## Vol. 64, No. 22 (2015)

##### 2015-11-20

###### SPECIAL ISSUE—Terahertz physics

2015, 64 (22): 224212.
doi: 10.7498/aps.64.224212

Abstract +

We present a quasi-three-dimensional efficient model for simulating and designing the terahertz quantum cascade laser with nonlinear axial waveguide structure, based on the finite difference beam propagation method. The traditional beam propagation method is widely used to simulate the beam profile of the passive waveguide. In order to study the active device, however, the current induced variation in the active region should also be considered in the numerical simulation model. In the model presented in this paper, the phase and the amplitude of the propagating confined field in the active waveguide are determined by a few linear and non-linear effects. The parameters relating to the linear effects, such as the intrinsic refractive index profile and the intrinsic losses of the waveguide under zero current injection, are calculated by using COMSOL-Multiphysics. While the non-linear effects, such as the modal gain and the refractive index variation induced by current injection, are considered in a rigorous way by including the rate-equation set for calculating the carrier dynamics in the active region. The parameters used in the rate-equation set are obtained by referring to the literature and fitting the experimental results of the considered terahertz lasers. By adding the current induced gain and refractive index variation, the presented beam propagation model is able to simulate many current-dependant properties of a laser, such as the output power, the gain guiding effect, and the self-focusing effect. We show in this paper that the latter two effects have influence on inner-waveguide beam profile, and the competitive balance between them determines the output beam quality.
By utilizing this numerical model, the terahertz quantum cascade laser with tapered waveguide structure is simulated, and the influences of the taper angle on output power and beam quality are investigated. According to the simulation results, we find that there is an obvious increase in the output power when the taper angle is increased from 0 to 3 degree, while the increment in the output power decreases rapidly when the taper angle is further increased. Besides, we observe that for the far field the full width at half maximum of the output beam decreases sharply with increasing the taper angle. However, when the taper angle equals 8 degree, multiple lateral modes are observed, which indicates poor output beam quality of this device and poor beam coupling efficiency between this device and the power meter.Therefore, although the simulation results show that the output power of this device is higher than that of the device with 5 degree taper angle, the experiment results show that the measured output power is lower. So the taper angle is not the larger the better, but there exists an optimum value, at which the terahertz quantum cascade laser can achieve the highest effective output power.

2015, 64 (22): 228501.
doi: 10.7498/aps.64.228501

Abstract +

The terahertz regime, as a last radio window, remains to be fully explored, and astronomical and atmospheric observations in this regime are scientifically important. Like other frequency regimes, developing high-sensitivity detectors (coherent and incoherent) is of particular significance for both ground-based and space-borne facilities. As the coherent detector of choice below 1.4 THz, superconductor-insulator-superconductor (SIS) heterodyne mixers have achieved as high a sensitivity as five times the quantum limit around 1.4 THz. It is, however, still a challenge to developing SIS mixers at frequencies beyond 1.4 THz with considerable transmission loss in superconducting circuits due to the Cooper-pair breaking by energetic photons and increased many difficulties in designing and fabricating.
So far, superconducting hot electron bolometer (HEB) mixers have been the most sensitive heterodyne detectors at frequencies above 1.5 THz, and successfully used to detect molecular spectral lines up to 2.5 THz from ground-based and space telescopes. Although spiral-antenna coupled NbN HEB mixers show a good sensitivity in the whole THz frequency range, the directly measured spectral response with Fourier transform spectrometer falls quickly as frequency increases, especially above 3 THz.
The terahertz band is also of particular importance to observe astronomical objects such as cosmic microwave background, early distant objects, cold objects and dusty objects. Aiming at such objects, we develop a terahertz imaging array system by combining advanced superconducting detectors such as transition edge sensor (TES) and microwave kinetic inductance detectors (MKIDs), thus the system has a frequency band centred at 350 m, an operational temperature of 0.3 K, and a sensitivity reaching background limit performance for ground-based applications. In addition, it is expected to have some breakthroughs in ultra-sensitive superconducting TES and MKID, low noise multi-channel readout and multiplexing, efficient terahertz-wave coupling technology, and large-scale array system integration. The developed terahertz imaging array system will serve as the next-generation instrument of Dome A 5 m terahertz telescope, conducting a 350 m-band legacy survey for studying the planets, stars, galaxies and cosmology. Besides the application in astronomy, the developed terahertz imaging array system can also be applied to some areas requiring rapid detection such as security, deep space exploration, and biomedical imaging.
In this paper, we mainly introduce the superconducting detectors developed at Purple Mountain Observatory and those for international collaborative projects.

2015, 64 (22): 228701.
doi: 10.7498/aps.64.228701

Abstract +

In the past decades, terahertz metamaterials have attracted considerable attention due to the capability of realizing essential terahertz functional devices and potential applications in sensing, imaging, spectroscopy and monitoring. In this review, we first present a brief introduction to the theory and development of terahertz metamaterials, and then focus on some terahertz devices including both triple-band and broadband metamaterial absorbers, the spoof surface plasmon polaritons (SPP) waveguides, the SPP bend, the SPP beam splitter, and the SPP ring resonator. The metamaterial absorbers are fabricated and measured in THz band, while the SPP devices are verified through numerical simulations. All the designs are easy to fabricate and favorable for practical applications.

2015, 64 (22): 228702.
doi: 10.7498/aps.64.228702

Abstract +

GaAs photoconductive switch illuminated by a femto-second laser has been widely used in a terabertz (THz) time domain spectroscopy system as a THz wave emission antenna. Now, all of the GaAs photoconductive switches are used in linear mode. However, when the GaAs photoconductive switch operates in an avalanche multiplication mode, the power capacity of output ultrafast electric pulse is much higher than that in a linear mode. So far, nobody has proposed the idea of generating THz waves by using the GaAs photoconductive switches in the avalanche multiplication mode. In this paper, we report the feasibility and research progress of using the GaAs photoconductive switches in the avalanche multiplication mode as the THz sources. By theoretical analysis and experimental research, some results are obtained experimentally as follows. 1) The GaAs photoconductive antenna can operate in an avalanche multiplication mode when illuminated by a femto-second laser pulse with an energy on the order of nJ. 2) The maintaining time of the avalanche multiplication mode, i.e, lock-on period, can be reduced by the quenching mode of photo-activated charge domain. These results lay the foundation for generating the high intensity THz emission by the GaAs photoconductive antenna with the avalanche multiplication mechanism.

2015, 64 (22): 229501.
doi: 10.7498/aps.64.229501

Abstract +

Graphene has unique electronic properties stemming from a linear gapless carrier energy spectrum, and has dominant advantages in the research of devices such as lasers, detectors and modulators in terahertz region due to its tunable energy gap and extremely high carrier mobility. In this review, we summarize its latest progress in applications of terahertz devices such as lasers, detectors and modulators. Terahertz lasers based on graphene can reach a gain as high as 104 cm-1, and terahertz detectors with different structures such as a bilayer graphene field-effect transistor with top gate and buried gate can achieve NEP (noise equivalent power) ～ m nW/Hz. Graphene terahertz modulators, which are equipped with transmission configuration and reflection configuration, can have a very high modulation depth. These results may be helpful for developing the high-efficiency graphene terahertz devices.

