all results, the ITO film with (100) preferred orientation can be an ideal candidate for transparent electrode in MAPbH3 perovskite solar cells.">In the case of methylammonium lead halide (MAPbH3) perovskite solar cells, the indium tin oxide (ITO) film has been widely used as the transparent electrode. In the preparation process and service process of MAPbH3 perovskite solar cells, the MAPbH3 perovskite layer can decompose into the methyl, amino, methylammonium, halide ion/group, etc. Thus, the diffusion of ammonia ion/group into ITO film is inevitable, which can seriously deteriorate the electrical property of ITO transparent electrode. In this study, the ITO films with and without (100) preferred orientation are bombarded by a low-energy ammonia (NHx) ion beam. After the bombardment, the electrical properties of ITO film without preferred orientation are deteriorated seriously, especially for carrier concentration, which is deteriorated down to an extent of about 5–6 orders of magnitude. The bombardment of low-energy NHx ion/group has little influence on the electrical properties of ITO film with (100) preferred orientation. Such phenomena can be explained by the following reasons. Based on XPS measurement results, the low-energy NHx ion/group diffuses into the ITO film surface after the bombardment. In the diffusion process, the low-energy NHx ion/group is mainly bonded with O in ITO lattice, which results in the formation of In/Sn—O—N bond. Based on the crystal structure of ITO, the (100) lattice of ITO consists of In/Sn, and the calculated value of surface energy ${\gamma }_{\left\{100\right\}/\left\{010\right\}/\left\{001\right\}}$ = 1.76 J/m2. While the (110) and (111) lattices of ITO consist of In/Sn/O, in which the O atom percent on (110) and (111) lattices are 56 at.% and 25 at.% respectively. Besides, the calculated values of surface energy ${\gamma }_{\left\{110\right\}/\left\{101\right\}/\left\{011\right\}}$ and ${\gamma }_{\left\{111\right\}}$ are 1.07 and 0.89 J/m2, respectively. Combining the XPS measurement results and crystal structure of ITO, it can be understood that in the diffusion process of low-energy NHx ion/group into ITO film without preferred orientation, lots of In/Sn—O—N bonds are formed in the ITO lattices, which are rich in O and have lower surface energy $\gamma$. Then, after the low-energy NHx ion/group bombardment, the electrical properties of ITO film without preferred orientation are deteriorated seriously. On the contrary, because of the absence of O and the highest surface energy $\gamma$, it is hard for the low-energy NHx ion/group to diffuse into ITO (100) lattice. Then, after the low-energy NHx ion/group bombardment, the electrical properties of ITO film with (100) preferred orientation have little change. With all results, the ITO film with (100) preferred orientation can be an ideal candidate for transparent electrode in MAPbH3 perovskite solar cells.

## EDITOR'S SUGGESTION

2020, 69 (23): 237101. doi: 10.7498/aps.69.20200990
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2020, 69 (23): 237401. doi: 10.7498/aps.69.20201125
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Iron-based superconductor FeSexTe1–x has attracted attention because of its high upper critical field, low anisotropy, and high critical current density. Also, it is predicted to have nontrivial topological properties, so that it is a candidate of realizing Majorana fermion, when the superconductivity is combined with topological features. However, its flux pinning behavior and mechanism in superconducting state with varying Se/Te ratio have not been systematically studied . We use self-flux method to grow single crystal samples of FeSexTe1–x with different x values (0.3, 0.4, 0.5 and 0.6). The structural and morphological properties of the monocrystalline samples are characterized by XRD and SEM. All samples show that they possess the expected crystalline structures and their lattice parameters vary with x value. The magnetic properties at low temperatures are also measured, showing that all samples have good superconductivity. Superconducting properties, such as critical current densities and flux pinning force densities, are extracted from the magnetic measurements and analyzed, and the flux pinning behavior is discussed. The best Se:Te ratio is determined to be nearly 0.4/0.6 based on the comparison among these properties of different samples. By utilizing the Dew-Hughes theory and analyzing the pinning force density peak, the flux pinning mechanism in the best samples (x = 0.4, 0.5) can be regarded as the mixture of normal point pinning and Δκ volume pinning. This work provides important information for the further study of the topological and superconducting properties of FeSexTe1–x.
