bluuebunny/crossref_metadata_2025 huggingface dataset:
Detection of atmospheric multi-component based on a single quantum cascade laser
Authors: Zhou Chao  , Zhang Lei  , Li Jin-Song  ,   Key Laboratory of Opto-Electronic Information Acquisition and Manipulation of Ministry of Education, Anhui University, Hefei 230601, China
Published: December 2020

Quantum cascade lasers (QCLs) are relatively new sources of mid-infrared radiation (between 2.5 m and 25 m), and are very well suited to the application of in-field trace gas sensing, mainly due to their superiority of being robust, compact, wavelength-versatile, narrow line width and low power consumption. All these advantages make the laser absorption spectroscopy based on QCL light sources become one of the most popular technologies for the quantitative chemical detection in a variety of fields including atmospheric environmental monitoring, chemical analysis, industrial process control, medical diagnostics, security or bio-medical studies, etc.
In the present work, a highly sensitive mid-infrared gas sensor employing a single continuous-wave distributed feedback QCL and an astigmatic multi-path optical absorption cell is demonstrated for the simultaneous measurement of atmospheric carbon monoxide (CO), nitrous oxide (N2O) and water vapor (H2O). By combining with an adaptive Savitzky-Golay (S-G) filter signal processing algorithm, the detection sensitivity and spectral resolution of the QCL sensor system are significantly improved. Compared with the traditional wavelet transform based signal de-noising technique, the developed adaptive S-G smoothing filter shows obvious advantages in terms of computational efficiency and selection of the optimal filter parameters, namely only two filter parameters (the width of the smoothing window and the degree of the smoothing polynomial) need to be considered. Currently, the QCL sensor system is estimated for the long term measurement of ambient air in laboratory environment. The results show that measurement precisions of 8.20 ppb (1 ppb=10-9) for CO, 7.90 ppb for N2O, and 64.00 ppm (1 ppm=10-6) for H2O at 1 s time resolution and 1 atmospheric pressure (atm) are obtained by using the quadratic differential detection scheme, which can be further improved to 1.25 ppb (for CO), 1.15 ppb (for N2O) and 35.77 ppm (for H2O) by increasing average time up to 85 s, respectively. On the whole, the QCL sensor system has significant features of portability and low-cost, moreover, it can be easily modified for the real-time analysis of other gas molecules through the choosing of corresponding QCL light sources. The QCL gas sensor can be widely used in the field of atmospheric chemistry and other applications. Future work will focus on H2O induced broadening coefficients for CO and N2O transitions near 4.57 m, which will be updated for the developed multi-species QCL sensor system, thus resolving the influence of water vapor broadening effect and achieving the measurement of gas concentration in a high humid environment with sub-percent precision.
Electromagnetic control and optimization of high power impulse magnetron sputtering discharges in cylindrical source
Authors: Cui Sui-Han  , Wu Zhong-Zhen  , Xiao Shu  , Liu Liang-Liang  , Zheng Bo-Cong  , Lin Hai  , Ricky K Y Fu  , Tian Xiu-Bo  , Paul K  , Tan Wen-Chang  , Pan Feng  ,   School of Advanced Materials, Peking University Shenzhen Graduate School, Shenzhen 518055, China;,   Department of Physics and Materials Science, City University of Hong Kong, Hong Kong 999077, China
Published: December 2020

High-power impulse magnetron sputtering (HiPIMS), a new physical vapor deposition technique which combines the advantages of the high ionization rates of the sputtered materials and control of electromagnetism, has been widely used to deposit high-performance coatings with a large density and high adhesion. However, HiPIMS has some intrinsic disadvantages such as the low deposition rate, unstable discharge, and different ionization rates for different materials thereby hampering wider industrial adoption. We have recently designed an optimized cylindrical source based on the hollow cathode effect to circumvent the aforementioned limitations. However, during the operation of the cylindrical source, the discharge is inhomogeneous and the etching stripes are nonuniform. In order to determine the underlying mechanism and optimize the electromagnetic control, the discharge in the HiPIMS cylindrical source is simulated. The tangential magnetic field distribution on the target surface of the cylindrical sputtering source is inhomogeneous and electron runaway is serious, resulting in a relatively low plasma density. Two solutions are proposed to improve the situations. The first one is electrical improvement by installing an electron blocking plate, and the second one is magnetic improvement by adding compensating magnets. Our simulation results of the first method show that a potential well is produced by the electron blocking plate to suppress electron runaway and the plasma density is improved significantly, especially around the central cross-section of the cylindrical sputtering source. The discharge becomes homogeneous, and the etching stripes are uniform albeit not full enough. The second method of magnetic improvement significantly improves the homogeneity of the tangential magnetic field distribution on the target surface and the target utilization rate. After adding the optimized compensating magnets, the shape of the effective area (the value of the tangential magnetic field in a range of 25-50 mT) on the target surface can be controlled and made zonal. The target utilization rate increases to over 80% from 60%. In order to obtain the optimal conditions, the two techniques are combined. A larger and more homogeneous etching ring is observed by adopting both the electrical and magnetic improvements as predicted and explained by the simulation results. It can be concluded that the combination of the two improvement techniques can improve and optimize the HiPIMS cylindrical source.
Design and electronic transport properties of organic thiophene molecular rectifier with the graphene electrodes
Authors: Zu Feng-Xia  , Zhang Pan-Pan  , Xiong Lun  , Yin Yong  , Liu Min-Min  , Gao Guo-Ying  ,   School of Science, Wuhan Institute of Technology, Wuhan 430205, China;,   School of Physics, Huazhong University of Science and Technology, Wuhan 430074, China
Published: December 2020

