## Vol. 65, No. 1 (2016)

##### 2016-01-05

## INVITED REVIEW

2016, 65 (1): 010701.
doi: 10.7498/aps.65.010701

Abstract +

With the continuous development of terahertz (THz) technology in recent years, many kinds of THz functional devices including switchers, filters, modulators, isolator and polarizers have been demonstrated. However, researches of the focusing devices in the terahertz frequency range are rarely reported. In this paper, we propose a subwavelength metal-air-InSb-metal periodic array structure to perform terahertz wave focusing. The dependence of permittivity of InSb in the THz regime on external magnetic field and temperature is calculated theoretically. Based on the magneto-optical effect of the semiconductor material InSb and asymmetrical waveguide structure, the influences of external magnetic field and temperature on the focusing and transmittance characteristics of the device are studied in detail. Numerically simulated results show that the structure proposed above can not only improve the transmittance greatly but also perform focusing perfectly. Calculations on the transmission properties show that in a certain range of temperature, the power flow transmittance at the focus point increases with the increase of temperature. In the meantime, for a certain temperature, with increasing the external magnetic field, the power flow continuously increases as well and reaches a maximum value at a certain magnetic field. For example, for a temperature of 172 K and a magnetic field of 0.6 T, the maximum power flow transmitted at the focus point is 10200 W/m2 at 0.8 THz, which is about 28 times larger than that without magnetic field at the same temperature. In addition, the simulation results also show that when the temperature and external magnetic field are fixed at 172 K and 0.5 T, respectively, the power flow transmittances for the incident waves at different frequencies are different. There is a peak value of the transmittance appearing at a specific frequency of 0.8 THz. Moreover, when the incident wave frequency is far from 0.8 THz, the transmittance decreases dramatically. It is worth noting that by choosing different temperatures and external magnetic fields, the structure proposed can not only enhance the transmittance over 20 times at the focus point, but also manipulate effectively the THz wave in a broad operating bandwidth of 400 GHz from 0.4 THz to 0.8 THz. These properties indicate that the proposed structure can act as an ideal tunable, broadband, and high transmittance focusing device in the terahertz regime.

## INVITED REVIEW

2016, 65 (1): 014207.
doi: 10.7498/aps.65.014207

Abstract +

Photonic microwave generation has attracted much attention in recent years due to its potential applications in various fields such as radio-over-fiber communication, signal processing and radar systems. So far, different photonic microwave generation schemes have been proposed and investigated, such as the optical heterodyne method based on the beat of two independent lasers with a certain wavelength difference, the external modulation method based on electro-optical modulator, the dual-mode beat method based on the monolithic dual-mode semiconductor lasers, and the optoelectronic microwave oscillator method based on optoelectronic feedback loops. These schemes have their own advantages and deficiencies. Unlike the above schemes, in this paper we propose an all optical scheme for generating high-quality microwave based on a 1550 nm vertical-cavity surface-emitting laser (1550 nm-VCSEL). For such a scheme, high frequency microwave can be obtained based on a 1550 nm-VCSEL subjected to external optical injection, where the polarization of the injected light is the same as that of the dominant mode of the free-running 1550 nm-VCSEL (named parallel-polarized optical injection) and its wavelength is adjusted to being close to the wavelength of the suppressed polarization mode of the free-running 1550 nm-VCSEL. With the aid of double optical feedback, the linewidth of the obtained microwave can be narrowed. In this work, firstly, the feasibility of microwave generation based on parallel-polarized optically injected 1550 nm-VCSEL is analyzed theoretically by using the spin-flip model. Next, a corresponding experimental system is constructed, and the performance of microwave generation is preliminarily investigated experimentally. The experimental results show that 30 GHz microwave signals could be obtained based on a parallel-polarized, optically injected 1550 nm-VCSEL under suitable injection parameters, but the linewidth of microwave signal is relatively wide (hundreds of MHz). Finally, after introducing double optical feedback, the linewidth of microwave signal can be reduced by more than two orders of magnitude and narrowed to less than 1 MHz, meanwhile the signal-noise ratio is larger than 40 dB. This work is helpful to develop relevant techniques to acquire high-performance narrow linewidth photonic microwave.

## INVITED REVIEW

2016, 65 (1): 014502.
doi: 10.7498/aps.65.014502

Abstract +

Granular materials are widely spread in nature and in industry. Owing to the inelastic collisions between particles and frictions among particles, granular systems are dissipative in nature. This intrinsic dissipative nature causes local clustering in granular gas systems. This is a unique phenomenon compared with the molecular gases. Understanding and predicting the condition and parameter values when this phenomenon happens will be helpful for us to gain knowledge of the conditions of clustering or pattern formations in non-equilibrium complex systems. The clustering phenomenon in granular gas is analyzed using phase-separation modeling of van der Waals-like molecules. The results from the model are verified by molecular dynamics numerical simulations. However, due to the influence of the gravity, experimental verification is difficult in laboratory. In this work, we perform an experiment in micro-gravity environment provided by the drop tower of National Microgravity Laboratory Chinese Academy of Science. In the experiment we for the first time observe the phase-separation clustering phenomenon. Comparing the observation condition with the model prediction, we are able to indirectly obtain the restitution coefficients of particles used in the experiment. A model calculation for the spinodal regime under experimental conditions is performed for possible particle restitution coefficients, and a comparison with the experimental observation allows us to justify the values of the restitution coefficients. It is found that the coefficient is larger for bigger particles. For d=2.5mm titanium particles, the restitution coefficient is higher than 0.8; for d=1mm titanium particles, the restitution coefficient is about 0.8, and for d=0.5mm titanium particles, the restitution coefficient is between 0.6 and 0.8. This useful result can be essential for comparing experimental observation with the theoretical and the numerical results, and is crucial to the success in the SJ-10 satellite experiments.

###### REVIEW

2016, 65 (1): 010702.
doi: 10.7498/aps.65.010702

Abstract +

The third-generation synchrotron radiation sources are widely used in physics, chemistry, material science, etc. due to their light beams with high brilliance and low emittance. In order to efficiently utilize such light beams for scientific research, reflective mirrors with excellent figure quality are required. The reflective mirrors on the beamlines of synchrotron radiation sources consist of fixed polished shape mirrors and bendable mirrors. Bendable mirrors have been attracting the attention of the synchrotron radiation community because their curvatures can be varied to realize different focusing properties. Classical bendable mirrors are realized by applying mechanical moment at the ends of the mirror substrates. In this paper, we introduce a new concept of bendable mirrors, X-ray adaptive mirrors which are based on the adaptive optics technology and the properties of piezoelectric bimorph systems. X-ray adaptive mirrors exhibit many advantages over the classical bendable mirrors, such as mechanics-free, figure local corrections, and good focusing properties. The piezoelectric bimorph mirrors have been used in astronomy to correct the wavefront distortions introduced by atmospheric turbulence in real time. The piezoelectric bimorph mirror was first introduced into the field of synchrotron radiation by European Synchrotron Radiation Facility (ESRF) in the 1990s for making an X-ray reflective mirror. Compared with astronomy community, synchrotron radiation community is not interested in high-speed wavefront correction, but looking for the ultimate precision of the surface shape of piezoelectric bimorph mirror.
In the second part of this paper, the usual structure and working principle are briefly described. Piezoelectric bimorph mirrors are laminated structures consisting of two strips of an active material such as zirconate lead titanate (PZT) and two faceplates of a reflecting material such as silicon. A discrete or continuous control electrode is located between the interfaces of PZT-PZT, while two continuous ground electrodes are located between the interfaces of Si-PZT. The PZTs that are polarized normally to their surface, any voltage applied across the bimorph results in a different change of the lateral dimensions of two PZTs, thereby leading to a bending of the whole structure. The relationship between the curvature of the bending mirror and voltage is given.
In the third part of this paper, the technical issues as well as the design concepts are discussed in detail. Several Si-PZT-PZT-Si bimorph mirrors are first fabricated and tested by ESRF. The dimensions of each of them are 150 mm in length, 4045 mm in width, and 1518 mm in thickness. PZT is selected as an active material because of its high coupling factor, high piezoelectric coefficient, and high Curie temperature. The faceplates need to be easy to polish such as silicon and silica. Owing to the symmetrical layered structure Si-PZT-PZT-Si, the mirror is less sensitive to temperature variations from the process of bonding and polishing. The bimorph mirrors are confirmed to be promising by experimental tests. As the state-of-art polishing technique, elastic emission machining (EEM) becomes available commercially, and diamond light source brings EEM into the bimorph mirror to achieve a novel adaptive X-ray mirror coupling adaptive zonal control with a super-smooth surface. This super-polished adaptive mirror becomes the first optics with a bendable ellipse with sub-nanometer figure error. Spring-8 fabricates an adaptive mirror with different structures, and two strips of PZTs are glued to the side faces of the mirror. This mirror shows a diffraction-limited performance.
Finally, the wavefront measuring methods and control algorithm are introduced. Wavefront measuring devices used in the metrology cleanroom include long trace profiler, nanometer optics component measuring machine, and interferometer. At-wavelength measuring methods used on the beamline include pencil-beam method, phase retrieval method, X-ray speckle tracking technique, and Hartmann test. The wavefront control algorithm is aimed at obtaining the voltages applied according to the inverse of the interaction matrix.

2016, 65 (1): 010704.
doi: 10.7498/aps.65.010704

Abstract +

The terahertz applications of bio-materials and energetic materials are hindered by the low power-intensity of the terahertz output and the narrow band of terahertz emission. So the crucial part of the development of terahertz time-domain spectroscopy (TDS) systems is the new terahertz source with broadband frequency range and high power output. As to the free-space TDS system, the system is necessarily purged by dried nitrogen gas to remove the absorbed water vapor. In addition, the low detection sensitivity also exists because of the free-space interactions between the terahertz emission and the substances. To address these problems, terahertz lab on-chip system is proposed. The local field effect in the nano-structures of on-chip system can contribute to the detection of low concentration of the substance. The present paper is composed of two sections. Firstly, a new terahertz source based on the metal nano-film can produce an intense and broad-band terahertz-infrared emission, which is comprised of incoherent terahertz-infrared signals and coherent terahertz signals. This emission can cover more than 100 THz and has an output power of up to 10 mW. This optical phenomenon mainly arises from the incoherent thermal radiation effect. Secondly, the terahertz lab on-chip systems with different transmission lines and different substrates are clarified. There exists lower loss on the on-chip system with coplanar stripline structure and copolymer substrate. High sensitivity of biological detection in terahertz band of up to 2 THz can be achieved by using this system.