###### GENERAL

2015, 64 (22): 220201.
doi: 10.7498/aps.64.220201

Abstract +

The point kinetic equations are the system of a couple stiff ordinary differential equations. Many studies have focused on the development of more advanced and efficient methods of solving the equations, such as the high order Taylor polynomials method, the Haar wavelet operational method, the fractional point-neutron kinetic model method, the basis function method, the homotopy analysis method, and other methods. Most of these methods are successful in some specific problems, but still have, more or less, disadvantages. For example, the accuracy of the Haar wavelet operational method is limited by the collocation points, and it needs more computing time for a high precision. Aiming at the requirements that some numerical calculation results must have the higher precision and only the positive error in the nuclear reactor safety engineering and ship reactor for the maneuverability, in this paper we try to look for a new numerical method to satisfy that the calculation value is slightly higher than the real value when the actual curve is upward convex or downward concave, and the error is not greater than that by the Euler and improved Euler method. The new method is so-called the curvature weight (CW) method, which is based on the curvature circle method and considers the contributions of two curvatures at the interval step point to the average curvature inside the interval step. Using the decoupling method to remove the stiffness of equations and the instantaneous jump approximation to derive the neutron differential equations, the first and second derivative of neutron density are obtained. Then the CW method is used to solve the point reactor neutron kinetic equations, and thus obtaining the numerical solution. Compared with the results by the Euler and improved Euler method, the numerical calculation results by the CW method are always higher than the real value, and the calculation accuracy and speed are improved significantly. When this new method is used to solve the point reactor neutron differential equations with the step and linear reactivity inserted into the subcritical reactor, the numerical results which satisfy the requirements of positive calculation error and high precision can be obtained quickly. After improving the calculation step length, the precision reduction by the CW method is significantly lower than that by the Euler and improved Euler method. So the CW method can greatly shorten the total computing time, and it is also effective for most of differential equation systems.

2015, 64 (22): 220301.
doi: 10.7498/aps.64.220301

Abstract +

Quantum correlation is an important resource in quantum information, quantum computation, and quantum metrology. Quantum entanglement, Einstein-Podolsky-Rosen (EPR) quantum steering and Bell nonlocality are the major quantum correlations. For quantum entanglement and Bell nonlocality, two subsystems play the same significant roles. EPR quantum steering is stronger than entanglement and weaker than Bell nonlocality. It represents the ability of one subsystem to nonlocally affect another subsystem's states through local measurements. In this paper, the dynamic quantum correlation between the modes in the two-site Bose-Hubbard model is investigated. According to Hillery-Zubairy entanglement criterion and based on maximum mean quantum Fisher information, the influences of initial states on the quantum entanglement evolutions are explored. If the coupling between the modes is much greater than that of the particles at the same site, and the initial states are symmetric or anti-symmetric SU(2) coherent states, the quantum correlations show simple periodic evolutions. The oscillation amplitudes of the evolutions increase with the interaction between the particles at the same site. The oscillation period decreases with the coupling strength between the modes. The dependence of the period on the interaction of the particles at the same site is related to the initial states. In other words, the time evolutions of quantum correlation are closely related to the symmetry of the initial states. In the case of symmetric (anti-symmetric) SU(2) coherent state and repulsive (attractive) interaction of the particles at the same site, the system presents two-way quantum steering. When the subsystem exchange symmetry of the initial states is broken, the collapse and revival of quantum correlation appear, moreover one-way quantum steering emerges in the infancy. One-way quantum steering is asymmetric for two subsystems. So exchange asymmetry of the initial state is necessary condition of one-way quantum steering when the Hamiltonian of the system is symmetric for two subsystems.

2015, 64 (22): 220501.
doi: 10.7498/aps.64.220501

Abstract +

The fractional over-damped ratchet model with thermal fluctuation and periodic drive is introduced by using the damping kernel function of general Langevin equation in the form of power law based on the assumption that cytosol in biological cells has characteristics of power-law memory. On basis of the Grunwald-Letnikov definition of fractional derivative, the numerical solution of this ratchet model is obtained. And furthermore, according to the numerical solution, the transport behaviors of stochastic ratchet and corresponding deterministic ratchet (especially when the deterministic ratchet has chaotic trajectory) are investigated, based on which we try to analyze how chaotic properties of the deterministic ratchet and the actions of noise influence the transport properties of molecular motors and moreover find the possible mechanism of current reversal of fractional molecular motor. Numerical results show that, as barrier height, barrier asymmetry and memorability of model change, the current reversal in deterministic ratchet is not necessarily required to appear when happening indeed in corresponding stochastic ratchet; moreover, with the decrease of order p, there exists a chaotic regime in deterministic ratchet model before current reversal, but with the disturbance of noise, current reversal will happen more earlier, namely, chaotic current direction in deterministic ratchet model can be reversed when disturbance of noise exists. This also demonstrates that noise can essentially change the transport behavior of a ratchet; current can change from chaotic state in a ratchet with no noise to directed transport with noise. This is a possible mechanism of current reversal of a fractional stochastic ratchet, and also a reflection that noise plays an active role in directed transport.

2015, 64 (22): 220502.
doi: 10.7498/aps.64.220502

Abstract +

In this paper, the Levy noise is combined with a power function type monostable stochastic resonance system for the first time. In order to ensure the reliability of the experimental data, the average signal-to-noise ratio gain is regarded as an index to investigate the stochastic resonance phenomenon stimulated by Levy noise. Potential function form of the monostable system and the method of generating Levy noise are presented in detail. The pulse characteristic and smear characteristic of Levy noise are also presented in detail. The laws for the resonant output of monostable system, governed by parameters a and b, the intensity amplification factor D of Levy noise, are explored under different values of characteristic index and symmetry parameter of Levy noise. Results show that no matter whether it is under any different characteristic index or symmetry parameter of Levy noise, the weak signal can be detected by adjusting the system parameters a and b. The intervals of a and b which can induce stochastic resonances are multiple, and do not change with nor . Moreover, the same rule is founded which by adjusting the intensity amplification factor D of Levy noise can also realize synergistic effect when studying the noise-induced stochastic resonance, and the interval of D does not change with nor ; the best value of characteristic index is =1 under any system parameter, and the best value of symmetry parameter is =1 under any system parameter. So, the system performance is best when =1 and =1. Finally, the interaction relationship between system parameters a and b is investigated, and it is found that the interval of a or b will change with b or a when characteristic index , symmetry parameter and the intensity amplification factor D of Levy noise are fixed. These results will contribute to reasonably choosing the system parameters and intensity amplification factor of power function type monostable stochastic resonance system under Levy noise, and provide a reliable basis for practical engineering application of weak signal detection by stochastic resonance.