Molecular electronics offers new possibilities for continually miniaturizing the electronic devices beyond the limits of standard silicon-based technologies. There have been significant experimental and theoretical efforts to build thiophene molecular junctions and study their quantum transport properties. However, in most of these studies Au is used as lead material. It is well known that the fabrication of the traditional molecular device is now hindered by technological difficulties such as the oxidation of metallic contacts, and the interface instability between the organic molecule and the inorganic metallic electrodes. In this paper, we use the graphene electrodes to construct a series of thiophene-based devices. The graphene electrodes proposed in this paper are able to avoid such problems. Moreover, the stability of graphene electrodes at room temperature paves the way to studying the electron transport through a single molecule under the ambient conditions. Firstly, we design a series of molecular rectifying devices based on thiophene dimer and its derivatives, in which the hydrogen atom on the thiophene monomer is substituted with a representative electron-donating group (NH2) and electron-withdrawing group (NO2). Secondly, we investigate systematically the electronic transport properties through these molecular junctions by performing the first principles calculations based on density functional theory and nonequilibrium Green's function. The calculated results show that these thiophene molecular devices substituted with NH2 and NO2 possess the rectifying behavior and negative differential resistance properties. Furthermore, we also find that the position of substituent group NH2 or NO2 has a major influence on the electronic transport properties. In order to explore the physical mechanism behind these transport properties, the electronic structures of the molecules, the transmission spectrum, and the molecular projected eigenstates are analyzed. The results reveal that the position of NH2 can adjust the intensity of the negative differential resistance. When the NH2 group is close to the molecular end, the negative differential resistance behavior in this molecular device is more prominent than in other molecules. In addition, the position of NO2 can change the direction of the rectification. When the NO2 group is close to the molecular end, the current in negative bias is larger than in positive bias, resulting in a negative rectification. In contrast, when the NO2 group is close to the molecular centre, a positive rectification occurs. Our results can provide a worthy complement to thiophene molecular experiment, and also has a guiding significance for designing other molecular electronic devices.
Research progress and application prospect of Fe-based soft magnetic amorphous/nanocrystalline alloys
Authors: Yao Ke-Fu  , Shi Ling-Xiang  , Chen Shuang-Qin  , Shao Yang  , Chen Na  , Jia Ji-Li  ,   School of Materials Science and Engineering, Tsinghua University, Beijing 100084, China
Published: December 2020

Amorphous alloy is a kind of metallic materials prepared by rapidly cooling the alloy melt through hindering crystallization in cooling process. Due to the unique structure of atomic random packing, Fe-based amorphous alloys exhibit not only structural and property isotropy, but also small structural correlation length, small magnetic anisotropic constant, and then small coercivity Hc. Like crystalline Fe-based alloys, Fe-based amorphous alloys also possess high saturation induction Bs. As a result, research on engineering applications of Fe-based amorphous alloys has been promoted by their excellent soft magnetic properties. Now Fe-based soft magnetic amorphous/nanocrystalline alloys have been produced and applied to various areas on a large scale. Here in this paper, the processes of discovery, development and application of Fe-based soft magnetic amorphous alloys are reviewed, and the effects of chemical composition, structure and preparation technology on the soft magnetic properties are introduced and discussed. The obtained theoretic results and the technological innovation show that the great contributions have been made to the development and application of Fe-based soft magnetic amorphous/crystalline alloys. Based on the progress of structure and soft magnetic property and our understanding, the development process of the fundamental research and the application progress of Fe-based soft magnetic amorphous alloys could be divided into three periods. In addition, the present challenge topics in their researches and applications are proposed.
Influence of asymmetrical angle on crystal lattice strain analysis using Voigt-function method
Authors: Zhu Jie  , Ji Meng  , Ma Shuang  ,   Key Laboratory of Advanced Micro-Structured Materials MOE, Institute of Precision Optical Engineering, School of Physics Science and Engineering, Tongji University, Shanghai 200092, China
Published: December 2020