2016, 65 (1): 018102.
doi: 10.7498/aps.65.018102

Abstract +

Recently, two-dimensional (2D) layered molybdenum disulfide (MoS2) has attracted great attention because of its graphene-like structure and unique physical and chemical properties. In this paper, physical structure, band gap structure, and optical properties of MoS2 are summarized. MoS2 is semiconducting and composed of covalently bonded sheets held together by weak van der Waals force. In each MoS2 layer, a layer of molybdenum (Mo) atoms is sandwiched between two layers of sulfur (S) atoms. There are three types of MoS2 compounds, including 1T MoS2, 2H MoS2, and 3R MoS2. As the number of layers decreases, the bad gap becomes larger. The bad gap transforms from indirect to direct as MoS2 is thinned to a monolayer. Changes of band gap show a great potential in photoelectron. Preparation methods of 2D MoS2 are reviewed, including growth methods and exfoliation methods. Ammonium thiomolybdate (NH4)2MoS4, elemental molybdenum Mo and molybdenum trioxide MoO3 are used to synthesize 2D MoS2 by growth methods. (NH4)2MoS4 is dissolved in a solution and then coated on a substrate. (NH4)2MoS4 is decomposed into MoS2 after annealing at a high temperature. Mo is evaporated onto a substrate, and then sulfurized into MoS2. MoO3 is most used to synthesize MoS2 on different substrates by a chemical vapor deposition or plasma-enhanced chemical vapor deposition. Other precursors like Mo(CO)6, MoS2 and MoCl5 are also used for MoS2 growth. For the graphene-like structure, monolayer MoS2 can be exfoliated from bulk MoS2. Exfoliation methods include micromechanical exfoliation, liquid exfoliation, lithium-based intercalation and electrochemistry lithium-based intercalation. For micromechanical exfoliation, the efficiency is low and the sizes of MoS2 flakes are small. For liquid exfoliation, it is convenient for operation to obtain mass production, but the concentration of monolayer MoS2 is low. For lithium-based intercalation, the yield of monolayer MoS2 is high while it takes a long time and makes 2H MoS2 transform to 1T MoS2 in this process. For electrochemistry lithium-based intercalation, this method saves more time and achieves higher monolayer MoS2 yield, and annealing makes 1T MoS2 back to 2H MoS2. The applications of 2D MoS2 in field-effect transistors, sensors and memory are discussed. On-off ratio field effect transistor based on MoS2 has field-effect mobility of several hundred cm2V-1-1 and on/off ratio of 108 theoretically.

###### GENERAL

2016, 65 (1): 010501.
doi: 10.7498/aps.65.010501

Abstract +

The void fraction wave is a special physical phenomenon in a gas-liquid two-phase flow system. Understanding the propagation of the void fraction wave is of great significance for uncovering the physical mechanisms in both flow pattern transition and the fluid velocity measurement. In this study, detrended cross-correlation analysis (DCCA) is used to investigate the multi-scale cross-correlation characteristics of the coupled ARFIMA processes. It is found that the DCCA can effectively reveal the multi-scale cross-correlation dynamical behaviors of complex system. Then, we carry out the experimental test in a vertical gas-liquid two-phase flow pipe with small inner diameter. The DCCA is used to detect the cross-correlation characteristics of the void fraction wave on multiple time scales, and the growth rate of the cross-correlation level for the void fraction wave is observed on low time scales. Additionally, the spatial attenuation factor (SAF) of the void fraction wave is calculated to investigate the instability of the wave propagation. The SAF is close to zero under the transitional flow patterns, which means that the void fraction wave is in a stable propagating state. For bubble flows, the void fraction wave presents the attenuation characteristics, whilst the void fraction wave shows the amplification characteristics under the slug and churn flow patterns. Interestingly, the instability behaviors of the void fraction wave are always associated with its multi-scale cross-correlation characteristics. Specifically, the increasing rate of the wave cross-correlation level on low scales is much higher for transitional flow patterns, which is corresponding to the stable propagating characteristic of the void fraction wave. However, when the void fraction wave exhibits attenuation or amplification characteristics under other flow patterns, the increasing rate of the wave cross-correlation level on low scales is much lower.

2016, 65 (1): 010502.
doi: 10.7498/aps.65.010502

Abstract +

Recently, the research on traffic flow system based on some classical models, such as cellular automata and car-following models, has attracted much attention. Some meaningful achievements have been obtained in the past few years by scholars from various fields. This paper starts with literature review on traffic flow theory studies. Car-following models, including the initial model proposed by Newell in 1961 (Newell G F 1961 Oper. Res. 9 209) and some later modified ones (e.g. full velocity difference model, or FVD model for short) have been deeply investigated. Based on Newell's car-following model, an extension of car-following model with consideration of vehicle-to-vehicle (V2V) communication is then developed. The vehicle-to-vehicle communication technology, which was proposed in the early 2000s, enable vehicles to collect traffic condition information from other vehicles (e.g. speed, headway, position, acceleration, etc.) and provide them for drivers in almost real time. Compared with those without V2V devices, drivers with information from V2V devices can react to traffic flow fluctuation timelier and more precisely. To represent the pre-reaction of drivers to traffic flow information provided by V2V devices, a parameter, , is newly introduced into Newell's car-following model. Then by second-order Taylor series expansion, a new car-following model with the influence of V2V (called V2V model) is proposed. Neutral stability condition of V2V model as well as phase diagram is derived theoretically with linear analysis method. The phase diagram of linear stability condition is divided into stable and unstable regions. By analyzing stability performance of the proposed model, it is evident that V2V communication technology can improve the stability of traffic flow system. Numerical simulation is demonstrated to study the influence of V2V devices on traffic flow on the one hand, and to acquire density waves as well as hysteresis loops under different values of parameter on the other hand. The sensitive analysis method are adopted as well.The numerical simulation results indicate that: 1) when compared with FVD model, V2V model can make vehicles react to traffic flow fluctuation earlier and reduce the speed changes under start-up, brake and incident conditions; this indicates that the consideration of V2V devices can improve the safety and ride comfort of traffic flow system; 2) the V2V model is sensitive to the value changes of parameter and T; the stability of traffic flow can be improved if the value of parameter increases, or parameter T decreases; this outcome precisely agrees with the above theoretical analysis; 3) the characteristics of traffic flow can influence the performance of V2V technology: compared with under low density condition, V2V communication technology can significantly increase the average speed of traffic flow under high density condition.

2016, 65 (1): 010703.
doi: 10.7498/aps.65.010703

Abstract +

X-ray communication, which was first introduced by Keith Gendreau in 2007, is potential to compete with conventional communication methods, such as microware and laser communication, against space surroundings. Researchers have spent much time and effort on the mission making the initial idea into reality in recent years. Eventually, the X-ray communication demonstration system based on the grid-controlled X-ray source and single-photon detection technique can deliver both audio and video information in a 6-meter vacuum tunnel, and the bit-error-rate performance of the communication system is analyzed. But it is difficult to implement applications in industries. The point is to find a way to reduce the signal divergence geometrical attenuation and increase the distance of the communication which can be regarded as an important foundation of future deep-space X-ray communication applications. Therefore, it is urgent to study the X-ray communication system. By using a nested X-ray focusing optics as transmitting and receiving antennas of the communication system, the signal gain and the distance of X-ray communication can be greatly improved. Specifically, the nested X-ray focusing optics is similar to the Wolter type I telescope, which is widely used in the field of X-ray astronomy. The difference between them is that the Wolter type I optics is originally proposed based on a paraboloid mirror and a hyperboloid mirror, but X-ray focusing optics, the simplified Wolter type I optics, provides a single reflection by a conical approximation mirror, and it is more suitable for X-ray communication. In this paper, aiming at the future demand of X-ray communication, the optimization and analysis of the nested X-ray focusing optics are carried out, and the recurrence relations between the layers of mirrors are derived. Reasonable initial structural parameters and structure of the optics are designed. In addition, the theoretical effective collection area is calculated. Feasibility of using the X-ray focusing optics as transmitting and receiving antennas is analyzed, and the theory and structural design of the X-ray focusing optical are discussed. Signal divergence of transmitting antenna, effective area of receiving antenna, the focal spot size, and the signal gain properties are preliminary studied. The results show that the signal divergence is about 3 mrad, and the transmit gain is 23 dB; the effective area of receiving antenna is 5700 mm2 at 1.5 keV. Moreover, the focal spot diameter and the receive gain are 4.5 mm and 25 dB, respectively, and the total gain of this communication system can reach up to 48 dB.

###### ELECTROMAGENTISM, OPTICS, ACOUSTICS, HEAT TRANSFER, CLASSICAL MECHANICS, AND FLUID DYNAMICS

2016, 65 (1): 014101.
doi: 10.7498/aps.65.014101

Abstract +

The research on a Smith-Purcell device becomes active since it holds promise in developing a high power, tunable, and compact terahertz radiation source. In this paper, a dielectric loaded grating for Smith-Purcell device is proposed. By investigating the interaction between the sheet electron beam and surface wave above the grating, the dispersion equation with electron beam is derived, in which the electron beam has a finite thickness. And then the growth rate of the beam-wave interaction is numerically calculated from the dispersion equation. In addition, the current threshold for oscillators, known as a start current, is carefully estimated from the dispersion equation by considering the boundary conditions of electromagnetic field. The effects of structure length, electron beam parameters and dielectric constant on start current are analyzed at length. The results reveal that the start current decreases as the structure length increases. This is because as the structure length becomes greater, the distance of the beam-wave interaction becomes longer, which can strengthen the beam-wave interaction. And with increasing beam thickness and beam-grating distance, the start current increases. Because the electric field of the surface wave decreases exponentially with the increase of distance from the grating, the electron beam far from the grating cannot be bunched by the field, which makes it harder for Smith-Purcell device to oscillate. However, as the beam voltage becomes greater, the start current decreases first quickly and then slightly. Compared with the case of metal grating, it can be seen that the use of dielectric can improve the growth rate and reduce the start current for the operation of a Smith-Purcell backward wave oscillator. The start current decreases quickly when the dielectric constant is greater than 1. Then it increases slightly when dielectric constant is between 2 and 3, and finally the start current continues to decrease. But it cannot be helpful to choose a very big value of dielectric in order to obtain a low start current, because the operation frequency decreases as dielectric constant increases. It is more appropriate to choose a dielectric constant in a required frequency range. The predictions of our theory and the results from the particle-in-cell simulation are consistent with each other, which verifies the validity and accuracy of the theory in this paper.

2016, 65 (1): 014201.
doi: 10.7498/aps.65.014201

Abstract +

There are intrinsic phase errors in swept source optical coherence tomography (SS-OCT), which severely influences the functional imaging. To overcome this difficulty, a numerical correction method is presented in this paper to correct the phase artifacts due to wavenumber shift among the spectral interferograms, resulting from the random delay variance between the sampling trigger and the clock of the swept source laser. This correction method is based on the linear relationship of phase difference to the depth of the sample and the wavenumber shift. The detailed procedure to eliminate the phase artifacts is as follows. Firstly, we figure out the complex OCT signals through inverse Fourier transform of the initial interferograms. Then we fit the upper surface of the sample with the intensity information of the B-scan by setting a floating threshold. After that the wavenumber shifts of each A-line are determined by two steps with the phase information of the sample surface: the relative wavenumber shifts between adjacent A-lines are first obtained according to the phase difference between the adjacent A-lines, the signal depth, and the linear relationship mentioned above; then we figure out the absolute wavenumber shifts between each A-line and the first A-line of the B-scan by an iteration algorithm. With the information about the wavenumber shift, we align the initial interferograms, and obtain the corrected complex signal through re-inverse Fourier transform of the aligned interferograms. This method introduces no extra noise, realizing phase measurement limited by the signal-to-noise ratio. It is noted that we take the average phase information of several axial pixels near the sample surface to diminish the noise influence when calculating the wavenumber shifts. Besides, this corrected algorithm acquires oversampling along the scanning direction to ensure the signal correlation between adjacent A-lines.
The SS-OCT system in the paper is set up with a vertical cavity surface emitting laser with a center wavelength of 1297 nm. The system measurement range is 12 mm in lateral direction, the axial resolution is 17 m, and the lateral resolution is 24 m. And the feasibility of this method is verified by Doppler imaging of a mirror, an infra-red detection card and the cerebral cortex of a mouse.

