Vol. 66, No. 8 (2017)
Symmetry reductions, exact equations and the conservation laws of the generalized (3+1) dimensional Zakharov-Kuznetsov equation
2017, 66 (8): 080201. doi: 10.7498/aps.66.080201
Because the nonlinear evolution equations can describe the complex phenomena of physical, chemical and biological field, many methods have been proposed for investigating such types of equations, and the Lie symmetry analysis method is one of the powerful tools for studying the nonlinear evolution equations. By using the Lie symmetry analysis method, we can obtain the symmetries, reduced equations, group invariant solutions, conservation laws, etc. In the reduction process, we can reduce the order and dimension of the equations, and a complex partial differential equations (PDE) can be reduced to ordinary differential equations directly, which simplifies the solving process. Meanwhile, the symmetries, conservation laws and exact solutions to the nonlinear partial differential equations play a significant role in nonlinear science and mathematical physics. For example, we can obtain a lot of new exact solutions by the known symmetries of the original equation; through the analysis of the special form of solution we can better explain some physical phenomena. In addition, the studying of conservation laws and symmetry groups is also the central topic of physical sciencein both classical mechanics and quantum mechanics. Lie symmetry analysis method is suitable for not only constant coefficient equations, but also variable coefficient equations and PDE systems. By using Lie symmetry analysis method, the symmetries and corresponding symmetry reductions of the (3+1) dimensional generalized Zakharov-Kuzetsov (ZK) equation are obtained. Combining the homogeneous balance principle, the trial function method and exponential function method, the group invariant solutions and some new exact explicit solutions are obtained, including the shock wave solutions, solitary wave solutions, etc. Then, we give the conservation laws of the generalized (3+1) dimensional ZK equation in terms of the Lagrangian and adjoint equation method.
2017, 66 (8): 080501. doi: 10.7498/aps.66.080501
Extracting the signals from non-stationary time series is a difficult task in many fields such as physics, economics, and atmospheric sciences. The theory of hierarchy suggests that varying driving force leads to the non-stationary behavior, so extracting and analyzing the slowly varying features can help to study non-stationary dynamical system, which has become a compelling question recently. Slow feature analysis (SFA) is an effective technique for extracting slowly varying driving forces from quickly varying non-stationary time series. The basic idea of SFA is to nonlinearly extend the reconstructive signal into a combination form with one or higher order polynomials, and to apply the principal component analysis to this extended signal and its time derivatives. The algorithm is guaranteed to seek an optimal solution from a group of functions directly and can extract a lot of uncorrelated features that are ordered by slowness. A series of studies has shown its superiority in extracting the driving force of non-stationary time series. The extracted signal is found to be highly correlated with the real driving force. Results based on ideal models show that either the slow driving force itself or a slower subcomponent can be detected by SFA. Yet despite all that, the further investigating of SFA is still needed to reduce its uncertainty. In this study, we create two types of non-stationary models by the logistic map with time-varying parameters: one includes two varying driving forces with different time periods constraining the evolution of time series in a non-stationary way; and the other is a three-layer structure encompassing two superimposed signals in which the slower signal of driving force is modulated by the lowest one. According to the ideal model and SFA, we conduct the numerical experiments to develop corresponding analysis method and discuss its application prospect in extracting driving force signals. We find that for the system of first kind, either the slowest signal or the combination of two driving forces constructed by SFA contains some uncertain information. However, we can detect the two independent driving forces from the constructed signal by wavelet analysis. For the three-hierarchy system that includes two superimposed signals of driving force, successive applications through SFA on the original time series and the constructed SFA signal will in turn detect the slower varying driving force signal and the slowest varying driving forces signal. The successful application of SFA shows its promising prospect in analyzing the external driving forces in non-stationary system and understanding relevant dynamic mechanism.
Self-reliance and independently developed high-finesse spherical ultrastable optical reference cavity
2017, 66 (8): 080601. doi: 10.7498/aps.66.080601
Ultra-stable reference cavity with high finesse is a crucial component in a narrow-linewidth laser system which is widely used in time and frequency metrology, the test of Lorentz invariance, and measure of gravitational wave. In this paper, we report the recent progress of the self-made spherical reference cavity, aiming at the future space application. The main function of cavity is the reference of ultra-stable laser, which is the local reference oscillation source of space optical clock. The diameter of the designed spherical cavity spacer made of ultra-low expansion glass is 80 mm, and the cavity length is 78 mm, flat-concave mirrors configuration, and the radius of the concave mirror is 0.5 m. The support structure is designed to have two 3.9 mm-radius spherical groves located at the poles of the sphere along the diameter direction (defined as support axis), and a 53 angle between the support axis and the optical axis. The mechanic vibration sensitivities of the cavity along and perpendicular to the optical axis are both calculated by finite element analysis method to be below 110-10/g. Five-axis linkage CNC machining sphere forming technology is applied to S80 mm spherical surface processing with spherical contour degree up to 0.02. After a three-stage surface polishing processes, the fused silicamirror substratessurface roughness is measured to be less than 0.2 nm (rms). Implementing double ion beam sputtering technique for mirror coating, the reflection of the coating achieves a reflectivity of 99.999% and a loss of 4 ppm for 698 nm laser. The coating surface roughness is measured to be 0.3 nm (rms). The cavity spacer and the mirror are bonded by dried optical contact. In order to improve the thermal noise characteristics of the cavity, an ultra low expansion ring is contacted optically to the outer surface of the mirror. The cavity is characterized by ring-down spectroscopy, and the finesse is around 195000. With the help of a home-made 698 nm ultra narrow line-width laser, the cavity line-width is measured to be 9.8 kHz by sweeping cavity method. A 698 nm semiconductor laser is locked to this spherical cavity by PDH technology, and the cavity loss is measured to be5 ppm.
Effect of reduction temperature on structure and hydrogen sensitivity of graphene oxides at room temperature
2017, 66 (8): 080701. doi: 10.7498/aps.66.080701
As precursors exfoliated from graphite oxide gels, graphene oxide thin films are annealed in a temperature range of 100 ℃ to 350 ℃ to obtain a series of reduced graphene oxide samples with different reduction degrees. For the gas sensing experiments, the reduced graphene oxide thin film gas sensing element is prepared by spin coating with Ag-Pd integrated electronic device (Ag-Pd IED). The functional groups, structures, and gas sensing performance of all the samples are investigated by X-ray diffraction, Raman spectroscopy, Fourier transform infrared spectroscopy, and gas sensing measurement. The results show that the structure of the graphene oxide samples are transformed to the graphitic structure after reduction at different thermal treatment temperatures. When the reduction temperature is lower than 150 ℃, materials exhibit features of graphite oxide. When the reduction temperature reaches about 200 ℃, the samples show characteristics transformed from graphite oxide to reduced graphite oxide gradually. When the temperature is higher than 250 ℃, materials show features of reduced graphite oxide. During the reduction process, the disorder degree increases from 0.85 to 1.59, and then decreases slightly to 1.41 with the rise of temperature. Additionally, the oxygen containing functional groups are removed with the increasing reduction temperature, and these functional groups can be removed at specific temperatures. In the lower temperature stage (100-200 ℃), the first kind of oxygen containing functional group removed is the hydroxyl group (C-OH) and the epoxy group (C-O-C) is the second. In the higher temperature stage (250-350 ℃), the main removed oxygen containing functional groups are the epoxy group (C-O-C) and the carbonyl group (C=O). The materials treated at 150, 200, 350 ℃ exhibit n-type, ambipolar, and p-type behaviors, respectively, while rGO-200 exhibits considerable increase in resistance upon exposure to hydrogen gas. rGO-200 exhibits very small decrease of resistance at room temperature and moderate increase of resistance at elevated temperatures upon exposure to hydrogen gas, while rGO-350 exhibits considerable decrease of resistance at room temperature upon exposure to hydrogen gas. These results indicate that the reduction temperature affects the distribution of density of states (DOS) in the band gap as well as the band gap size. The graphene oxide and the reduced products at low temperature show good sensitivity to hydrogen gas. With the increasing reduction temperature, the sensitivity fades while the response time and recovery time increases. The gas sensor exhibits high sensitivity (88.56%) and short response time (30 s) when exposed to the 10-4 hydrogen gas at room temperature.
2017, 66 (8): 082801. doi: 10.7498/aps.66.082801
The nuclear nonproliferation is a common objective for the international society, of which one of the most important issues is the nonproliferation of weapon-grade nuclear material. Plutonium is a by-product when nuclear reactors are operated. If a commercial power nuclear reactor operates without counting its economic benefits, it is possible that weapon-grade plutonium (WGPu) would be produced in the nuclear reactor with using uranium as nuclear fuel. In the paper, we quantitatively study the plutonium isotopic composition and yield of the WGPu produced in a pressurized water reactor (PWR), and thereby investigate the proliferation risk of commercial nuclear reactors. The properties of plutonium produced in the PWR are calculated by MCORGS, which is developed by us to link MCNP and ORIGENS for calculating the transport-burnup. For evaluating the changing behavior of plutonium isotopic ratio dependent on the cooling time after being discharged from a PWR, we add the model of calculating the depletion and decay properties of nuclear fuel into the MCORGS code system. In order to calculate the yield of WGPu produced in the PWR, we carry out the neutron and burnup calculations by using five reactor models. The simulation models and operation history are based on the configuration and parameters of Japanese Takahama-3 unit. According to the positions and proportions of UO2 fuel rods, burnable poison rods and guide tubes in Takahama-3 PWR, we build a PWR model of an infinite heterogeneous 66 pin cell lattice, carry out simulation calculation and explore the condition for WGPu existing in the two kinds of fuel rods. When the burnup of a UO2 fuel rod is no more than 4.7 MWd/kgU, it contains WGPu. When the burnup of a burnable poison rod is no more than 2.7 MWd/kgU, it contains WGPu. Therefore, the issue of WGPu production in PWR is transformed into the research of the spatial distribution of PWR burnup. In order to obtain the axial PWR burnup, we build an infinite fuel pin cell model in which the PWR is divided into 20 equal zones in the axial direction, and calculate PWR axial burnup distribution when it is operated at 9 typical powers of Takahama-3 PWR. It is found that the burnup value of the two ends of 1/20 section is worth 1/3 of the two middle ones. Based on the principle of neutron leakage in a PWR and the simulation results of a fuel assembly, we build a special PWR mode, in which the PWR is divided into 10 zones in radial direction, and obtain the radial distribution of PWR burnup after the first, the second and the third fuel cycle. Based on the WGPu existing condition and the spatial distribution of a PWR burnup, in this paper we present the exact position of WGPu contained in PWR core and the yield of WGPu in UO2 fuel rods. The calculation results indicate that the spent nuclear fuel with low burnup brings huge proliferation risk, of which the supervision should be strengthened.