2015, 64 (22): 220503.
doi: 10.7498/aps.64.220503

Abstract +

In this paper, by using the Terman-Wang small-world neuronal network with electrical synapse coupling, we investigate the synchronous dynamics of neuronal network system subjected to spatially correlated white noise. First, the dynamical mean-field approximation theory is extended to the small-world network system under spatially correlated white noise, through which the original 2N-dimensional stochastic differential equations of the network system are transformed to 11-dimensional deterministic moment differential equations. Then, based on this set of moment differential equations, the key effects of spatially correlated noise and network structure on the synchronous firing property are discussed in the Terman-Wang neuronal network system. The results show that the synchronization ratio of this considered neuronal network system becomes higher not only as the noise correlation coefficient is increased but also as the coupling strength and the average vertex degree are added. Those results imply that the noise spatial correlation coefficient, the coupling strength, and the average vertex degree can play a positive role in inducing synchronous neuronal behaviors. Furthermore, the synchronous dynamics of the original neuronal network system, obtained by direct numerical simulations, is compared with those obtained by the dynamical mean-field approximation theory, and good consistence between them is revealed.

2015, 64 (22): 220504.
doi: 10.7498/aps.64.220504

Abstract +

It is of great significance to study the weak harmonic signal detection from strong chaotic background. Current detection methods mainly use the chaotic phase space reconstruction method based on Takens theory, among which the neural network method has attracted the most attention. However, these methods require high signal-to-interference-plus-noise ratio (SINR) and are sensitive to Gaussian white noise, etc.
Noticing the fact that the second-order statistical properties of chaotic signals are stationary, we propose a harmonic signal detection method from strong chaotic background based on optimal filter. We first construct a data matrix, whose rows are the detection signal and reference signals. The reference signals only contain chaotic interference. Then we calculate the one-dimensional fast Fourier transformation of the data matrix to make each column of the matrix form a frequency channel. The harmonic signal can be detected by searching each frequency channel in the frequency domain, thus the signal detection problem is converted into an optimization problem. Further, we use the optimization theory to design a filter such that it can maintain the gain of the signal from the current frequency channel and suppress signals from other frequency channels as far as possible. Finally, the harmonic signal can be obtained by calculating the output SINR of each frequency channel.
In order to reduce the calculation, we can further design a local region optimal filter. We choose part of frequency channels to constitute a local area, thus the dimension of the chaotic interference covariance matrix is greatly reduced. Theoretically speaking, the more the number of auxiliary frequency channels, the better the detection results are. However, in the practical application, choosing two channels on the left and right side of current channel each can obtain a very good detection effect. After obtaining the chaotic interference covariance matrix, we can further achieve the output SINR of each frequency channel.
Compared with the traditional methods, the proposed method has the following advantages: 1) it can detect a weak harmonic signal under lower SINR; 2) it can detect a greater range of signal amplitude; 3) it is robust against white Gaussian noise. The simulation results with taking Lorenz system as the strong chaotic background show that the proposed method still has a very good detection effect when SINR =-81.03 dB, and the stronger the harmonic signal, the better the detection effect is, while the neural network method can work under the condition of SINR higher than -67.03 dB; the proposed method still can correctly detect the target signal in the case that the SNR is as low as -20 dB, but the neural network method has a poor detection effect under the same condition.

2015, 64 (22): 220505.
doi: 10.7498/aps.64.220505

Abstract +

It is of fundamental importance to investigate the evacuation process from a room with obstacles. The typical case is the evacuation of students from a classroom. Based on evacuation experiments from a classroom, the essential features of evacuee are concluded. In the original floor field model, the dynamic floor field is introduced in order to reflect the interaction among pedestrians. A pedestrian may follow the virtual trace of another one in front. The static floor field does not consider the influence of pedestrians. In this paper, the original dynamic floor field is ignored. These desks and chairs are treated as impassable and passable static obstacles, respectively. The static and passible obstacles, such as chairs, lead to the delay of movement of pedestrians. Furthermore, pedestrians are regarded as movable obstacles. The effect of static obstacles on floor field does not change with time. However, the effect of movable obstacles on floor field is dynamic. Therefore, the whole floor field is updated dynamically according to the movement of crowd. Pedestrians may try to find another uncongested path or exit when they find the crowd in front. It provides a better description of the influence of downstream congestions on upstream crowd. The cellular automaton model based on the dynamic floor field is used to investigate the evacuation process in the case of four layouts and three exit widths. The spatial distributions of evacuation time in different conditions and also the average and maximum evacuation times are obtained. Numerical simulations reproduce the evacuation process observed in the experiment quite well. The evacuation time depends on arrangement of these desks and the exit width. For a given layout, the smaller exit leads to longer evacuation time. It is found that the evacuation time does not decrease monotonically with increasing the number of aisles, which depends on the width of aisle as well. When the aisle is not wide enough, the conflict of pedestrians from both sides reduces the efficiency of evacuation. It is helpful for coping with crowd evacuation with an aisle close to the exit side of the wall. The reasons of the differences between experimental and simulation results are also discussed in more detail.

###### ATOMIC AND MOLECULAR PHYSICS

2015, 64 (22): 223301.
doi: 10.7498/aps.64.223301

Abstract +

Coherent anti-Stokes Raman scattering (CARS) microscopy can break through the optical diffraction limit by applying the additional probe beam induced phonon depletion (APIPD). Using this method, we can obtain a spatial resolution beyond the optical diffraction limit by introducing a doughnut additional probe beam to deplete phonons at the periphery of the focal spot. To achieve higher spatial resolution and better phase matching conditions, it is necessary to use high numerical aperture objectives, whereas scalar diffraction theory is no longer valid. According to the full vector diffraction theory, we calculate the intensity distributions at the focal plane when the linearly and circularly polarized lights pass through a spiral phase plate and an objective with high numerical aperture successively. The result shows that the circular polarization can generate the perfectly doughnut-shaped focal spot, which is more suitable for the additional beam than the linear polarization induced beam. Furthermore, we analyze the APIPD induced CARS process with the full quantum theory. Simulations indicate that a spatial resolution as high as 45 nm could be realized when the ratio between the intensities of additional probe and probe is 80. And the spatial resolution turns higher with increasing the power of additional probe.

## EDITOR'S SUGGESTION

2015, 64 (22): 223302.
doi: 10.7498/aps.64.223302

Abstract +

In this paper, we prepare several GexSb20Se80-x glasses (x=5 mol%, 10 mol%, 15 mol%, 17.5 mol%, 20 mol%, and 25 mol%), and measure their Raman and X-ray photoelectron spectra (Ge 3d, Sb 4d, and Se 3d) in order to understand the evolution of the glass structure with chemical composition. We further decompose the spectra into different structural units according to the assignments of these structural units in the previous literature. It is found that the structural units of SeSeSe trimers exist in the Se-rich glasses, but the number of the structural units of trimers decreases rapidly with the increase of Ge concentration and finally becomes zero in Ge15Sb20Se65 glass. With the increase of Ge concentration, the quantity of GeSe4/2 tetrahedral structures increases, but the number of SbSe3/2 pyramidal structures remains almost unchanged in the Se-rich glasses. On the other hand, the numbers of GeGe and SbSb homopolar bonds increase with the increase of Ge concentration, but those of the GeSe4/2 tetrahedral and SbSe3/2 pyramidal structures decrease in the Se-poor glasses. Moreover, the SeSe homopolar bonds exist in all the glasses, and they cannot be completely suppressed. When the composition is close to stochiometric value, the glass is dominated by heteropolar GeSe and SbSe bonds, but has negligible quantities of GeGe, SbSb and SeSe homopolar bonds. The transition threshold, rather than the transition predicted by the topological constraint model, occurs at the chemically stoichiometric glasses. This suggests that chemical order, rather than topological order, is a main factor in determining structures and physical properties of GeSbSe glasses.