2016, 65 (1): 014202.
doi: 10.7498/aps.65.014202

Abstract +

The polarization singularities in vector wavefields have been extensively studied analytically and experimentally. The polarization singularities can be analyzed by using electromagnetic theory or Stokes parameters, or be described in terms of complex Stokes scalar fields. In some practical applications, partially coherent beams have more advantages than fully coherent beams. Recently, the concept of the polarization singularities has been extended from fully coherent beams to partially coherent beams. In this paper, using the representation of cross-spectral density matrix propagation, the explicit propagation expressions for the partially coherent edge dislocation beams are derived in free space, and based on the spectral Stokes parameters the spectral singularities are studied in detail. It is shown that there exist spectral s12, s23 and s31 singularities of partially coherent edge dislocation beams in free-space propagation. s12 singularities correspond to circular polarization (C-points) of the partially coherent edge dislocation beams, and s30 (s30) means right-(left-) handedness, where the orientations of the major and minor axes of the polarization ellipse become undefined. s23 and s31 singularities must be located on L-lines, where the handedness of the polarization ellipse is undetermined (linear polarization). The motion, creation and annihilation of spectral Stokes singularities may appear in the variation of a controlling parameter, such as off-axis distance, slope of edge dislocation, spatial correlation length, or in the variation of the propagation distance. By suitably varying the spatial correlation length or propagation distance the V-point, the handedness reversal of C-point, creation and annihilation for a pair of oppositely charged spectral singularities take place. The creation and annihilation occur for a pair of s12 singularities with opposite topological charge but same handedness. The critical points of the controlling parameters and propagation distance, at which pairs of different spectral singularities annihilate, are not the same. The collision of the C-point and L-line results in a V-point (vector singularity), which is unstable. A small perturbation leads to the handedness reversal. At such a point the state of polarization is undetermined and the degree of polarization P=0. The results obtained in this paper would be useful for a deep understanding of polarization singularities of stochastic electromagnetic beams.

2016, 65 (1): 014203.
doi: 10.7498/aps.65.014203

Abstract +

To simulate the atmospheric polarization pattern for small solar elevation angle, we develop a the vector radiative transfer model VSPART (vector pseudo-spherical radiative transfer model considering refraction), and use it to calculate the polarization state of downwelling diffuse light. In this model, the propagation trajectory, transmittance rate and polarization states of directly transmitted light are tracked by ray-tracing method for spherical refractive atmosphere. Based on the matrix algorithm, an improved method to solve the radiative transfer equation is proposed. Output of this model includes not only the Stokes vector and degree of polarization of diffuse light, but also the polarized irradiance. The precision of VSPART is validated against the benchmark results, literature results and SPDISORT, and excellent agreement is achieved. DOP (degree of polarization) and AOP (angle of polarization) are simulated for pure Rayleigh scattering atmosphere and atmosphere with aerosol, and the characteristics of their angular distributions are analyzed. In addition, the influences of atmospheric spherical geometry and refraction effect on the sky DOP are discussed as well. Simulation results show that for low solar elevation angle, with the increasing of wavelength, DOP increases gradually, and the Arago and Babinet neutral points move towards the horizon when Rayleigh scattering atmosphere is considered. Although the existence of aerosol does not change the basic distribution of DOP, it has a significant influence on AOP. With the increasing of aerosol optical depth, DOP decreases gradually, and the distribution of AOP changes dramatically. By comparing the sky distribution of DOP, it could also be concluded that the neutral points might arise from low order scattering. The area affected by atmospheric spherical geometry and atmospheric refraction effect mainly includes the area near horizontal directions, the area near the neutral points and the area perpendicular to the ground. For pure Rayleigh scattering atmosphere, the influence is reduced with the increasing of the wavelength of incident light, especially for the areas near the neutral points, where the influence gradually disappears as wavelength increases. For atmosphere with aerosol, with their optical depth increasing, the effects of atmospheric spherical geometry and atmospheric refraction are gradually enhanced.

2016, 65 (1): 014204.
doi: 10.7498/aps.65.014204

Abstract +

Ptychography is a new kind of lens-less imaging technology. What restricts the technique is the assumption of a multiplicative interaction between the illuminating coherent beam and the specimen, i.e., and the ptychography cannot be applied to samples no thicker than a few tens of micrometers in the case of visible-light imaging at micron-scale resolution. In the present work, we split a sample into axial sections, thereby realize three-dimensional ptychographic imaging of thick samples at the millimeter level in a series of computer simulations and optical experiments. Our simulation results reveal that by using single wavelength we cannot achieve good-quality images of thick samples. Thus it is necessary to introduce more wavelengths for illumination. With increasing the number of wavelengths, the imaging quality of three-dimensional thick samples can be enhanced continually. Then we make further study on the relationship between the imaging quality and the magnitude of wavelength in optical experiments by using two groups of samples with different thickness values. The results demonstrate that our experimental results are highly consistent with simulations. For our concrete configuration in this paper, the best results of imaging and separation may be obtained for the case of tri-wavelength. At the same time we make a reasonable explanation for the phenomenon of fold-over in the experiment. Our results are important and meaningful for the practical utilizing of three-dimensional ptychography of thick samples.

2016, 65 (1): 014205.
doi: 10.7498/aps.65.014205

Abstract +

Electromagnetically induced transparency (EIT) effect is an effective means to store light field into the atom ensemble. The extra noise introduced in the stored procedure can be suppressed greatly under the condition of large one-photon detuning and proper two-photon detuning. In this paper, we experimentally investigate the slow light and light storage in 87Rb vapor by using EIT effect, and study the effects of the two-photon detuning on light pulse delay and light memory at 650 MHz one-photon red detuning. In order to avoid some unwanted effects under the high optical depth condition, such as four-wave mixing, etc., the temperature of the atomic cell is controlled at 65 degrees Celsius. The experimental results show that the delay and the retrieval signals are significant in a two-photon detuning range from 0 to 0.5 MHz. The pulse delay decreases with the increase of two-photon detuning. The delay is 0.36 ups at two-photon resonance, and it is 0.07 ups at 1 MHz two-photon detuning. We simulate the delayed light pulse by using a three-level -type EIT model. The shapes of the measured slow light are in agreement with the theoretical results. The retrieval signals are observed at different two-photon detunings. The shapes of the retrieval pulses change with the two-photon detuning. The shape variations of the retrieval pulses cannot be explained by the three-level EIT theoretical model. By considering the atomic Zeeman sublevels interacting with the left-circular and right-circular polarized components of probe and coupling fields, multiple -type EIT systems will be formed. The interference between the retrieval signals from multiple EIT subsystems causes the shape distortions of retrieval pulses. The retrieval efficiency is measured as a function of two-photon detuning. The retrieval efficiency oscillates, and multiple peaks appear with the increase of two-photon detuning. The first peak appears at two-photon resonance, and the second peak appears at 0.48 MHz two-photon detuning. Finally, we measure the retrieval efficiency as a function of the coupling power at 0.48 MHz two-photon detuning. The optimal retrieval efficiency reaches 25% when the coupling power is 100 mW. These results provide experimental reference for the quantum memory of continuous variables in the hot atom ensemble.

2016, 65 (1): 014206.
doi: 10.7498/aps.65.014206

Abstract +

In an adaptive optics system, proportion-integration-differentiation (PID) controller is widely used for correcting wave front, but the controller is strictly dependent on the response model of deformable mirror. In this paper, a novel wave front correction method is proposed. The method, combining fuzzy control and PID control, does not depend on the response model of the deformable mirror. Based on rapid wave front reconstruction, the wave front evaluation indexes, extracted from the reconstructed wave front, are employed for the input of fuzzy controller and PID controller. Thus, the model response matrix of deformable mirror is not required. Each actuator of deformable mirror corresponds to an independent fuzzy PID controller. By designing the fuzzy controller, including fuzzy rule base selection and fuzzy reasoning, the three parameters of PID controller, the proportional kp, the integral ki and the differential kd, are adjusted automatically. A high rapid DSP hardware platform is constructed to verify the method. Test results show that the method can be used to correct the diffraction limit multiplication factor of the light spot from 10-12 to 3-4, which is basically the same as the traditional PID control, but its stability is better. Because the model does not need to calibrate the deformable mirror, the installation of the deformable mirror is easier.

2016, 65 (1): 014208.
doi: 10.7498/aps.65.014208

Abstract +

In this paper, the propagation of a few-cycle femtosecond pulse in a nonlinear Kerr medium is studied by the method of time-transformation. The time-transformation approach can greatly improve the computational efficiency. Because the width of electric field of the few-cycle femtosecond pulse is less than the characteristic time of Raman response in a nonlinear medium, it is observed that the electric field of the pulse experiences a significant deformation and breaks into a Raman soliton and the dispersion waves during the propagation, which can be attributed to strongly nonlocal nonlinearity. A deeper investigation of the time-frequency distributions for both the Raman soliton and the dispersion waves is also included. Since the pulse contains only few cycles, the carrier-envelope phase (CEP) of the pulse plays an important role in the process of nonlinear propagation. The numerical results show the CEP-dependence in the process of nonlinear propagation: the phase changes for both the Raman soliton and the dispersive waves are just equal to the CEP change of the initial pulse, which indicates that the CEP of the pulse is linearly transmitted in the process of nonlinear propagation. This phenomenon can be attributed to the fact that the phase change due to the nonlinearity is only dependent on the intensities of the fields of both the Raman soliton and the dispersion wave, which are unchanged for all the CEPs.

2016, 65 (1): 014209.
doi: 10.7498/aps.65.014209

Abstract +

Optical parametric frequency up-conversion and amplification, used for ultra-weak infrared image detection, is investigated both numerically and experimentally. A circum-circle model of spatial filtering is proposed to suppress the parametric fluorescence background noise. Combining the spatial filter, image relay module and band-pass filter, the fluorescence noise is suppressed by 70%. Accordingly, the image quality is significantly improved and the peak signal-to-noise ratio is increased by 22%. Based on the 10 Hz, mJ-level, 355 nm picosecond pump laser, the weak infrared image is amplified and up-converted into the visible region, and over 1.3108 (82 dB) optical gain is obtained at the same time. Our experiment demonstrates that the detection sensitivity of the conventional uncooled CCD can be as high as 7.4 photons per pixel when the high gain OPA is used as an image preamplifier. The scheme in this paper is expected to be used in single-photon-level sensitive infrared imaging.