2017, 66 (8): 082802. doi: 10.7498/aps.66.082802
In the world there have been built five reactor based slow positron sources producing very intense beams, of which, the NEPOMUC source generates the highest intensity about 3109 e+/s after updated. The beam intensity depends on the power of the core, the converter material, and the moderator geometry. It is important to have good knowledge of the influencing factors and relevant processes for building a positron source in China Mianyang Research Reactor (CMRR). In this paper, the basic mechanism and several pivotal processes are studied and modeled, including the high energy ray induced fast positron generated in target, the moderation of fast positron to slow positron, the emission of slow positron from surface, the extraction of slow positron from surface to external grid, and finally the focusing and transport by beam optic system. The beam intensity at the end of the solenoid can be deduced as I = Emth 12, where 1 is the slow positron extraction efficiency from moderators, 2 is the efficiency of lens extraction and solenoid transportation, and Emth is the slow positron emission rate from surface. The value of Emth can be expressed as Emth= AP 2L+e+pbmod, where A is the effective surface area of the moderator, P is the generating rate of the fast positron in unit volume, L+ is the slow positron diffusion length, e+ is the branching ratio of surface positron ( 0.25), i.e. the ratio of positrons reaching the surface to that emitted freely, pbmod ( 0.4) is the probability of the emitted moderated positron. Therefore, attention should be paid to the values of P, L+, 2 and A to enhance the beam intensity. P is in proportion to the neutron absorption rate by cadmium, which requires higher neutron flux of incidence. L+ is sensitive to the moderator material and its annealing condition. For the well annealed single crystal tungsten, the value of L+ is about 100 nm, while for that annealed at 1600 ℃, it decreases to only 40 nm. The value of 1 is related to the moderator depth/width ratio, the extraction voltage, and the moderator back layout. Although deeper ring can enlarge the moderator area A, the average extraction efficiency 1 decreases obviously. Considering the product of 1 and A, the recommended depth/width ratio is 3 : 1. Validations are performed by employing two types of experimental results, including several isotope slow positron sources and the PULSTAR reactor based source. The calculated efficiencies of isotope sources match well with the experimental measured results, which verifies our basic model and parameters. With these parameters and models, the intensity of PULSTAR reactor based positron source at system exit is calculated to be 5.8108e+/s, which matches well with the reported measured value of (0.5-1.1)109e+/s. Some suggestions are made and will be considered in our future design of positron source.
ATOMIC AND MOLECULAR PHYSICS
A rapid atomic beam of rubidium (87Rb) is produced by two-dimensional magneto-optical trap (2D MOT), and then trapped by three-dimensional magneto-optical trap (3D MOT) with high vacuum for further cooling. After a process of optical molasses cooling, atoms are reloaded into a magnetic trap, where radio frequency (RF) evaporation cooling is implemented. The precooled atoms in the magnetic trap are then transferred into a far detuning optical dipole trap, where Bose-Einstein condensate (BEC) appears by further evaporation cooling. The 3D MOT is loaded to its maximum within 25 s and then BEC is prepared in 16 s. Due to the linear intensity of magnetic trap, the frequency can be scanned fast in the RF evaporation cooling process. In our experiment, the frequency scans from 39 MHz to 15 MHz in 6 s and then scans to 2 MHz in 5 s. The number of atoms in 3D MOT is about 11010, and there are 5105 atoms in the BEC after a succession of cooling processes. To optimize the performances of 2D MOT, a special light path is constructed. And prisms with high reflectivity are used to reduce the imbalance between opposite propagating cooling +beams. Furthermore, quarter-wave plates are used to keep the polarization state of the cooling beam when reflected by prisms or mirrors. The atoms are cooled to a temperature about 15 K in the magnetic trap by RF evaporation. In such a low temperature, the loss of magnetic trap (Majorana loss) will prevent the atoms from reaching a high density, and the atoms cannot be cooled further. To reduce the loss rate of the magnetic trap, the far blue detuning light (532 nm, 18 W) is added to plug the zero point of the magnetic trap. In the optically plugged magnetic trap, atoms with high density are cooled down enough, which gives a good start for the loading of optical dipole trap.
ELECTROMAGNETISM, OPTICS, ACOUSTICS, HEAT TRANSFER, CLASSICAL MECHANICS, AND FLUID DYNAMICS
2017, 66 (8): 084101. doi: 10.7498/aps.66.084101
Photonic crystals are materials patterned with a periodicity in the dielectric constant, which can create a range of forbidden frequencies called as a photonic band gap. The photonic band gap of the photonic crystal indicates its primary property, which is the basis of its application. In recent years, photonic crystals have been widely used to design optical waveguides, filters, microwave circuits and other functional devices. Therefore, the study on the transmission properties in photonic crystal is significantly important for constructing the optical devices. The finite difference time domain (FDTD) is a very useful numerical simulation technique for solving the transmission properties of the photonic crystals. However, as the FDTD method is based on the second order central difference algorithm, its accuracy is relatively low and the Courant stability condition must be satisfied when this method is used, which may restrict its application. To increase the accuracy and the stability, considerable scientific interest has been attracted to explore the schemes to improve the performance of the FDTD. The fourth order Ronge-Kutta (RK4) method has been applied to the FDTD method, which improves the accuracy and eliminates the influence of accumulation errors of the results, but the stability remains very poor if the time step is large. An effective time domain algorithm based on the high precision integration is proposed to solve the transmission properties of photonic crystals. The Yee cell differential technique is used to discretize the first order Maxwell equations in the spatial domain. Then the discretized Maxwell equations with the absorption boundary conditions and the expression of excitation source are rewritten in the standard form of the first order ordinary differential equation. According to the precise division of the time step and the additional theorem of exponential matrix, the high precision integration is used to obtain the homogeneous solution. To obtain the discretized electric and magnetic fields, the particular solution must be solved based on the excitation and then be added to the homogeneous solution. The transmission properties of photonic crystals are obtained by the Fourier transform. Practical calculation of photonic crystals is carried out by the precise integration time domain, and the accuracy and the stability are compared with those from the FDTD and the RK4 methods. The numerical results show that the precise integration time domain has a higher calculation precision and overcomes the restriction of stability conditions on the time step, which provides an effective analytical method of studying the transmission properties of photonic crystals.
Propagation characteristics of terahertz waves in temporally and spatially inhomogeneous plasma sheath
2017, 66 (8): 084102. doi: 10.7498/aps.66.084102
The plasma sheath is produced by high-temperature heating during the reentry of a hypersonic vehicle to the Earth atmosphere. Temperature around the vehicle rises rapidly because of severe friction with air. The vehicle temperature behind friction is high enough to excite various real gas effects including chemical reactions of air, which contains ablation particles of vehicle, free electrons, and ions. The plasma sheath greatly affects the transmission of electromagnetic waves and has very strong interference on the communication signals, which results in interrupt between the target and the ground station, namely, blackout. The electron density of plasma sheath surrounding the aircraft is inhomogeneous and varies with time. Temperature and pressure will also change at different altitudes. Therefore, it is meaningful to investigate the propagation characteristics of electromagnetic waves in temporally and spatially inhomogeneous plasma sheath. The temporally and spatially inhomogeneous plasma sheath model is introduced and the electron density data of the National Aeronautics and Space Administration (NASA) reentry vehicle is employed. The relationships among temperature, pressure, and collision frequency are obtained with the empirical formula of collision frequency. Then, the reflection coefficient and transmission coefficient of time-varying single layer plasma are calculated with the shift operator finite-difference time-domain (SO-FDTD) method. These results are compared to verify the correctness of the proposed method. Finally, the LTJEC-FDTD method is used to calculate the reflection coefficient, transmission coefficient and absorptivity at different relaxation time, temperature, and pressure in the terahertz (THz) band. The results show that the higher temperature and pressure will enable the electromagnetic wave to penetrate the plasma sheath at high relaxation time of electron density. If the incident wave frequency is lower than the cut-off frequency of plasma, the reflection of electromagnetic wave will be more obvious. However, when the incident wave frequency is in the THz band, the effects of temperature and pressure on the propagation of electromagnetic wave are obviously weakened. The absorption of electromagnetic wave by plasma will be more obvious when the relaxation time, temperature, and pressure decrease. If the relaxation time of electron density is shorter than or equal to the period of THz wave, more energy of electromagnetic wave will be absorbed by the plasma sheath. Contrarily, if the relaxation time of electron density is much longer than the period of THz wave, the absorption of electromagnetic energy will decrease. This study gives some insight into the temporally and spatially inhomogeneous plasma sheath, and provides a theoretical basis for solving the blackout problem.
A method to diverge reflected beam uniformly using cube-corner retroreflector array with dihedral angle tolerances
2017, 66 (8): 084201. doi: 10.7498/aps.66.084201
The cube-corner retroreflector (CCR) is widely applied in the electro-optical tracking, satellite communication, interferometers and adjust-free solid state laser. In some applications, the incident beam emitted by a laser is reflected back by the CCR to a photoelectric detector. The distance between the photoelectric detector and the laser source on the ground is much larger than the diffraction-limited spot. Meanwhile, the attitude angle of the CCR would randomly vary for the jitter of the platform. Therefore, the reflected beam should be diverged uniformly at far-field, whereas the normal CCR cannot achieve the divergence on the reflected beam. The investigation indicated that six sub-spots are generated by a CCR with dihedral angle tolerances at far-field. According the characteristics of the CCR with dihedral angle tolerances, a structure and its design method are proposed to diverge the reflected beam with a CCR array. The azimuthal angles of the every CCR of the array should be specially designed to generate an annular and uniform pattern. Due to the propagation distance is much larger than the size of the CCR array, the feasibility of the method is analyzed by the wave theory. A CCR array with a divergence half-angle of 0.5 mrad is designed, in which the dihedral angle tolerance of every CCR is 20. The influences of the beam and structure parameters on the diffraction characteristics of the reflected beam are investigated. The numerical results indicate the divergence half-angle of the CCR array varies quasi-linearly with the change of the dihedral angle tolerance, and the intensity distribution of the incident beam does not influence the divergence half-angle. The propagation distance does not affect the uniformity of the reflected beam when the CCR array satisfies the point source condition. When the number of the array element increases to a certain value, the increase of the number can strengthen the intensity and hardly influences the uniformity of the reflected beam. For the restriction of the machining and assembling technics, the dihedral angle tolerance of every CCR is hardly identical and the assembling azimuthal angles of the array element can not be identical with the design result. Therefore, the influence of the assemblage azimuth error and machining accuracy of the dihedral angle are studied. It reveals that the assemblage azimuth error does not remarkably the reflection pattern, whereas the machining accuracy can observably affect the uniformity of the reflection pattern, which can be resolved by the growth of the number of array element.