The Voigt function provides a rapid and easy method of explaining the breadths of diffraction profiles, and it defines two main broadening types: the domain size and strain component. The latter is caused by lattice imperfection (dislocation and different defects). Thus, diffraction can be used to measure crystal strain with very high precision and accuracy. However, each of all the crystals used in the present study has asymmetrical angle due to the processes of cutting grinding and polishing. This deviation angle is the angle between the considered lattice plane and crystal surface. The crystal with asymmetrical angle also satisfies Bragg's law but with different incident angle and reflected one. In the following, we investigate the crystal strain as a function of asymmetrical angle to evaluate the lattice distortion in detail. The single crystal silicon samples with different asymmetrical angles (in a range from 0.008 to 5.306) are prepared in this experiment. The lattice plane is (111). After grinding and polishing, the surface and subsurface damage are almost wiped off to remove internal stress which comes from cracks and grain refinement. Only broadening from lattice strain depends on the nature of imperfection, and the shape of crystallite can be left. It is convenient to acquire the full width at half maximum (FWHM) and integral breadth of diffraction curve by high resolution X-ray diffraction technique. Using the Voigt function method, diffraction line is characterized by all three parameters of the half-width integral breadth and form factor. The crystal lattice strains are calculated by analyzing the experimental line profile composed of Cauchy and Gaussian parts. Simulation of coherence diffraction of asymmetric crystal silicon is achieved by ray tracing code SHADOW. Both the theoretical calculation and experimental results show that if asymmetrical angle reaches 0.749, the half-width and integral breadth of diffraction curve change obviously compared with the situation where asymmetrical angle reaches 0.008. This is why the calculation error of crystal strain will be beyond 5% by the Voigt function method no matter whether we use theoretical value or experimental data. It is shown that the precise crystal cut is extremely important for device application. And this conclusion will also be helpful in other crystal studies by using X-ray diffraction parameters.
Surface oxidation of as-deposit uranium film characterized by X-ray photoelectron spectroscopy
Authors: Yang Meng-Sheng  , Yi Tai-Min  , Zheng Feng-Cheng  , Tang Yong-Jian  , Zhang Lin  , Du Kai  , Li Ning  , Zhao Li-Ping  , Ke Bo  , Xing Pi-Feng  ,   Research Center of Laser Fusion, China Academy of Engineering Physics, Mianyang 621900, China
Published: December 2020

Uranium film is a main functional component to realize the high efficiency conversion of laser to X-ray in the study on laser inertial confinement fusion. It also has important applications in relevant physics experiments. Due to its active chemical properties, the metal uranium film is extremely difficult to preserve in the atmosphere. A variety of methods may help to avoid the oxidation of uranium film, such as coating protective layer, vacuum or inert atmosphere protection. But under these conditions, the oxidation property of uranium film still needs experimental investigation. In this paper, the oxidation processes of uranium films under different atmospheres are studied by X-ray photoelectron spectroscopy (XPS) and depth profiling. Firstly, uranium films and gold-uranium multilayer films are prepared by ultra-high vacuum magnetron sputtering deposition, and then they are exposed to atmosphere, high purity argon and ultrahigh vacuum for a period of time. Finally, the distributions and valence states of oxygen and uranium elements are investigated by XPS depth profiling. The oxidation mechanism is analyzed according to the oxidation products and the microstructure characteristics of samples. The results show that the oxygen element is undetectable in the initial films. In the Au-U multilayer film, the protective ability of Au layer is greatly weakened by the micro-defects. The defect is not only the micro channel of oxygen entering into the sample directly, but also the origin of the interlaminar cracks at the Au/U interface. In about three weeks, the uranium layer is severely oxidized and large area lamination occurs. The oxidation products consist of a dense oxide thin film on uranium surface and corrosion pitting around the defects, which are mainly UO2. For the pure uranium film, the surface of the film is completely oxidized when it is exposed to high purity argon only for 6 h. The UO2 layers with different thickness values are formed on their surface, which is due to the rapid diffusion of oxygen atoms at the columnar grain boundaries of the film. After the sample is exposed to the ultra-high vacuum for 12 h, UO2 layer with a thickness of less than 1 nm is generated on the surface of the pure uranium film. In the etching of oxide by argon ion beams, the preferential sputtering effect of O is produced, and UO2 is reduced into non-stoichiometric UO2-x. The effect of preferential sputtering is weakened with the decrease of oxide content. When the oxide content is less than 10%, the reduction of UO2 can hardly be detected.
Secure transmission mechanism based on time reversal over wireless multipath channels
Authors: Zhu Jiang  , Wang Yan  , Yang Tian  ,   Chongqing Key Laboratory of Mobile Communications Technology, Chongqing University of Posts and Telecommunications, Chongqing 400065, China
Published: December 2020