2016, 65 (1): 014210.
doi: 10.7498/aps.65.014210

Abstract +

The problem of detecting the biological aerosol at low elevation angle is analyzed and discussed by using MODTRAN model. First of all, the atmospheric model and profile of MODTRAN model are introduced for the biological aerosol detection by Fourier transform infrared (FTIR) spectroscopy. According to the passive detection of biological aerosol by FTIR spectroscopy, the radiation brightness difference L between the background and the target biological aerosol is calculated by using the radiative transfer theory and the simplest three-layer model. The signal value 2Lt under the actual circumstance is derived from the derivative of L combined with the noise equivalent spectral radiance value of the spectrometer. Finally, the detection limit of biological aerosol for each atmospheric mode is predicted with the passive remote sensing method. The limit concentration of detection for each atmospheric mode is different due to the differences in boundary layer temperature, transmittance, and background radiation brightness of atmospheric model, and it is also related to the absorption coefficient of biological aerosol. It is shown that the passive remote sensing of FTIR technology can detect the presence of the biological aerosol. Therefore, the detection of the biological aerosol is feasible. It presents a method of detecting the biological aerosol cloud under the actual circumstance.

2016, 65 (1): 014211.
doi: 10.7498/aps.65.014211

Abstract +

In order to obtain new-type laser crystals, SrY2O4 is chosen as a host material. Because Y3+ ions in SrY2O4 occupy two non-equivalent sites, it might be possible to realize dual-wave laser and broadband emission at 1.06 m by partially replacing Y3+ with Nd3+. In this work, (3 at.%) Nd3+ doped SrY2O4 phosphor is synthesized by the conventional solid state reaction. The structure and luminescence properties in the visible and near-infrared ranges are studied. The peaks in the X-ray powder diffraction pattern of (3 at.%) Nd3+:SrY2O4 can be well indexed according to ICSD#25701. The lattice parameters, atomic coordinates, atomic temperature factors etc., are obtained by the Rietveld refinement with R_p of 4.68% and R_wp of 5.91%. According to the excitation spectra in a range of 220-380 nm, it can be seen that Nd3+:SrY2O4 is efficiently excited by 353 nm which is assigned to the 4I9/24D3/2+4D5/2+2I11/2+4D1/2 transition of Nd3+ ions. Under the 353 nm light excitation, Nd3+:SrY2O4 exhibits the strongest emission at 419 nm corresponding to the 2D15/24I9/2 transition of Nd3+ ions. What is more, Nd3+:SrY2O4 can be excited effectively by 824 nm light, which matches well with the commercial 830 nm diode laser. When excited with 824 nm, the strongest fluorescence peak is located at 1083 nm with a wide bandwidth of about 90 nm. Compared with that at 8~K, the bandwidth in the fluorescence spectrum at 300 K is broadened because of the homogeneous broadening induced by the increase of temperature. Additionally, the peaks corresponding to the 4F3/24I11/2 transition are split into two groups at 8~K, which results from the two non-equivalent sites of Nd3+ ions. Compared with Nd3+:YAG, Nd3+:SrY2O4 has more potential applications in the tunable and ultrashort lasers. The fluorescence lifetime of the 4F3/24I11/2 transition of (3 at.%) Nd3+:SrY2O4 is 281.7 s, which shows slight concentration quenching compared with that of (0.5 at.%) Nd3+:SrY2O4. The fluorescence lifetime of (3 at.%) Nd3+:SrY2O4 is much longer than that of (0.6 at.%) Nd3+:YAG which is beneficial to the energy storage. In conclusion, the wide emission band and the long decay time of 1.08 m indicate that Nd3+:SrY2O4 is a very promising new-wavelength and ultrashort laser material pumped by laser diode.

2016, 65 (1): 014212.
doi: 10.7498/aps.65.014212

Abstract +

The binding energies, electronic structures and optical properties of LiNbO3 and Cu/Fe doped LiNbO3 crystals are investigated by first principles based on the density functional theory in this paper. The supersell structures of crystals are established each with 60 atoms, including five models: pure LiNbO3, LN1 (Cu2+ occupy Li+ site), LN2 (Fe3+ occupy Li+ site), LN3 (Cu2+ occupy Li+site and Fe3+ occupy Li+ site) and LN4 (Cu2+ occupy Li+ site and Fe3+ occupy Nb5+ site). The optimized results show that the total energies of all models can achieve certain stable values, which means that the models accord with the actual crystal structures. The impurity energy levels of Cu and Fe doped LiNbO3 crystals appear within the band gaps, which are contributed by Cu 3d orbital, Fe 3d orbital and O 2p orbital; in co-doped LiNbO3, Cu offers deep energy level and Fe offers shallow energy level within the band gaps. There are two wide absorption peaks appearing respectively at 445 nm and 630 nm in co-doped LiNbO3 crystal, which correspond to the electron transitions from Eg orbital of Cu to Nb 4d orbital and T2g orbital of Fe to Nb 4d orbital respectively; the absorption edge of Cu, Fe mono and co-doped LiNbO3 crystals are red-shift successively, which coincides with the variation of band gape. The light absorption intensity of co-doped LiNbO3 crystal is stronger than that of mono-doped LiNbO3 crystal. The co-doped sample light absorption property is related to Fe site occupation. In this paper, it is suggested that the co-doped sample with Fe at Nb site is more competitive than that with Fe at Li site in optical volume holographic storage applications, and that reducing properly [Fe2+]/[Fe3+] value may be conducible to the formation of this advantage.

2016, 65 (1): 014213.
doi: 10.7498/aps.65.014213

Abstract +

In a zero index material, the phase velocity of light is much greater than the speed of light in vacuum and can even approach to infinity. Thus, the phase of light throughout a piece of zero-index material is essentially a constant. The zero index material has recently been used in many areas due to its extraordinary optical properties, including beam collimation, cloaking and phase matching in nonlinear optics. However, most of zero index materials usually have narrow operating bandwidths and the operating frequencies are not tunable. In this work, the model of tunable near-zero index photonic crystal is established by using colloidal magnetic fluid. Magnetic fluid, as a kind of easy-made mature nanoscale magnetic material, has proved to be an excellent candidate for fabricating self-assembled photonic crystal, especially the band-tunable photonic crystal with fast and reversible response to external magnetic field. The band structure can be calculated using the plane wave expansion method. For TE mode, it can be seen that a triply-degenerate point (normalized frequency f=0.734) at point under external magnetic field H=147 Oe, forms a Dirac-like point in the band structure, which is called an accidental-degeneracy-induced Dirac-like point. The effective permittivity eff and permeability eff are calculated using an expanded effective medium theory based on the Mie scattering theory. The calculated results show that both eff and eff are equal to zero at Dirac-like point, which means that the effective index neff is zero and the effective impedance Zeff is 1. The lattice structure of such a self-assembled photonic crystal will change with the external magnetic field, leading to the disappearance of Dirac-like point. However, when 143.6 OeH 152.4 Oe (1 Oe=79.5775 A/m), |neff | can keep less than 0.05 under the condition of Zeff = 1. Correspondingly, the operating frequency will change from 0.75 to 0.716. The model is verified by the numerical simulations (COMSOL Multiphysics) and the theoretical results agree well with the numerical ones.

2016, 65 (1): 014214.
doi: 10.7498/aps.65.014214

Abstract +

Based on the idea of multiple photonic bandgap (PBG) overlapping for a one-dimensional photonic crystal heterostructure, a novel hybrid quasiperiodic heterostructure is proposed to enlarge the omnidirectional photonic bandgap (OPBG). The heterostructure is formed by combining Fibonacci and Thue-Morse quasiperiodic structure. The results show that the OPBG of the heterostructure is enlarged obviously, which increases about three times compared with that of Fibonacci quasiperiodic structure, and twelve times compared with that of Thue-Morse quasiperiodic structure. The influences of structural parameters, such as period number and generation number, on PBGs of Fibonacci and Thue-Morse quasiperiodic structure are studied respectively. The results show that the parameters have little effects on PBG widths of the two quasiperiodic structures. The influences of the refractive indexes and thickness values of the high and low refractive index materials on OPBG of the heterostructure are also investigated. The results show that the OPBG of the heterostructure can be further broadened by increasing the refractive index ratios and thickness values of the high and low refractive index materials. The reason why the quasiperiodic structure can easily realize the multiple band gap overlapping is analyzed by comparing the bandgap properties of periodic structure. The number of PBGs of the quasiperiodic structure in the same wavelength range is more than that of the periodic structure. Moreover, with the increase of generation number of the quasiperiodic structure, due to the occurrence of PBG split, the number of PBGs increases obviously, and each PBG width is less than that of the periodic structure. Owing to this kind of PBG characteristic of the quasiperiodic structure, the heterostructure formed by cascading the two quasiperiodic structures is more prone to realizing the multiple PBG overlapping than other heterostructures, thus more easily achieving the expansion of OPBG. These results lay the design foundation for the compensation and broadening of the more complex bandgap structure.

2016, 65 (1): 014215.
doi: 10.7498/aps.65.014215

Abstract +

The Talbot effect is a self-imaging phenomenon of near-field diffraction. When a plane wave is incident on a periodic diffraction grating, the image of the grating is repeated at regular distances away from the grating plane. A Talbot array illuminator is a device that splits singular light beam into an array of beams with periodical optical intensity based on Talbot effect. LiNbO3 (LN) crystal is a kind of practicable material for a Talbot array illuminator due to its perfect optical characteristics. MgO-doped LiNbO3 (MgLN) crystal shows shorter absorption edge wavelength and higher resistance to photorefractive damage than LN. Up to now, the usefulness and simplicity of Talbot effect have still aroused the interest of many scholars.In the conventional method, a Talbot array illuminator is fabricated by using high external electric field to modulate the phase difference. However, essentially, high external electric field restricts the Talbot array illuminator to applications in optical integration and optical micro structure devices. Now we are looking forward to a new way which avoids using high external electric field.In this paper, we systematically study the two-dimensional (2D) hexagonal tunable array beam splitter, which is fabricated by domain-etching in MgLN crystal, and its fractional Talbot effect. The self-imaging phenomenon caused by Talbot effect in the Fresnel field for this phase array coherently illuminated is theoretically analyzed according to Fresnel diffraction theory. We numerically simulate the light intensity distributions of Talbot diffraction image under different values of Talbot coefficient and different values of domain-etching depth. The simulation results show that can change the array period and the structure distribution of the fractional Talbot diffraction image, and the domain-etching depth can modulate the light intensity distribution of diffraction image. Based on the numerical simulation results, the 2D hexagonal array beam splitters are fabricated with different values of domain-etching depth. The fractional Talbot diffraction images of array splitters are obtained at different values of through the optical experiments. The results show that domain-etching depth can effectively modulate the intensity distribution of diffraction image, becoming a tunable array beam splitter successfully. The experimental results agree well with the simulation results. The theoretical and experimental results show that the optimal self-image visibility can be obtained at a Talbot coefficient of 0.5 and a domain-etching depth of 0.39 m, while the duty cycle is 52%. Moreover, a good self-image pattern is also observed under thinner domain-etching depth, which is beneficial to optical integration and micro optical devices.