Analysis of the influence of diattenuation on optical imaging system by using the theory of vector plane wave spectrum
2017, 66 (8): 084202. doi: 10.7498/aps.66.084202
In most of the researches of polarization aberration, the influence of diattenuation is not large enough to affect imaging quality evidently. However, the modulation transfer function decreases when optical elements with complex planar dielectric structures and low transmittance, such as beam-splitter and optical modulator, are introduced into an imaging system. In this paper, a vector optical model in Descartes coordinate system is proposed based on the concept of vector plane wave spectrum (VPWS). The results of calculation show that the VPWS model is consistent with Debye model. Compared with Debye vector diffraction integral, the VPWS method is more suitable to the description of the PA introduced by planar optical device with opaque mask, such as larger surface quantum-confined-stark-effect electro-absorption modulator, which is used to modulate the light collected by optical antenna of time-of-flight (TOF) depth system or modulating-retroreflector free-space-optical communication system. In order to simplify the calculation and obtain the conclusion of the change in imaging quality directly, the formula of optical transfer function is decomposed into three parts (TE component, TM component and the correlation of them) instead of polynomial expansion of pupil function. The influences of diattenuation on MTF is studied globally and locally in a range of cut-off frequency of optical imaging system (2NA/ ). Allowance of diattenuation is analysed by numerical calculation, and a mathematical expression is derived. The result shows that the change of diattenuation can be neglected when the spatial frequency v is less than 0.2NA/, and the range of allowance decreases with the increase of spatial frequency. According to numerical calculation shown in Fig.7 and the derived formulas (15) and (16), the ratios of reflection/transmission coefficient of s-light and p-light D should range respectively from 0.63 to 1.6(0.2NA/ v 0.8NA/) and from 0.9 to 1.11(v0.8NA/ ) when the MTF is required to be not less than 90% of the value in ideal diffraction-limited system. The range of allowance becomes larger gradually with the increase of angle n between the normal of optical interface n and the optical axis of imaging system z. If a polarization beam splitter is considered, D,n sin-1 NA should be greater than 1-3.
2017, 66 (8): 084203. doi: 10.7498/aps.66.084203
To overcome the limitation of existing algorithms for detecting moving objects from the dynamic scenes, a foreground detection algorithm based on optical flow field analysis is proposed. Firstly, the object boundary information is determined by detecting the differences in optical flow gradient magnitude and optical flow vector direction between foreground and background. Then, the pixels inside the objects are obtained based on the point-in-polygon problem from computational geometry. Finally, the superpixels per frame are acquired by over-segmenting method. And taking the superpixels as nodes, the Markov Random field model is built, in which the appearance information fitted by Gaussian Mixture Model is combined with spatiotemporal constraints of each superpixel. The final foreground detection result is obtained by finding the minimum value of the energy function. The proposed algorithm does not need any priori assumptions, and can effectively realize the moving object detection in dynamic and stationary background. The experimental results show that the proposed algorithm is superior to the existing state-of-the-art algorithms in the detection accuracy, robustness and time consuming.
2.5 kW average power, two-channel spectral-beam-combined output based on transmitting volume Bragg grating
2017, 66 (8): 084204. doi: 10.7498/aps.66.084204
Spectral beam combination based on volume Bragg gratings is an effective approach to obtaining high power laser output. In spectral beam combining system, spectral channel spacing will affect the number of non-combined sub-beams and the overall combined output power due to the finite available gain bandwidth. Based on coupled wave theory, a two-channel high power spectral beam combining model is proposed. By appropriately relaxing the requirements for the spectral channel spacing and line-width of sub-beams, the higher combined output power can be obtained but the spectral density does not significantly decrease. In this work, a 2-channel spectral beam combining system is demonstrated to present a 2.5 kW combined power with combining efficiency 85% by employing a transmitting volume Bragg grating. The combining system has a high spectral density of 0.51 kW/nm with 5 nm spectral spacing between channels. The output can keep a good beam quality when the combined power is less than 1 kW, while the significant degradation of combined beam quality occurs when output power is 1.5 kW and is restricted mainly by the dispersion properties and thermal effects of volume Bragg gratings. During this 2-channel beam combining process, no special active cooling measure is used. Interactions between laser radiation and the grating are verified. Thermal absorption of high power laser radiation in the grating will cause the temperature to remarkably increase, resulting in the thermal expansion of the grating period, which leads to the degradations of diffraction efficiency and the spectral selectivity. Research is also focused on the surface distortion, and the results indicate that the thermal-induced wave-front aberrations of the non-combined sub-beams lead to the deterioration of beam quality. Transmitted and diffracted beams experience wave-front aberrations to different degrees, leading to distinct beam deterioration.
2017, 66 (8): 084205. doi: 10.7498/aps.66.084205
A 852 nm ridge waveguide edge emitting laser has important applications. But lateral mode instability leads to its poor beam quality because of its ridge structure. Such a structure gives rise to two guidance mechanisms (gain-guide and index-guide), whose change leads to kink effect. So, the control of the single fundamental lateral mode is more difficult. There is no well-informed study in these aspects for ridge waveguide edge emitting lasers. In this paper we study how to improve the beam quality for achieving a stable fundamental lateral mode output experimentally. We are to investigate the influence of lateral mode characteristics of the laser with different ridge depth-to-width ratios in detail by waveguide theory and equivalent refractive index method. Depth and width of the ridge are two key parameters influencing lateral mode. The depth can control lateral guidance mechanism, and the width can control lateral mode order. We find that the ratio must be in a limited range to ensure the single fundamental lateral mode steady. Through theoretical analysis of waveguide theory and equivalent refractive index method, we obtain a limited range of depth-to-width ratio. Then we conduct an experimental comparison, where we adjust the ridge depth, with the width fixed, to control the ratio. Meanwhile we improve the fabrication technology to ensure the accuracy of the structure. We design and fabricate an asymmetric waveguide ridge waveguide edge emitting laser with isolation grooves, whose active region is the core of asymmetric waveguide epitaxy structure. The key structural parameters are 5 m in ridge width, 500 nm in ridge depth, 2 m in isolation grooves depth, 10 m in width, 30 m in spacing between the grooves, and 1 mm in cavity length. Isolation grooves are very useful for improving the performance of the laser: threshold decreased by 50%, output power raised by 44%, and slop efficiency increased by 17%. And the equally crucial role of grooves is to avoid being damaged at packaging process to maintain laser structure. Finally we achieve a stable single fundamental lateral mode output and an accurate tuning wavelength at 852 nm of ridge waveguide edge emitting laser without cavity surface coated at working current 150 mA, working temperature 30 ℃ (working conditions can be changed in a small range). The slope efficiency is on average 0.7 mW/mA (its maximum value is 0.89 mW/mA), and the full wave at half maximum is less than 1 nm. Although we improve the performance of ridge waveguide edge emitting laser and beam quality for stable output, there is still a need to further study the stable output over a wide range. The results in this paper will provide a useful reference for realizing the stable output ridge waveguide edge emitting lasers and the ultra-narrow line-width lasers.
2017, 66 (8): 084206. doi: 10.7498/aps.66.084206
We observe special scattering light by using a simple experimental device and record the dynamic behavior with a camera. A laser beam from an Nd:YAG laser, which is expanded by the spatial filter (SF) and collimated by the lens L1 (f1=300 mm), is focused into a line light through a pair of cylindrical lenses L2, L3 (f2=f3=200 mm) and irradiates the LiNbO3:Fe crystal. On condition that the directions of line beam (f) and c-axis of the crystal are both parallel to the horizontal direction, we observe experimentally that the scattering light spreads gradually in the horizontal direction in the far field as irradiation time goes on. Then the scattering light reaches a steady state after 10 min. The scattering light beam is composed of vertical filaments. When the line beam is horizontal and the c-axis is vertical, the scattering light composed of horizontal filaments firstly appears in the vertical direction. About 30 min later, the scattering light appears and spreads along the horizontal direction to the far field as irradiation time goes on. At this time, the scattering light is also composed of vertical filaments. That is to say, we observe the scattering light whose direction is inconsistent with the c axis of the crystal. We also give the corresponding theoretical explanation to the phenomenon. We suppose that the line beam consists of many close-set thread-like sub-beams, which are vertical to the direction of the line beam. When the line beam irradiates the photorefractive crystal, the sub-beams record the gratings in the crystal according to photorefractive nonlinear effect. The gratings diffract the input beam. The scattering light and the incident beam interfere with each other, thereby recording the new grating. At the same time, the new gratings also diffract the incident beam. It goes full circle. So energy transfers from incident beam to the scattering light beam. The direction of the scattering light beam spreads along the direction of the line beam.
2017, 66 (8): 084207. doi: 10.7498/aps.66.084207
A novel design of optical receiver for visible light communication system in indoor environment is proposed in this study. The compound parabolic concentrator is coupled with a photo-detector as the receiving unit due to its optical properties. The composite optical receiver is composed of seven receiving units inserted in a hyper-hemispherical lens aligned with geometry configuration based on angle diversity. The composite optical receiver has fields of view of 360 in the horizontal direction and 180 in the vertical direction respectively, while the field of view of each receiving unit is 30. Model of indoor visible light communication is built through measurement in a room of a 5 m 5 m 3 m size. The received power and SNR distribution are acquired through MATLAB scripts. The received power of each receiving unit is treated by different algorithms. At a lower data rate, the sum of the received power from all receiving units is the final received power, while at a higher data rate, the final received power is the highest value of power collected by the each unit. The results show that the received powers of the composite receiver by using two different algorithms increase 11.58 and 7.47 dB, respectively, while the gains of the receiver are 15.31 and 5.98, respectively. The mean values of the signaltonoise ratio are 79.17 dB from the sum algorithm and 72.26 dB from maximum algorithm, respectively. It is concluded that signaltonoise ratio is high and the distribution fluctuation is weak. This usually means a good and stable communication performance. It is proved that the composite receiver designed in this study gives high quality communication performance and presents a wide field of view, thereby avoiding the blind area in communication.