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

2015, 64 (22): 224201.
doi: 10.7498/aps.64.224201

Abstract +

Metamaterials, composed of subwavelength building blocks with artificial electric/magnetic response, have attracted the intensive interest due to the unprecedented controllability of electromagnetic (EM) waves and the potential applications. Nonetheless, the resonance of the metallic building block induces a strong loss, severely limiting the performance. Dielectric particle based subwavelength structures provide an alternative choice for the manipulation of EM waves, meanwhile, circumventing the loss problem inevitable for metallic metamaterials, in particular, in optical regime. It is shown that this kind of metamaterial can be used to guide the surface wave with the dielectric particle chain, which is similar to the surface plasmon mediated wave guiding. The structure is also shown to be capable of implementing negative refraction with negligible loss theoretically and experimentally. In addition, the single-layer dielectric rod array can be used to achieve omnidirectional total reflection at subwavelength scale. To further extend the functionality of dielectric based metamaterials and make them more appropriate for integrated optics, a variety of experimentally feasible configurations should be designed. In this work, based on the Mie scattering theory and the multiple scattering theory, we investigate the manipulation of EM waves through a single-layer subwavelength dielectric rod array (SDRA) and particle coupled system. Our results show that by removing the central dielectric rod in the SDRA and at the beam focus, like a vacancy defect, a normal incident transverse electric polarized Gaussian beam is weakly transmitted with an efficiency of less than 12 percent. By further introducing a dielectric rod with optimized parameters on the incident side of the vacancy defect, an enhanced transmitted EM wave with an efficiency of 36 percent is exhibited, nearly triple that with a solely vacancy defect. By adding another identical dielectric rod symmetrically on the outgoing side of the vacancy defect, the transmitted EM field pattern can be clearly tailored due to the dual-particle coupling so that the forward scattering is intensified, similar to the beaming effect, although the total transmittance is not further improved. Interestingly, by use of dual-particle system composed of metallic rods a similar effect can be realized as well near the surface plasmon resonance, adding flexibility to design. It should be pointed out that one-way beam propagation can be possibly achieved by constructing an asymmetric dual-particle coupling system. More importantly, the proposed systems are simple and experimentally realizable, which makes them favorable for the on-chip beam steering, offering a possibility to improve the optical element design of the integration photonic circuit in the terahertz and optical range.

2015, 64 (22): 224202.
doi: 10.7498/aps.64.224202

Abstract +

With the rapid development of wavelength division multiplexing technology, narrow bandpass filters have drawn widespread public attention. In this paper, a compact narrow bandpass filter based on two-dimensional photonic crystals is proposed. The transfer characteristics of the filter with a single mode resonator and two reflectors are analyzed by using coupled mode theory. Research results show that the bandwidth of the filter can be controlled by adjusting the distance between the resonator and the two reflectors, which can be applied to the realization of narrow bandpass filters, and even ultra-narrow bandpass filters. Based on the theoretical model mentioned above, we design a narrow bandpass filter based on two-dimensional photonic crystals, which is composed of silicon rods with square lattice in air. Two single mode waveguides are formed by removing two rows of rods. Meanwhile, a point cavity is formed by removing a dielectric column. In order to precisely control the phase change between the resonator and the two reflectors, a phase adjustment region is introduced. We study the transmission spectrum of the structure by the finite-difference time-domain (FDTD) method. We find that the bandwidth of the filter can be narrowed when the phase change between the resonator and the two reflectors satisfies the specific conditions, and the transmission ratio is still high as well. These are consistent with the theoretical analyses. But it is worth noting that there is a difference between the simulation result and theoretical result. This is because in the theoretical analysis, we consider that the propagation constants of the frequencies close to the central frequency are the same. In fact, the propagation constant increases with the increase of frequency, however, this does not affect the central frequency nor its transmission. The performance of the designed filter is analyzed by FDTD, showing that the working frequency is close to 193.40 THz, the bandwidth is smaller than 5.9 GHz, the peak transmittance is up to 94%, and the length of the working area is only 9 m. Compared with the conventional photonic crystal filters, the designed narrow bandpass filer is very compact, and the performance is suitable for dense wavelength division multiplexed communication systems.

2015, 64 (22): 224203.
doi: 10.7498/aps.64.224203

Abstract +

Polarization difference imaging technique can effectively solve the underwater image deterioration problem that is caused by the interaction between light and water. Therefore, it has a significant application value in detecting and recognizing underwater target. In a traditional polarization difference imaging system, the object image is carried out by the common-mode rejection of background scattering light. However, the polarization state of the background scattering light is unknown, so the polarization difference imaging is realized by the irregular mechanical rotation of the optical polarization analyzer with two orthogonal polarization orientations. Therefore, it needs more time to determine the optimum detection angle of the polarization analyzer and cannot perform real-time underwater imaging, which restricts the rapid detecting function in the process of underwater imaging. In this paper, the detection principle of underwater polarization difference imaging is considered to exploit the difference in the polarization angle between background scattering light and target light. According to Marius's law, the physical model of polarization difference imaging is that the common-mode rejection of background scattering light will be achieved when the angles between the vibration direction of background and the two orthogonal polarization orientations are 45. Because the Stokes vector can be used to express the polarization angle of light, we propose the principle and construction of a computational polarization difference imaging system for real-time underwater imaging by incorporating the Stokes vector into the established model. It replaces the mechanical rotation of the polarization analyzer in a traditional polarization difference imaging system with the information processing of the Stokes vector. The experimental results show that the proposed method not only has the same effective performance as the conventional polarization difference imaging compared with the regular imaging, but also can improve the blurred imaging performance caused by an underwater scattering effect as well as increase the underwater detection distance. This method realizes rapid underwater target detection and recognition because it saves a large amount of time compared with the traditional one. Further, if we combine this method with the current polarization imaging instruments that capture the Stokes vector instantaneously, then a real-time automatic underwater polarization imaging can improve the efficiency of the underwater target detection and recognition. These findings are helpful for designing and developing the underwater polarization difference imaging systems.