2016, 65 (1): 014301.
doi: 10.7498/aps.65.014301

Abstract +

The interaction of bubbles must be taken into consideration in the investigation of sound wave in the liquid containing gas bubbles, particularly in the case where the gas content is high. The force between two air bubbles due to the secondary sound fields radiated by the bubbles is called the secondary Bjerknes force, which makes the dynamics and scattering of bubbles different from a single bubble's. In order to investigate the influence of secondary Bjerknes force on bubbles' pulsation and scattering, we obtain the universal expression of bubbles' pulsation under the secondary Bjerknes force by Lagrange's equation. The influences on volume amplitude and initial phase of different parameter under the second Bjerknes force are discussed, and the scattering of bubbles with phase differences of and 0 is studied. The results show that the radius of neighbouring bubble, distance between two bubbles, polytropic coefficient and the phase can change the volume amplitude of pulsation under the secondary Bjerknes force. The mean radius of bubbles, distance and the frequency of sound have a significant effect on initial phase; the scattering of two bubbles of small distance and phase difference of is directional and decreases with distance r, which is related to the volume amplitude, initial phase and distance between two bubbles. The mean scattering power of bubble pairs of phase difference is 1/6(kd12)2 of single bubble's. The scattering of two bubbles with small distance and same phase also decreases with the distance r and relates to the volume amplitude, initial phase and distance between two bubbles. The mean scattering power of bubble pairs of same phase is 4 times as bigger as the mean scattering power of single bubble. It is expected that the mean radiuses, driving frequency and distance between bubbles can be used to change the scattering of bubbles.

2016, 65 (1): 014302.
doi: 10.7498/aps.65.014302

Abstract +

The fluctuation of sea surface and the Doppler effect cause carrier phase fluctuations of received signal, and the multipath expansion of the underwater acoustic channel makes received signal distorted, which can seriously influence the decoding performance of the cyclic shift spread spectrum system. In this paper, we propose cyclic shift energy detector (CS-ED) algorithm for cyclic shift keying spread spectrum system, which can solve the problem about the influence of carrier phase fluctuation by detecting the output energy of cyclic shift matched filter. The CS-ED combined with time reversal mirror is further proposed and analyzed in this paper by using a time-updated channel impulse-response estimate as a (match) filter to do time reversal processing to mitigate the multipath-induced interferences. Time reversal CS-ED method is simple and robust, which can make the system work in low SNR. Dalian Sea test validation and Lianhua lake test validation are carried on, showing that the low bit error rate underwater acoustic communication is achieved under the condition of multi-path extension, carrier phase fluctuation and low signal-to-noise ratio.

2016, 65 (1): 014303.
doi: 10.7498/aps.65.014303

Abstract +

Variation of bathymetry has a large effect on the sound propagation in deep water. An acoustic propagation experiment is carried out in the South China Sea. Some different propagation phenomena are observed for two different tracks in the flat bottom and the sloping bottom environments. Numerical analysis based on the parabolic equation model RAM (range-dependent acoustic model) is performed to explain the causes of the differences. The experimental and numerical results show that the transmission losses (TLs) decrease down to about 5 dB above the slope due to the reflection of the bottom, with a high-intensity region appearing below the sea surface. When a sea hill with a height of 320 m, which is less than 1/10 of water depth, exists in the incident range of sound beams on bottom first time, the sound beams are blocked due to the reflection of the sea hill. Then their propagating directions are changed, which makes an inverted-triangle shadow zone appearing in the reflection area of the sea hill. Compared with the TL results in the flat bottom environment, TLs increase up to about 8 dB in the corresponding area of the first shadow zone, and the abnormal TL effects can reach a maximal depth of 1500 m. Consequently, the shadow amplification effect caused by a small variation of bathymetry in deep water for long-range/large-depth sound propagation should receive enough attention. Furthermore, the convergence-zone structure in the sloping environment is different from that in deep water with flat bottom. The first convergence zone caused by refractions from the water above the axis of sound channel disappears. There are only the sound beams refracted back from water below the axis of sound channel. The numerical simulations show that the reflection-blockage of sound beams caused by the sloping bottom is significant. When the source is located somewhere above the slope, sound beams with large grazing angles can be reflected by the sloping bottom, and only some sound beams with small grazing angles can be refracted in the water without touching the slope and then come into the depth range of the vertical line array (VLA), forming the first part of the convergence zone refracted back from water. As the source moves farther from the VLA, the reflection-blockage of the sloping bottom becomes stronger. Sound beams are all reflected by the slope at a depth of about 3000 m, and they go through below the VLA, which leads to the absence of the first convergence zone caused by refractions from the water above the axis of sound channel. Therefore, the accuracy of bathymetry is meaningful for the sound propagation and target detection in deep water.

2016, 65 (1): 014304.
doi: 10.7498/aps.65.014304

Abstract +

The interdigital transducer (IDT) of the traditional Mach-Zehnder (MZ) acousto-optic modulator on a silicon-on-insulator (SOI) platform is located outside its two arms. The crest and trough of the surface acoustic wave (SAW) are used to modulate the two arms of the MZ interferometer so as to achieve a high modulation efficiency. Therefore, the distance between the two arms must be odd multiples of half acoustic wavelength. However, since the substrate is usually not uniform, the wavelength of the SAW changes as it transmits through the surface of the device. As a result, the exact distance between the two arms is difficult to choose. On the other hand, the SAW losses a portion of energy after passing through the first arm of the MZ interferometer, so the modulation of the second arm becomes much weaker. To solve these problems, we propose a new structure where its IDT is situated in the middle of the two arms of the MZ interferometer. With this scheme, the two arms of the MZ interferometer are located exactly at the crest and the trough of the SAW, while they are modulated with equal strength. In this paper, we first use the finite element method to simulate the acoustic frequency and the surface displacement of the excited SAW. Then we deduce the refractive index variations of all layers according to their acousto-optic effects. After that, we analyze the influences of different factors on the acousto-optic modulation efficiency, including the type and size of waveguide, the thickness of zinc oxide (ZnO) layer, and the area it covers, the number of electrodes, etc. These parameters are accordingly optimized to enhance the modulation efficiency. Modeling result based on COMSOL Multiphysics indicates that when the width of the strip waveguide is 6 m, the ZnO layer covers only the area under the IDT and has a thickness of 2.2 m, and the number of the electrodes is 50, the effective refractive index variation of the waveguide reaches 4.0810-4 provided that the amplitude of the driving voltage is 1 V. This value is 12% higher than that of the traditional structure.

2016, 65 (1): 014305.
doi: 10.7498/aps.65.014305

Abstract +

Anechoic coating attached to the surface of an underwater object is used for absorbing sound wave thereby reducing the reflection. The anechoic coating is often made of viscoelastic materials embedded with designed acoustic substructures, such as air cavities. The prediction of sound scattering on underwater object coated with such materials can be challenging due to the complex geometry of the anechoic coating, and it has been a research subject of interest in underwater acoustics. In this paper, we study the sound scattering on an infinite cylindrical shell coated with anechoic coating. Two types of coatings are considered: one is a layer of homogeneous isotropic material, and the other is a layer of homogeneous isotropic material with periodically embedded cylindrical air cavities. We use an equivalent method, in which the anechoic coating with air-filled cavities is regarded as a homogeneous isotropic material with equivalent material properties. The key point of the equivalent method is to ignore the internal structure of the anechoic coating, and the anechoic coating is considered as a homogeneous isotropic layer with the same complex reflection coefficient. These equivalent material properties are acquired based on the data of complex reflection coefficient obtained from either the physical experiment using water-filled impedance tube or the numerical experiment using the finite element method with COMSOL Mutiphysics software. Then a genetic algorithm is developed to inversely calculate the equivalent Young's modulus, Poisson's ratio, and damping loss factor of the coating which has the same reflection coefficient as the original coating. The results of the equivalent material properties show that 1) the three properties are all frequency dependent; 2) in general, equivalent Young's modulus increases with the increase of frequency, meanwhile the equivalent damping loss factor tends to decrease; 3) there is a wide variation in the results of equivalent Poisson's ratio. Despite that, the reflection coefficient of the equivalent homogeneous isotropic coating accords well with that of the original coating.Based on the above, the sound scattering on the infinite cylindrical shell coated with the equivalent coating is calculated by using the finite element method based on COMSOL Mutiphysics software. In order to verify the accuracy of the equivalent model, we use COMSOL Mutiphysics software to build up the full geometrical model of the coated shell to calculate the sound scattering. This can be considered as the benchmark. The results of morphic function show that the scattering calculated using equivalent material properties accords well with that obtained from the full finite element model with a mean error of about 1 dB in all frequency spectrum range.

2016, 65 (1): 014501.
doi: 10.7498/aps.65.014501

Abstract +

Rotor-pendulum systems are widely applied to aero-power plants, mining screening machineries, parallel robots, and other high-speed rotating equipment. However, the investigation for synchronous behavior (the computation for stable phase difference between the rotors) of a rotor-pendulum system has been reported very little. The synchronous behavior usually affects the performance precision and quality of a mechanical system. Based on the special background, a simplified physical model for a rotor-pendulum system is introduced. The system consists of a rigid vibrating body, a rigid pendulum rod, a horizontal spring, a torsion spring, and two unbalanced rotors. The vibrating body is elastically supported via the horizontal spring. One of unbalanced rotors in the system is directly mounted in the vibrating body, and the other is fixed at the end of the pendulum rod connected with the vibrating body by the torsion spring. In addition, the rotors are actuated with the identical induction motors. In this paper, we investigate the synchronous state of the system based on Poincar method, which further reveals the essential mechanism of synchronization phenomenon of this system. To determine the synchronous state of the system, the following computation technologies are implemented. Firstly, the dynamic equation of the system is derived based on the Lagrange equation with considering the homonymous and reversed rotation of the two rotors, then the equation is converted into a dimensionless equation. Further, the dimensionless equation is decoupled by the Laplace method, and the approximated steady solution and coupling coefficient of each degree of freedom are deduced. Afterwards, the balanced equation and the stability criterion of the system are acquired. Only should the values of physical parameters of the system satisfy the balanced equation and the stability criterion, the rotor-pendulum system can implement the synchronous operation. According to the theoretical computation, we can find that the spring stiffness, the installation title angle of the pendulum rod, and the rotation direction of the rotors have large influences on the existence and stability of the synchronous state in the coupling system. Meanwhile, the critical point of synchronization of the system can lead to no solution of the phase difference between the two rotors, which results in the dynamic characteristics of the system being chaotic. Finally, computer simulations are preformed to verify the correctness of the theoretical computations, and the results of theoretical computation are in accordance with the computer simulations.