2017, 66 (8): 084208. doi: 10.7498/aps.66.084208
With the development of infrared detection technology, the survival of military target is now under serious threat. Therefore, new infrared stealth technologies and materials are now in an urgent demand. The photonic crystal (PhC) possesses regularly repeating structure which results in band-gap and diffraction satisfying Bragg's law of diffraction. The PhC presents unique optical properties and functionality. The PhC with band-gap located in visible band is used widely as biosensor, chemical sensor, optical filter, reflector, modulator, metasurface and solar cell. The PhC with band-gap located in infrared band can be used to control the propagations of the electromagnetic waves of infrared band, and could be used as a promising material in the infrared stealth technology. Photonic structure used to tune the infrared radiation usually has a one-dimensional layer-by-layer stack or three-dimensional wood pile structure. However, the poor flexibility, low strength, small area coverage, complicated fabrication process and high cost can prevent this new infrared stealth technology from being applied and developed. In this report, a simple and cost-effective method of preparing the opal PhC materials is proposed, and this infrared stealth material forbids electromagnetic waves of infrared band to propagate on account of band-gap.In this paper, opal PhCs materials with high quality are assembled from SiO2 colloidal microspheres with micrometer size by using optimized vertical deposition method. We calculate the relation between the diameter of SiO2 colloidal microsphere and the frequency of opal PhCs band-gap in theory and verified in experiment, which operates in the working band of infrared detector. The results show that the diameters of SiO2 colloidal microspheres should be 1.33-2.22 m and 3.56-5.33 m. A series of monodispersed micrometer SiO2 colloidal microspheres is prepared by the modified Stber method, and bigger microspheres are prepared by using the seeded polymerization method. Then, we choose the diameters of 1.5 m and 4.3 m SiO2 microspheres to prepare the opal PhCs materials. The PhCs materials assembled by 1.5 m SiO2 microspheres are prepared in alcohol under 60 ℃ or in acetone under 40 ℃; while the PhCs material assembled by 4.3 m SiO2 microspheres is prepared in alcohol/dibromomethane =3:1 under 60 ℃. Finally, the opal PhC materials with band-gap located in 2.8-3.5 m and 8.0-10.0 m are successfully prepared, and the widths of band-gap are 0.7 m and 1.9 m, respectively. These opal PhCs materials could change the infrared radiation characteristics of the target in infrared waveband, and meet the requirements of wide band-gap for infrared stealth materials.
2017, 66 (8): 084301. doi: 10.7498/aps.66.084301
Distinguishing and recognizing water targets and underwater targets has been the focus of passive sonar detection. The depth of the target is closely related to the physical characteristics of the signal. In the shallow water waveguide, the normal mode theory can be used to give a good explanation to the acoustic signal physical properties. In this paper, a new method of beam forming in horizontal array modal domain is proposed. Under the condition of predicting target azimuth, the difference in acoustic path between the horizontal array elements corresponding to the direction of the target signal can be calculated according to the azimuthal information, and the phase delay of each normal mode component of the acoustic signal can be obtained. The horizontal wave number varies with order of normal mode, so each order of the normal mode has a specific phase delay. By using the beam forming principle, when the phase of a certain order of normal mode is compensated for, the output of the superposition of the signal on each element is the modal intensity of the normal mode. After obtaining the target signal modal intensity of each order, based on the shallow water condition, the modal intensities of sound source excitation at different depths are obtained as the reference mode intensities of the sound source at corresponding depths in the shallow water waveguide by simulating on Kracken software. Then, calculating the correlation coefficient between the target signal modal intensity of each order and the reference modal intensity of the sound source at each depth, we search for the maximum value of the correlation coefficient. The reference depth corresponding to the maximum value of the correlation peak is the estimated value of the target depth calculated by the method. Based on physical causes and characteristics of the normal modes, in this paper, the influences of the parameters such as the element number of horizontal array, depth of receiving array, signal-to-noise ratio, velocity profile, waveguide depth, azimuthal estimation accuracy, effective array length and application frequency band on the performance of this method are analyzed. The simulation results show that the algorithm can estimate the depth of the sound source effectively by using the signal sample with a bandwidth of 300 Hz when the signal-to-noise ratio is -10 dB. The wider the frequency band, the longer the effective array length, and the more the array element number, the higher the accuracy of azimuth estimation will be, which will bring beneficial effects to the depth estimation with the method. In addition, the depth estimation performance of the proposed method is still robust when the waveguide conditions such as the velocity profile and the seafloor parameters are disturbed.
2017, 66 (8): 084302. doi: 10.7498/aps.66.084302
The interface waves propagating along liquid-solid interface are widely studied and used in a lot of fields, especially in ocean acoustics, ocean engineering, and ocean geophysics. The dispersion characteristics of this kind of interface wave are closely related to the seafloor medium parameters, which is an effective means for the inversion of the seafloor sediments. However, the interface wave is difficult to use for ultrasonic nondestructive material characterization, especially for stiff and dense solid materials, owing to the mode shape or wave structure of the liquid-solid interface waves.The fraction of the total wave energy that travels in the fluid compared with the solid depends on the properties of the solid material. Usually, for a stiff and dense solid compared with the fluid, most of the energy travels in the fluid, while for a soft solid more energy travels in the solid. Therefore, it is difficult to use this kind interface wave for stiff solid material characterization. However, in the case of liquid-coated solid interface, the behaviors and properties of interface waves are quite different.In this paper, we use pulsed laser to generate the interface waves at the water-coated solid interfaces. The theoretical analysis of the laser-induced excitation of acoustic waves propagating along a plane interface between liquid and layered elastic solid is perforemd first. The general solution for the interface motion is derived. The analytic expression of the transient response is then obtained. Based on this expression, the dispersion characteristics of the interface waves, which propagate along the fluid-coated solid interface for the cases of slow coating on fast substrate and fast coating on slow substrate, are calculated and analyzed. The transient response signals are further calculated. In the case of slow coating on fast substrate, the interface wave shows an evident dispersion, in which its phase velocity is larger than its group velocity. In the case of fast coating on slow substrate, the interface wave also shows a remarkable dispersion within a smaller frequency-thickness product range, in which its phase velocity is less than its group velocity. The theoretical transient signals show the same properties.In order to verify the theoretical results, an experimental system is set up, and the interface waves are generated and measured. The experimental system mainly consists of pulsed laser, hydrophone, oscilloscope, and movable translation stage. The pulsed laser is used to excite the interface waves, and the hydrophone mounted on the movable translation stage is placed near the interface to receive the signals. Two kinds of samples of slow coating on fast substrate and fast coating on slow substrate are made and measured. The recorded testing signals are then processed and analyzed.The theoretical results and the experimental ones are in good agreement. The research results presented in this paper can provide theoretical basis for ultrasonic nondestructive characterization of coating and film material in immersion testing mode, and also for seafloor sediment parameter inversion.
Discrete optimal control for Birkhoffian systems and its application to rendezvous and docking of spacecrafts
2017, 66 (8): 084501. doi: 10.7498/aps.66.084501
In general, optimal control problems rely on numerically rather than analytically solving methods, due to their nonlinearities. The direct method, one of the numerically solving methods, is mainly to transform the optimal control problem into a nonlinear optimization problem with finite dimensions, via discretizing the objective functional and the forced dynamical equations directly. However, in the procedure of the direct method, the classical discretizations of the forced equations will reduce or affect the accuracy of the resulting optimization problem as well as the discrete optimal control. In view of this fact, more accurate and efficient numerical algorithms should be employed to approximate the forced dynamical equations. As verified, the discrete variational difference schemes for forced Birkhoffian systems exhibit excellent numerical behaviors in terms of high accuracy, long-time stability and precise energy prediction. Thus, the forced dynamical equations in optimal control problems, after being represented as forced Birkhoffian equations, can be discretized according to the discrete variational difference schemes for forced Birkhoffian systems. Compared with the method of employing traditional difference schemes to discretize the forced dynamical equations, this way yields faithful nonlinear optimization problems and consequently gives accurate and efficient discrete optimal control. Subsequently, in the paper we are to apply the proposed method of numerically solving optimal control problems to the rendezvous and docking problem of spacecrafts. First, we make a reasonable simplification, i.e., the rendezvous and docking process of two spacecrafts is reduced to the problem of optimally transferring the chaser spacecraft with a continuously acting force from one circular orbit around the Earth to another one. During this transfer, the goal is to minimize the control effort. Second, the dynamical equations of the chaser spacecraft are represented as the form of the forced Birkhoffian equation. Then in this case, the discrete variational difference scheme for forced Birkhoffian system can be employed to discretize the chaser spacecraft's equations of motion. With further discretizing the control effort and the boundary conditions, the resulting nonlinear optimization problem is obtained. Finally, the optimization problem is solved directly by the nonlinear programming method and then the discrete optimal control is achieved. The obtained optimal control is efficient enough to realize the rendezvous and docking process, even though it is only an approximation of the continuous one. Simulation results fully verify the efficiency of the proposed method for numerically solving optimal control problems, if the fact that the time step is chosen to be very large to limit the dimension of the optimization problem is noted.