2020, 69 (23): 237402. doi: 10.7498/aps.69.20201116
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NiFe2O4 (NFO) nanoparticle doped YBCO bulk superconductors are fabricated by using a novel top-seed infiltration growth (TSIG) technique. The growth morphology, microstructure and superconducting properties are investigated. The results show that at low doping levels, the normal growth of YBCO single domain is not affected by the NFO doping, but at high doping levels, obvious random nucleation appears at the edge of the sample. The measurement of levitation force indicates that the maximum levitation force on the sample first increases and then decreases with the increase of the NFO doping amount, and the largest levitation force is obtained to be 33.93 N for the sample with a doping level of 0.2% (weight percent). Low-temperature magnetization measurement shows that the YBCO sample exhibits that Tc value decreases with NFO amount increasing, and the superconducting transition width (ΔTc) also broadens gradually. The sample with the optimal doping (0.2% weight percent) presents an enhanced zero-field Jc value of 8.68 × 104 A/cm2, which is 31% higher than the sample without dopant. In addition, a more obvious secondary peak of 4.37 × 104 A/cm2 at a field of 1 T is observed for the 0.2 wt.% NFO doped sample, which indicates the existence of enhanced δTc pinning in the bulk. The SEM measurement shows that two types of particles are trapped in the Y-123 matrix for YBCO sample doped with 0.2% weight percent NFO: one is the large particle with a size mainly ranging from 0.5 μm to 2.0 μm, and the other is small nano-inclusion mainly ranging from dozens of nanometers to about several hundreds of nanometers. Such a microstructure is very similar to the microstructure of the undoped sample we reported earlier. So whether the NFO nanoparticles exist in the microstructure cannot be judged just from the morphology of the nano-inclusions. The electron probe microarea analysis (EPMA) result shows that different concentration distributions of Ni and Fe elements are observed in the sample doped with 0.2% weight percent NFO, which indicates the separation of NFO nanoparticles in the heat treatment process, and the dissolved Ni and Fe ions finally exist in the form of element substitutions in the YBCO bulk. Such element substitutions can introduce local lattice distortions and weak-superconducting regions into the superconducting matrix, which can act as effective flux pinning centers, and hence improving the properties of the samples.
2020, 69 (23): 237801. doi: 10.7498/aps.69.20200897
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At present, there are several kinds of broadband antireflection coatings (ARCs). For the flat multilayer ARC, it usually contains double, triple, or up to 4 layers. It has been demonstrated that the performance of a single layer coating is not good enough across the desired spectral range. Multiple layer ARCs have much better performance for broadband solar cells (SCs). When inspecting the antireflection structure of Cu2ZnSnS4 solar cells (CZTSSCs), it is shown that the transparent conductive oxide (TCO) of traditional CZTSSCs does not have an satisfactory antireflective performance. This paper aims to investigate a way to increase the incident light transmitted into CZTSSCs, and thus improving the efficiency of solar cells by studying the use of the antireflective effect of a TCO film. It introduces a new type of TCO film with better antireflective properties across a wide wavelength range. An SiO2/ZnO antireflective TCO (ATCO) is designed under AM1.5 illumination. In order to measure the antireflective effect over the 300–800 nm wavelength range, an effective average reflectance method (EAR) is introduced. Considering the effect of the refractive index dispersion and the coupling of the TCO or ATCO films with the active layer, in this paper we use a multi-dimensional transfer matrix to optimize the thickness of each key layer to accurately confirm the best antireflective effect. In addition, the optimized TCO film and the optimized ATCO film in CZTSSCs are compared and analyzed by means of EAR. The result shows, through the comparison of the antireflection between conventional TCO CZTSSCs and ATCO CZTSSCs, that there are considerable differences in final optimal reflectivity between TCO layer and ATCO film. For the conventional CZTSSC, the optimal effective average reflectance of TCO layer is 5.6%, and the lowest reflectivity in the waveband from 400 nm to 500 nm is 6.9%. In addition, the corresponding values obtained in the new ATCO CZTSSC are 3.8% and 1.6% respectively. These apparent changes in reflectivity are appealing in that the new ATCO films can effectively reduce light loss and improve the efficiency of photovoltaic conversion.