2016, 65 (1): 014701.
doi: 10.7498/aps.65.014701

Abstract +

Boundary element method (BEM) is widely used in engineering analysis, especially in solving the transient heat conduction problem because of the advantage that only boundary of the problem needs to be discretized into elements. The general procedure of solving the variable-coefficient transient heat conduction problem by using the BEM is as follows. First, the governing differential equations are transformed into the boundary-domain integral equations by adopting the basic solution of the linear and homogeneous heat conduction problemGreen function. Second, domain integrals in the integral equation are converted into boundary integrals by the radial integral method or the dual reciprocity method. Finally, the time difference propulsion technology is used to solve the discrete time differential equations. A large number of practical examples verify the correctness and validity of the BEM in solving the variable coefficient of transient heat conduction problem. However, two deficiencies are encountered when the system of time differential equations is solved with the time difference method, i.e., one is the stability of the algorithm, which is closely related to the time step size, and the other is time-consuming when the freedom degree of the problem is large and all specified time steps are considered, because a system of linear equations needs to be solved in each time step. Therefore, in this paper we present a reduced order model analysis method of solving the variable-coefficient transient heat conduction problem based on BEM by using the model reduction method of proper orthogonal decomposition (POD). For variable-coefficient transient heat conduction problems, the discrete integral equations which are suitable for order reduction operation are deduced by using the BEM, the reduced order model is established by using the model reduction method of POD, and a lowdimensional approximate description of the transient heat conduction problem under time-varying boundary condition is obtained by projection of the initial discrete integral equations on some few dominant POD modes obtained from the problem under constant boundary conditions. First, for a variable coefficient transient heat conduction problem, boundary-domain integral equations are established and the domain integrals are transformed into boundary integrals by using the radial integration method. Second, the time differential equations with discrete format which is suitable for order reduction operation are obtained by reorganizing the integral equations. Third, the POD modes are developed by calculating the eigenvectors of an autocorrelation matrix composed of snapshots which are clustered by the given results obtained from experiments, BEM or other numerical methods for transient heat transfer problem with constant boundary conditions. Finally, the reduced order model is established and solved by projecting the time differential equations on reduced POD modes. Examples show that the method developed in this paper is correct and effective. It is shown that 1) the low order POD modes determined under constant boundary conditions can be used to accurately analyze the temperature field of transient heat conduction problems with the same geometric domain but a variety of smooth and time-varying boundary conditions; 2) the establishment of low order model solves the problem of heavy workload encountered in BEM where a set of large linear equations will be formed and solved in each time step when using the time difference method to solve the large time differential equations.

2016, 65 (1): 014702.
doi: 10.7498/aps.65.014702

Abstract +

The flow and diffusion of miscible fluid in a porous medium with a high Plcet number (Pe) and large viscosity ratio widely exist in industrial processes, such as oil recovery, geological sequestration of carbon dioxide, and chemical engineering process. When these problems are studied by numerical methods, the key point is to accurately describe the flow dynamics and diffusion process in a porous medium at the same time. As an alternative to conventional numerical methods, the lattice Boltzmann method based on kinetic theory is well suited to pore-scale simulations of miscible fluid flows and molecular diffusion. However, most of the existing lattice Boltzmann models have many difficulties (e.g. robustness and numerical stability) in simulating such systems at high Pe and large viscosity ratio. In this paper, in order to overcome the above difficulties, we propose a coupled lattice Boltzmann model based on the multiple-relaxation-time model and the lattice kinetic scheme for the fluid flow and diffusion, respectively. It can be shown that the incompressible Navier-Stokes equations and the convection-diffusion equation can be derived from the presented coupled model through the Chapman-Enskog procedure. The proposed model is validated by simulating a concentration gradient driven flow in a porous channel. Numerical results demonstrate that the model is of second-order accuracy in space. We further simulate a flow through two types of artificial porous media. The robustness of the presented model is investigated by measuring the permeability and diffusivity under different relaxation times. It is found that the model is insensitive to relaxation parameters. In addition, the miscible viscous displacement in two parallel plates is simulated to test the numerical stability of the model. It is observed that the results accord well with those reported in previous work, and the model is very stable at high Pe and large viscosity ratio in comparison with the standard lattice Bhatnagar-Gross-Krook model. Overall, the coupled lattice Boltzmann model can serve as an effective tool for directly simulating the fluid flow and diffusion at high Pe and large viscosity ratio in the pores of a porous medium.

2016, 65 (1): 014703.
doi: 10.7498/aps.65.014703

Abstract +

With the rapid development of micro-electro-mechanical systems (MEMS), microscale rarefied gas flows have received considerable attention in the past decades. Recently, the lattice Boltzmann method (LBM) emerges as a promising way to study the flow in MEMS for its kinetic nature and distinctive computational features. Various LBM models have been used to simulate the microscale and nanoscale flow, among which the two-dimensional and nine-velocities (D2Q9)-based LBM is most widely accepted due to its extremely simplicity and high efficiency. However, the D2Q9-based LBM encounters great difficulties in the transition regime due to the rarefaction effects on mean free path and gas viscosity. An effective way to improve the capability of the existing LBM model is to incorporate an effective viscosity into the relaxation time, which can improve the accuracy of LBM model while keeping the simplicity and efficiency of LBM. However, the existing D2Q9-based LBM models with effective viscosity cannot give satisfactory predictions of the none-equilibrium phenomenon at moderate or high Knudsen (Kn) number both in accuracy and efficiency. To solve the above problem, in this study, an effective mean free path function proposed by Dongari et al. (Dongari N, Zhang Y H, Reese J M 2011 J. Fluids Eng. 133 071101) via modular dynamics mean is introduced into the D2Q9 multi-relaxation-time lattice Boltzmann model (MRT-LBM) to account for the effect of Knudsen layer in transition flow regime, and the viscosity in the MRT-LBM model is modified correspondingly. The combination of the bounce-back and specular reflection boundary condition is used to deal with the velocity slip, and the relaxation time and the reflection coefficient are properly set to eliminate the numerical artifact on the boundaries as the kinetic boundary condition is used. Micro Couette flow at Kn=0.1-6.77, and periodic Poiseuille flow at Kn=0.1128-2.2568, respectively, are numerically investigated by using the proposed MRT-LBM model, and the numerical results, including the non-dimensional velocity profile and the mass flow rate, are verified by the direct simulation Monte~Carlo (DSMC) data, the linearized Boltzmann solutions and the existing LBM model. The calculation results demonstrate that in transition regime, with the increase of Knudsen number, the dimensionless slip velocity at the wall significantly increases. It is shown that the velocity profiles predicted by the present MRT-LBM model agree well with the DSMC data and linearized Boltzmann solutions up to Kn=4.5 in Couette flow, which is much more accurate than that obtained from the existing LBM model. And the present LBM model gives at least the same order of accuracy in the prediction of velocity profile and mass flow rate as the existing LBM model in periodic Poiseuille flow. What is more, the Knudsen minimum phenomenon of flow in the microchannel is successfully captured at around Kn=1. The results demonstrate that the proposed model can enhance the ability of LBM in capturing the non-equilibrium phenomenon in micro flow in the transition regime both in accuracy and efficiency.

2016, 65 (1): 014704.
doi: 10.7498/aps.65.014704

Abstract +

The objective of this present study is to address the cavitating flow patterns and regimes in the water-entry cavity. For this purpose, an experimental study of vertical water-entry cavity of an end-closed cylindrical shell is investigated by using high-speed video cameras and visualization technique. According to the cavitating flows as observed in the experiments, two flow pattern forms of fluctuation cavitation and cloud cavitation are found around the body. A further insight into the characteristics of the cavity shape and the variation in the cavity fluctuations parameters is gained by analyzing the image data. Furthermore, the experiments at different impact velocities are conducted to analyze the effects of impact velocity on the flow patterns and parameters. Finally, the formation mechanisms of cavitation fluctuations and cavitation clouds are studied based on the basic theory of fluid mechanics. The obtained results show that the cavitation flow pattern form of fluctuation cavitation occurs under the impact velocity condition of low speed, and the cloud cavitation occurs under the velocity condition of high speed. As fluctuation cavitation, the maximal extension diameters of cavitation fluctuate periodically along the water depth, and the speeds of extension and shrinkage are both proportional to the extension diameter. The collapses are different for the two flow pattern cavitations, i.e., the fluctuation cavitation, which is of deep closure and closed at the trough of wave cavitation more than once, and the cloud cavitation, which falls off and forms the leading edge of the cylindrical shell. The frequency fluctuation is independent of the impact velocity, the corresponding pinch-off time decreases with increasing the impact velocity, and the pinch-off time decreases in a nearly linear relation with Froude number. The water poured to the cylindrical shell causes the internal air to compress and expand, and as a consequence of these effects, periodic disturbances of pressure distribution and velocity field occur around the leading edge of the cylindrical shell, then the extended intensity of the cross section of the cavity shows variation in this process, which can be defined as fluctuation cavitation pattern. It appears that the re-entrant flow after the pinch-off at the trailing edge of cavity, then the laminar-turbulent transition is waken as a consequence of the re-entrant flow moving upstream, which flow pattern involved in this structure occurs as cloud cavitation.

###### PHYSICS OF GASES, PLASMAS, AND ELECTRIC DISCHARGES

2016, 65 (1): 015201.
doi: 10.7498/aps.65.015201

Abstract +

Graphene is a single atomic layer of carbon atoms forming a dense honeycomb crystal lattice. Now tremendous results of two dimensional (2D) graphene have been obtained recently in the electronic properties both experimentally and theoretically due to the massless energy dispersion relation of electrons and holes with zero (or close to zero) bandgap. In addition, through the process of stimulated emission in population inverted graphene layers, the coupling of the plasmons to interband electron-hole transitions can lead to plasmon amplification. Recently, research results have also shown that at moderate carrier densities (109-1011/cm2), the frequencies of plasma waves in graphene are in the terahertz range.In this paper, based on the Maxwell's equations and material constitutive equation, the gain characteristics of the surface plasmon in graphene are theoretically studied in the terahertz range. In the simulations process we assume a nonequilibrium situation in graphene, where the densities of the electron and the hole are equal. And the gain characteristics for different carrier concentrations, graphene temperature and the momentum relaxation time are calculated. The calculated results show that the peak gain positions shift towards the higher frequencies with the increase of the quasi Fermi level of electron and hole associated with electron-hole concentrations. The reason may be that the change rate of the electron quasi Fermi level is higher than the hole's and thus the distributions of electrons and holes in energy are broader, resulting in the peak gain frequency shifting towards higher frequencies. However, the results also indicate that the temperature of the graphene has little effect on both the peak gain value and the peak gain position of the plasmon. It is maybe because in the simulation process the temperature is taken to be less than 50 K, which is corresponding to the energy of the 1 THz. However the calculated results show that the frequencies of the gain peak positions are all larger than 1 THz, hence, the effects of the temperature on the peak gain value and peak position both could be neglected. Moreover, it is obviously seen that the peak gain value is a function of momentum relaxation time in graphene. This is because when the momentum relaxation time increases, more electrons will be excited, and this will increase the plasmon gain probability in graphene. However, the momentum relaxation time has no effect on the position of the gain peak. It is maybe because the momentum relaxation time has little effect on radiation frequency in the whole momentum relaxation period.