2017, 66 (8): 084701. doi: 10.7498/aps.66.084701
Pulsed actuation is one of the most fundamental control types to study regularity of flow structures in supersonic mixing layers, which helps to predict the aero-optical effects caused by the supersonic mixing layer where the different-sized vortices dominate the flow field. However, the knowledge about the evolution mechanism of vortices in the supersonic mixing layer which is controlled by the pulsed forcing is limited. Based on the large eddy simulation (LES), the visualized flow field of a supersonic mixing layer controlled by the pulsed forcing is presented and the unique growth mechanism of the vortices in such a case is revealed clearly. The method of position extraction of the vortex core in the supersonic mixing layer, which is a quantitative technique to obtain the instantaneous location of a vortex in flow field, is employed to calculate the dynamic characteristics (e.g., instantaneous convective speed and size) of the vortices quantitatively. The pulsed forcings of different frequencies are imposed on the same supersonic mixing layer respectively, and the instantaneous convective speed and size of the vortices for each pulse frequency considered in this study are then computed. By comparing the dynamic characteristics of the vortices between cases, the evolution mechanism of the vortices in the supersonic mixing layer controlled by the pulsed forcing is revealed.as follows. 1) Growth of the vortices in the supersonic mixing layer controlled by the pulsed forcing no longer depends on the pairing nor merging between adjacent vortices, which is just the growth mechanism of vortices in a free supersonic mixing layer. Actually, the size of a vortex in the controlled supersonic mixing layer is dominated by the imposed pulse frequency, so the size of each vortex in such a flow field is approximately identical. 2) The number of vortices in the controlled supersonic mixing layer is proportional to the pulse frequency, whereas the size of vortex is inversely proportional to the pulse frequency. That is, the higher the pulse frequency, the bigger the number of vortices in the controlled flow field is and the smaller the size of every vortex. 3) The average convective speed of vortices in the controlled supersonic mixing layer gradually decreases with pulse frequency increasing because the pulsed forcing essentially drags on the movement of vortices in flow field. Finally, an equation which describes the quantitative relationship between the dynamic characteristics of a vortex and the pulsed forcing frequency is derived, that is, the product of the average convective speed of vortices in the controlled supersonic mixing layer and the imposed pulse period is approximately equal to the mean diameter of vortices in the flow field.
Numerical solution procedure for Hall electric field of the hypersonic magnetohydrodynamic heat shield system
2017, 66 (8): 084702. doi: 10.7498/aps.66.084702
Magnetohydrodynamic (MHD) heat shield system is a novel-concept thermal protection technique for hypersonic vehicles, which has been proved by lots of researchers with both numerical and experimental methods. Most of researchers neglect the Hall effect in their researches. However, in the hypersonic reentry process, the Hall effect is sometimes so significant that the electric current distribution in the shock layer can be changed by the induced electric field. Consequently, the Lorentz force as well as the Joule heat is varied, and thus the efficiency of the MHD heat shield system is affected.In order to analyze the influence of Hall effect, the induced electric field must be taken into consideration. According to the weakly-ionized characteristics of hypersonic flow post bow shock, the magneto-Reynolds number is assumed to be small. Therefore, the Maxwell equations are simplified with the generalized Ohm's law, and the induced electric field is governed by the potential Possion equation. Numerical methods are hence established to solve the Hall electric field equations in the thermochemical nonequilibrium flow field. The electric potential Poisson equation is of significant rigidity and difficult to solve for two reasons. One is that the coefficient matrix may not be diagonally dominant when the Hall parameter is large in the shock layer, and the other is that this matrix including the electric conductivity is discontinuous across the shock. In this paper, a virtual stepping factor is included to strengthen the diagonal dominance and improve the computational stability. Moreover, approximate factor and alternating direction implicit method are employed for further improving the stability. With these methods, a FORTRAN code is written and validated by comparing the numerical results with the analytical ones as well as results available from previous references. After that, relation between the convergence property and the virtual stepping factor is revealed by theoretical analysis and numerical simulations. Based on these work, a local variable stepping factor method is proposed to accelerate the iterating process. Results show that the convergence property is closely related to the mesh density and Hall parameter, and there exists a best stepping factor for a particular mesh as well as a particular Hall parameter. Since the best stepping factor varies a lot for different meshes and different Hall parameter, its appropriate value is hard to choose. The best value of stepping factor coefficient still exists in the local step factor method, but its value range is relatively smaller. More importantly, the local stepping factor method yields better convergence property than the regular constant one when employing a locally refined mesh.
2017, 66 (8): 084703. doi: 10.7498/aps.66.084703
Studies of the driving force of the self-propulsion Janus particles are very important in the fields of micro-power and nano-motor. In this paper, we choose the micron Pt-SiO2-type Janus particle as a research object, which is propelled by self-generated concentration gradient in the dilute solution of H2O2, focusing on the self-propulsion of the single particle. According to the force analysis of the Janus particle, the surface force can be decomposed into the viscous resistance of the fluid, the Brownian force derived from the molecular thermal fluctuation, and the diffusiophoresis caused by the diffusion of the solute component. The main aim of this paper is to find the way to accurately simulate the diffusiophoresis generated by the huge concentration gradient on a microscale. The lattice Boltzmann method (LBM) is a modern mesoscopic method based on the microscopic particle characteristics of the fluid, which makes it more intuitive to deal with the interaction between the fluid and solid. It is more advantageous than the traditional numerical method in the description of this micro-interface dynamic problem, i.e., the self-propulsion of Janus particle. On a certain time scale, when the Janus particle shows the directional motion, the influence of the Brownian force can be ignored. Thus, the analytical process can be simplified. Based on the momentum theorem, the method of calculating the diffusiophoresis produced by concentration diffusion is proposed. We introduce the momentum exchange in the half-way bounce-back scheme of LBM into the model of the multicomponent diffusion and reaction. Through counting the surface force we can obtain the diffusiophoresis acting on the Janus particle. Moreover, this diffusiophoresis model is modified by comparing the experimental fluid resistance with simulated one. This comparision verifies the validity of the diffusiophoresis model. Then, the analysis of the variation of diffusiophoresis proves that the value of diffusiophoresis is independent of the fluid velocity. Through the further application of this model, the different shapes of Janus particles with the same volume are compared in simulations. The results show that the self-diffusiophoresis is mainly determined by the axial projection area. In addition, the reaction area of the particle also affects the value of the diffusiophoresis.
Lorentz force filtering and fast steering mirror optical compensation in optical axis stability control for photoelectric mast
2017, 66 (8): 084704. doi: 10.7498/aps.66.084704
The photoelectric mast equipped on the underwater vehicle is the key equipment for photoelectric tracking. While the vehicle moves under water, especially, at high speed, more complex vortexes are generated at the surface, which will give rise to great disturbance to the stability of optical axis. In this paper, firstly, based on the basic control equations of electromagnetic field and fluid mechanics, the effects of the Lorentz force on flow field structure and vortex induced vibration are numerical simulated with using the finite volume method with hierarchy grids. Secondly, the structural characteristics, transfer functions and PID control strategies of fast steering mirror (FSM) are analyzed. Finally, combining the transfer function of FSM and the force characteristics, the effect of the composite control on the stability of submarine photoelectric tracking system is discussed by MATLAB. The results show that the Lorentz force can adjust the boundary layer and suppress vortex induced vibration, based on which the FSM can be used to further improve the accuracy of the optical tracking system. This research offers a new exploration in the field of electromagnetic fluid control, as well as a novel development of the traditional research direction of fluid mechanics. Therefore it appears to have a certain scientific significance and practical value.
2017, 66 (8): 084705. doi: 10.7498/aps.66.084705
Control of shock wave/boundary layer interaction (SWBLI) is of high practical importance for supersonic aircraft drag reducing. Lots of flow control strategies including passive and active control techniques have been put forward to minimize negative effect of SWBLI.Plasma aerodynamic control technique is considered as a potential one due to its flexibility in manipulating the supersonic flow. The goal of this research is to investigate the control effect of the novel actuator called plasma synthetic jet on the SWBLI.The effect of counter-flow plasma synthetic jet actuator on the SWBLI is investigated experimentally in this paper. The experiments are conducted in a supersonic wind tunnel at Mach number Ma=3.1. The test model is a blunt body with a plasma synthetic jet actuator installed inside its head which is used to create aerodynamic perturbations, and with a conical compression ramp in the rear, enabling the creation of SWBLI flow configuration. The plasma synthetic jet actuator is designed to inject pulsed hot gas by arc discharge into a small cavity in the direction perpendicular to the normal shock wave induced by the blunt body. The schlieren method is used for flow measurement and the flow characteristics are studied according to a sequence of schlieren images (1024512 pixel resolution) captured by a high speed charge-couple device camera with a framing rate of 58 kHz, triggered externally, and an exposure time of 1 s. Additionally, the mechanism of this control strategy on the SWBLI induced by the ramp is revealed by using the numerical method.The characteristics of the plasma synthetic jet in quiescent air are firstly studied. The results show a sudden reduction of averaged jet velocity under the resistance of the air. In addition, some small-scale flow structures in the jet are observed which may enhance the turbulence in the upstream boundary layer. The flow topology of interaction modified by actuation with frequencies of f=1 kHz and f=3 kHz are respectively analyzed. It is shown that by using this type of control strategy, the attached shock is locally degraded with the attachment point moving upward. The separation bubble is suppressed, hence making the separation shock move downstream. In addition, an extensive impact effect is exerted to the interaction region by actuation at f=1 kHz because more hot gas is produced by the actuator. Therefore, the actuator is found to be capable of significantly mitigating the negative effects induced by the SWBLI. The numerical work focuses on the interaction between the jet and the flow after the normal shock. The results show that large-scale vortex is induced by the interaction which increases turbulence and accelerates the flow near the wall during its moving downstream and dissipation, demonstrating turbulence enhancement in the boundary layer and a variation of upstream flow characteristics are the key factors for separation reduction and shock wave mitigation.
PHYSICS OF GASES, PLASMAS, AND ELECTRIC DISCHARGES
2017, 66 (8): 085201. doi: 10.7498/aps.66.085201
Laser-driven helium ion source with multi-MeV energy has an important application in the field of fusion reactor material irradiation damage. At present, the generating of high energy helium ions by relativistic ultraintense laser interacting with helium gas jet is the main scheme of laser-driven helium ion source. However, so far, this scheme has been hard to generate the helium ion beam with the characteristics, i.e., it is forward and quasi-monoenergetic and has multi-MeV in energy and high yield. These characteristics of helium ion beam are important for studying the material irradiation damage. In this paper, we propose a new scheme in which an ultraintense laser interacting with foil-gas complex target is used to generate helium ions. With this method, we perform an experiment on XingGuang III laser facility which has three laser beams with different laser durations (nanosecond, picosecond and femtosecond). In our experiment, we use a picosecond laser beam. The wavelength of this laser beam is 1054 nm and its duration is 0.8 ps. We use an off-axis parabola mirror to focus the 100 J energy of this laser beam onto a focal spot of 25 m far away. The laser intensity reaches 51018 W/cm2. The foil-gas target is composed of a copper foil with 7 m in thickness and a helium gas nozzle which is behind the copper foil. The helium gas nozzle can generate a helium gas jet with a full ionization electron density of 51019/cm3. We use the Thomson Parabola Spectrometer to record the helium ion signals and the Electron Magnetic Spectrometer to diagnose the hot electron temperature. In the experiment, the laser pulse interacts with the front surface of the copper foil and generates lots of hot electrons. These hot electrons result in the expansion of the rear surface of the copper foil. The expanding plasma accelerates the helium ions behind the copper foil. The experimental results show that the obtained helium ions are forward and quasi-monoenergetic (the peak energy is 2.7 MeV), and the total energy of the helium ions whose energies are all higher than 0.5 MeV is about 1.1 J/sr, and correspondingly the yield of helium ions is about 1013/sr. The helium ion spectrum and hot electron temperature given by particle in cell (PIC) simulation with using the experimental parameters are consistent with the experimental results. In addition, the PIC simulations also show that helium ions are accelerated by target normal sheath acceleration and collisionless shock acceleration-like mechanisms, and the maximum helium ion energy is proportional to the hot electron temperature.