2015, 64 (22): 224204.
doi: 10.7498/aps.64.224204

Abstract +

In this article, we take partially coherent electromagnetic Gaussian-Schell model (EGSM) beam as a research object. Based on the unified theory of coherence and polarization and the generalized Stokes parameters of random electromagnetic beams, the expressions of degree of polarization (DoP) and orientation angle of EGSM beam that propagates in the slant atmospheric turbulence, are derived. And the expressions are used to investigate the difference in polarization property of EGSM beam between propagating along an uplink path and a downlink path in the atmospheric turbulence. The results show that under the same conditions, the DoP distribution of EGSM beam propagating along a downlink path is more concentrated than that along an uplink path. When EGSM beam propagates along a downlink path, the maximum DoP corresponding to the propagation distance is larger than along an uplink. Therefore, when EGSM beam propagates along a downlink path, the detector can receive beam information at the longer distances.

2015, 64 (22): 224205.
doi: 10.7498/aps.64.224205

Abstract +

The influence of aspect ratio on the light scattering properties of ellipsoidal particles is studied by using T-matrix method and discrete ordinate method in this paper. The light scattering characteristic quantities including extinction efficiency, asymmetrical parameter, single scattering albedo, scattering phase matrix, and bidirectional reflectance distribution function (BRDF) are computed. It is found that light scattering properties of ellipsoidal particles are sensitive to aspect ratio. The aspect ratio can influence the oscillation frequencies and amplitudes of the scattering parameters except some special size parameters. The value of asymmetrical factor could be as large as 0.3 in wave crest value of size parameter while it is no more than 0.1 at the balance location. As for the multiple scattering, the characteristics of BRDF for different aspect ratios in different incident angles and optical thickness values are analyzed, and for a further study, the relative differences of BRDF influenced by aspect ratio, optical thickness, and incident angle are analyzed. The results show that the variation trends of BRDF for the ellipsoidal particles with various aspect ratios are basically the same. However, the curve of BRDF of spherical particles (i.e., with their aspect ratios being 1) is more variable. With the increasing of aerosol optical thickness and aspect ratio, the curves of BRDF at different aspect ratios for ellipsoidal particles tend to a steady and similar value and the relative difference of BRDF decreases. But with the increasing of aerosol optical depth, the relative difference of BRDF increases with the increasing of incident angles, especially in large optical thickness, the value of the relative difference of BRDF can be as large as 15%.

2015, 64 (22): 224206.
doi: 10.7498/aps.64.224206

Abstract +

The principle of beam splitting of interference imaging spectrometer based on Savart plates is presented. The propagation track of light wave in crystal and the exit aperture is analysed by combining wave normal-tracing method with ray-tracing method at random incidence angle and azimuth angle. The relationship among exit aperture, incident angle, incident position and azimuth angle is deduced in detail. The requirements that the propagation track of light remains in Savart plates and would not exit from the top surface, bottom surface and side are discussed in detail. The area and the position of exit aperture are simulated by computer, which proves the correctness of the deduction for normal incidence of light. It is shown that the lateral shear of single Savart plate restricts the boundary of clear aperture, and the area of the clear aperture is only 85.73% against the total incident surface. The parameter of experimental prototype is introduced and the clear aperture is in detail analysed and discussed by the method mentioned above. The results show that the accurate and the approximate values of exit aperture area of experimental prototype are greatly different, and the position of exit aperture are shifted into the lower right, which may reduce the image quality and even cannot generate the double-beam interference fringes in some specific areas. The effective clear aperture as a function of azimuth angle is also presented. It shows that the used clear aperture area is between 0.8005 and 0.8547 while changes from 0 to 2, in order to match the conditions that the value of area availability decreases to 0.6976 when the light always propagates inside the Savart plates. The article shows that the change of clear aperture caused by crystal birefringence phenomenon cannot be ignored when selecting the instrument aperture stop and parameter of interference imaging spectrometer. The clear aperture of the two beams, o-light and e-light, which propagate in Savart plates should be calculated respectively and then they are used to determine the last clear apertures of plates. This study provides a theoretical and practical guidance for study, design, modulation, experiment and engineering of interference imaging spectrometers.

2015, 64 (22): 224207.
doi: 10.7498/aps.64.224207

Abstract +

Wavefront coding technique is a powerful technique which overcomes the defects of traditional way to extend depth of field. By inserting a phase mask into the traditional incoherent imaging system, wavefront coding technique does not reduce the resolution and the light gathering power of the optical system but enlarges the depth of field of incoherent imaging system. Although several kinds of phase masks have been reported, cubic phase mask is still of a classical type which has been investigated widely both in spatial and frequency domain. Since the phase profiles of phase masks adopted in classical wavefront coding systems are predefined with specific optical systems, the extension of depth of field is not tunable. Tunable wavefront coding systems are introduced by using a pair of detachable phase masks, which is possible to control the depth of field and bandwidth of system by changing the position of each component with respect to the pupil center. Ojeda-Castaeda [Ojeda-Castaeda J, Rodrguez M, Naranjo R 2010 Proceedings of Progress in Electronics Research Symposium, Cambridge, July 58, 2010 p531] proposed to use a pair of cosine phase masks to make defocus sensitivity tunable. Zhao [Zhao H, Wei J X 2014 Opt. Commun. 326 35] investigated an improved version of Ojeda- Castaneda's design in frequency domain and found that the proposed system realized tunable bandwidth. The present study, based on the work of Zhao, analyzes the tunable characteristics of a pair of simple modified detachable cubic phase masks in spatial domain and frequency domain. Firstly, the ray aberration theory is adopted to give mathematical analyses and ray aberration maps of the proposed tunable phase mask. Based on the mathematical derivations, the size of point spread function (PSF) of system can be changed not only by profile of each cubic mask but also by the each mask displacement relative to pupil center. Secondly, a mathematical PSF based on the stationary phase method is derived in spatial domain. Simulations indicate that the positions of PSF translate in the image plane with the displacements of phase mask profile and the position of each component with respect to the pupil center. By analyzing the oscillations of PSF, the effective bandwidth is obtained. Through the expression, we can conclude that the effective bandwidth can be changed by the position, mask profile of each component and defocus. Only when the addition of two mask profiles is large enough, can the effective bandwidth be simplified without adding the influence of defocus. In addition, though the approximate expression of magnitude transfer of function (MTF) has been given by adopting stationary phase method in the appendix of previous work, it cannot give an intuitive grasp of the effective bandwidth in MTF map. Unlike the MTF expression derived before, the exact optical transfer function (OTF) expression is derived by adopting Fresnel integral in frequency domain. Exact MTF and phase transfer function (PTF) can be derived from OTF. Based on the exact MTF expression, simulations give an intuitive effective bandwidth in MTF map. Simulations also show the nonlinear property of PTF. The effective bandwidth and MTF can be changed by different phase mask profiles and positions, which indicate that the effective bandwidth and defocus sensitivity can be tuned. Analyses are conducted both in spatial domain and in frequency domain to verify the tunable property of the proposed phase mask, which provides theoretical foundation for tunable wavefront coding system design.