2020, 69 (23): 237802. doi: 10.7498/aps.69.20201056
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Black phosphorus(BP) is a kind of two-dimensional (2D) material with direct bandgap. Its adjustable bandgap fills the gap between graphene and transition metal dichalcogenides(TMDCs). At the same time, the black phosphorusalso has a higher charge carrier mobility. The unique fold-like crystal structure of the black phosphorus leads to in-plane anisotropy and it makes the photoelectric response anisotropic. It shows that the properties of black phosphorus can be dynamically adjusted by various methods. These characteristics make black phosphorus a two-dimensional material with great potential applications in the visible light to mid-infrared region and even terahertz bands. In view of this, this paper focuses on the magneto-optical response of black phosphorus. In this paper, we design a magneto-optical device in Au grating/black phosphorus/silicon hybrid plasmonic structures. The inducing of abnormal transmission through the metal grating significantly enhances the transmittance, while the Faraday rotation effect is enhanced through the mode coupling between the TE and TM in the THz range. The rigorous coupled wave analysis (RCWA) is used to calculate the transmittance of the grating. The finite element software COMSOL Multiphysics is used to calculate the transmittance and simulate the electric field distribution of the magneto-optical device. Under the optimal parameters, the Faraday rotation can increase 14.434 times, reaching to 2.7426°, and the transmittance is more than 85% with an external magnetic field of 5 T at the operation frequency (1.5 THz). We plot the electric profiles of the magneto-optical device with and without BP to prove that the Faraday rotation is a result of the magneto-optical property of the monolayer phosphorus and that the enhancement is due to the mode coupling between the TE and TM. Moreover, we extract the tunable character of the magneto-optical device with the external magnetic field and the carrier density of the black phosphorus. The external magnetic field can effectively tune the Faraday rotation angle while keeping the working wavelength and the transmittance substantially unchanged. The increasing of the carrier density will not improve the Faraday rotation angle, for the changes in surface conductivity under fixed structural parameters will disrupt the mode coupling. At the same time the transmittance will decrease, because the larger carrier density will enhance the absorption of the BP. Therefore, to obtain a higher FR angle with apparent transmittance, the carrier density should not be too high. Finally, the effects of the spoof surface plasmons on the waveguide mode and the Faraday magneto-optical effect are also discussed.

## EDITOR'S SUGGESTION

2020, 69 (23): 238102. doi: 10.7498/aps.69.20201041
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An asymmetric graphene-coated elliptical dielectric nano-parallel wires’ waveguide is proposed. By using the multipole method, in the two elliptic cylindrical coordinate systems, firstly, the longitudinal component of the electric field and the magnetic field are expressed by Mathieu functions, then the corresponding angular and radial components are obtained by Maxwell’s equations. The graphene is regarded as a zero-thickness interface with surface conductivity, and the boundary conditions are applied to these interfaces by the point-matching method. A linear algebraic equation system is obtained finally. The effective refractive indices and the field distributions of modes can be obtained by numerically solving the equation. The six lowest order modes supported by the proposed structure are classified, and the dependence of the characteristics of these modes, separately, on the working wavelength, the graphene Fermi energy and waveguide structure parameters are studied. The real part of the effective refractive index, the propagating length, and the quality factor are used to judge the performance of the waveguide. The results reveal that the characteristics of these modes can be greatly changed by altering the working wavelength of the waveguide, the Fermi energy of graphene, and the spacing between nanowires. When the length of the semi-major and the semi-minor axes of the nanowires are modified, the real part of the effective refractive index, the propagating length, and the quality factor can only be changed finely. At the same time, the results obtained by the multipole method are completely consistent with the results from the finite element method. By comparing the performances among the fundamental mode supported by the single graphene-coated elliptical dielectric nanowire, the symmetric graphene-coated elliptical dielectric nano-parallel wires, and the asymmetric graphene-coated elliptical dielectric nano-parallel wires by the means of the FEM based on commercial software (COMSOL), we find that the performances of the proposed waveguide in this paper are superior to those of the other two waveguides. This work can provide a theoretical basis for the design, fabrication, and application of asymmetric graphene-coated elliptical dielectric nano-parallel wires’ waveguide. The proposed structure is expected to be used in the mode conversion and coupling in the future devices.
2020, 69 (23): 238201. doi: 10.7498/aps.69.20201301
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The diffusive transport in complex confined media is ubiquitous such as diffusions of micro- or nano-particles in glassy liquids and polymer solutions, protein diffusions under crowded conditions, and deliveries of drugs in the biological media. Therefore, the understanding of the diffusive transport arouses the great interest of researchers in the physics, materials science, and biology circles. Despite the fact that the shape of the colloidal particles acts as one of the important physical factors influencing their dynamic behaviors, the study of the anisotropic particles diffusing in confined media is still lacking. In this work, we propose a simple experimental model to investigate the confined diffusion of shape-anisotropic particles. The diffusion of an ellipsoid at different area fractions (ϕ) of colloidal spheres is investigated through video microscopy. At low ϕ, ellipsoid exhibits a random trajectory and free diffusion in translational and rotational degree of freedom; while at high ϕ, the trajectory is in a small spatial range with a nearly constant orientation of the particle, indicating that the arrested diffusion takes place in translational and rotational degree of freedom. The translational and rotational mean square displacement decrease with the increase of ϕ. By power-law fitting (~tβ), it is found that β decreases from 1 to a small value at high ϕ, demonstrating that the ellipsoid experiences a transition from normal diffusion to sub-diffusion. Moreover, β for rotational motion decreases faster than that for translational motion at high ϕ, which signifies that the the rotational motion decouples from the translational motion with increasing ϕ. The results from the van Hove correlation function show that the translational displacement along the major axis of the ellipsoid is always larger than that along the minor axis, manifesting the ellipsoid prefers to diffuse along its major axis independent of ϕ. Significant non-Gaussian tail is observed in the distribution of the translational displacement along the major axis with increasing ϕ. However, the distribution of the translational displacement along the minor axis presents a nearly Gaussian behavior independent of ϕ. This indicates that the translational motion along the major axis decouples from the translational motion along the minor with increasing ϕ. For the rotational displacement, the non-Gaussian tail is only observed at the intermediate ϕ. These non-Gaussian behaviors are confirmed by calculating the non-Gaussian parameter (α2). Our experiments demonstrate that the confinements give rise to the anomalous diffusion behaviors of the anisotropic colloids, which is conducive to the understanding of transportations of anisotropic objects in complex environments.