CONDENSED MATTER: STRUCTURAL, MECHANICAL, AND THERMAL PROPERTIES
Precise control of branch-length of light irradiated gold nanostars and associated thermal performance
2017, 66 (8): 086101. doi: 10.7498/aps.66.086101
Gold nanostars are multi-branched nanoparticles with tip structures. Nanostars have excellent photoelectric properties, which make them able to be used in a variety of optoelectronics devices. Moreover, these stars have good biocompatibility and low toxicity, which opens broad application prospect in the biomedical field. Gold nanostars with admirable optical as well as thermal properties, are thought as a good candidate in cancer treatment that is a hot research topic in recent years. Gold nanostars with different branch-lengths were prepared by the photo-assisted method, and the effect of light was well studied in relation with gold nanostar branch-length. In the solution system, HEPES was used as the reducing agent, stable agent and shape-inducing agent. Under light irradiation, a certain amount of chloroauric acid solution (HAuCl4) was added to the HEPES solution. After a period of time, gold nanostars were prepared. Different wavelengths of irradiating light were selected in this experiment. The wavelength has different effects on the growth of branches associated with gold nanostars. The transmission electron microscope and the ultraviolet-visible-near infrared spectrophotometer were used to analyze the morphology and absorption spectra of gold nanostars. Meanwhile, a nano-measurer software was used to determine branch-lengths of gold nanostars under light irradiation of different wavelengths. The results indicate that the branches of the nanostars under irradiation were shorter than those of nanostars without irradiation. Different branch lengths correspond to different irradiation wavelengths. Based on these results, the physical process of shortening nanostars branches was analyzed, and a theoretical model of changing branch-length in the process of light-induced nanostars growth was proposed. The model indicates that there are two steps when the branch-length is changing. Firstly, the branch-length grows longer with the overall growth of the nanostar. Secondly, the nanostar becomes shorter because of the insatiability of HEPES molecules that are adsorbed on the nanostar surface with the increasing solution temperature. Through a photothermal measurement, a xenon lamp (wavelength 670 nm) was used as a light source to measure the temperature change within 30 min, and then the photothermal conversion efficiency of the gold nanostars was calculated. The results show that the branch-length of gold nanostars can be precisely controlled by light irradiation with slight variation in wavelength. The photothermal conversion efficiency of gold nanostars can also be regulated.
Experimental investigation on the influence of Mg content on Portevin-Le Chatelier effect in Al-based alloys by using digital image correlation
2017, 66 (8): 086201. doi: 10.7498/aps.66.086201
The Protein-Le Chatelier (PLC) effects are investigated by using digital image correlation at a constant applied strain rate of 5.0010-3 s-1 and room temperature in Al-Mg alloys with Mg content values (wt.%) of 2.30, 4.57, 6.10 and 6.91 respectively in this study. Both the yield strength and the ultimate strength increase with increasing Mg content, which is generally called solution strengthening. Type of PLC band changes from A to B with increasing Mg content. In low Mg content (2.30%) alloy, the serration amplitude almost remains 1 MPa, while in each of high Mg content (4.57%, 6.10%, 6.91%) alloys it linearly increases with the strain increasing. The serration amplitude is found to increase with increasing Mg content and gradually reaches a saturated state. With the increase of Mg content, the period of PLC band for continuous propagation gradually reduces and the time when the PLC band location sudden jumps increases in the process of propagation. When the strain is small, the out-of-band deformation of alloy is inhomogeneous obviously. And the deformation inhomogeneity slightly decreases with increasing Mg content. DIC results indicate that the PLC bandwidth does not change with Mg content, while the maximum strain increment in the PLC band increases with increasing both Mg content and strain. Additionally, special periodic damped serrations are observed in the stress-time curve of the low Mg content (2.30%), the corresponding PLC band shows that the periodic changed serrations in the stress-time curve correspond to the transformation of the PLC band orientation. Besides, the PLC band propagates upward continuously both before and after the shift.
Atomic-scale simulation study of structural changes of Fe-Cu binary system containing Cu clusters embedded in the Fe matrix during heating
2017, 66 (8): 086301. doi: 10.7498/aps.66.086301
Nano-size Cu precipitates are the main products of irradiation embrittlement of nuclear reactor pressure vessel steels. Molecular dynamics simulation within the framework of embedded atom method is performed to study atomic packing change in Fe-Cu binary system, where the small Cu clusters are embedded in the crystal body centered cubic (BCC) Fe lattices. As the temperature increases, atomic packing change occurs in the Fe-Cu binary system. The mean square displacement of Cu atom, pair distribution function of the Cu atoms, and the atomic density profile along the radial direction are calculated. The atom packing structures in pure Cu region, Fe-Cu interface region, and pure Fe matrix are analyzed. The simulation results show that the packing structures in the Cu cluster and the Fe matrix are greatly affected by the sizes of these clusters and the volume of the Fe matrix containing these clusters. The structural changes present apparent differences, for the Fe matrixes contain these confined Cu clusters with different atom numbers during heating. As the Fe matrix can only provide small space to accommodate the Cu atoms, packing patterns in many Cu atoms are disordered for the Febulk-Cu135 system. In this binary system, strain region in the Fe matrix is adjacent to the Cu cluster. In the meantime, there are a lot of vacancy defects and strain regions in the matrix. For the Febulk-Cu141 system, although the Cu cluster contains more atoms, the Fe matrix can accommodate Cu atoms in a larger space, and the majority of these Cu atoms are located at the BCC crystal lattices. With increasing the temperature, the changes can be observed that the number of the strain regions decrease, whereas the sizes of some strain regions increase.
2017, 66 (8): 086801. doi: 10.7498/aps.66.086801
With the development of modern industrial technology, tungsten products prepared from normal tungsten powder cannot meet the demands of industry. The tungsten product produced from ultra-fine tungsten powder exhibits high strength, high toughness, and low metal plasticity-brittleness transition temperature, which greatly improves the performance of materials. Hence, it is necessary to carry out theoretical research on the micro adsorption dynamics during hydrogen reduction of tungsten trioxide to prepare ultra fine tungsten powder. In order to understand crystal characteristics of WO3 and WO3(001) surface characteristics, and to provide beneficial theoretical support for reaction law of hydrogen reduction on the WO3(001) surface, the mechanisms of H atom adsorption on cubic WO3 and WO3(001) surface are studied by the first-principles calculation based on the density functional theory (DFT) plane wave pseudo-potential method. The results show that theoretically calculated band gap of the cubic crystalline WO3 is 0.587 eV. There are two kinds of WO3(001) surfaces, WO-terminated (001) surface and O-terminated (001) surface. The W-O bond length and the bond angle of W-O-W structure change after the geometric optimization of the surface, and thus the surface relaxation is realized. The WO-terminated (001) surface shows n-type semiconductor characteristics while the O-terminated (001) surface shows p-type semiconductor characteristics. Four adsorption configurations of H atoms on the WO-terminated (001) surface and the O-terminated (001) surface, including H-O2c-H, H-O2 cH-O2c, H-O1c-H, and H-O1cH-O1c, are calculated. Among them, the adsorption energy of the H-O1c-H configuration is the smallest (-3.684 eV) with the shortest bond length of H-O bond (0.0968 nm), and hydrogen atoms lose the most of electrons (0.55e), which indicates that the H-O1c-H adsorption configuration is the most stable one. The band gap of the H-O1c-H configuration increases from 0.624 eV to 1.004 eV after adsorption, while the bandwidth of valence band is almost unchanged. The results about the density of states (DOS) reveal that 1s state of the H atom interacts with 2p and 2s states of the O atom. Strong isolated electron peaks are formed to be at about -8 and -20 eV. The outermost O1c atoms of O-terminated (001) surface contain an unsaturated bond, facilitating the bonding between two H atoms and one O1c atom. Thus, two H atoms and one O1c atom form chemical bonds respectively, and an H2O molecule is generated, leaving an oxygen vacancy on the surface after adsorption reaction. By combining experimental observations with simulation results, the mechanism of hydrogen reducing tungsten trioxide can be elaborated profoundly from a micro view.
