x

COVER ARTICLE

Radioactive residual nuclides, which are usually closely related to radiation protection and personnel safety, will be generated when target materials are irradiated by high energy particles. Based on different nuclear reaction models, Monte Carlo code is a usual method to obtain residual nuclide production. The simulation accuracy needs to be evaluated by experimental data. In this paper, an irradiation experiment of thin copper target irradiated by 12C6+ particles with energy of 80.5 MeV/u is carried out. The radioactivities and cross-sections of 18 radioactive residual nuclides are obtained by gamma spectrometry analysis. Compared with the Monte Carlo simulation by PHITS, the results show that the spallation model of PHITS has a high reliability in estimating the types of radioactive residual nuclei, and it could be optimized in the aspect of the absolute yield. Ruan Xi-Chao Acta Physica Sinica.2021, 70(7): 072501.

EDITOR'S SUGGESTION

2021, 70 (7): 076802. doi: 10.7498/aps.70.20201677
Abstract +
Thermoelectric (TE) materials can directly realize the mutual conversion between heat and electricity, and it is an environmentally friendly functional material. At present, the thermoelectric conversion efficiencies of thermoelectric materials are low, which seriously restricts the large-scale application of thermoelectric devices. Therefore, finding new materials with better thermoelectric properties or improving the thermoelectric properties of traditional thermoelectric materials has become the subject of thermoelectric research. Thin film materials, compared with bulk materials, possess both the two-dimensional macroscopic properties and one-dimensional nanostructure characteristics, which makes it much easier to study the relationships between physical mechanisms and properties. Besides, thin film are also suitable for the preparation of wearable electronic devices. This article summarizes five different preparation methods of Cu2Se thin films, i.e. electrochemical deposition, thermal evaporation, spin coating, sputtering, and pulsed laser deposition. In addition, combing with typical examples, the characterization methods of the film are summarized, and the influence mechanism of each parameter on the thermoelectric performance from electrical conductivity, Seebeck coefficient and thermal conductivity is discussed. Finally, the hot application direction of Cu2Se thin film thermoelectrics is also introduced.
2021, 70 (7): 078701. doi: 10.7498/aps.70.20201632
Abstract +
The rapid development of artificial intelligence (AI) requires one to speed up the development of the domain-specific hardware specifically designed for AI applications. The neuromorphic computing architecture consisting of synapses and neurons, which is inspired by the integrated storage and parallel processing of human brain, can effectively reduce the energy consumption of artificial intelligence in computing work. Memory components have shown great application value in the hardware implementation of neuromorphic computing. Compared with traditional devices, the memristors used to construct synapses and neurons can greatly reduce computing energy consumption. However, in neural networks based on memristors, updating and reading operations have system energy loss caused by voltage and current of memristors. As a derivative of memristor, memcapacitor is considered as a potential device to realize a low energy consumption neural network, which has attracted wide attention from academia and industry. Here, we review the latest advances in physical/simulated memcapacitors and their applications in neuromorphic computation, including the current principle and characteristics of physical/simulated memcapacitor, representative synapses, neurons and neuromorphic computing architecture based on memcapacitors. We also provide a forward-looking perspective on the opportunities and challenges of neuromorphic computation based on memcapacitors.
GENERAL
2021, 70 (7): 070201. doi: 10.7498/aps.70.20201682
Abstract +
With the integration of electromagnetic devices, the modules that make up into the devices and the functions that the devices needed to achieve are becoming more and more diverse. The coupling between the modules is difficult to ignore, the difficulty in designing increases sharply, and the traditional design methods gradually become incompetent. It is urgent to find a new comprehensive electromagnetic design method. This paper is to use the spatiotemporally synchronous focusing characteristics of time-reversed electromagnetic waves to explore the possibility of applying time-reversal technique to device design. First, based on the general device inverse design process, using the time-reversal technique, dyadic Green's function and basic principle of electromagnetics, a method of converting the port field distribution into the internal field distribution of the device is proposed. It is also proved that the continuous equivalent source obtained by the time-reversed field at a certain position in space can produce a field distribution close to the desired field at the port. In the single frequency inverse design process, only the tangential component of the electric field or magnetic field of the port is needed to be known. Then, with the help of the reciprocity of Green's function, the above theory is transformed to facilitate the numerical simulation. This numerical simulation realizes the reconstruction of the amplitude distribution source and the phase distribution source. It should be noted that the amplitude distribution source and phase distribution source are both randomly constructed. The numerical simulation verification is completed in two different cases and a variety of different initial conditions. All the simulation results are consistent with the theoretical results, which proves that it is feasible to apply time-reversal technique to the inverse design of electromagnetic devices.

EDITOR'S SUGGESTION

2021, 70 (7): 070301. doi: 10.7498/aps.70.20201754
Abstract +
In an isolated two-body composite system, the discussion of how the change of one body affects the state of the other will help to know the relation of a single particle's mixed and pure states. Given 5 one-dimensional hydrogen-like atoms models, each Coulomb interaction potential keeps invariant, while the masses of the nuclei are different. These two-body composite systems stay in their respective entangled state, each electron stays in a mixed state. If we suppose a one-dimensional hydrogen atom model having infinite nuclear mass, the electron stays in a pure state. We select position representation. The wave function of the ground state of the atom has the form of the square root of a δ function. To avoid calculation trouble of δ function, we select the first excited state and the superposed state of the first and the second excited states. We compare the two pure states, the first excited state and the superposed state, with those corresponding mixed states by fidelity and l1 norm coherence, and calculate the purities of the mixed states. The summations become integrations due to the position variable having a continuous eigenvalue spectrum. We investigate the changes in these quantities with the increase of the nuclear mass. The results show that the purities of the mixed states and the fidelities increase with the nuclear mass increasing. However, the coherences of the mixed states decrease with the nuclear mass increasing. This can be explained as that a mixed state with non-zero coherence may approach to a pure eigenstate, while the latter has zero coherence in the eigenspace. These mean that the greater a nuclear mass is, the closer the mixed state approaches to the corresponding pure state. Therefore, the two pure states are the approximations of the corresponding mixed states. The entangled state of the electron and the nucleus is related with the nuclear mass and the Coulomb interaction potential. So, each electron mixed state and its coherence are related with the nucleus and their Coulomb interaction potential. If the nuclear mass is great, the nucleus is approximately motionless or its state is approximately unchanged, and the Coulomb interaction potential approximates to the external Coulomb potential of the electron. The electron approximately stays in a pure state. The state and its coherence are related with the nucleus and the Coulomb interaction. From other point of view, the entangled state of the nucleus and the electron approximates to the product state of the unchanged nucleus state and the electron state. In this case, an electron mixed state approximates to its corresponding pure state, and then these states and their coherences are all related with the nucleus and the Coulomb interaction.
2021, 70 (7): 070302. doi: 10.7498/aps.70.20201394
Abstract +
The Duan-Lukin-Cirac-Zoller (DLCZ) process in the atomic ensemble is an important means to generate quantum correlation and entanglement between photons and atoms (quantum interface). When a write pulse acts on atoms, the DLCZ quantum memory process will be generated, which has been extensively studied. In the process a spontaneous Raman scattering (SRS) of a Stokes photon is generated, and a spin-wave excitation stored in the atomic ensemble is created at the same time. The higher probability of the generation of Stokes photons will cause more noise and reduce entanglement. On the contrary, the low generation probability of Stokes photons affects the success probability of entanglement distribution on a quantum repeater. How to increase generation probability of Stokes photons without causing more noise is an urgent problem to be resolved. In this work, a 87Rb atomic ensemble is placed in a standing wave cavity which resonates with the Stokes photon. This cavity has a trip length of 0.6 m and a free spectral range (FSR) of 256 MHz. The optical loss of all the optical elements in this cavity is 9%, of which 4% loss originates from the other optical elements and 5% loss from the vacuum chamber of the magneto-optical trap (MOT). The fineness of the cavity with the cold atoms is measured to be ~19.1. By calculating the total probability of Stokes photon emission out of the cavity, we derive the enhancement factor of this standing wave cavity when the cavity loss is l. When this cavity is locked with PDH frequency locking technique, we observe that the production probability of the Stokes photons is 8.7 times higher than that without cavity due to the optical cavity enhancement effect. Under this condition, the relationship between the generation probability of Stokes photons and the power of write beam is studied. The write excitation probability changes linearly with the power of write beam. This work provides an experimental solution to reducing the noise caused by time multimode operation in DLCZ scheme.
2021, 70 (7): 070701. doi: 10.7498/aps.70.20201451
Abstract +
Moiré patterns formed by overlapping two circular gratings of slightly different pitches have been extensively used for measuring the two-dimensional (2D) and three-dimensional (3D) displacements. However, in the existing applications, Moiré patterns are analyzed based on geometric superposition, by which the 3D displacements cannot be instantaneously or simultaneously measured with a high accuracy. In this paper, radial shearing interferometry with double circular gratings of slightly different pitches is presented to realize the simultaneous measurement of 3D displacements. The measurement is based on the principle that Moiré patterns produced by radial shearing interferometry are determined not only by the 2D in-plane displacements, but also by the out-of-plane displacement that brings about a phase shift between Moiré patterns of +1 and –1 diffraction orders. First, the production mechanism of Moiré patterns by radial shearing interferometry is studied based on the scalar diffraction theory and the intensity distribution of Moiré fringes of +1 and –1 orders is derived to establish the exact analytic relations between Moiré patterns and 3D displacements. Second, on the basis of spectrum characteristics of circular grating, a semicircular ring filter is proposed for spatial filtering to realize the simultaneous imaging of Moiré fringes of +1 and –1 orders. Then, the algorithm to quantitatively extract 3D displacements from Moiré patterns is proposed and demonstrated by numerical simulation. In the algorithm, Moiré patterns in the rectangular coordinate system are transformed into the polar coordinate system and skeletons are extracted to determine the feature points of the bright fringes. The in-plane displacements can be solved by feature points of +1 or –1 diffraction order, and the out-of-plane displacement can be computed by the feature points of +1 and –1 diffraction orders in the same bright fringe. Finally, experimental results prove that the maximum absolute error and mean error for in-plane displacements are 4.8 × 10–3 mm and 2.0 × 10–4 mm respectively, and 0.25 mm and 8.6 × 10–3 mm for out-of-plane displacement. In conclusion, by using the Moiré patterns of +1 and –1 diffraction orders imaged by radial shearing interferometer with double circular gratings of slightly different pitches, the 3D displacement can be simultaneously measured. The method has the advantages of simple device, high measurement accuracy, non-contact and instantaneous measurement, which provides an important guidance for practically measuring the 3D displacements.