2015, 64 (22): 224208.
doi: 10.7498/aps.64.224208

Abstract +

Quantum imaging is such an technique that the total light intensity transmitted through or reflected by an object is collected by a bucket detector, which generally is a photodiode with a collection lens in front and with no spatial resolution, and an image of the object can be retrieved through the assistance of another spatially correlated reference beam which does not interact with the object. In this paper, Quantum imaging scheme is investigated, instead of using the conventional linear detector, and a single photon detector working in a photon-counting mode is used as a bucket detector, which is the most sensitive detector in the present. It is experimentally demonstrated that quantum imaging illuminating by pseudo-thermal light can be retrieved through using the single-photon detector working in the photon-counting mode, and the averaged power received by the bucket detector is only 2 femto-Watt. It is also experimentally and theoretically demonstrated that the image of the cooperative target can be recovered through the wake scattering medium, which cannot be realized by the classical imaging method. Furthermore, it is found that the wake scattering medium has the potential application in reducing the size of the collection lens of the bucket detector, in other words, enlarging the field of view. Besides, quantum imaging recovered by correlation of intensity fluctuations and compressive sensing algorithm are compared, and the most effective ways to retrieve the image are discussed. The scheme of our experiment which is different from the traditional ways, offers a novel method to make the quantum imaging technique step further toward its applications in wake light imaging or remote sensing.

2015, 64 (22): 224209.
doi: 10.7498/aps.64.224209

Abstract +

Mode-locked fiber lasers output ultra-short pulse trains with extremely high temporal stability, showing great potential in systems that require precise timing synchronization, such as pump-probe experiments, high-speed analog-to-digital conversion, large-scale timing distribution and coherent combination. The fiber lasers are usually simpler, less costly, more efficient and more robust to the environment than solid state lasers, making them a better option for real-world applications. With the atto second temporal resolution of the balanced optical cross-correlation (BOC) method, timing jitter of mode-locked fiber lasers has been carefully measured and optimized over the last decade. However, due to the inherently large amplified spontaneous emission noise in the long gain fiber and broad pulse width inside the laser cavity, the quantum-noise-limited timing jitter of mode-locked fiber lasers is still much higher than that of the solid state lasers. In order to further optimize the timing synchronization of mode-locked fiber laser, larger locking bandwidth is required to suppress the low-frequency timing jitter, which contributes significantly to the total amount of residual timing jitter. In this work, tight timing synchronization between two mode-locked Yb-fiber lasers is achieved via a feedback loop built on an intra-cavity electro-optic phase modulator. Both lasers work in the stretched-pulse regime, which has been proven to support the lowest quantum-noise-limited timing jitter of mode-locked fiber laser. The output of the BOC system provides a timing error discriminator of 40 mV/fs, corresponding to 13 as resolution within the integration bandwidth. When the pulse trains from both lasers are successfully synchronized, the residual timing jitter can be measured with the same signal as that used for timing synchronization Based on the residual timing jitter measurement, the intra-cavity dynamics of the laser and the locking parameters of the feedback loop can be further optimized and a tight synchronization with 400 kHz locking bandwidth is finally achieved. When performing the integration from 1 Hz to 10 MHz, the residual timing error is as low as 109 as, corresponding to 77 as averaged timing jitter of each laser. A parallel out-of-loop single-arm cross-correlation measurement is also performed to test the validity of the in-loop results, and both measurements agree with each other.

2015, 64 (22): 224210.
doi: 10.7498/aps.64.224210

Abstract +

In order to analyze the characteristic of the close optical axis (conjugate optical axis) and the existence condition of resonator composed of multiple reflective mirrors, the existence condition of conjugate optical axis of the multiple-resonator consisting of many flat mirrors is analyzed and derived from the angle of beam conversion coordinate transformation. The results show that a closed ray axis in resonator with odd number of mirrors can exist only if each mirror is suitably aligned, while a closed ray axis always exists in non-planar resonators with even number of mirrors, and the angle of the cavity conjugate axis direction changes due to the misalignment of different mirrors. Then from the point of view of the optical multi-pass matrix, the incidence direction of the self-conjugate ray of the resonator consisting of spherical mirrors is analyzed. A detailed analysis of conjugated axis of the resonator consisting of two flat mirrors and one spherical mirror is conducted, and the results show that when different mirrors have angle deviations, the closed conjugated optical axis remains in the cavity, the change of resonator axis occurs, and the position and orientation of new resonance surface are given, thereby indicating that in the case of resonator with spherical mirrors there is a self-conjugate ray irrespective of the other flat mirrors positions. All of these will provide theoretical guidance for achieving the high-accuracy alignment and improving the measurement accuracy of spectral measurement technology based on high-quality optical cavity.

2015, 64 (22): 224211.
doi: 10.7498/aps.64.224211

Abstract +

In order to reduce the influences of misalignment parameter and mismatch parameter on measurement based on optical resonator, the influence on the coupling efficiency of a source laser is stabilized to a fundamental cavity mode, and two limiting cases are analyzed and derived by using conversion of Gaussian beam, mode coupling theory and coordinate transformation theory, including the expression of coupling efficiency of fundamental cavity mode as two limiting cases emerge simultaneously. Analyses show that for mismatch parameter, only even-indexed Hermite-Gaussians beam is excited; for misalignment parameter, there exists an effect on the proportion of Hermite-Gaussians beam, which should bring about serious measurement error. These optical signals provide the error signals which are minimized. By taking the laser line width into account, we propose two methods for real time alignment of a Gaussian beam for an optical resonator perfectly coupled system: Fabry-Perot electro-optic sensors of a misadjusted system and control loops system depends on detecting emergent light of cavity via multi-dimensional quadrant detector. All of these will provide a theoretical direction for analyzing the measurement error and improving the measurement accuracy.

2015, 64 (22): 224213.
doi: 10.7498/aps.64.224213

Abstract +

The time-delay signature (TDS) and the bandwidth (BW) are two important performance indexes to assess the chaos signal from a delayed laser system. Based on the spin flip model of vertical-cavity surface-emitting laser (VCSEL), we numerically investigate the characteristics of chaos dynamics in a master-slave VCSEL system, where a chaotic signal generated by a master VCSEL (M-VCSEL) under external optical feedback is unidirectionally injected into a slave VCSEL (S-VCSEL). The influences of injection strength, frequency detuning between M-VCSEL and S-VCSEL, and feedback strength of M-VCSEL on chaos TDS (including intensity TDS (I-TDS) and phase TDS (P-TDS)) and BW are investigated. The results show that by adjusting the injection strength and the frequency detuning, both I-TDS and P-TDS of two polarization components (referred to as X-PC and Y-PC respectively) of the chaotic output from the system can be suppressed simultaneously. Through further analyzing the influences of the injection strength and frequency detuning on the BW of chaotic signal, we find that the BWs of both X-PC and Y-PC of chaotic outputs can simultaneously exceed 30 GHz within a large negative frequency detuning range. Furthermore, by combining the evolution characteristics of the TDS and BW of chaotic outputs in the parameter space of injection strength and frequency detuning, the parameter region for generating the chaotic signals with wide BW and low TDS can be determined. In addition, by reasonably adjusting feedback strength, the quality of chaotic signal from the system can be further optimized.