CONDENSED MATTER: ELECTRONIC STRUCTURE, ELECTRICAL, MAGNETIC, AND OPTICAL PROPERTIES
Interfacial cohesive interaction and band modulation of two-dimensional MoS2/graphene heterostructure
2017, 66 (8): 087101. doi: 10.7498/aps.66.087101
To improve the efficiency of water-splitting, a key way is to select suitable semiconductor or design semiconductor based heterostructure to enhance charge separation of photogenerated h+-e- pairs. It is possible for a two-dimensional (2D) heterostructure to show more efficient charge separation and transfer in a short transport time and distance. Among numerous heteromaterials, the 2D layered MoS2 has become a very valuable material in photocatalysis-driven field due to the appropriate electronic structure, peculiar thermal and chemical stability, and low-cost preparation. To couple with MoS2, layered graphene will be an ideal candidate due to extremely high carrier mobility, large surface area, and good lattice match with MoS2. At present, a lot of researches focus on the synthesis and modification of MoS2/graphene heterostructure. However, it is hard to detect directly the weak interaction between MoS2 and graphene through the experiment. Here, an effective structural coupling approach is described to modify the photoelectrochemical properties of MoS2 sheet by using the stacking interaction with graphene, and the corresponding effects of interface cohesive interaction on the charge redistribution and the band edge of MoS2/graphene heterostructure are investigated by using the planewave ultrasoft pseudopotentials in detail. Three dispersion corrections take into account the weak interactions between MoS2 and graphene, resulting in an equilibrium layer distance d of about 0.34 nm for the MoS2/graphene heterostructure. The results indicate that the lattice mismatch between monolayer MoS2 and graphene is low in contact and a van der Waals interaction forms in interface. Further, it is identified by analyzing the energy band structures and the threedimensional charge density difference that in the MoS2 layer in interface there appears an obvious electron accumulation, which presents a new n-type semiconductor for MoS2 and a p-type graphene with a small band gap ( 0.1 eV). In addition, Mo 4d electrons in the upper valence band can be excited to the conduction band under irradiation. And the orbital hybridization between Mo 4d and S 3p will cause photogenerated electrons to transfer easily from the internal Mo atoms to the external S atoms. The build-in internal electric field from graphene to MoS2 will facilitate the transfer and separation of photogenerated charge carriers after equilibrium of the MoS2/graphene interface. It is identified that the hybridization between the two components induces a decrease of band gap and then an increase of optical absorption of MoS2 in visible-light region. It is noted that their energy levels are adjusted with the shift of their Fermi levels based on our calculated work function. The results show that the Fermi level of monolayer MoS2 is located under the conduction band and more positive than that of graphene. After the equilibrium of the MoS2/graphene interface, the Fermi level shifts toward the negative direction for MoS2 and the positive direction for graphene, respectively, until they are equal. At this time, the conduction band and valence band of MoS2 are pulled to the negative direction a little, and then form a slightly upward band bending close to the interface between MoS2 and graphene. Combining the decrease of the band gap of MoS2 in heterostructure, the potential of the conduction band minimum of MoS2 in heterostructure will increase to -0.31 eV, which enhances its reduction capacity. A detailed understanding of the microcosmic mechanisms of interface interaction and charge transfer in this system can be helpful in fabricating 2D heterostructure photocatalysts.
2017, 66 (8): 087201. doi: 10.7498/aps.66.087201
The simulations of the structure and behavior of the molecule in the simulation software are an effective way to analyze the microscopic mechanism associated with performance change of space charge trap in the polymer. To achieve this, in this paper we first present the polyethylene molecular model which is developed by using the simulation software Materials Studio (MS). Then, the microstructure and property of space charge trap are analyzed by the changes with the energy and the free volume in the polyethylene due to the chain segment motion under the universal force field (UFF), respectively. Some important findings are extracted from simulation results. First, in the process of the temperature gradually increasing from 298 K to 363 K, the phenomena of slippage and diffusion of the molecule due to the enhanced thermal motion of molecules are observed. These phenomena lead to the free volume increasing and the space charge trap energy level decreasing gradually, whose maximum value is 1542.073 and the minimum value is 0.66 eV when the temperature is 363 K. Second, when an electrostatic field of 0.0007 Hartree/Bohr is applied to the polymer, molecular chain segments are oriented by the Maxwell stress that is generated by the electric effect. Molecular chain segment orientations induce the van der Waals interaction energy to increase to -360.18 kcal/mol (1 kcal/mol = 4.18 kJ/mol), the free volume to decrease by 279.77 3, and the space charge trap energy level to decrease by 0.45 eV. Third, by comparing the cases of applying the temperature field and the electric field to the polyethylene, it is found that the electric field has stronger effect on charge trap. Specifically, the space charge trap energy level of the polyethylene associated with 0.0007 Hartree/Bohr electric field is reduced by 0.17 eV compared with that associated with the temperature of 363 K. Moreover, simulation results and measured results are compared with each other and they are well consistent. Finally, it is concluded that using electric effect and molecular thermodynamic movement is an very effective way to analyze the microscopic mechanism of changes with free volume and van der Waals interaction energy. This analysis confirms that molecular motion changes the microstructure of the polyethylene and generates charge traps. In addition, it confirms that the influence of the electric field on the polyethylene generates the lower level of space charge trap than the effect of the temperature field.
2017, 66 (8): 087301. doi: 10.7498/aps.66.087301
The electrical characteristics of Ni electrode-based metal-insulator-metal (MIM) capacitors have been investigated with atomic layer deposited ZrO2/SiO2/ZrO2 symmetric stacked-dielectrics. When the thickness of the stacked-dielectrics is fixed at 14 nm, the resulted capacitance density decreases from 13.1 fF/m2 to 9.3 fF/m2, and the dissipation factor is reduced from 0.025 to 0.02. By comparison of current-voltage (I-V) curves of different MIM capacitors, it is found that the leakage current density in the high voltage region decreases gradually with the increasing thickness of SiO2, and it does not exhibit clear change in the low voltage region. Meanwhile, the capacitors show different conduction behaviors under positive and negative biases with increasing the thickness of SiO2 from 0 to 2 nm. Under the positive bias, different I-V characteristics are demonstrated at high and low electric fields, respectively. However, a single I-V characteristic is dominant under the negative bias. Further, the conduction mechanisms of the capacitors are investigated under the electron bottom and top injection modes, respectively. It is found that the Poole-Frenkel emission and the trap-assisted tunneling are dominant in the high and low field regions, respectively, for the electron bottom injection; however, the trap-assisted tunneling is dominant in the whole field region for the electron top injection. These are attributed to the formation of a thin NiOx interfacial layer between the Ni bottom-electrode and the ZrO2 dielectric layer, as well as the existence of both deep and shallow level traps (0.9 and 2.3 eV) in the ZrO2 dielectric. Therefore, the level trap plays a key role in the electron conduction in the MIM capacitor under different electron injection modes and different electric fields.
Study on ultrafast dynamics of low-temperature grown GaAs by optical pump and terahertz probe spectroscopy
2017, 66 (8): 087801. doi: 10.7498/aps.66.087801
Low-temperature-grown GaAs (LT-GaAs) possesses high carrier mobility, fast charge trapping, high dark resistance, and large threshold breakdown voltage, which make LT-GaAs a fundamental material for fabricating the ultrafast photoconductive switch, high efficient terahertz emitter, and high sensitive terahertz detector. Although lots of researches have been done on the optical and optoelectrical properties of LT-GaAs, the ultrafast dynamics of the photoexcitation and the relaxation mechanism are still unclear at present, especially when the photocarrier density is close to or higher than the defect density in the LT-GaAs, the dispersion of photocarriers shows a complicated pump fluence dependence. With the development of THz science and technology, the terahertz spectroscopy has become a powerful spectroscopic method, and the advantages of this method are contact-free, highly sensitive to free carriers, and sub-picosecond time resolved. In this article, by employing optical pump and terahertz probe spectroscopy, we investigate the ultrafast carrier dynamics of photogenerated carriers in LT-GaAs. The results reveal that the LT-GaAs has an ultrafast carrier capture process in contrast with that in GaAs wafer. The photoconductivity in LT-GaAs increases linearly with pump fluence at low power, and the saturation can be reached when the pump fluence is higher than 54 J/cm2. It is also found that the fast process shows a typical relaxation time of a few ps contributed by the capture of defects in the LT-GaAs, which is strongly dependent on pump fluence: higher pump fluence shows longer relaxation time and larger carrier mobility. By employing Cole-Cole Drude model, we can reproduce the photoconductivity well. Our results reveal that photocarrier relaxation time is dominated by the carrier-carrier Coulomb interaction: under low carrier density, the carrier-carrier Coulomb interaction is too small to screen the impurity-carrier scattering, and impurity-carrier scattering plays an important role in the photocarrier relaxation process. On the other hand, under high pump fluence excitation, the carrier-carrier Coulomb interaction screens partially the impurity-carrier scattering, which leads to the reduction of impurity-carrier scattering rate. As a result, the photocarrier lifetime and mobility increase with increasing pump fluence. The experimental findings provide fundamental information for developing and designing an efficient THz emitter and detector.
INTERDISCIPLINARY PHYSICS AND RELATED AREAS OF SCIENCE AND TECHNOLOGY
2017, 66 (8): 088101. doi: 10.7498/aps.66.088101
CdTe is a promising material for fabricating high-efficient and low-cost thin film solar cell. To achieve high energy conversion efficiency, polycrystalline CdTe films must go through an annealing process in an atmosphere containing chlorine. Numerous researches of the mechanisms of chlorine treatment have been conducted. It is generally believed that chlorine treatment can increase the quantum efficiency of CdTe, cause CdTe grain to recrystallize, and reduce the defect density. In 2014 a research discovered that after chlorine treatment, Cl atoms are segregated at grain boundaries of CdTe and form p-n-p junction, which can separate electrons and holes, thus inhibiting the carrier recombination at grain boundaries. Another first-principle calculation research claimed that Cl atoms form VCd-ClTe complex, which is also named A-center, and provide extra shallow p-energy level to improve shallow p-doping of CdTe. It seems that both segregation and doping of Cl atoms can enhance cell performance.To test whether chlorine doping can contribute to the enhancement of cell performance, in this work we study chlorine doping in CdTe absorption layer by experiment. We deposit chlorine doped CdTe (CdTe:Cl) film by well controlling the chlorine concentration ((1005) ppm) to investigate the effects of Cl doping on device performance. In this work, we also compare the lattice structure and electrical properties of CdTe:Cl films with those of conventional Cl treated CdTe films.The CdTe:Cl film deposited at low temperatures consists of both cubic and hexagonal phases. CdTe:Cl film deposited at high temperature consists of only cubic phase with (111) orientation. Phase structure remains stable after annealing. Serried twins can be observed in all CdTe:Cl rods and the twins each contain only several atom layers. The ultra-thin twins can be found in both as-deposited CdTe:Cl and post-annealing CdTe:Cl. There is neither separate conduction channel of electrons nor that of holes in CdTe:Cl. But for chlorine treated CdTe, grain boundaries are the conduction channels of electrons and holes traveling within grains. The resistivity of the CdTe:Cl film is found to increase drastically, and carrier density reduces to intrinsic state after annealing. The efficiency of CdTe:Cl cell is lower than that of chlorine treated CdTe cell. It seems that non-balanced heavy chlorine doping by magnetron sputtering is bad to CdTe absorption layer.