EDITOR'S SUGGESTION

2021, 70 (7): 070702. doi: 10.7498/aps.70.20201277
Abstract +
THE PHYSICS OF ELEMENTARY PARTICLES AND FIELDS
2021, 70 (7): 071401. doi: 10.7498/aps.70.20201744
Abstract +
Based on the three-dimensional spinor Gross-Pitaevskii (GP) equation, the dynamic behavior of the Bose-Einstein condensate under the action of a time-dependent periodic external magnetic field is studied. The results show that the Bose-Einstein condensate with spin-1 in a ferromagnetic state will undergo topological deformation under the action of an external magnetic field periodically varying with time. When the two zero points of the magnetic field enter into the condensate, the density pattern of the spin-up state forms small convexities protruding upward and downward on the z-axis, respectively. As the two zero points of the magnetic field gradually coincide in the condensate, the upward and downward protruding convexities are elongated. Finally, the spin-up state in the shape of a line is distributed on the z-axis, which is consistent with the scenario of the isolated Dirac string predicted by theoretical analysis. As far as we know, magnetic monopole can be divided into positive monopole and negative monopole. The positive magnetic monopole means that all magnetic induction lines are emitted from the center of the circle. And only the Dirac string points to the center of the circle. The negative monopole is that all the magnetic induction lines point from the outside to the center of the circle, and only the Dirac string emits from the center of the circle. Magnetic monopole is a topological defect in vector field, which accords with both quantum mechanics and gauge invariance of electromagnetic field. Single magnetic monopole has been studied a lot in theory, and its analogues have been observed in experiment. But multiple monopoles and the interaction between them are still rarely studied. In this paper, multiple monopoles are produced based on the fact that the periodic magnetic field has multiple zeros. We use a new periodic magnetic field to generate a positive and negative magnetic monopole. Due to the strong external magnetic field, the vorticity in the condensate is consistent with the magnetic field of the monopole. Finally, by calculating the superfluid vorticity of the condensate, the characteristic diagram of the magnetic monopole is obtained. The results show that the condensate forms a pair of positive and negative magnetic monopoles at the two zero points of the magnetic field, corresponding to the two small convexities protruding upward and downward on the z-axis of the spin-up state, respectively. As the two zero points of the magnetic field coincide, the two Dirac strings in the positive and negative magnetic monopole gradually approach to each other, and after about 5 ms, they are completely connected, finally forming an isolated Dirac string. This result provides a new idea for further studying the isolated Dirac strings.