2015, 64 (22): 224214.
doi: 10.7498/aps.64.224214

Abstract +

A coupled quantum system composed of cavity field and atoms is one of the main research contents of cavity quantum electrodynamics. It can be used to realize single atom manipulation and measurement, and has important significance for studying the interaction between light and the atom, preparing quantum states and quantum entanglement. Current research work mainly focuses on two aspects. One is to achieve the atom trapping via the feedback control of the trapping laser intensity. The other is to measure the single atomic motion in a Fabry-Perot cavity by using Hermite-Gaussian transverse modes. The detection of the atomic trajectories has been realized via the observation of transmission spectra of the strong coupling system composed of cold atoms and Hermite-Gaussian transverse modes in a Fabry-Perot cavity. In order to observe the atomic motion trajectories in the cavity, we theoretically study the transmission spectrum of a strong coupling system composed of cold atoms and Laguerre-Gaussian transverse modes in a Fabry-Perot cavity in this paper. We calculate the relationship between the coupling coefficient and the mode number of Laguerre-Gaussian transverse modes. The result shows that with the increase of Laguerre-Gaussian transverse mode number, the maximum coupling coefficient between the atoms and cavity fields is almost unchanged, so the contrast of the detected spectrum is nearly independent of the mode number. Analysis shows that Laguerre-Gaussian transverse mode provides more abundant information about atomic motion trajectory than Hermite-Gaussian transverse mode. The field distribution of Laguerre-Gaussian transverse mode is ring-shaped. Owing to the ring shape, the atoms dropped at different positions experience different electric field intensities, and the detected transmission spectra are changed. Therefore, we can implement the high precision distinguishment of the atomic trajectories by observing the features of the transmission spectra such as the number of the transmission peaks and their positions. Furthermore, a small deviation of the atomic motion trajectories, on the edges of the rings of the electric field, may induce great change in transmission spectrum, and then we can very accurately detect the atomic motion around these positions.

2015, 64 (22): 224215.
doi: 10.7498/aps.64.224215

Abstract +

Continuous-wave (CW) coherent sources emitting two wavelengths of 1.57 m and 3.84 m have aroused much interest of scientists due to their many applications such as military multiband composite guidance, remote monitoring of the special environment, etc. Quasi-phase matching (QPM) optical parametric oscillator (OPO) device with periodically inverted structure of nonlinear coefficient can implement an efficient and wavelength conversion at arbitrary wavelength in the transparent range of the QPM material. Nowadays, using MgO:PPLN for QPM, various MgO:PPLN-OPOs pumped by conventional 1.06 m laser source can produce 1.57 m and 3.84 m laser and also achieve good results. But as a result of the limitation of momentum conservation condition and periodically poled structure, 1.57 m and 3.84 m laser can only meet a single band. To obtain the two-wavelength laser output at the same time, the MgO:PPLN-OPO could not be applied. In this paper, a CW 1.57 m and 3.84 m intra-cavity multiple optical parametric oscillator based on MgO:APLN is reported. The cross period parameter light is obtained by using a folded type doubly cavity which consists of 1064 nm resonator and multiple optical parametric oscillator. Considering both its thermal stability under high power pump and the light spot mode matching of multiple optical parametric oscillation process, through numerical simulation and theoretical analysis of two sub cavities, the optimum parameters of the cavity structure are determined. On this basis, the influences of output coupler transmittance on oscillation threshold, the down-conversion efficiency, output power stability are investigated in experiment. With T=10% at 1.47 m and 3.3 m output coupler used, the maximum output powers of 3.13 W at 1.57 m and 0.85 W at 3.84 m are obtained, corresponding to slope efficiencies of 6.8% and 1.9%, respectively. The power stabilities are better than 1.8% and 3% at the maximum output power in half an hour. The experimental results show that the intra-cavity multiple optical parametric oscillator based on a single poled crystal MgO:APLN is an effective method of obtaining a 1.57 m and 3.84 m CW laser.

2015, 64 (22): 224216.
doi: 10.7498/aps.64.224216

Abstract +

In this article, the Von-Karman model of turbulence spatial correlation function which contains the parameter of outer scale is analyzed. Then, the experimental data of air refractive index variation obtained from a high quality fiber optical turbulence sensing array are used to evaluate the outer scale of atmospheric optical turbulence as well as its diurnal variation through the algorithm of nonlinear fitting. The results validate the suitability of the Von-Karman model. By combining the theory of correlation function with the spatially distributed and simultaneously measured data, three kinds of turbulence spatial scales described by correlation function are revealed as clearly as possible. Results show that the values of outer scale in atmospheric optical turbulence 1.8 m above the grassland tend to be larger in the daytime and smaller in the night. The mean value around noon is 0.44 m, while in the night it becomes 0.3 m. Here, three of the important views should be noted. Firstly, when the displacement of two points is just equal to the outer scale, their correlation coefficient is 0.26, and when it exceeds the outer scale, there is still a certain value of correlation coefficient. Secondly, the integral scale represents the averaged value of scale in the vortex of optical turbulence. And, it is slightly smaller than the outer scale. Thirdly, when the distance of two points equals the biggest scale of vortex in optical turbulence, the correlation coefficient tends to zero, and the value of biggest scale is slightly bigger than the outer scale. It is easy to find that the diurnal variation tendencies of the three spatial scales are similar to that of intensity in optical turbulence. The method of obtaining the characteristic scales by spatially arranged and simultaneously measured optical turbulence is direct, and the results can be considered as the evidence to prove the models of correlation function including the Von-Karman model. So, it promotes the research on the property of spatial structure to a certain extent.

2015, 64 (22): 224217.
doi: 10.7498/aps.64.224217

Abstract +

The generation of atmosphere turbulence wave-front is important for studying the light propagation and imaging through the atmosphere, and correcting the atmosphere turbulence, such as the adaptive optics system. The power spectral density method generates phase screens quickly for using the fast Fourier transform (FFT). The main drawback to this approach is that lower order aberrations such as tilt are often under represented. The reason is that the low frequency is sampled inadequately. Since the low order aberrations include a major percentage of the atmospheric energy spectrum, the error of simulated phase screens makes this method less desirable to use. To overcome this shortcoming, a non-uniform sampling method is proposed to generate phase screens accurately. Unfortunately, when the sampling is nonuniform, the FFT does not apply directly. Generating such a phase screen is computation intensive which greatly reduces simulation speed. In this paper, we develop a fast, more accurate method to generate atmospheric turbulence phase screens, according to non-uniforming sampling.
The nonequispaced fast Fourier transform (NUFFT) arises in a variety of application areas, ranging from medical imaging to radio astronomy to the numerical solution of partial differential equations. Speeding up the simulation of atmospheric turbulence phase screens is possible by using the non-uniform fast Fourier transform. In this paper, the atmospheric turbulence phase screen is decomposed into a series of harmonics. Then the non-uniform distributed harmonics are projected onto over-sampled uniform grid by using the Gaussian kernel function. Atmospheric turbulence phase screen will be generated using the standard fast Fourier transform on the over-sampled uniform grid. The atmospheric turbulence phase screens can be generated quickly. Using Kolmogorov spectrum model in this paper, the phase screens can be generated quickly. The performances of generated phase screens are analyzed through their phase structure functions. The statistical results are in very good agreement with the theoretical values. The relative error curve of simulation phase screens is calculated and analyzed. The more the oversampling grid, the more the relative error is. Compared with the result from the direct harmonics summation method, the error here mainly concentrates in high-frequency region where the sampling frequency points are sparse. However, the atmosphere turbulence phase screen is simulated in high accuracy on the whole. Compared with the time cost of the harmonics summation, the time using NUFFT is decreased to about 800 times. The simulated phase screens indicate that non-uniform fast Fourier transform is able to generate atmospheric turbulence phase screen with high accuracy and fast speed.