2016, 65 (1): 015202.
doi: 10.7498/aps.65.015202

Abstract +

It is important to improve the hohlraum radiation temperature for the research of high energy density physics, especially for study of inertial confinement fusion. Increasing the wall reemission ratio is an effective way to improve the temperature. It is found in theory that low density foam could reduce hohlraum wall energy loss, and then increase hohlraum temperature. In previous studies, experiments have shown that laser-to-X-ray conversion is enhanced by Au foam. However, improving reemission ratio is more important to increase hohlraum radiation temperature, because most of energy is lost in the wall.In this paper, we report our experiments carried out on SGⅢ prototype to compare the X-ray flux reemitted by Au foam and that by Au. For the experimental design, Au solid and Au foam are irradiated symmetrically along the axis by hohlraum radiation source Tr(t), which is assessed by broadband X-ray spectrometer flat-response X-ray diodes. The measured peak temperature is about 190 eV. Reemission flux from sample is measured by transmission grating spectrometer (TGS). The space-resolved image for pure Au sample shows that the hohlraum radiation is asymmetrical along the axis in the experimental conditions, temperature of top is higher than that at the bottom, which is consistent with simulation results obtained by using IRAD3D code. In order to compare the reemission flux from Au solid sample and that from Au foam sample in same conditions, we need to correct the symmetry of hohlraum radiation. By multiplying the ratio of top flux to bottom flux in pure Au target by the bottom flux in Au-Au foam target, where Au foam is on, we make sure that they are ablated by the same radiation source. The calculated results show that X-ray flux is increased by 20% by Au foam of 0.4 g/cc density when the hohlraum temperature is 190 eV. The typical observed time-integrated X-ray reemission spectra for Au solid and Au foam by TGS are also shown. We see that N-band and O-band reemission are clearly enhanced by Au foam, and the O-band reemission is almost the same as M-band reemission. The increased flux concentrates below 1 keV of the soft X-ray emission.The self-similar solution results and MULTI 1D simulation results show that the wall loss energy fraction is saved by Au foam, whose relation to reemission flux can be described by a simple expression. The theoretical solution shows that the emission flux increases about 10%, and the MULTI simulation indicates that the emission flux increases about 6.8%. They are in qualitative agreement with the experiments results. These results show an alluring prospect for Au foam to be used as hohlraum wall.

2016, 65 (1): 015203.
doi: 10.7498/aps.65.015203

Abstract +

The electrical explosion of single wire occurs in many application fields, such as wire-array Z-pinch, synthesis of the nanopowder, high-intensity magnetic field source, etc. The initial stage of the electrical explosion of single wire has a critical influence on the stagnation and X-ray yield in the wire-array Z-pinch. The impressive result of X-ray yield from wire-array Z-pinch is a major motivation to promote the research in this field. Although numerous studies have been carried out to gain a deep insight into the physics of the electrical explosion of single wire, more experimental investigations are necessary to optimize the energy deposition and expansion rate. It is important to investigate the characteristics of the electrical explosion of single wire under the negative polarity pulsed-current, which is adopted in many Z-pinch facilities. In this paper, the electrical explosion of aluminum wire under negative polarity pulsed-current in vacuum is investigated. In the present experiments, the light emission is measured by the photomultiplier and streak camera. A laser probe EKSPLA-PL2251C (30 ps, 532 nm) is adopted to perform the shadowgraphy, schlieren and interferometry diagnostics. The radial knife-edge schlieren scheme is employed to translate the regions with plasma refractivity and gas-type refractivity. The interferometry is constructed based on Mach-Zehnder interferometer. The shadowgram, schlieren image and interferogram are recorded by Canon cameras. The typical waveforms of the voltage, current and light emission from the electrical explosion of 15 m-diameter, 2 cm-long aluminum wire are derived. The energy deposition at the instant of voltage collapse is about 2.4 eV/atom (vaporization energy is about 4 eV/atom). In order to increase the energy deposited into the wire, the 15 m-diameter, 2 cm-long aluminum wire with 2 m polyimide coating is exploded with the same electrical parameters. The energy deposition in the coated wire is about 5 eV/atom. From the shadowgram of the electrical explosion of uncoated aluminum wire, the expansion velocity of the high-density region can be estimated to be about 2.2 km/s. However, the expansion velocity of the high-density region of the polyimide-coated aluminum wire is about 5 km/s. The schlieren images show that the wire is exploded into a binary structure, i.e., a high-density core surrounded by the low-density corona. It should be noted that the energy deposition in the coated wire is larger than the vaporization energy, indicating that the aluminum wire is totally in gaseous state. Thus, the plasma region in the schlieren image of electrical explosion of coated wire is not distinct. The core-corona structure is depressed by the insulating coatings to a certain extent. The configuration of the parallel wire is adopted to estimate the expansion velocity of the plasma shell. The expansion velocity of the low-density plasma is about 5.8 km/s. Two-dimensional distribution of the phase shift is derived through the interferogram. The central part of the gas-type material with a radius of 0.1 cm exhibits a large positive phase shift, while the peripheral plasma shows a small negative phase shift. The three-dimensional atomic density distribution is reconstructed in the gas-type distribution area in which the contribution of electrons is negligible. In our experiments, the energy deposition of the electrical explosion of uncoated wire ranges from 2 to 4 eV/atom. This may be caused by the initial conditions of the wire surface and the connection between the wire and electrode. Further research should be carried out for a better understanding of this phenomenon.

###### CONDENSED MATTER: STRUCTURAL, MECHANICAL, AND THERMAL PROPERTIES

2016, 65 (1): 016101.
doi: 10.7498/aps.65.016101

Abstract +

In this paper, electromagnetic properties of the zigzag graphene nanoribbon (ZGNR) with a single-row line defect are studied by using the first-principles method based on the density functional theory. The energy band structures, transmission spectra, spin polarization charge densities, total energies, and Bloch states of the ZGNR are calculated when the line defect is located at different positions inside a ZGNR. It is shown that ZGNRs with and without a line defect at nonmagnetic and ferromagnetic states are metals, but the reasons for it to become different metals are different. At the antiferromagnetic state, the closer to the edge of ZGNR the line defect, the more obvious the influence on electromagnetic properties of ZGNR is. In the process of the defect moving from the symmetrical axis of ZGNR to the edge, the ZGNR has a phase transition from a semiconductor to a half metal, and then to a metal gradually. Although the ZGNR with a line defect close to the central line is a semiconductor, its band gap is smaller than the band gap of perfect ZGNR, owing to the new band introduced by the defects. When the line defect is located nearest to the boundary, the ZGNR is stablest. When the line defect is located next nearest to the boundary, the ZGNR is unstablest. When the line defect is located nearest or next nearest to boundary, the ground state of the ZGNR is a ferromagnetic state. However, if the line defect is located at the symmetric axis of ZGNR (M5) or nearest to the symmetric axis, the ground state would be an antiferromagnetic state. At the antiferromagnetic state, the phase transition of M5 from a semiconductor to a half metal can be achieved by applying an appropriate transverse electric field. Without a transverse electric field, M5 is a semiconductor, and the band structures of up-and down-spin states are both degenerate. With a transverse electric field, band structures of up-and down-spin states near the Fermi level are both split. When the electric field intensity is 2 V/nm, M5 is a half metal. These obtained results are of significance for developing electronic nanodevices based on graphene.

2016, 65 (1): 016801.
doi: 10.7498/aps.65.016801

Abstract +

Droplet spreading behavior on a substrate is closely bound up with the wettability of the substrate, and plays a critical role in many industrial applications, such as lubrication, painting, coating, and mineral flotation. In this paper, a dynamical model of droplet spreading on a smooth substrate is established through a mechanical analysis. According to the lubrication approximation theory and Navier-Stokes equation, a general nonlinear evolution equation or equations are derived, including the momentum equation, the continuity equation, and the evolution equation of film thickness. We adopt numerical methods to solve these equations, and also quantitatively analyze the relation among film thickness, spreading radius, speed of wetting contact line and time in detail. The results show that the droplet spreading process is mainly divided into two phases, namely expansion phase and contraction phase. Moreover, the spreading process is along with mutual transformation among surface energy, kinetic energy, and different kinds of potential energies. In addition, the final spreading radius Rf of droplet is determined by the inherent wettability of solid surface, and the collapse effect, which emerges at t=0.006 s in the spreading process, is related to Laplace pressure difference of curved liquid surface. Finally, by controlling the droplet size, we obtain the scaling law of droplet spreading radius with time, which approximately meets R ～ t1/7. The scaling law is validated both experimentally and numerically. The results of this study are expected to enhance our knowledge of the movement of wetting contact line and also provide some guidance for the wetting theory.

2016, 65 (1): 016802.
doi: 10.7498/aps.65.016802

Abstract +

Fe-doped high-resistivity GaN films and AlGaN/GaN high electron mobility transistor (HEMT) structures have been grown on sapphire substrates by metal organic chemical vapor deposition. The lattice quality, surfaces, sheet resistances and luminescent characteristics of Fe-doped high-resistivity GaN with different Cp2Fe flow rates are studied. It is found that high resistivity can be obtained by Fe impurity introduced Fe3+/2+ deep acceptor level in GaN, which compensates for the background carrier concentration. Meanwhile, Fe impurity can introduce more edge dislocations acting as acceptors, which also compensate for the background carrier concentration to some extent. In a certain range, the sheet resistance of GaN material increases with increasing Cp2Fe flow rate. When the Cp2Fe flow rate is 100 sccm, the compensation efficiency decreases due to the self-compensation effect, which leads to the fact that the increase of the sheet resistance of GaN material is not obvious. In addition, the compensation for Fe atom at the vacancy of Ga atom can be explained as the result of suppressing yellow luminescence. Although the lattice quality is marginally affected while the Cp2Fe flow rate is 50 sccm, the increase of Cp2Fe flow rate will lead to a deterioration in quality due to the damage to the lattice, which is because more Ga atoms are substituted by Fe atoms. Meanwhile, Fe on the GaN surface reduces the surface mobilities of Ga atoms and promotes a transition from two-dimensional to three-dimensional (3D) GaN growth, which is confirmed by atomic force microscope measurements of RMS roughness with increasing Cp2Fe flow rate. The island generated by the 3D GaN growth will produce additional edge dislocations during the coalescence, resulting in the increase of the full width at half maximum of the X-ray diffraction rocking curve at the GaN (102) plane faster than that at the GaN (002) plane with increasing Cp2Fe flow rate. Therefore, the Cp2Fe flow rate of 75 sccm, which makes the sheet resistance of GaN as high as 1 1010 /\Box, is used to grow AlGaN/GaN HEMT structures with various values of Fe-doped layer thickness, which are processed into devices. All the HEMT devices possess satisfactory turn-off and gate-controlled characteristics. Besides, the increase of Fe-doped layer thickness can improve the breakdown voltage of the HEMT device by 39.3%, without the degradation of the transfer characteristic.