COVER ARTICLE

2021, 70 (7): 072501. doi: 10.7498/aps.70.20201503
Abstract +
Radioactive residual nuclides, which are usually closely related to radiation protection and personnel safety, will be generated when target materials are irradiated by high energy particles. Based on different nuclear reaction models, Monte Carlo code is a usual method to obtain residual nuclide production. The simulation accuracy needs to be evaluated by experimental data. In this paper, an irradiation experiment of thin copper target irradiated by 12C6+ particles with energy of 80.5 MeV/u is carried out. The radioactivities and cross-sections of 18 radioactive residual nuclides are obtained by gamma spectrometry analysis. Compared with the Monte Carlo simulation by PHITS, the results show that the spallation model of PHITS has a high reliability in estimating the types of radioactive residual nuclei, and it could be optimized in the aspect of the absolute yield.
ATOMIC AND MOLECULAR PHYSICS
2021, 70 (7): 073101. doi: 10.7498/aps.70.20201789
Abstract +
Thermoelectric materials, which can convert wasted heat into electricity, have attracted considerable attention because they provide a solution to energy problems. The Si/Ge superlattices have shown tremendous promise as effective thermoelectric materials. The period lengths of the Si/Ge superlattices can effectively tailor the phonon's transport behaviors and control their thermal conductivities. In this paper, three kinds of Si/Ge superlattices with different period length distributions (uniform, gradient, random) are constructed. The non-equilibrium molecular dynamics (NEMD) method is used to calculate the thermal conductivities of Si/Ge superlattices under the different period length distributions. The effect of the sample’s total length and temperature on the superlattice's thermal conductivity are studied. The simulation result shows that the thermal conductivity of gradient and random periodical Si/Ge superlattices are significantly reduced at room temperature compared with that of the uniform period Si/Ge superlattices. Phonons are transported by wave or particle properties in the different periodical superlattices. The thermal conductivity of uniform period superlattices has an obvious size effect with the increasing of the sample total length. In contrast, the thermal conductivity of gradient, random periodical Si/Ge superlattices are weakly dependent on the sample’s total length. At the same time, temperature is an important factor affecting the heat transport properties. We find that the temperature affects the thermal conductivities of the three kinds of superlattices in different ways. With the increase of the temperature, (i) the thermal conductivity of uniform periodical superlattices shows an obvious temperature effect; (ii) the thermal conductivity of the gradient and random periodical Si/Ge superlattices are nearly unchanged due to the competition between phonon localization weakness and phonon-phonon scattering enhancement. In addition, the phonon densities of states of superlattices with three different periodical length distributions are calculated. We find that in the picture of uniform periodical Si/Ge superlattices, the number of pronounced peaks quickly decreases as the period length increases, particularly at higher frequencies. This indicates that as the period length increases, fewer coherent phonons will be formed over the superlattices. Moreover, the scattering mechanisms of phonons for gradient and random periodical Si/Ge superlattices are basically the same at 100 K and 500 K. These findings provide a developmental way to further reduce the thermal conductivity of superlattices.
2021, 70 (7): 073102. doi: 10.7498/aps.70.20201483
Abstract +
The study of warm dense matter is very important for the evolution of celestial bodies and inertial confinement fusion, which often contains a mixture of multiple elements and different charge-state ions. The ionic structure and distribution of different charge-states directly affect the diagnosis and physical properties of warm dense matter. At the same time, the influence of high-temperature dense plasma on the ionic structure should be considered when we study the physical properties from the first-principle calculation of electron structure. In the present work, the radial distribution functions of multiple charge-state ions (gold, carbon-hydrogen mixture, and aluminum) are developed in the hypernetted-chain approximation, and elastic x-ray scattering of different charge-state ions are calculated in the warm dense matter regime. Firstly, the electron structure of different charge-state ions is self-consistently computed in the ionic sphere, in which the ion-sphere radii are determined by the plasma density and their charges. And then the ionic fraction is obtained by solving the modified Saha equation, with the interactions among different charge-state ions taken into account, and ion-ion pair potentials are obtained by Yukawa model. Finally, the ion features of x-ray elastic scattering for Al are calculated on the basis of electronic distribution around the nuclei and ionic radial distribution function. By comparing the results of different charge-sate ions with the result of mean charge-sate ion, it is shown that different statistical methods can affect the physical properties which are dependent on the electronic and ionic structure.
2021, 70 (7): 073201. doi: 10.7498/aps.70.20201672
Abstract +
The CO2, CH4 and other greenhouse gases are measured by using a home-made Fourier transform infrared spectrometer at the Longfengshan atmospheric background station. Compared with the measurement results of the instrument from the background station which meets the standards of the World Meteorological Organization, the correlation coefficient and the root mean square error of the CO2 concentration value are 0.9576 and 18.6015, so the measurement results from the home-made instrument are reliable. In the home-made instrument the calibration spectrum of standard temperature and the calibration spectrum of stand pressure are used to invert the concentration. With the temperature changing, the temperature of the measured gas will vary, thus resulting in error. The research of environmental variable factors can improve the accuracy of concentration inversion. For example, compared with CO2 absorption spectrum under 296 K, the CO2 absorption spectrum under 297 K will have 1.8% spectrum deviation and its inversion concentration error is 0.41%. This is the main cause of inversion concentration error. Based on the above analysis, the absorption cross section is calculated by using the high-resolution transmission molecular absorption database parameters. Combining with the instrument line shape, the calibration spectra at different temperatures and pressures can be obtained. The calibration spectra at different temperatures and pressures are used to calibrate the concentration inversion. After calibration, compared with the measurement results of the background station instrument, the correlation coefficient and the root mean square error of the CO2 concentration value are 0.9637 and 6.7803. The correlation coefficient of CO2 concentration value measured by self-developed instrument is improved and root mean square error is reduced. The result shows that the calibration algorithm enhances the accuracy of the measurement results to a certain extent. The above results illustrate the reliability of the home-made FTIR instrument and this experiment provides important data, which lay the foundation forstudying the home-made Fourier transform infrared spectrometer. Of course, improvement can be made in the following areas. Other minor factors may affect the effect of the inversion algorithm. The concentration inversion will have subtle differences at different bands of calibration spectra. So in order to improve the measurement accuracy, we need to choice more reasonable band inversion and more precise parameters from the high-resolution transmission molecular absorption database.
2021, 70 (7): 073701. doi: 10.7498/aps.70.20201721
Abstract +
In an experimental system of 87Sr atomic optical lattice clock, the free-running 698 nm diode laser is locked in an ultra-stable optical reference cavity to obtain the ultra-stable narrow linewidth laser with good short-term frequency stability. The ultra-stable optical reference cavity, which is usually composed of glass material doped with titanium dioxide for ultra-low thermal expansion coefficient and two highly reflective fused quartz mirrors, is called ULE cavity. The cavity length is prone to being affected by mechanical vibration, temperature change, airflow, etc. The stability of the cavity length determines the stability of the final laser frequency. Near the room temperature, there exists a special temperature point for the ultra-low expansion glass material, at which temperature its thermal expansion coefficient becomes zero, which is called the zero-crossing temperature. At the zero-crossing temperature, the length of the ULE cavity is not sensitive to the temperature fluctuation, reaching a minimum value, and the laser locked to the ULE cavity has a minimum frequency drift. In order to reduce the influence of temperature on the laser frequency instability, the zero-crossing temperature of the ultra-stable optical reference cavity of 698 nm ultra-stable narrow linewidth laser system is measured by using the clock transition spectrum of the strontium atomic optical lattice clock. The frequency drift and frequency instability of the 698 nm ultra-stable narrow linewidth laser system at zero-crossing temperature are measured by using the change of the in-loop locked clock frequency of strontium atomic optical lattice clock. By scanning the atomic clock transition frequencies at different temperatures, the clock transition spectra at different temperatures are obtained. The second order polynomial fitting of the central frequency of the clock transition spectrum with the change curve of temperature is carried out, and the zero-crossing temperature of the 698 nm ultra-stable narrow linewidth laser system ULE cavity is measured to be 30.63 ℃. At the zero-crossing temperature, the 698 nm ultra-stable narrow linewidth laser frequency is used for in-loop locking of 87Sr atomic optical lattice clock. The linear drift rate of the ULE cavity in the 698 nm ultra-stable narrow linewidth laser system is measured to be 0.15 Hz/s, and the frequency instability of the 698 nm ultra-stable narrow linewidth laser system is 1.6 × 10–15 at an average time of 3.744 s. The determination of ULE cavity zero-crossing temperature for the 698 nm ultra-stable narrow linewidth laser system is of great significance in helping to not only improve the instability of the laser system, but also increase the instability of 87Sr optical lattice clock system. In the future, we will improve the temperature control system of the ULE cavity in the 698 nm clock laser system, enhancing the temperature control accuracy of the ULE cavity and reducing the measurement error, thus achieving a more accurate zero-crossing temperature and further improving the frequency instability of the 698 nm ultra-stable narrow linewidth laser system.