Broadband wireless communication is implemented primarily in a complicated environment. The complex environment with time-varying multi-path propagation characteristics will seriously affect the performance of communication. To solve the problem of insecurity in information transmission in wireless channels, in this paper a system is modeled by using the multi-input single output eavesdropping channel model and the security of information transmission through time reversal technology is ensured. Another problem is that the information focuses on the receiving point. Owing to the temporal and spatial focusing characteristics of the time reversal technology the information near the receiving point can be eavesdropped easily. To solve this problem, a secure transmission scheme based on time reversal technology with artificial noise interference on the transmitter side is proposed. One of the core technologies to solve this problem is to introduce the environment adaptive technique–time reversal in the wireless link. Further, the problem of a wiretap channel in physical layer security research has become a popular research topic in recent years. To solve the problems about the physical layer wiretap channel and multi-path fading in wireless channels, a novel concept combining time reversal technology with physical layer security technology is proposed. In this paper, a physical layer secure transmission scheme based on the joint time reversal technique and artificial noise at the sending end is proposed for the wireless multi-path channel. First, in a typical wiretap channel model the time reversal technique is used to improve the security of the information transmission process by using the properties of spatial and temporal focusing. It refers to the fact that information can be focused at a given moment and in space. Second, as the information is easily eavesdropped near the focus point, artificial noise is added to the sending end to disrupt the ability of the eavesdropper to eavesdrop. The artificial noise has no effect on legitimate user due to the use of null-space artificial noise in legitimate user. Based on this scheme, a closed expression, such as secure signal-to-interference and signal-to-noise ratio, an achievable secrecy rate and bit error rate are obtained, and the influences of the number of antennas, signal-to-noise ratio, and artificial noise are analyzed. The theoretical analysis and simulation results show that the proposed scheme has a higher secrecy signal-to-noise ratio, a higher rate of secrecy, and a lower bit error rate of the legitimate user than the the existing physical layer security schemes.
Spontaneous generation of spiral wave in the array of Hindmarsh-Rose neurons
Authors: Wang Peng  , Li Qian-Yun  , Tang Guo-Ning  ,   College of Physical Science and Technology, Guangxi Normal University, Guilin 541004, China
Published: December 2020

Spiral waves have been reported to be existent in the neocortex, during pharmacologically induced oscillations and sleep-like states. In the last decades, theoretical studies have demonstrated an underlying mechanism of the generation of spiral waves in a heart system. Nevertheless, how can a neural system produce spontaneous spiral wave and whether this behavior is sensitive to the dynamics of isolated neurons have not been systematically studied yet. In this paper we propose a modified Hindmarsh-Rose (HR) neuron model to study whether spiral wave can occur spontaneously in a two-dimensional array of HR neurons, which evolves from the initial state with a random phase distribution. The simulation results show that whether spiral wave can occur spontaneously in the system depends on the state of the single HR neuron, initial state of system and coupling strength. Especially, the state of the single HR neuron plays a central role. When the single HR neuron is in the state of period 1 spike, multiple spiral waves and spiral pairs can be generated spontaneously in the system for a certain range of coupling strength. In this case, the formations of spiral waves are completely independent of the initial state of the system, and as long as choosing an appropriate coupling strength, a single spiral wave can be found in the system. Furthermore, when the coupling strength exceeds a certain threshold value, the system will exhibit three kinds of dynamical behaviors, and correspond to three kinds of the different initial states, respectively. When system evolves from the first kind of initial state, the single spiral wave can be found occasionally in the system. When the system evolves from the second or third kind of initial state, the oscillation with intermittently global synchronization and oscillation death can be observed in the system, respectively. When a single HR neuron is in the state of period 2 spike, the spiral wave can appear spontaneously in the system only when the phase distribution of the initial state approaches to a uniform distribution. Moreover, the range of coupling strength on the generation of spiral wave is smaller than that of period 1 spike. When the single HR neuron is in a higher periodic state, it is difficult to generate spontaneously spiral wave in the system. These results are useful in understanding the spontaneous generation of spiral waves in the neocortex.
Progress of laser-driven quasi-isentropic compression study performed on SHENGUANG III prototype laser facility
Authors: Xue Quan-Xi  , Jiang Shao-En  , Wang Zhe-Bin  , Wang Feng  , Zhao Xue-Qing  , Yi Ai-Ping  , Ding Yong-Kun  , Liu Jing-Ru  ,   Research Center of Laser Fusion, CAEP, Mianyang 621900, China;,   State Key Laboratory of Laser Interaction with Matter, Northwest Institute of Nuclear Technology, Xi'an 710024, China
Published: December 2020