###### CONDENSED MATTER: ELECTRONIC STRUCTURE, ELECTRICAL, MAGNETIC, AND OPTICAL PROPERTIES

2016, 65 (1): 017401.
doi: 10.7498/aps.65.017401

Abstract +

Magnesium diboride is a binary compound with a simple AlB2 type crystal structure and a high-Tc (nearly 40 K) superconductor. The rather high Tc value and the specific properties make it a potential material for electronic applications. The key structure for the application is a Josephson junction. The growth of tri-layer structure consisting of MgB2 film and tunneling barrier layer is a key technology for a Josephson junction. Boron is a kind of good insulating medium. Preparation of MgB2/B/MgB2 tri-layer structures by chemical vapor deposition (CVD) method is investigated. The experimental results indicate that the depositing temperature will influence the microstructure of boron film significantly and different crystal structures of boron films are obtained at different temperatures.The boron film is an amorphous film while the deposition temperature is lower than 500 ℃, and the amorphous B film can be transformed into MgB2 superconducting film by annealing in Mg vapor. For precursor B films deposited at 470 ℃ and 500 ℃, the critical temperatures of the relevant MgB2 films are 39.8 K and 38.5 K, respectively. As the deposition temperature is higher than 550 ℃, the boron film becomes crystallized, and increasing deposition temperature will increase the crystallinity of the B film as can be seen from the samples deposited at 550 ℃, 600 ℃, 650 ℃ and 680 ℃. The boron film turns out to be of -phase crystalline texture, which is verified by X-ray diffraction and scanning electron microscope. What is more, the crystalline boron film is a kind of inert film, and it does not react with Mg in Mg vapor, thus it cannot be transformed into superconducting film in the subsequent annealing steps. By utilizing the property of the crystallized boron film, a square-shaped Josephson junction with a size 100 m100 m of MgB2/B/MgB2 structure is prepared. The thickness of boron dielectric layer is about 10 nm, and the DC Josephson effect is observed by the I-V measurement of the junction. Compared with other tri-layer structure based on MgB2 material, such as the MgB2/MgO/MgB2, the structure in which B film serves as a barrier layer eliminates the oxygen and can be fabricated in-situ easily by CVD method, and reliable Josephson junctions can be expected by such a technology.

###### INTERDISCIPLINARY PHYSICS AND RELATED AREAS OF SCIENCE AND TECHNOLOGY

2016, 65 (1): 018101.
doi: 10.7498/aps.65.018101

Abstract +

Recently, metamaterials have attracted considerable attention because of their unique properties and potential applications in many areas, such as in bio-sensing, imaging, and communication. Among these researches, the metamaterial absorber has aroused much interest of researchers. The metamaterial absorber is important due to a broad range of potential application to solar energy, sensing, coatings for reducing the reflection, and selective thermal emitters. As a two-dimensional honeycomb structure composed of a single layer carbon atom, graphene is a promising candidate for tuning metamaterials and plasmonic structures due to its unique properties which differ substantially from those of metal and semiconductors. In this paper, we propose a tunable terahertz absorber based on graphene complementary metamaterial structure by removing periodic cut-wires on the graphene meta-surface. On the basis of the tunability of graphene conductivity, the absorber possesses a frequency tunable characteristic resulting from the change of graphene Femi level by altering the applied voltage. Here, we mainly study the influences of Fermi level of graphene and the size of the structure on the absorption characteristic of this metamaterial absorber. We finally obtain the corresponding Femi level and structural size under the perfect absorption condition. In addition, we utilize the multiple reflection theory to explore the physical mechanism, and verify the feasibility of the simulation method at the same time. The research indicates that the absorber can achieve 99.9% perfect absorption at 1.865 THz when the graphene Femi level is 0.6 eV, the thickness of substrate is 13 m, and the length and width of slit are 2.9 m and 0.1 m, respectively. When graphene Femi level increases from 0.4 eV to 0.9 eV, the resonance frequency of the absorber is blue-shifted from 1.596 THz to 2.168 THz. Meanwhile, the absorption rate increases from 84.68% at 0.4 eV to a maximum value of 99.9% at 0.6 eV, then gradually decreases to 86.63% at 0.9 eV. The maximum modulation of the absorption rate is 84.55% by varying the Femi level. When the thickness of substrate increases, the resonant frequency is red-shifted. The resonant frequency is blue-shifted when both the width and the length of the cut-wire on graphene increase. On the basis of the proposed graphene meta-surface absorber, one can gain different resonant frequencies by adjusting the structure geometric size and graphene Femi level. The graphene complementary structure can also be designed into different patterns to achieve the purpose of practical application.

2016, 65 (1): 018103.
doi: 10.7498/aps.65.018103

Abstract +

Magnetic thin films are widely used in magnetic recording and magnetorheology, and also in magnetic lubrication such as ferromagnetic fluids. Polymethylmethacrylate (PMMA) is used as a coating material on the surface of the magnetic material in an electromagnetic system because of its good dielectric properties. In this study, the tribological behavior of reciprocating motion between ferromagnetic films coated with PMMA films under a magnetic field is evaluated. The system of ferromagnetic films coated with PMMA films based on glass is called ferromagnetic/PMMA double membrane in this paper. Two pieces of membranes in each tribological experiment are absolutely the same. Two kinds of experimental conditions, that is, under dry friction and silicone oil lubrication, are used to investigate the influences of load and magnetic field strength on the friction performance of ferromagnetic/PMMA double membranes. Experimental results show that the magnetic field directly affects the friction performance of a ferromagnetic /PMMA double-film system, and the performance changes with the normal load and intensity of the magnetic field. However, the influence of magnetic field on the tribological property in the dry friction mode is different from that in the silicone oil lubrication mode. The influences of magnetic force and the changes of the physical properties of the friction pair on friction and friction coefficient, which are both induced by the magnetic field, are analyzed. The theoretical analysis results are in good agreement with the experimental date. This work provides a basis for designing and controlling magnetic film interface media.

2016, 65 (1): 018201.
doi: 10.7498/aps.65.018201

Abstract +

Over past years, the excessive use of fossil fuel has posed serious problems such as greenhouse effect and environmental pollution, which threaten human life. Regarded as an ideal substitution for traditional internal combustion engine, low temperature proton exchange membrane fuel cell (PEMFC) converts chemical energy through electrode reaction directly into electrical energy with high efficiency and low pollution. However, the main problem behind the industrialization of PEMFC, is that oxygen reduction reaction (ORR) occurring on the cathode needs precious metal platinum (Pt) as catalyst, which has a limited reserve and is costly. Owing to high activity and stability, the graphenes doped with non-metal B and P, have proven to be excellent alternatives to Pt experimentally. However, the relevant theoretical work is scarce.Adsorptions of the ORR intermediates, i.e., O, O2, OH, and OOH, of doped graphenes are essential for the cathode reaction, which also bring some difficulties to the next step reaction. Therefore, in this paper, based on density functional theory, the adsorption characteristics of O, O2, OH, and OOH of B-doped, P-doped and B, P-codoped graphenes are studied using first-principles calculation code VASP first. By analyzing the adsorption energies, bond lengths, densities of states and charge transfers, the influences of the different dopants on the intermediates are evaluated. Then, the ORR steps are discussed, and the free energy change of each step is further given. The results show that for B-doped and P-doped graphenes, the adsorption energies of various intermediates exhibit similar linear relationships. The adsorption energy of OOH of P-doped graphene (3.26 eV) is much larger than that in B-doped grapheme (0.73 eV). The large adsorption energy of P-doped graphene is beneficial to the fracture reaction of OO bond in OOH, while the small adsorption energy of B-doped graphene can promote the reaction of OH converting into water. Owing to the synergistic effect, the graphene codoped with B and P possesses better catalyzing ability than single B-and P-doped ones. The results are helpful for understanding the excellent performances of codoped graphenes.

2016, 65 (1): 018501.
doi: 10.7498/aps.65.018501

Abstract +

The performance of a Si metal-oxide-semiconductor field-effect transistor can be enhanced effectively by the strain technology and the orientation engineering. For example, the [110] direction is usually used as the channel direction in the Si p-channel metal-oxide-semiconductor (PMOS) on 100 oriented substrate. While SunEdison company rotates the channel direction 45 degrees to the [100] direction, its hole mobility is 1.15 times larger than the hole mobility of the former.The orientation engineering is based on the anisotropy of the hole effective mass along different directions. The enhancement of carrier mobility naturally occurs when we choose the direction with the smaller carrier effective mass as the channel direction.However, according to the reported results in the literature, the hole effective mass values along the [110] and [100] orientation are about 0.6m0 and 0.29m0, respectively. The former is twice larger than the latter, which cannot explain that the experimental result increases 1.15 times.We find that the effective mass values along both the long axis and the short axis should be taken into consideration, and the value of 0.6m0 can only represent the long axis term by observing the equivalent energy diagram of the first sub-band in Si PMOS.In view of this, the double ellipsoid model is given for the conductivity effective mass along the [110] direction in (100) Si PMOS, which explains the reason why the hole mobility along the [100] direction is 1.15 times larger than that along the [110] direction in Si PMOS. And then, based on the E-k relation of the inversion layer in Si-based strained PMOS, we study the conductivity effective mass along the [110] direction in (100) Si-based strained PMOS by the above method.The results show that 1) the [110] oriented hole conductivity effective mass of biaxially strained Si PMOS can be calculated directly by its spherical equivalent energy diagram; 2) in the case of biaxially strained Si1-xGex PMOS, its conductivity effective mass needs to be calculated by the double ellipsoid method; 3) the [110] oriented hole conductivity effective mass of uniaxially strained Si PMOS should be solved approximately by two different sizes of ellipsoid.Our valid models can provide the valuable references for studying and designing the Si-based strained PMOS device.

2016, 65 (1): 018502.
doi: 10.7498/aps.65.018502

Abstract +

Scannerless (flash) lidar system based on streak camera is able to realize three-dimensional (3D) multi-spectral fluorescence imaging and 3D imaging polarimetry. Compared with conventional lidar system, the flash lidar system overcomes image distortions caused by the motion between the target and the sensor platform. Other advantages of the flash lidar system are higher image update rates and the potential for creating a miniaturized lidar system. To meet the requirements for developing this new technology, a super small-sized, large photocathode area and meshless streak tube with spherical cathode and screen is designed with the aid of computer simulation technology (CST) software. The tube with nearly 28 mm wide photocathode work area contains two electrostatic focusing lens, a pair of deflection plates, and a 50 mm diameter output screen. The external dimension of the tube is merely 50 mm100 mm. And its electromagnetic fields are calculated in the CST Particle Studio based on the finite integration theory. Some dynamic properties of the tube are analyzed via observing different electron trajectories launched from a number of different points on the cathode. The influences of the deflector position on deflection sensitivity and spatial resolution are analyzed. Increasing the distance between the deflector and the anode pin hole leads to a worse deflection sensitivity but a better spatial resolution. As for the temporal resolution, three electron pulses separated by 30 ps can be well resolved by the streak tube in the dynamic mode. Thus, the dynamic temporal resolution of the streak tube is better than 30 ps. And a 10 lp/mm spatial resolution across the 28 mm long slit on the photocathode can be obtained by estimating modulation transfer functions of the electron trajectories. Temporal distortions at the entire photocathode working area are evaluated, and the data reveal that the larger the photocathode working area, the bigger the temporal distortions are. Also, the temporal distortion is present mainly in the photocathode-to-deflection plates region. In addition, the slit image of the streak tube working in the dynamic mode is simulated and presented. The phenomenon that the slit image is curved due to the temporal distortion is analyzed. Two rectangular electron pulses separated by 50 ps are well resolved by the streak tube. Therefore, the temporal resolution of this small-size steak tube is better than 50 ps with a rectangular slit dimension of 30 mm50 m on the photocathode, and its electron-optic magnification is 1.2.