ELECTROMAGNETISM, OPTICS, ACOUSTICS, HEAT TRANSFER, CLASSICAL MECHANICS, AND FLUID DYNAMICS
2021, 70 (7): 074201. doi: 10.7498/aps.70.20201495
Abstract +
With its rapid development, the terahertz technology is widely used in radar, imaging, remote sensing and data communication. As one of terahertz wave devices, the terahertz phase shifter has become a research hotspot. The existing phase shifters have the disadvantages of large volume, high power consumption and small phase shifting. In the present work, a tunable terahertz phase shifter with liquid crystal and vanadium dioxide is proposed. It is composed of an upper vanadium dioxide embedded metal layer, a liquid crystal, a lower vanadium dioxide embedded metal layer, and a silicon dioxide substrate in sequence from top to bottom. The liquid crystal is sandwiched between the upper and lower vanadium dioxide embedded metal layer. The phase of the device can be controlled by both the phase transition characteristics of vanadium dioxide and the birefringence of liquid crystal. By changing the external applied temperature, the conductivity of vanadium dioxide is changed, and the phase of the device shifts accordingly. Likewise the refractive index of the liquid crystal changes under different externally applied voltages. Finally, the phase of the proposed device can be effectively controlled in a terahertz range by both externally applied temperature and voltage. The phase shift characteristics of the device are analyzed by using software CST studio. The results verify that the terahertz phase shifter can achieve a maximum phase shift of 355.37° at f = 0.736 THz and a phase shift is larger than 350° in a range of 0.731–0.752 THz (bandwidth 22 GHz). Therefore, compared with the traditional phase shifter, this kind of phase change material-metasurface composite structure provides a new idea for flexibly manipulating the terahertz beam. And it is expected to be widely used in terahertz imaging, terahertz wireless and other fields.
2021, 70 (7): 074202. doi: 10.7498/aps.70.20201648
Abstract +
Semiconductor single-photon avalanche detectors (SPADs) have played an important role in practical quantum communication technology due to their advantages of small size, low cost and easy operation. Among them, InGaAs/InP SPADs have been widely used in fiber-optic quantum key distribution systems due to their response wavelength range in a near-infrared optical communication band. In order to avoid the influence of dark count and afterpulsing on single photon detection, the gated quenching technologies are widely applied to the InGaAs/InP SPADs. Typically, the duration of gate pulse is set to be as short as a few nanoseconds or even less. As the detection of the arrival of single photons depends on the coincidence between the arrival time of gate pulse and the arrival time of photon, the gate pulse duration of the InGaAs/InP SPADs inevitably affects the effective detection of the single photons. Without the influence of dispersion, the temporal width of the transmitted photons is usually on the order of picoseconds or even less, which is much shorter than the gate width of the InGaAs/InP SPAD. Therefore, the gate width normally has no influence on the temporal measurement of the detected photons. However, in quantum systems involving large dispersion, such as the long-distance fiber-optic quantum communication system, the temporal width of the transmitted photons is significantly broadened by the experienced dispersion so that it may approach to or even exceed the gate width of the single-photon detector. As a result, the effect of the gate width on the recording of the arrival time of the dispersed photons should be taken into account. In this paper, the influence of the gate width coupled to the InGaAs/InP single photon detectors on the measurement of the two-photon coincidence time width is studied both theoretically and experimentally. The theoretical analysis and experimental results are in good agreement with each other, showing that the finally measured coincidence time width of the two-photon state after dispersion is not more than half of the effective gate pulses width. The maximum observable coincidence time width based on the gated single photon detector is fundamentally limited by the gate width, which restricts its applications in quantum information processing based on the two-photon temporal correlation measurement.
2021, 70 (7): 074203. doi: 10.7498/aps.70.20201564
Abstract +
Narrow-linewidth femtosecond optical frequency comb plays an important role in the fields, such as optical clock comparison, time frequency transfer, ultrastable microwave generation, absolute distance measurement, high precision spectroscopy, etc. Due to the influence of the lifetime of the upper energy level in the gain medium, the linewidth of Er-fiber combs is generally on the order of several hundred kilohertz. In order to narrow the linewidth of comb teeth, an effective method is to insert a fast response electro-optic modulator (EOM) into the laser cavity, so that the servo bandwidth of fiber comb is extended to several hundred kilohertz, which provides a feedback mechanism for fast servo locking. Among them, a high quality femtosecond laser is the core. Based on this, the influence of the EOM on the parameters of Er-fiber femtosecond laser is studied in this paper. By calculating the refractive index, group velocity dispersion, and phase delay of the electro-optic crystal, the influence of the introduction of the EOM on the laser performance is analyzed. A LiNbO3 (LN) crystal with a length of 3 mm and x-cut is selected as the EOM and inserted into the laser cavity. The influence of the applied voltage of the EOM on the repetition rate and carrier envelope offset frequency of the laser are obtained experimentally. When the voltage on the LN crystal changes from -200 to 200 V, the adjustment of repetition rate is 60 Hz and the carrier envelope offset frequency is 25 MHz. Then the two parameters are phase locked through the EOM. Furthermore, by phase locking the beat note between the fiber comb and a narrow-linewidth continue wavelength laser at 1542 nm, it is verified that the introduction of the EOM can expand the servo bandwidth of the laser to more than 236 kHz, which provides a technical basis for establishing narrow linewidth femtosecond optical frequency combs. The following work will verify the performance of comb line, that is, when the comb is locked to a narrow-linewidth laser (such as 1542 nm), the performance of comb line at wavelength (such as 698, 729 nm, and so on) of distant place will be analyzed in detail.
2021, 70 (7): 074204. doi: 10.7498/aps.70.20200607
Abstract +
Phase anisotropy in laser resonant cavity will bring about an influence on laser frequency and polarization, such as laser frequency splitting, of which the frequency difference is determined by their introduced phase retardation. For a helium-neon laser with a small phase retardation in the cavity, the two split modes are very close to each other whose burned holes are overlapped. Then only one mode oscillates while the other is always in lock-in state due to strong mode competition, which forms hidden frequency split. Meanwhile the spacing between adjacent longitudinal modes deviates from original value and produces a certain variation equal to twice the hidden splitting frequency difference. As a result the longitudinal modes spacing variation is dominated by the phase retardation. On the other hand, by applying transverse magnetic field to a laser tube along the polarization direction, the neon atoms will undergo transverse Zeeman effect and be divided into two groups to provide the gain for polarized light beams parallel to the magnetic field and perpendicular to the magnetic field respectively. Then the laser mode competition is greatly weakened so that the two split modes can oscillate simultaneously to obtain the frequency difference. In order to make profound study of the consistency between longitudinal mode spacing variation and splitting mode frequency difference in the presence of transverse magnetic field, the samples of tilted quartz plate or half wave plate is placed into laser cavity to produce phase retardation. By the two mentioned methods, the splitting frequency difference varying with phase retardation of samples is deduced to make a comparison. Two measurements show that the average relative deviation is less than 1%, while the experimental results accord with theoretical analyses quite well. In this way splitting frequency difference of Zeeman dual-frequency laser can be determined accurately, and a new method to measure the phase retardation of half wave plate is provided.
2021, 70 (7): 074205. doi: 10.7498/aps.70.20201661
Abstract +
When a powerful laser beam propagates in a Kerr nonlinear medium, the Kerr effect on the beam propagation characteristics is very significant. The astigmatic laser beams are often encountered in practice. Until now, much work has been carried out on the propagation characteristics of astigmatic laser beams in linear media, but a few researches have been reported about the propagation of astigmatic laser beams through nonlinear media. To the best of our knowledge, the propagation or the transformation of astigmatic laser beams through an optical system in a Kerr nonlinear medium has not been investigated. In this paper, the propagation characteristics of focused astigmatic Gaussian beams in a nonlinear Kerr medium are studied. The Kerr effect on the beam astigmatism and the focal shift of focused astigmatic Gaussian beams are investigated in detail, and the self-focusing focal length and focus control of focused astigmatic Gaussian beams in the Kerr nonlinear medium are also studied. For the beam spreading case, the analytical formula for each of the beam width, the beam waist position, and the focal shift of focused astigmatic Gaussian beams in the Kerr nonlinear medium is derived. It is shown that in the self-focusing medium, as the beam power increases (i.e. the self-focusing effect becomes stronger), the beam astigmatism becomes stronger, but the focal shift decreases. However, in a self-defocusing medium, as the beam power increases (i.e. the self-defocusing effect becomes stronger), the beam astigmatism becomes weaker, but the focal shift increases. On the other hand, for the beam self-focusing case, the analytical formula of the self-focusing focal length of focused astigmatic Gaussian beams in the Kerr nonlinear medium is derived. It is found that the number of foci can be controlled by applying beam astigmatism. The results obtained in this paper are of theoretical and practical significance.