The equation of state for solid at extreme pressure and relatively low temperature is an important topic in the study of astrophysics and fundamental physics of condensed matter. Direct laser-driven quasi-isentropic compression is a powerful method to achieve such extreme states which have been developed in recent years. A lot of researches have been done in Research Center of Laser Fusion in China since 2012, which are introduced in this article. The researches include an analytical isentropic compression model, a developed characteristic method, techniques for target manufacture, and experiments performed on SHENGUANG Ⅲ prototype laser facility. The analytical isentropic compression model for condensed matter is obtained based on hydrodynamic equations and a Murnaghan-form state equation. Using the analytical model, important parameters, such as maximum shockless region width, material properties, pressure pulse profile, and pressure pulse duration can be properly allocated or chosen, which is convenient for experimental estimation and design. The characteristic method is developed based on a Murnaghan-form isentropic equation and characteristics, which can be used for experimental design, simulation, and experimental data processing. Based on the above researches, several rounds of experiments have been performed to obtain better isentropic effect by upgrading the target configurations. Five kinds of target configurations have been used up to now, which are three-step aluminum target, CH-coated planar aluminum target, CH-coated three-step aluminum target, planar aluminum target with Au blocking layer, and three-step aluminum target with Au blocking layer. The rear surface of three-step aluminum target is found to be destroyed when the loading pressure rises up to 194 GPa, and weak shock appears in CH-coated planar aluminum target and CH-coated three-step aluminum target. Besides, velocity interferometer system for any reflector (VISAR) fingers are found to decrease when the pressure rises up to about 400 GPa and disappears at 645 GPa. By reducing laser intensity, the whole interface velocities on three steps are obtained in the CH-coated three-step aluminum target and a stress-density curve is calculated. In order to eliminate the weak shock, the target configurations are upgraded by changing the ablation layer and putting a gold blocking layer after it. The experimental results show that the weak shock is eliminated and much clearer VISAR fingers are obtained when pressure rises to as high as 570 GPa.
Numerical simulation on particle density and reaction pathways in methane needle-plane discharge plasma at atmospheric pressure
Authors: Zhao Yue-Feng  , Wang Chao  , Wang Wei-Zong  , Li Li  , Sun Hao  , Shao Tao  , Pan Jie  ,   School of Physics and Electronics, Shandong Normal University, Jinan 250014, China;,   Institute of Electrical Engineering, Chinese Academy of Sciences, Beijing 100190, China;,   Department of Chemistry, University of Antwerp, Wilrijk 2610, Belgium;,   State Grid Jinan Power Supply Company, Jinan 250012, China
Published: December 2020

Methane needle-plane discharge has practical application prospect and scientific research significance since methane conversion heavy oil hydrogenation is formed by coupling methane needle-plane discharge with heavy oil hydrogenation, which can achieve high-efficient heavy oil hydrogenation and increase the yields of high value-added light olefins. In this paper, a two-dimensional fluid model is built up for numerically simulating the methane needle-plane discharge plasma at atmospheric pressure. Spatial and axial distributions of electric intensity, electron temperature and particle densities are obtained. Reaction yields are summarized and crucial pathways to produce various kinds of charged and neutral particles are found out. Simulation results indicate that axial evolutions of CH3+ and CH4+ densities, electric intensity and electron temperature are similar and closely related. The CH5+ and C2H5+ densities first increase and then decrease along the axial direction. The CH3 and H densities have nearly identical spatial and axial distributions. Particle density distributions of CH2, C2H4 and C2H5 are obviously different in the area near the cathode but comparatively resemblant in the positive column region. The CH3+ and CH4+ are produced by electron impact ionizations between electrons and CH4. The CH5+ and C2H5+ are respectively generated by molecular impact dissociations between CH3+ and CH4 and between CH4+ and CH4. Electron impact decomposition between electrons and CH4 is a dominated reaction to produce CH3, CH2, CH and H. The reactions between CH2 and CH4 and between electrons and C2H4 are critical pathways to produce C2H4 and C2H2, respectively. In addition, the yields of electron impact decomposition reactions between electrons and CH4 and reactions between CH2 and CH4 account for 52.15% and 47.85% of total yields of H2 respectively.
Effects of external electric field and Al content on g factor of wurtzite AlGaN/GaN quantum wells
Authors: Li Ming  , Yao Ning  , Feng Zhi-Bo  , Han Hong-Pei  , Zhao Zheng-Yin  ,   College of Electrical and Information Engineering, Xuchang University, Xuchang 461000, China
Published: December 2020

In this paper, we study the effects of external electric field and Al content on the transverse and longitudinal g-factor (g┴ and g//) and its anisotropy (δg) of wurtzite AlGaN/GaN quantum wells (QWs). The Δg┴=(g┴-g0)=g┴bulk + gw and Δg//=(g//-g0)=g//bulk are mainly contributed by the bulk structure (g//bulk and g┴bulk) respectively, but the difference between g//bulk and g┴bulk is small and almost remains unchanged when the external electric field and Al content are varied. So the anisotropy of the g factor in AlGaN/GaN QWs induced by the bulk wurtzite structure is small, while the anisotropy induced by the quantum confined effect (gw) is considerable. When the direction of the external electric field is the same as (opposite to) the polarization electric field, the magnitudes of g//bulk and g┴bulk both increase (decrease) with increasing external electric field. This is induced mainly by the variations of envelope function and confined energy with the electric field. With the external electric field changing from -1.5×108 V·m-1 to 1.5×108 V· m-1, the confined energy ε1 increases slowly, and the magnitude of the envelope function at the left heterointerface increases. So the contribution to Δg┴ from the heterointerface ΓInter is positive and increases slowly, and that from the well ΓW is negative and increases slowly in magnitude. The magnitude of ΓInter is larger than that of ΓW, but the magnitude of the latter increases more rapidly. All the above factors make the g-factor anisotropy δg>0 and decrease in magnitude with electric field increasing. With increasing Al content of the barrier, both β>1 (g┴bulk) and γ>1 (g//bulk) decrease if the strain effects are ignored (S1, 2=0), because the confined energy decreases and the peak of the envelope function shifts towards the left heterointerface. By considering the strain effects (S1, 2 ≠ 0), the magnitude of β>1 (g┴bulk) and γ>1 (g//bulk) increase with Al content increasing. The strain effect has a great influence on the confined potential V(z), leading to the rapid increase of β(z) when z > zp, which the situation for γ (z) is similar to. With increasing Al content, the magnitudes of ΓInter and ΓW both increase, but the magnitude of ΓInter is larger and increases more rapidly. Therefore δg increases slowly. The magnitude of Δ g┴ first decreases with increasing Al content, then it increases with Al content increasing, and since g┴bulk g-factor and its anisotropy in AlGaN/GaN QWs can be greatly modulated by the external electric field, the Al content in the barrier, the strain effects and the quantum confined effect. Results obtained here are of great importance for designing the spintronic devices.
Effects of adding Ti on microstructure and properties of Al-Bi immiscible alloy
Authors: Man Tian-Nan  , Zhang Lin  , Xiang Zhao-Long  , Wang Wen-Bin  , Gao Jian-Wen  , Wang En-Gang  ,   Key Laboratory of Electromagnetic Processing of Materials(Ministry of Education), Northeastern University, Shenyang 110819, China;,   School of Materials Science and Engineering, Northeastern University, Shenyang 110819, China
Published: December 2020