2020, 69 (23): 238701. doi: 10.7498/aps.69.20200908
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Laser scanning confocal microscope (LSCM) is one of the most important tools for biological imaging due to its strong optical sectioning capability, high signal-to-noise ratio, and high resolution. On the basis of LSCM, line-scanning fluorescence microscopy (LSFM) uses linear scanning instead of point scanning to improve the speed of image acquisition. It has the advantages of simple system structure, fast imaging speed, and weak phototoxicity, and in addition, it is more suitable for high-resolution and fast imaging of living thick samples. It is of great significance for studying the life science, biomedicine, and others. However, the current LSFM technology still faces many urgent problems in terms of system flexibility, imaging speed, resolution and optical sectioning capabilities. Therefore, based on the existing multifocal structured illumination microscopy (MSIM) in our laboratory, a digital line-scanning fluorescence microscopy (DLSFM) based on digital micromirror device(DMD) is presented in this paper. In the illumination path, a high-speed spatial light modulator DMD is adopted to realize multi-line parallel scanning excitation, which simplies the optical system and improves the flexibility and scanning speed of the system. A DLSFM image reconstruction algorithm based on the standard deviation of fluorescence signal is proposed, which is combined withthree-dimensional (3D) Landweber deconvolution algorithm to achieve 3D high-resolution optical slice image reconstruction. On this basis, the imaging experiments on fluorescent beads and standard samples of mouse kidney section are carried out by using DLSFM. The experimental results show that the resolution of DLSFM in the x, y and z directions is 1.33 times, 1.42 times and 1.19 times that of wide field microscope, respectively, and the fast 3D high-resolution optical sectioning imaging of biological samples is realized, which lays a technical foundation for further developing the rapid high-resolution imaging of the whole cells and tissues in vivo.

## COVER ARTICLE

2020, 69 (23): 238702. doi: 10.7498/aps.69.20200690
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1,3,5-trinitro-1,3,5-triazacyclohexane (RDX) or hexogen, a high-insensitivity explosive, the accurately description of its energy and properties is of fundamental significance in the sense of security and application. Based on the machine learning method, high-dimensional neural network is used to construct potential function of RDX crystal. In order to acquire enough data in neural network learning, based on the four known crystal phases of RDX, the structural global search is performed under different spatial groups to obtain 15199 structure databases. Here in this study, we use nearby atomic environment to build 72 different basis functions as input neurons, in which the 72 different basis functions represent the interaction with nearby atoms for each type of element. Among them, 90% data are randomly set as training set, and the remaining 10% data are taken as test set. To obtain the better training effect, 9 different neural network structures carry out 2000 step iterations at most, thereby the 30-30-10 hidden layer structure has the lower root mean square error (RMSE) after the 1847 iterations compared with the energies from first-principles calculations. Thus, the potential function fitted by 30-30-10 hidden layer network is chosen in subsequent calculations. This constructed potential function can reproduce the first-principles results of test set well, with the RMSE of 59.2 meV/atom for binding energy and 7.17 eV/Å for atomic force. Especially, the RMSE of the four known RDX crystal phases from 1 atm to 6 GPa are 10.0 meV/atom and 1.11 eV/Å for binding energy and atomic force, respectively, indicating that the potential function has a better description of the known structures. Furthermore, we also propose four additional RDX crystal phases with lower enthalpy, which may be alternative crystal phases undetermined in experiment. In addition, based on molecular dynamics simulation with this potential function, the α-phase RDX crystal can stay stable for a few ps, further proving the applicability of our constructed potential function.