EDITOR'S SUGGESTION

2021, 70 (7): 074206. doi: 10.7498/aps.70.20201693
Abstract +
The ranging based on the chaotic lidar (CLR) generated by using the nonlinear dynamic of semiconductor with optical feedback or optical injection exhibits many advantages over the ranging using pulse lasers and CW lasers, such as low probability of intercept, strong anti-interference ability and low cost. Moreover, it has high resolution, benefiting from the broad bandwidth of the optical chaos. Finally, it is easily be generated and controlled due to the sensitivity of chaotic radar to laser parameters. The resolution of the correlated chaotic lidar (CLR) ranging which has been reported in many literatures is largely limited by the bandwidth of the chaotic laser. An ultra-fast chaotic laser with large modulation bandwidth is required to further improve the ranging resolution. The recently proposed optically pumped spin-VCSEL has attractive features such as flexible spin control of lasing output, fast dynamics with femtosecond magnitude and large modulation bandwidth. The ultra-fast chaos radar wave emitted from the optically pumped spin-VCSEL with optical injection or optical feedback is expected to be used for improving the resolution and accuracy of target ranging. In addition, since the multi beams of CLRs were utilized in the previous works, the number of ranging targets is limited to a small number of targets. The reported CLR ranging technology cannot completely detect the distance of different regions in the target, and it is not suitable for the accurate ranging of the whole area in the complex shape target. The detection waveform based on the correlation CLR has not been designed before the target ranging, which affects the further improvement of the resolution and accuracy of the target ranging. To overcome these problems, it is necessary to further explore the theoretical and physical mechanism of the CLR ranging for the multi-region in complex shape target, and explore the new scheme and method for its realization. Motivated by these, in this paper, based on the optically pumped spin vertical cavity surface emitting laser with optical injection, we present a novel scheme for the accurate ranging of the multi regions in two complex shape targets, using two chaotic polarization components modulated by the bipolar sinc waveform. Here, these two modulated chaotic polarization probe waveforms possess the attractive features of the uncorrelation in time and space, fast dynamic with femtosecond magnitude. Utilizing these features, the accurate ranging to the position vectors of the multi regions of two complex-shape targets can be achieved by correlating the multi beams of the time-delay reflected chaotic polarization probe waveforms with their corresponding reference waveforms. The further investigations show that the ranging to the multi-region small targets possesses the very low relative error that is less than 0.94%. If the bandwidths of the photodetectors are large enough, their range resolutions are achieved as high as 0.4 mm, and exhibit excellent strong anti-noise performance and strong stability. The multi area target ranging proposed in our scheme has the following attractive advantages: stable and high range resolution, strong anti-noise ability and very low relative error. These characteristics can meet the needs of the position vector ranging of the multi regions in complex shape targets.
2021, 70 (7): 074207. doi: 10.7498/aps.70.20201477
Abstract +
Microstructured fiber (MF) sensors based on surface plasmon resonance (SPR) have been widely investigated because they have many merits including high sensitivity, label-free and real-time detection and so on, thus they possess extensive applications such as in food safety control, environmental monitoring, biomolecular analytes detection, antibody-antigen interaction, liquid detection and many others. However, most of reported SPR-based MF sensors can only work in the visible or near-infrared wavelength region. Hence, the investigation of high-performance mid-infrared SPR-based MF sensors is a challenge task. In this paper, with the aim of overcoming the above limitation, a new type of high-sensitivity SPR-based MF sensor coated with indium tin oxide (ITO) layer is proposed. The proposed sensor can work in both the near-infrared and mid-infrared wavelength region. Benefitting from its two-core and single analyte channel structure, our proposed sensor can effectively eliminate the interference among neighboring analyte channels, improving its signal-to-noise ratio, and achieving high-sensitivity detection in ultra-broadband wavelength range. By using the full-vector finite method with the PML boundary conditions, the sensing properties of our proposed sensor are numerically studied in detail. The numerical results show that the resonance wavelength of the proposed sensor shifts toward a long wavelength region as the refractive index of analyte increases from 1.423 to 1.513, and a similar phenomenon can be found if the thickness of the ITO layer increases from 40 nm to 60 nm. Nevertheless, the wavelength sensitivity of the proposed sensor decreases with the increase of the diameter of the hole located in the fiber core region. On the other hand, when the refractive index of analyte varies in a large range of 1.423–1.513, the proposed sensor can operate in an ultra-broad wavelength range of 1.548–2.796 μm, and the average wavelength sensitivity is as high as 13964 nm/refractive index unit (RIU). Moreover, the maximum wavelength sensitivity and refractive index resolution increase up to 17900 nm/RIU and 5.59 × 10–7 RIU, respectively. Hence, our proposed SPR-based MF sensor can be applied to environmental monitoring, biomolecular analyte detection and chemical detection.
2021, 70 (7): 074208. doi: 10.7498/aps.70.20201524
Abstract +
The large aperture deuterated potassium dihydrogen phosphate (DKDP) is an important frequency conversion crystal in a large power laser device. There are many defects inside the DKDP bulk material, including the varying element impurities and electronic defects. Comparing with the defect-free material, these bulk defects can easily absorb incident laser energy and pose the risks of initiating damage sites when exposed to high-energy lasers. Beside bulk defects, there are surface defects originating from the DKDP machining process, including cracks, scratches and protuberances. These surface defects affect the damage performance of DKDP crystal by increasing light absorption and weakening local mechanical strength. Due to the defects from both bulk and surface, the actual damage threshold of DKDP crystal is much lower than the expected theoretical value. The lack of its laser damage resistance seriously restricts the laser output power. In this study, the off-line sub-nanosecond laser conditioning technology is proposed to effectively improve the laser damage performance of large aperture DKDP crystal. Its principle is to irradiate DKDP with a mild laser fluence in advance. Although the laser pretreatment cannot directly eliminate the impurities, dislocations, grain boundaries or other macro structural defects in crystals, it indeed changes the distribution and density of intrinsic point defects on a micro-scale. It suggests that the complicated reactions of electron-hole, atom-vacancy and the intrinsic point defect annihilation caused by the microstructural transformation of crystal materials under laser conditioning are the possible reasons for reducing absorption and improving the damage resistance. In this experiment, the result of the damage to high-power laser device shows that the mean surface damage density of DKDP crystal at 9 J/cm2 decreases from 5.02 pp/cm2 to 0.55 pp/cm2 after sub-nanosecond laser conditioning. The laser conditioning can remove the protuberance defects on the surface, thus reducing the surface damage density. In addition, the damage size probably decreases after laser conditioning. There is a leftward shift in the damage size curve after laser conditioning, and its peak decreases from 25 μm to 18 μm–20 μm. In addition, due to the removal effect of laser conditioning on defects, the spatial distribution of damage points after sub-nanosecond laser irradiation turns more uniform. This study provides a foundation for the applications of off-line sub-nanosecond laser conditioning technology in high-power laser facility.
2021, 70 (7): 074401. doi: 10.7498/aps.70.20201628
Abstract +
With the improvement of the integration and power density of three-dimensional integrated microsystem, it is imperative to simultaneously investigate the multi-field coupling analysis of electrical design and thermal management. This paper is to investigate a three-dimensional integrated microprocessor system and realize the rapid electrothermal analysis of the system through an improved dual cell method (DCM). This method decomposes the constitutive matrix into a constant matrix and a temperature-dependent matrix by introducing the coupling of leakage power and material coefficients with temperature. In the calculation, only the temperature-dependent matrix needs to be updated and assembled, which makes the calculation speed faster than the traditional finite element method. The simulation results show that the speed of the proposed algorithm is improved by about 30% compared with that of the traditional finite element method. After considering the thermal coupling factors of material coefficient and leakage power, the hot spot temperature of the system increases by 20.8 K compared with before coupling. Finally, the algorithm proposed in this paper is used to study the layout of three-dimensional integrated microprocessor system. The influence of TSV array conventional layout and centralized layout under the processor core(core-layout) on the hot spot temperature of upper and lower chips are compared, and the influences of uneven power distribution on the two layouts are studied. The results show that compared with the conventional layout of TSV array, the core-layout can reduce the hot spot temperature of processor, but it will aggravate the hot spot problem of DRAM at the same time. And when the power is not evenly distributed on the four cores, the hot spot of DRAM under the core-layout will be more seriously affected. In conclusion, the algorithm model proposed in this paper can quickly analyze the electrothermal coupling problem of 3D integrated microsystem, realize the hot spot prediction of the system, and provide theoretical guidance for designing the chip layout of 3D integrated microsystem.
2021, 70 (7): 074402. doi: 10.7498/aps.70.20201354
Abstract +
The phenomenon of small droplets impacting on the wall is widely used in fields such as spray cooling. In recent years, the research on the behavior of droplets impacting on a wall has been extensively concerned by scholars. However, current research mainly focuses on the flow and heat transfer characteristics of large droplets (millimeters) impacting on a hot wall. Since the dynamic and thermodynamic characteristics of small droplets (micrometers) impacting on the hot wall are significantly different from those of large droplets, the research on the behavior of small droplets impacting on the hot wall will further facilitate the understanding of the heat transfer mechanism. In order to study the heat transfer process of small droplets impacting on the hot wall (non-boiling zone), a two-dimensional transient model of droplets impacting on the wall is established, and the phase field method is used to analyze the convective heat flux and heat conduction heat flux in the heat transfer process of small droplets. The effect of velocity, wettability and droplet size on the heat transfer characteristics of droplets impacting on the wall are explored. The simulation results show that the phase field method is feasible in studying the behavior of small droplets impacting on the wall. Furthermore, the research results show that the initial stage of droplet impacting on the wall is “cold spot”, which is conducive to the heat transfer between the small droplet and the wall. The peak heat flux during the small droplet impacting on the wall is near the three-phase contact point, and it is on the order of 105–106 W/m2. The influence of wall wettability and droplet size on the conductive heat flux during the small droplets impacting on the wall are more significant, while velocity and droplet size have a significant influence on convective heat flux. In most cases, the conduction heat flux of small droplets impacting on the wall is about 103–105 W/m2, and the convective heat flux is about 104–106 W/m2. The convective heat flux is larger than the conduction heat flux, and it plays a dominant role in the whole process of heat transfer. The above conclusions are helpful in enriching the heat transfer mechanism of small droplets impacting on the hot wall, and implementing the spray cooling and other technologies.
2021, 70 (7): 074701. doi: 10.7498/aps.70.20201171
Abstract +
Asymmetric droplet splitting is a common method to obtain micro-droplets of different sizes. The study of droplet asymmetric splitting behaviors is of great significance to the fields of biomedicine, energy, chemical industry and food engineering. In this paper, the control flow is introduced into a branch of the T-shaped microchannel to control the pressure distribution in the channel and precisely control the size of the daughter droplets. The method is simple to operate and is a preferred method for asymmetric microfluidic splitting. Existing studies have analyzed droplet splitting modes, critical conditions for flow pattern transitions, and splitting dynamics, but the theoretical prediction of droplet asymmetric splitting behaviors needs to be strengthened. Moreover, compared with tunnel splitting and obstructed splitting, which are more abundantly studied, neither semi-obstructed splitting as an intermediate state of tunnel splitting nor obstructed splitting is analyzed sufficiently. Therefore, a microfluidic T-junction chip is designed and fabricated, with which asymmetrical splitting behaviors of droplets with a tunnel in a microfluidic T-junction are investigated experimentally. The influence of flow rate regulation on the droplet splitting ratio is studied. And a theoretical model is also established to predict the splitting ratio. The results are concluded as follows: 1) the process of asymmetrical droplet splitting is divided into three stages i.e. early squeezing, late squeezing and rapid pinch-off stage. In the early stage of squeezing, the radius of curvature of the droplet neck is sizable, and the additional pressure of interfacial tension is minor. Compared with the additional pressure that hinders neck contraction, the upstream continuous phase driving force is dominant, and the width of the neck changes linearly with time; in the process of late squeezing, the upstream pressure driving effect is still greater than the hindering effect of the additional tension, and the neck width changes exponentially with time; However, in the rapid pinch-off stage, the interfacial tension pointing to the center of the cross section of droplet neck dominates the pinch-off stage. Then, the droplet neck shrinks sharply. 2) Adjusting the flow rate of the branch channel can effectively control the asymmetric splitting ratio of the droplets, and under the current semi-obstructed asymmetric splitting of the droplets, the regulation effect is less affected by the size of the mother droplet, but more affected by the capillary number. 3) The prediction model of droplet splitting ratio based on the pressure drop model can effectively predict the droplet splitting ratio.