Immiscible alloy, as a kind of special metallurgy characteristic alloy, has been investigated for decades. The fabrication of immiscible alloy with a homogeneous microstructure remains a challenge due to the liquid-liquid phase separation. The microstructure and the properties of Al-Bi immiscible alloy with an addition of Ti are investigated, and the effect of adding Ti on mechanical behavior for self-lubricating performance is measured. The pure Al and Ti are first melted in graphite crucible under argon gas protection. An appropriate amount of Bi is added into the melt. After melting and homogenizing the immiscible alloy, the melt is maintained at 1150 ℃ for 10 min, and then it is quenched. The scanning electron microscope analysis results show that the addition of Ti leads to a significant reduction of Bi-rich droplet size and an increase of particle number. The Bi-rich droplets of the ternary Al-Bi-Ti alloy are more homogeneously distributed throughout the Al matrix than the microstructure of binary Al-Bi alloy. The results from X-ray diffraction and energy disspersive spectrometer indicate that Al3Ti compounds, which are the transformation products between Al and Ti elements, disperse in the Al matrix. The needle-like Al3Ti compounds suspend in Al-Bi melt and impede the Bi phase in the liquid miscibility gap from being segregated. This is conducible to refining the microstructure of Al-Bi alloy. The Al3Ti compounds form before the initial nucleation of the Bi phase in the Al matrix, and impede the Bi phase from being segregated. Al-Bi immiscible alloy is effectively fabricated with dispersed fine second phase droplets by the addition of Ti. For the Al-Bi alloy, the coarse and non-uniform distribution of Bi-rich droplets can be easily broken. The improvement in the wear resistance of Al-Bi immiscible alloy by adding Ti can be attributed not only to the dispersion and size of the Bi soft phase but also to the in-situ formation of Al3Ti compounds. The addition of Ti is effective for refining the microstructure and improving the wear properties, which simultaneously improves the practical applications of self-lubrication bearing material with low coefficient of friction i.e., reducing the energy loss.
Coherence of digital phase conjugation for implementing time reversal in scattering media
Authors: Zhang Hong-Bo  , Zhang Xi-Ren  ,   School of Aeronautics and Astronautics Engineering, Air Force Engineering University, Xi'an 710038, China;,   Sichuan Jiuzhou Electric Group Co., Ltd., Mianyang 621000, China;,   School of Optoelectronic Information, University of Electronic Science and Technology of China, Chengdu 610054, China
Published: December 2020

The strong light scattering in complex media, due to the highly inhomogeneous distributions of refractive indexes, is regarded as a fundamental impediment in numerous optical applications such as optical communications, biophotonics, and optical tweezer. Recently, many optical techniques based on the coherence of light source with long coherent length have been developed and widely used to suppress and control light scattering and propagation in complex media. Here, we propose and experimentally demonstrate the control and time reversal of only one part instead of all of light passing through complex media and different optical paths by combining digital phase conjugation and coherence gating based on partially coherent light source. Interference of reference and objective beams and corresponding phase maps are measured by the charge coupled device (CCD) and four-step phase-shift measuring technique only when the optical path difference between two beams is less than coherence length. Time reversal is achieved by spatial light modulator (SLM). In the experiment we further analyze the phase map and time reversal with different optical path differences and different coherence lengths of source. The experimental results demonstrate that for each optical path difference, the time reversal of only the part of light coming from the same scattering> and identical optical path is achieved by digital phase conjugation and coherent gating of broadband light source.
Numerical research of selectively photoionization in thick atom vapor medium
Authors: Lu Xiao-Yong  , Zhang Xiao-Zhang  , Zhang Zhi-Zhong  ,   Department Engineering of Physics, Tsinghua University, Beijing 100084, China;,   Research Institute of Physics and Chemical Engineering of Nuclear Industry, Tianjin 300180, China
Published: December 2020