Interface electronic structure and the Schottky barrier at Al-diamond interface: hybrid density functional theory HSE06 investigation
2017, 66 (8): 088102. doi: 10.7498/aps.66.088102
Diamond is regarded as one of the most promising semiconductor materials used for high power devices because of its superior physical and electrical properties, such as wide bandgap, high breakdown electric field, high mobility, and high thermal conductivity. Highpower diamond devices are now receiving much attention. In particular, Schottky diode based on a metal/diamond junction has promising applications, and high breakdown voltage has been achieved, though unfortunately its forward resistance is high. In this paper, the first principles calculations are performed to study the electronic structure of interface and the Schottky barrier height of Al-diamond interface. The projection of the density of states on the atomic orbitals of the interface atoms reveals that the typical Al-induced gap states are associated with a smooth density of states in the bulk diamond band gap region, and these gap states are found to be localized within three atom layers. At the same time, electronic charge transfer makes the Fermi level upgrade on the side of diamond. Besides, the typical Al-induced gap state model gives a simple picture about what determines Schottky barrier height at Al-diamond interface, by assuming an ideal, defect-free and laterally homogeneous Schottky interface in which the only interaction comes from the decay of the electron wave function from the metal into the semiconductor, which in turn induces electronic charges to be rearranged in the region close to the interface. As for the electronic charge transfer, this potential shift can be extracted by subtracting the superimposed planar or macroscopically averaged electrostatic potentials of the Al and diamond surfaces (at frozen atomic positions), from the planar or macroscopically averaged potential of the relaxed Al-diamond interface. The electronic charge transfer suggests that the formation of an interface should be associated with the formation of new chemical bonds and substantial rearrangements of the electron charge density. Especially, we obtain the Schottky barrier height of 1.03 by the first principle, which is in good agreement with the results from phenomenological model and experiment. The research results in this paper can provide a theoretical basis for the research of the metal diamond Schottky junction diode, and can also give a theoretical reference for the research of the metal-semiconductor highpower device based on diamond material.
Electrochemical batteries for wind and solar renewable energy storage have attracted world-wide attention, due to their merits of flexibility, modularity and being environmental friendly. Nowadays, new rechargeable battery systems are highly desired for large-scale electrical energy storage. Here in this paper, we report that alkali metal can be dissolved into aromatic compound-ether solvent to obtain a dark blue solution with high conductivity. This solution consists of alkali metal cation and radical anion generated by electron transfer reaction between alkali metal and aromatic compounds. For instance, sodium and biphenyl can be dissolved into 1,2-dimethoxyethane to obtain a dark blue solution which exhibits high electronic conductivity (8.410-3Scm-1), high ionic conductivity (3.6 10-3Scm-1), low potential 0.09 V vs. Na/Na+ and low cost. Using this solution as the anode, we demonstrate a new rechargeable battery with quinone liquid cathode. It is found that the battery with anthraquinone (AQ) liquid cathode displays long cycle ability, low cost properties. This work proposes a new strategy for designing the electrode materials and rechargeable battery systems. Furthermore, this kind of liquid may possess other unique physical properties and might be used in other devices, like thermoelectric battery.
Wavelet transform in the application of three-dimensional terahertz imaging for internal defect detection
2017, 66 (8): 088701. doi: 10.7498/aps.66.088701
Spatial resolution and spectral contrast are two major bottlenecks for non-destructive testing of complex samples with current imaging technologies. We use a three-dimensional terahertz (THz) imaging system to obtain the internal structure of the sample, and exploit the wavelet transform algorithm to improve the spatial resolution and the spectral contrast. With this method, the longitudinal resolution of terahertz imaging system can be improved to the wavelength comparable thickness, while the x-y plane resolution can be as high as 0.2 mm0.2 mm, which benefits from the point-to-point scanning on the x-y plane. In this three-dimensional terahertz imaging system, the Syn View Head 300 with light source/detector frequency of 0.3 THz is used for two-dimensional scanning (x-y direction) of the sample, and the linear frequency modulated continuous wave technique is used to obtain the reflected terahertz light intensity at different depths (z axis) of the sample. When the sample is thin, the upper and lower interface reflection peaks are difficult to distinguish due to broad peak width of the THz source. To solve this problem efficiently, continuous wavelet transform (CWT) is used. In recent years, CWT is applied widely because of its particular mathematical properties in the feature signal recognition. Since the Gaus2 wavelet basis is better to highlight the peak signal, we choose it for CWT. After CWT, one scale of the wavelet coefficients is chosen for three-dimensional data reconstruction, for which the widths of the reflection peaks are narrower and the noise signals are weaker. That means if we reconstruct the three-dimensional wavelet coefficient data on the chosen scale, the three-dimensional image of the tested sample will be enhanced. In order to demonstrate that, the three-dimensional images reconstructed by wavelet coefficients are compared with those by original data. The tested sample has holes inside with different depths. Based on the original three-dimensional THz image, it is hard to locate the top of 4 mm deep hole (1 mm deep photosensitive material plate), while the top of the inner 4 mm deep holes (the bottom of the 1 mm deep photosensitive material plate) can be distinctly located and the noises are greatly reduced based on the three-dimensional images reconstructed by wavelet coefficients. With this method, the longitudinal resolution of terahertz detection systems can be improved to 1 mm that is comparable to the wavelength, which demonstrates advantages of this method.
GEOPHYSICS, ASTRONOMY, AND ASTROPHYSICS
2017, 66 (8): 089201. doi: 10.7498/aps.66.089201
Mesoscale weather research and forecasting model with high resolution is used to investigate the landfall process of typhoon Mujigae (2015). The simulation well reproduces the path, intensity and rainfall of the typhoon, especially before and after the landfall. The fine thermal and dynamical structures of the typhoon Mujigae and its macroscopic characteristics of rain bands are examined with the simulation output. The rain band regions from the eyewall outward are composed of mixing rain band, secondary rain band, principal rain band and distant rain band. The lower-level inflow and upper-level outflow are observed in the eyewall. The maximum tangential wind, strong updraft and positive temperature anomaly are located in the eyewall and tilted outward with height. The convective systems in the eyewall with high radar reflectivity are much deeper than those in the principal rain band, secondary rain band and distant rain band.In order to analyze the vortex Rossby waves, the fast Fourier transform is performed to decompose the model output variables into perturbations with different wavenumbers. The vorticity perturbations in the wavenumbers 1 and 2 have significant features in the azimuthal and radial propagation. The amplitude of wavenumber 1 is larger than that of wavenumber 2, while the wavenumber 2 propagates much faster than the wavenumber 1 both in azimuthal and radial directions. The waves propagate with a speed less than 10 m/s, which are in consistent with the magnitudes of the radial velocities in spiral rain band. The amplitude of vortex Rossby waves decreases quickly beyond the stagnation radius which is about 90 km from the cyclone center. For the perturbations of wavenumbers 1 and 2, there are some intrinsic relations among the vertical vorticity, divergence and vertical velocity. The positive values of vertical vorticity with the two wavenumbers are associated with the strong reflectivity indicating deep convections. When the dipole patterns of positive vorticity in the upper level and negative vorticity in the lower level over the rainfall region are coupled with the pattern of divergence, the upper-level divergence and lower-level convergence are promoted. Then, updrafts are enhanced, which is favorable for the development of convective system and the increase of precipitation. On the other hand, the updrafts can be weakened in two cases: i) the vertical distribution of negative vorticity in the upper level and positive vorticity in the lower level is similar to the divergence distribution; ⅱ) the vertical distribution of vorticity is opposite to that of divergence. Consequently, the convective systems are inhibited and less rainfall is produced. The dynamical structures of vortex Rossby waves with wavenumbers 1 and 2 affect the development of deep convective system and precipitation in the typhoon Mujigae.
Investigation of the absolute detection method of atmospheric temperature based on solid cavity scanning Fabry-Perot interferometer
2017, 66 (8): 089202. doi: 10.7498/aps.66.089202
measurement methods based on Rayleigh scattering are employed to relatively detect atmospheric temperature profiles. That is to say, the definition of response functions and calibration procedures is required for temperature retrieval. Because the thermal motion rate of gas molecule complies with Maxwell distribution, and gas molecule is always in motion state, the frequency of scattering return signal generates Doppler spectral broadening. There is a positive correlation between the full width at half maximum of widened Doppler spectrum and T1/2, atmospheric absolute temperature can be obtained by measuring the Doppler spectrum shape. In this paper, the fine detection method of the spectrum shape of Rayleigh scattering and residuary Mie-scattering correction method based on solid cavity scanning Fabry-Perot (F-P) interferometer are investigated. According to the characteristics of Rayleigh scattering spectrum, the free spectral range, the geometric length of solid cavity, the type of cavity media, the full width at half maximum, the reflectivity of cavity, and the scanning step are designed. When the electro-optical crystal of KD*P with the length of 8.5 mm acts as solid cavity medium of scanning F-P interferometer, the designed free spectral region and 3 dB bandwidth are 11.5 GHz and 60 MHz at the central wavelength of 354.7 nm, respectively. The energy datum of 185 discrete points at Rayleigh scattering spectrum are obtained by using an optimized solid cavity scanning F-P interferometer with the scanning voltage of 23.5 V. A fitting spectrum is generated by employing polynomial interpolation method at the atmospheric temperature of 300 K. The maximum absolute error and full width at half maximum error of Rayleigh scattering spectrum are 22 MHz and 337 kHz, respectively. In order to verify the results, a numerical simulation of Rayleigh scattering spectrum based on standard atmosphere model and S6 model is performed. The detection uncertainty of atmospheric temperature is up to 0.8 K. As SNR (signal to noise ratio) is 10, the detection distance is 4.5 and 7.9 km at day-time and night-time, respectively. The research provides a new solution of filter system for the achievement of all-time, high-precision, and absolute detection of atmospheric temperature in the future. In meteorology, in order to investigate the temporal and spatial characteristics, the change rules and physical mechanism of weather processes, the temperature in the boundary layer of urban atmosphere is absolutely detected, where human activities are frequent and the changes of weather elements are obviously at day and night. In addition, the absolute detection method of atmospheric temperature can provide the valid means to research urban heat island, weather forecast for urban environment, and high temperature alert. In environmental studies, the absolute detection of atmospheric temperature can provide the big amount of scientific data for establishment of numerical model and research on air pollution diffusion. There is reference significance for the investigation of filter system of similar lidar. Simultaneously, the scanning filter method provides a feasible solution for the filter system with the characteristics of miniaturization, high anti-interference and high stability in the space-based platform.