EDITOR'S SUGGESTION

2021, 70 (7): 075201. doi: 10.7498/aps.70.20200944
Abstract +
2021, 70 (7): 075202. doi: 10.7498/aps.70.20201450
Abstract +
When a hypersonic vehicle flies, it will have friction with the atmosphere, ionizing the surrounding air, and producing a plasma sheath containing a large number of free electrons. The plasma sheath will cause the electromagnetic wave to seriously attenuate, which will result in communication interruption and other problems. With the gradual realization of terahertz wave technology, its high penetrability and anti-interference performance provides an important way to solve the blackout problem. Thus, the using of the terahertz wave to solve the blackout problem encountered during vehicle reentry is of great significance to studying the transmission performance of terahertz wave in the plasma sheath. This article refers to the public data of the plasma sheath during the reentry of the RAM vehicle. Considering the asymmetry of the sheath density distribution, a double Gaussian distribution is used to simulate the longitudinal electron density distribution. Based on the SMM algorithm, the article uses the magnetization direction, electron density, external magnetic field strength, collision frequency of the non-uniformly magnetized plasma as variables, and their effects on left-hand and right-hand polarized terahertz wave under normal incidence are studied. The results show that these parameters have obvious effects on the transmission performance of terahertz wave in high-density plasma sheath. The right-hand polarized terahertz wave will produce a power absorption peak near the cyclotron frequency due to cyclotron resonance. Changing the magnetization angle in a certain direction will bring an opposite effect on the transmission rate to left-hand polarized and right-hand polarized terahertz wave. Reducing the magnetization intensity can avoid the absorption peak of the right-hand polarized wave by the plasma to a certain extent. Increasing the magnetization can increase the transmission power of the left-hand polarized wave to a certain extent. Moreover, reducing the collision frequency can narrow the absorption band of the right-hand polarized wave in the plasma and increase the transmission power of left-hand polarized wave. In general, the transmission performance of left-hand polarized terahertz wave in non-uniformly magnetized plasma is better than that of right-hand polarized terahertz wave. These results provide a theoretical basis for investigating the blackout phenomenon. The adjusting of these parameters studied in the article is expected to be able to alleviate the blackout problem to a certain extent.
2021, 70 (7): 075203. doi: 10.7498/aps.70.20201368
Abstract +
In order to solve the problems of unstable discharge, low deposition rate and large difference in ionization rate between different targets in high power impulse magnetron sputtering, a novel cylindrical cathode with annular magnetic field based on hollow cathode effect is proposed, which can be used to produce ion beam with high ionization rate, high plasma density and no large particles. However, the traditional channel structure could not guarantee its high efficiency and uniform heat dissipation. The sealing ring may be damaged by ablation due to high power density, which restricts the further improvement of power density. Therefore, it is necessary to optimize the design of the channel structure. SolidWorks flow simulation software is used to simulate the cooling channel of plasma source. The influence of water hole structure parameters on cooling effect is analyzed, including distribution angle, hole number, diameter and inlet hole height. And the channel structure parameters are optimized. The results show that the increasing of the circumferential distribution range of the water hole is beneficial to the uniformity of heat dissipation, ensuring a large temperature difference between cooling water and copper sleeve, and strengthening heat exchange. The water inlet hole set in the upper layer of the structure is conducive to alleviating the temperature stratification phenomenon of the cooling water, so that the copper sleeve and sealing ring are in good cooling condition. Appropriately reducing the aperture is beneficial to increasing the cooling water jet velocity, enhancing the jet impact effect, and then increasing the turbulence degree, strengthening the heat transfer and improving the heat transfer efficiency. By systematically studying the influencing factors, the optimized cooling flow field structure of cylindrical cathode with an annular magnetic field is obtained. The distribution angle is 30°, the number of holes is 6, the aperture is 4 mm, and the height of water inlet hole is 36 mm. The optimized channel structure can improve the utilization rate of cooling water, obtaining better cooling effect at the same flow rate, and improving the discharge stability of the plasma source, which provides a basis for designing the cooling structure of the cylindrical cathode with annular magnetic field.
2021, 70 (7): 075204. doi: 10.7498/aps.70.20201667
Abstract +
The cathode-less miniature electron cyclotron resonance ion thruster (ECRIT) has the advantages of long-life and simple-structure. In the ECRIT ion source, the plasma distribution will affect the beam extraction, and the relative position of the ECR layer determined by the magnetic field structure and the flat-ring antenna together affect the plasma distribution. Due to the sheath, the ions or electrons in the plasma will be accelerated to sputter the surface of wall and induce plasma loss. It is important to investigate the wall currents and plasma characteristics. Therefore, particle-in-cell with Monte Carlo collision (PIC/MCC) model is established in this article to study the influence of the magnetic field structure on the plasma and wall current characteristics of 2-cm ECRIT ion source. The calculation results show that the electrons are confined near the ECR layer of antenna by the magnetic mirror, which leads the plasma to be distributed near the ECR layer. When the ECR layer is located on the upstream side of the flat-ring antenna, the plasma is concentrated between the antenna and magnet rings, and the ion density in front of the grid is lowest, which results in a lower ion beam current extracted from ion source and a lower current on the surface of magnetic ring and antenna. When the ECR layer is located on the downstream side of the flat-ring antenna, the plasma density near the upstream side of the antenna and grid is high, which results in higher ion beam current extracted from the ion source and higher current on the surface of antenna and magnetic ring. The plasma distribution and the total wall current of the ion source are affected weakly by the magnetic field structure. In this magnetic field structure, the ion sputtering on the flat-ring antenna is serious. Although such a magnetic field design can increase the extracted ion beam current, it will shorten the working life of the ion source. In the future, when designing a new thruster, it is necessary to weigh the ion current of extraction and lifetime to select the appropriate magnetic field structure.