The pulse deformation and time delay, which appear when the laser propagates in a thick atom vapor, influence the ionization yield and selectivity of atomic multi-step photoionization process directly. In this paper, we study the propagation of laser pulse and atomic photoionization in a thick atom vapor medium according to the atom vapor laser isotope separation. The process of atomic multi-step photoionization and the propagation of laser in a thick medium are described by density matrix equation and Maxwell equations, respectively. The medium consists of target isotope and non-target isotope, which is non-resonantly excited. Through numerical solution of the coupled equations we illustrate the propagation characteristics of laser and the influences of atom vapor parameters and laser parameters on average ionization yield and average selectivity in a thick medium. The important results of calculation are as follows:when the atom vapor medium is rather thick, the average ionization yield increases while average selectivity decreases with the increase of laser power. When the atom vapor is relatively thin, the average ionization yield and average selectivity increase with the decrease of laser power simultaneously. Besides, there is a positive time delay between two laser pulses in which case the average ionization yield of target isotope reaches its maximum value. Moreover, when the parameters of atom vapor are constant, extending the width of laser pulses as great as possible can not only increase average ionization yield and average selectivity simultaneously, but also loosen the control accuracy of time delay between laser pulses.
Design and analysis of novel two-stage optical receiving antenna for indoor visible light communication technology
Authors: Peng Xing  , Kong Ling-Bao  ,   Shanghai Engineering Research Center of Ultra-Precision Optical Manufacturing, School of Information Science and Technology, Fudan University, Shanghai 200433, China
Published: December 2020

White light emitting diode has become the next generation of light source because of its high illuminance efficiency, low power consumption, and long life, and it has also been adopted in the application of indoor visible light communication (VLC) system. The VLC has great development potentials, however, there is a lack of research on optical receiving antenna which is a key component of VLC. Therefore, in this paper we design a novel two-stage optical receiving antenna for indoor VLC system. In the designed antenna, the lens wall of a compound parabolic concentrator with a certain rotation angle and thickness is obtained through rotating the parabolic reflector cross-section reference curve. Furthermore, a novel two-stage optical receiving antenna is designed by taking advantage of lens-wall compound parabolic concentrators and hemispherical lenses. This significantly increases the view angle by nearly 20 in the case of gain retention. The analytical model of the optical antenna in a 5 m5 m3 m open room is established by using a software of TraceProTM. The indoor VLC system is also modelled and implemented by using a software of MatlabTM. The results show that the growth rate of average received power is 757.2%, which is 5.62 times that of the compound parabolic concentrator, and the signal-noise-ratio is increased by 28.07%, on average, which is 1.67 times that of the compound parabolic concentrator. The optical gain of the two-stage optical receiving antenna is 11.49, which is 2.81 times that of the compound parabolic concentrator. The spot radius is only 2.5 mm, which is reduced by nearly 37.5% compared with the spot radius of the compound parabolic concentrator, and the energy concentration is evenly distributed at the same time. This further confirms that the designed two-stage optical receiving antenna is suitable for indoor VLC system. Finally, the performance analysis and experimental verification of the new two-stage optical receiver antenna are also given.
An adaptive denoising algorithm for chaotic signals based on collaborative filtering
Authors: Wang Meng-Jiao  , Zhou Ze-Quan  , Li Zhi-Jun  , Zeng Yi-Cheng  ,   College of Information Engineering, Xiangtan University, Xiangtan 411105, China;,   School of Physics and Optoelectric Engineering, Xiangtan University, Xiangtan 411105, China
Published: December 2020

Chaos is a seemingly random and irregular movement, happening in a deterministic system without random factors. Chaotic theory has promising applications in various areas (e.g., communication, image encryption, geophysics, weak signal detection). However, observed chaotic signals are often contaminated by noise. The presence of noise hinders the chaos theory from being applied to related fields. Therefore, it is important to develop a new method of suppressing the noise of the chaotic signals. Recently, the denoising algorithm for chaotic signals based on collaborative filtering was proposed. Its denoising performance is better than those of the existing denoising algorithms for chaotic signals. The denoising algorithm for chaotic signals based on collaborative filtering makes full use of the self-similar structural feature of chaotic signals. However, in the parameter optimization issue of the denoising algorithm, the selection of the filter parameters is affected by signal characteristic, sampling frequency and noise level. In order to improve the adaptivity of the denoising algorithm, a criterion for selecting the optimal filter parameters is proposed based on permutation entropy in this paper. The permutation entropy can effectively measure the complexity of time series. It has been widely applied to physical, medical, engineering, and economic sciences. According to the difference among the permutation entropies of chaotic signals at different noise levels, first, different filter parameters are used for denoising noisy chaotic signals. Then, the permutation entropy of the reconstructed chaotic signal corresponding to each of filter parameters is computed. Finally, the permutation entropies of the reconstructed chaotic signals are compared with each other, and the filter parameter corresponding to the minimum permutation entropy is selected as an optimal filter parameter. The selections of the filter parameters are analyzed in the cases of different signal characteristics, different sampling frequencies and different noise levels. Simulation results show that this criterion can automatically optimize the filter parameter efficiently in different conditions, which improves the adaptivity of the denoising algorithm for chaotic signals based on collaborative filtering.
Fiber-type difference frequency generation infrared optical frequency comb based on the femtosecond pulses generated by a mode-locked fiber laser
Authors: Ma Jin-Dong  , Wu Hao-Yu  , Lu Qiao  , Ma Ting  , Shi Lei  , Sun Qing  , Mao Qing-He  ,   School of Environmental Science and Optoelectronic Technology, University of Science and Technology of China, Hefei 230026, China;,   Anhui Provincial Key Laboratory of Photonics Devices and Materials, Anhui Institute of Optics and Fine Mechanics, Chinese Academy of Sciences, Hefei 230031, China;,   Division of Optics, National Institute of Metrology, Beijing 100029, China
Published: December 2020

Optical frequency comb (OFC) is a new type of high-quality laser source. The visible and near-infrared OFCs have become mature, and it has been widely used in optical frequency metrology, time/frequency transfer, precision laser spectroscopy and other fields. Since the mid and far-infrared spectral regions contain a large number of baseband absorption lines for molecules and the absorption intensities are several orders of magnitude higher than those in the visible and near-infrared spectral region, one has made great efforts to develop the mid and far-infrared OFCs in recent years. Although a variety of approaches to achieving infrared OFCs directly have been proposed, the method of difference frequency generation (DFG) infrared OFC based on the optical rectification technique is still more efficient. DFG infrared OFCs with widely tuning ability have been demonstrated based on fiber lasers so far. However, how to obtain the broadband spectrum for a DFG infrared OFC with widely tuning ability still needs to be solved. In this paper we report a fiber-type DFG infrared OFC by using the femtosecond pulses from a mode-locked erbium-doped fiber laser as the fundamental light. Based on the self-developed mode-locked fiber laser oscillator with repetition rate locked, the two-color fundamental pulse trains with the central wavelengths of 1.5 and 2.0 m are respectively achieved after the chirped pulse fiber amplification and all-fiber supercontinuum (SC) generation techniques have been utilized. With a time-domain synchronous detection system based on the intensity autocorrelation principle, the accurate synchronization with the fundamental two-color pulses is obtained by optimizing the OFS compensated fiber length and adjusting a tunable optical delay line. Finally, by using the optical rectification technique, a fiber-type DFG infrared OFC is successfully generated with the help of a suitable designed GaSe nonlinear crystal. Our experimental results also show that the spectral location of the DFG infrared OFC can be tuned by controlling the spectral shape of the SC combined with the adjustment of the phase-matching for the nonlinear crystal. The measured tuning range of the DFG infrared OFC is from 6 to 10 m, and the maximum spectral width is 1.3 m. This fiber-type DFG infrared OFC may play an important role in the molecular spectroscopy, the atmospheric environmental monitoring, and other fields.
Fast solution of near-field time reversal electromagnetic field of sub-wavelength perfect conducting ball arrays
Authors: Gong Zhi-Shuang  , Wang Bing-Zhong  , Wang Ren  ,   Institute of Applied Physics, University of Electronic Science and Technology of China, Chengdu 610054, China
Published: December 2020

To solve the near-field time reversal electromagnetic fields of sub-wavelength perfect conducting ball arrays rapidly, an analytical formulation is presented based on the equivalent dipole model. As is well known, the efficient use of evanescent information is the key to the realization of sub-wavelength focusing and imaging. However, evanescent components always suffer exponential decays with the increase of propagating distance. Therefore, in order to effectively control the evanescent waves, some measures should be taken in the near field region of the scatters before their amplitudes are reduced to an undetectable level. Since small perfect conducting ball is the basic component of large scatter, the first step should be to study the scattering properties of small perfect conducting ball. The far-field scattering fields of perfect conducting balls have been analyzed for plane waves. However, for spherical waves, the analytical results are not convenient to extend to multi-ball situation since they are all expressed by series. In this paper, the analytical solution to scattering field of the small perfect conducting balls irradiated by spherical radiative waves is analyzed. The result shows that the scattering fields can be approximately equivalent to the superposition of the radiation fields of electrical and magnetic dipoles in some restrictive conditions. The intensity of the equivalent dipole is proportional to the magnitude of the original excitation source dipole. Therefore all the equivalent dipole moments can be calculated easily by setting up the coupling equations between different equivalent dipoles and source dipole. Then, the forward dyadic Green's function can be obtained by combining the vacuum electrical and magnetic Green's function. At the same time, the time reversal dyadic Green's function can be derived through the time reversal cavity theory. Afterwards, the near-field time reversal electromagnetic field of the perfect conductive ball arrays can be calculated directly by the time reversal dyadic Green's function. The results obtained from the proposed method and a numerical software are compared, which shows that a coincidence extent reaches more than 0.95. This confirms the correctness and high efficiency of the proposed method. After that, an imaging experiment is implemented and the result shows that an imaging resolution of 0.3 can be obtained by loading small conducting balls in the near field. All these experiments show that combined with near-field loading of sub-wavelength scatterer arrays, the time reversal technique has the potential to realize super-resolution focusing and imaging.