EDITOR'S SUGGESTION

2021, 70 (7): 075205. doi: 10.7498/aps.70.20201716
Abstract +
Micro-beam radio-frequency (RF) capacitive discharges have been widely used in the plasma enhanced chemical vapor deposition of nanocrystalline particles such as nano silicon crystal. However, the plasma column shrinks radially at a sufficiently high gas pressures as manifested by their glow not entirely filling the radial cross-section of the discharge tube. This greatly limits the dissociation rate of gas in plasma. In order to obtain the information about the plasma column varying with gas pressure, the formation of different gas discharge mode under different pressure is discussed. In this paper the spatial characteristics of micro-beam RF capacitive discharges are investigated by using an intensified charged-coupled device (ICCD) and a single lens reflex camera (SLR camera). Furthermore, high voltage probe and current probe are used to record the electrical characteristics of the high voltage electrode. The results indicate that in a pure argon discharge, the discharge mode evolves from a glow discharge into a filament discharge with the increase of pressure. As the pressure continues to increase, the filament is split: a single channel of plasma is split into two or more filaments at a certain gas pressure. However, the glow discharge in a mixture of 99% argon and 1% hydrogen at a low pressure is observed: the plasma spreads throughout the tube. As the pressure increases, the filament disappears, and the plasma column still can be observed in the center of quartz tube. The glow shrinks in the radial center at a moderate pressure. At a high pressure, the "annulus" glow discharge is achieved as manifested by a glow ring on the surface of the discharge tube. In addition, in pure hydrogen discharges, the discharge mode evolves from the full-space glow discharge into an "annulus" glow discharge with pressure increasing. Finally, through the interaction between the electron heating by the radio frequency electric field and heat conduction of gas, the filament discharge in a low thermal conduction gas is explained. In addition, special attention is paid to the pure argon filamentation, which is the splitting of a single channel of plasma into two or more smaller filaments as a result of the skin effect.
2021, 70 (7): 075206. doi: 10.7498/aps.70.20201700
Abstract +
Recently, atmospheric non-equilibrium plasma has been proposed as a potential and novel type of “reaction carrier” for the activation and conversion of greenhouse gases (methane and carbon dioxide) into value-added chemicals, due to its unique non-equilibrium characteristics. In this paper, a zero-dimensional plasma chemical reaction kinetic model in CH4/CO2 gas mixture is constructed, with an emphasis on reaction mechanism for plasma dry reforming of methane to syngas and oxygenates. Especially, the effect of the CH4 molar fraction (5%–95%) on plasma dry reforming of methane is investigated. First, the time evolution of electron temperature and density with initial methane content is presented, and the results show that both the electron temperature and electron density vary periodically with the applied triangular power density pulse, and the higher initial methane content in gas mixture is favored for a larger electron temperature and density. Subsequently, the time evolution of number densities of free radicals, ions and molecules at different CH4/CO2 molar fraction are given. The higher the initial methane content, the greater the number densities of H, H, H2, and CH3, leading to insufficient oxygen atoms to participate in the reaction for oxygenates synthesis. The conversions of inlet gases, the selectivities of syngas and important oxygenates are also calculated. The conversion rate of carbon dioxide increases with the increasing methane content, but the conversion rate of methane is insensitive to the variation of methane content. As methane mole fraction is increased from 5% to 95%, the selectivities of important oxygenates (CH3OH and CH2O) are relatively low (<5%), and the selectivity of H2 gradually increases from 13.0% to 24.6%, while the selectivity of CO significantly decreases from 58.9% to 9.7%. Moreover, the dominant reaction pathways governing production and destruction of H2, CO, CH2O and CH3OH are determined, and CH3 and OH radicals are found to be the key intermediate for the production of valuable oxygenates. Finally, a schematic overview of the transformation relationship between dominant plasma species is summarized and shown to clearly reveal intrinsic reaction mechanism of dry reforming of methane in atmospheric non-equilibrium plasma.
2021, 70 (7): 075207. doi: 10.7498/aps.70.20201491
Abstract +
Cryogenic target is one of the key components of inertial confinement fusion. The removal degree and efficiency of impurity gas in cryogenic target are of great significance to the on-line preparation of ice layer for cryogenic target fuel. According to the design requirements of cryogenic target physics for impurity content in ice layer, the influence factors of upper limit of partial pressure are analyzed, based on the derivation of the calculation formula of maximum allowable partial pressure of impurity gas in the target. Then the flow field of air and hydrogen in microchannels is investigated, and the filling and evacuation model of gas flow in a microscaled filling tube is established. The dynamic simulations of microtubules with different lengths and diameters are carried out. The results show that the microtubules with a length of 5 mm could save 80% of the time compared with the microtubules of 50 mm in length when the microtubule is 5 μm in diameter. At the same time, the total flow washing time decreases by 46% when the diameter of 2 μm is doubled under the condition of 20-mm-long microtubule. Considering the requirements for efficiency and fusion stability, four kinds of tubes are proposed and simulated. The results indicate that the conical transition tube has a strong flow capacity and high flow evacuation efficiency, and is suitable for use as a filling microtubule. On the basis of the best tube shape, the comparison between the two processes under different intermediate pressures is carried out with the time and number of filling and evacuating serving as evaluation criterion. Ultimately, the intermediate pressure of 52000 Pa is selected, the total number of evacuation is 10 and the time is 758 s. Finally, the effect of temperature on the evacuation efficiency is studied in a temperature range of 113 K－293 K in steps of 60 K. The results show that the total time of filling and evacuation will be reduced by 15% on the basis of normal temperature when the temperature is reduced by 60 K, which proves the feasibility of evacuation at low temperature in practical operation.
CONDENSED MATTER: STRUCTURAL, MECHANICAL, AND THERMAL PROPERTIES
2021, 70 (7): 076101. doi: 10.7498/aps.70.20201409
Abstract +
In order to understand further the micro-mechanism of helium bubble punching out of the dislocation loop in α-Fe, it is necessary to study the ultimate pressure characteristics of helium bubble punching out of the dislocation loop. In this paper, a cubic representative volume element (RVE) model of the metal-helium bubble is established. For eight types of spherical helium bubbles with different initial radii, molecular dynamics simulations are carried out with the initial helium-to-vacancy ratio serving as a variable and the ultimate pressure of helium bubble and the critical helium-to-vacancy ratio at the beginning of dislocation loop formation in each model are obtained. The results show that for helium bubbles with dimensionless radius ranging from 2 to 10, both the ultimate pressure and the critical helium-to-vacancy ratio of helium bubble punching out of the dislocation loop decrease nonlinearly with the increase of initial helium bubble radius. The relationships of the ultimate pressure and the critical helium-to-vacancy ratio with the initial radius of helium bubble are fitted respectively according to the simulation results and the fitted two equations are in good agreement with the results of previous theoretical studies. The critical helium-to-vacancy ratio required for helium bubble punching out of the dislocation loop in α-Fe has an obvious size effect. For the helium bubble in the late nucleation stage (e.g. helium bubble with an initial radius of 0.81 nm) and non-ideal gas stage (e.g. helium bubble with an initial radius ranging from 1.00 nm to 2.50 nm), the critical helium-to-vacancy ratios for punching out of the dislocation loop are the same as the initial helium-to-vacancy ratio corresponding to the peak pressure point of helium bubble. But for early or middle nucleation stage, such as helium bubble with an initial radius of 0.50 nm, the critical helium-to-vacancy ratio for punching out of the dislocation loop is about 13.46% greater than the initial helium-to-vacancy ratios corresponding to the peak pressure points. At the initial moment (0 ps), in the cross section passing through the center of cubic RVE, the shear stress is concentrated, and the maximum shear stress of Fe atom array around the helium bubble is located at the intersection points (i.e. at 45°) of diagonal and helium bubble boundary, and it is distributed symmetrically with respect to the double fold lines of the cross section parallel to the sides. Both the range of shear stress concentrating area and the maximum shear stress increase with the initial helium-to-vacancy ratio increasing. The dislocation loop’s punching direction corresponds to the direction of the maximum shear stress. The research in this paper can deepen the understanding of the physical properties of helium bubbles in metals and lay a useful foundation for the subsequent analyzing of the effects of helium bubbles on the macroscopic physical and mechanical properties of materials.
2021, 70 (7): 076801. doi: 10.7498/aps.70.20201771
Abstract +
In-depth understanding is limited to the oscillation properties of a droplet on a superhydrophobic surface, which are closely related to the contact line movement, droplet volume, and substrate amplitude, to name only a few factors. In the present work, we investigate the characteristics of droplet resonance amplitude, mode range, and resonance frequency, as well as their correlations with droplet volume (from 20 to 500 μL). In particular, the theoretical resonance frequency is mainly concerned and addressed. To this end, a model based on general hydrophobic surfaces proposed by Noblin et al. is employed, with its applicability to superhydrophobic surfaces examined. We propose a concept “virtual stationary point” for analyzing the errors from this model, with which we modify the model through using the correction coefficients. The main results are concluded as follows. 1) Under resonance, the change rate in droplet height rises with the increase of droplet volume and reduces with the increase of oscillation mode number. 2) Each number of oscillation mode corresponds to a frequency range, and the ends of adjacent mode ranges are connected to each other. These frequency ranges decrease with the increase of droplet volume. 3) Resonance frequency, f, decreases with the increase of droplet volume, V, and they are related approximated by f -V–0.4 under high mode numbers, which is different from f -V–0.5 as found on general hydrophobic surfaces. 4) Direct application of Noblin model to a superhydrophobic surface results in nonnegligible errors, because geometric characteristics in this case are different from those on a general hydrophobic surface, which leads to inaccuracy in counting the number of surface wave segments. In contrast, results from modified Noblin model accord well with experimental results.
2021, 70 (7): 076803. doi: 10.7498/aps.70.20201502
Abstract +
The detection of intensity peaks, which correspond to atom positions, in high-resolution (scanning) transmission electron microscopy images is of great practical significance. By quantitatively determining the locations of these peaks, it is possible to obtain important information such as the structural deformation and the electric dipole distribution inside a material on the nanoscale. The detection of the peak positions in image processing can be regarded as a target detection problem, for which breakthroughs have been made with deep-learning neural networks. Comparing to the traditional target detection algorithms, which are based on specifically designed feature extractor and classifier, the deep-learning approach can obtain the features at multiple levels of abstraction automatically, thus improving the robustness of the detection process. In this paper, we realize the automatic detection of the intensity peaks in high-resolution electron microscopy images by building a high-quality atomic image sample set and using the YOLOv3 target detection framework. With its accuracy and speed, which are superior over other target detection neural networks, the YOLOv3 is suitable for image processing as the number of images increases explosively. The YOLOv3 network converges well in the training process using our atomic image sample set, with the loss function reaching a minimum after 500 epoches; the trained neural network can detect almost all the major atoms in the images, showing its excellent ability. With the aid of YOLOv3, we also develop a program to organize the detected atoms, enabling the detection of all the other atoms within each unit cell. It is found that, combining YOLOv3 with the newly developed algorithm, almost all the atoms can be successfully determined, showing its advantages over previous algorithms based on bravis lattice construction, especially for high-resolution transmission electron microscopy images with lattice defects. Our results show that this image processing technique has the potential in overcoming the bottleneck in the fast processing of high resolution electron microscopy images.
CONDENSED MATTER: ELECTRONIC STRUCTURE, ELECTRICAL, MAGNETIC, AND OPTICAL PROPERTIES
2021, 70 (7): 077201. doi: 10.7498/aps.70.20201355
Abstract +
$\ln(I/E)\text{-}E^{1/2}$ data points obey a good linear relationship, whose current slope and corresponding activation energy are close to the values predicted by the Frenkel-Poole (FP) model, indicating the dominant role of the FP emission mechanism. 3) In the high-bias region, the $\ln(I/E^2)\text{-}E^{-1}$data points also follow a linear dependence, but the current slope is a weak function of temperature, indicating that the Fowler-Nordheim tunneling mechanism should be mainly responsible for the leakage current. 4) The current barrier height is extracted to be about 0.60 eV, which is much lower than the value of 2.91 eV obtained from the TE model, confirming the primary leakage path of the conductive dislocations, where the localized barrier is significantly reduced due to the ionization of shallow donor-like traps.">In this paper, the temperature-dependent current-voltage (T-I-V) characteristics of lattice-matched InAlN/GaN heterostructure Schottky contact in a reverse direction are measured, and the voltage dependence and temperature dependence of the leakage current are studied. The obtained results are as follows.1) The reverse current is a strong function of voltage and temperature, and the saturation current is much larger than the theoretical value, which cannot be explained by the classical thermionic emission (TE) model. 2) In the low-bias region, the $\ln(I/E)\text{-}E^{1/2}$ data points obey a good linear relationship, whose current slope and corresponding activation energy are close to the values predicted by the Frenkel-Poole (FP) model, indicating the dominant role of the FP emission mechanism. 3) In the high-bias region, the $\ln(I/E^2)\text{-}E^{-1}$data points also follow a linear dependence, but the current slope is a weak function of temperature, indicating that the Fowler-Nordheim tunneling mechanism should be mainly responsible for the leakage current. 4) The current barrier height is extracted to be about 0.60 eV, which is much lower than the value of 2.91 eV obtained from the TE model, confirming the primary leakage path of the conductive dislocations, where the localized barrier is significantly reduced due to the ionization of shallow donor-like traps.
2021, 70 (7): 077202. doi: 10.7498/aps.70.20201444
Abstract +
Since the breakthrough by Tang et al. in 1987, organic light-emitting devices (OLEDs) have attracted extensive attention in the industries and academic research communities. OLEDs have many promising characteristics, such as self-illumination, lower power consumption, easy fabrication and so on. It has a broad development prospect in high resolution display and other fields. For RGB color OLED display technology, blue light organic material is very important. Polyfluorene (PFO) is a kind of rigid planar biphenyl structure compound in all kinds of OLEDs blue light materials. However, PFO has a very big disadvantage: the long wave shift of the light-emitting peak of the electroluminescent device will produce the green light-emitting band that should not have appeared. This seriously affects the saturation color purity of PFO devices, and also seriously restricts the industrialization process. In this paper, the molecular magnetic material [Fe(NH2trz)3· (BF4)2] is used to solve this problem. ITO/PEDOT:PSS (30 nm)/PFO:Fe(NH2trz)3·(BF4)2 (65 nm)/CsCl (0.6 nm)/Al (120 nm) devices were fabricated on ITO glass substrate. It is the first time to report the strong pure blue emission of PFO by using the special electronic spin state modulation of Fe(NH2trz)3·(BF4)2. The influence of Fe(NH2trz)3·(BF4)2 on the photoelectric properties of PFO was studied in detail by analyzing the PL and EL characteristics of PFO and PFO:Fe(NH2trz)3·(BF4)2. Under the bias voltage of 4 V to 9 V, the device without doping Fe(NH2trz)3·(BF4)2 emits very strong green light. The central peak wavelength is 553 nm, and the color coordinates are (0.33, 0.45). Moreover, with the constant change of voltage, the green light-emitting band is always much larger than the blue light-emitting band. However, the obvious difference is that Fe(NH2trz)3·(BF4)2 doped device emits strong blue light, the peak wavelength is 438 nm, and the color coordinates (0.23, 0.22), which is completely consistent with the peak wavelength of the PL spectrum of the PFO film; the green light-emitting band of the PFO is successfully suppressed; with the change of the electric voltage, the proportion of the blue light part of the device spectrum in the whole EL spectrum is almost unchanged. The photoconductivity effect of undoped Fe(NH2trz)3·(BF4)2 device is further studied by means of the integrated opto-electro-magnetic measurement technology. Under different bias voltage, it is found that there is almost no excimer in PFO:Fe(NH2trz)3·(BF4)2. This study solves the problem of green light of polyfluorene, which has puzzled the industry for many years, and provides a reliable way for the industrialization of polyfluorene used in blue OLED. The mechanism of Fe(NH2trz)3·(BF4)2 blocking the abnormal green emission of PFO was discussed by using the theory of luminescence dynamics.

EDITOR'S SUGGESTION

2021, 70 (7): 077501. doi: 10.7498/aps.70.20201737
Abstract +
Rare-earth iron garnet films with perpendicular magnetic anisotropy could open new perspectives for spintronics. Holmium iron garnet (Ho3Fe5O12, HoIG) films with thickness ranging from 2 to 100 nm are epitaxially grown on (111) orientated gadolinium gallium garnet single crystal substrate doped with yttrium and scandium (Gd0.63Y2.37Sc2Ga3O12, GYSGG) by ultra-high vacuum magnetron sputtering. A 3-nm Pt film is further deposited on each of the HoIG films. The magnetic anisotropy and magneto-transport properties of heterostructures at room temperature are investigated. It is shown that the HoIG film as thin as 2 nm (less than two unit cells in thickness) exhibits the ferromagnetic properties at room temperature, and perpendicular magnetic anisotropy is achieved in the 2-60 nm thick films, and a maximum effective perpendicular anisotropy field reaches 350 mT due to the strain induced magnetoelastic anisotropy. The HoIG/Pt heterostructure shows significant anomalous Hall effect (AHE) and appreciable spin-Hall magnetoresistance (SMR) and/or anisotropic magnetoresistance (AMR). Remarkably, the AHE starts to decline gradually when the HoIG thickness is less than 4 nm, but the magnetoresistance decreases rapidly with the HoIG layer becoming less than 7 nm in thickness. The fact that the AHE in the heterostructure is less sensitive to the HoIG thickness suggests that the interface effect is more dominant in the AHE mechanism, whereas the bulk magnetic properties of the HoIG plays a more important role for the observed magnetoresistance. In addition, the spin Seebeck effect decreases exponentially with the decrease of HoIG thickness till the ultrathin limit, which was previously validated in the micrometer-thick YIG/Pt stacks in the frame of thermally excited magnon accumulation and propagation. The present results show that the nanometer HoIG/Pt heterostructure with tunable perpendicular magnetic anisotropy and efficient interfacial spin exchange interaction could be a promising candidate for insulating magnet based spintronic devices.
2021, 70 (7): 078901. doi: 10.7498/aps.70.20201516
Abstract +