2015, 64 (22): 224218.
doi: 10.7498/aps.64.224218

Abstract +

The lattice parameters and band-gap of native rutile TiO2 are investigated by the first-principles calculations of local density approximation+U method with different U values for Ti-3d (0 U 9 eV). The electronic structures and optical properties of different content C doped rutile TiO2 systems are also studied by the same method with appropriate U values. The calculations results show that the lattice parameters and band-gaps of TiO2 increase with the increase of U and the U =3 eV is fitted for the corrected band-gap. For the doped systems, the impurity energy level is introduced due to the coupling between O-2p and C-2p, which can increase the TiO2 absorption edge to the visible region, and therefore enlarge the absorption region of TiO2. Moreover, the 8.3% C is an optimal doped density, which can lead to the red-shift of optical absorption edge obviously and increase the coefficient of light absorption, therefore facilitate the enhancement of the photocatalytic efficiency.

2015, 64 (22): 224219.
doi: 10.7498/aps.64.224219

Abstract +

A novel circularly polarized patch antenna, which can achieve low radar cross section (RCS) and high gain performance simultaneously, is designed on the basis of metamaterial superstrate. The novelty of the design is that this antenna can possess the absorbing characteristic and the partially reflective characteristic simultaneously in an integrated structure. The proposed superstrate is composed of two metallic layers with different periodic patterns on both sides of a dielectric substrate. Through constructing different metallic patterns on the two sides of the superstrate, the upper and bottom surfaces of the superstrate will have different transmission and reflection performances when illuminated by an incident plane wave. The low RCS characteristic is dependent on the upper surface, while the gain enhancement of the resonator antenna relies on the reflection coefficient of the bottom surface. The upper surface consisting of a periodic metallic square loop with four lumped resistances on the four sides of the loop is of low reflection and transmission, and the bottom surface composed of a metallic plane with periodic slots is of high reflection and low transmission. When the superstrate is located at approximately half a wavelength above the ground plane of the circularly polarized patch antenna, the upper surface will absorb most of the incident wave by converting the electromagnetic wave into heat as Ohm loss to reduce the antenna RCS, and the bottom surface will form a Fabry-Perot resonance cavity with the ground plane of the antenna to achieve high gain and high directivity by multiple reflections between the bottom surface and the ground plane. The measured results show that with using the superstrate, the relative axial ratio bandwidth of the circularly polarized patch antenna extends from 5.9% to 7.1%, and the high gain performance is achieved in the whole working frequency band, which can be enhanced by 6.61 dB at most. Meanwhile, the RCS of the proposed antenna is dramatically reduced in a wide angle range and a broad frequency band covering a range from 2 to 14 GHz. The measured results are in good agreement with the simulated ones, which further verifies the correctness and effectiveness of the proposed method.

## EDITOR'S SUGGESTION

2015, 64 (22): 224220.
doi: 10.7498/aps.64.224220

Abstract +

We report Bi-doped fibers prepared by modified chemical vapor deposition combination with solution doping process. The fibers are divided into three groups under 60Co radiations with different doses. The absorption spectra and fluorescence spectra of the fiber before and after irradiation are investigated. The dependence of fluorescence intensity of the fiber on temperature (-40-70 ℃) are measured. Experimental results show that the radiation-induced absorptions (RIAs) of the fibers increase significantly at 700 nm and 800 nm with the increase of the irradiation dose. We ascribe the great enhancement of the RIA of the fiber to the generation of more Bi near-infrared (NIR) active centers. Because near infrared nonluminous valance state like Bi3+ captures free electrons and converts into Bi2+, and further into Bi+under the 60Co radiations with different doses. We also find that the NIR fluorescence spectra are stable before and after irradiation under 976 nm LD excitation. The possibility of communication in a radiation environment is proved, such as in outer space is proved. In addition, the fluorescence intensity dependence on temperature in a full-temperature range is analyzed, and we find that the fluorescence intensity decreases with the increase of temperature. It is contributed to the Bi active center that Bi+ may gradually turn into nonluminous Bi metallic colloids during thermal activation. The variation law of fluorescence intensity is analyzed in the whole range of temperature. We believe that the variation law of fluorescence intensity provides data and basis for the stable operation of bismuth-doped fiber laser in the future.

2015, 64 (22): 224221.
doi: 10.7498/aps.64.224221

Abstract +

The photonic crystal fiber has received the widespread attention in the sensing field because of its flexible structure and unique features. A refractive index and temperature sensor based on the D-shaped photonic crystal fiber is designed and analyzed. In the side section of the D-shaped photonic crystal fiber, a coat with a gold film is used as a surface plasmon resonance (SPR) sensing channel for measuring the refractive index of liquid determinand. Temperature sensitive liquid-toluene is filled in an air hole A as a directional coupling sensing channel to realize the temperature measurement. When the SPR mode and guided mode satisfy the phase matching condition, the SPR effect is produced. Most of the core energy is transferred to the metal film layer in the surface, and then the loss of guided mode in the fiber core will grow. Therefore, the shift of the SPR peak position can be used to measure the refractive index of the determinand indirectly. When the wave mode in the thermosensitive liquid-toluene can achieve phase matching with the guided mode, the directional coupling effect occurs, and then the wavelength of the absorption peak position can be used to measure the change of temperature indirectly. Based on further numerical simulation, the peak position of directional coupling is not changed by the refractive index of the determinand, and the SPR peak position is not shifted by the temperature change either. As these two sensing mechanisms can be distinguished easily, the refractive index and temperature sensing are simultaneously realized. The characteristics of the sensor are simulated numerically by using a full vector finite element method under the boundary condition of anisotropic perfectly matched layer. From the analysis of the D-shaped photonic crystal fiber structure parameters, we find that the diameter d of air hole plays an important role in the directional coupling absorption peak position and temperature sensitivity. For the SPR peak, its position is only affected by the thickness t of gold film, and its relative intensity is changed with the diameter d of air hole and grinding depth d1. The results show that when the temperature ranges from -10 ℃ to 80 ℃, the temperature sensitivity reaches 11.6 nm/℃, and when the refractive index is in a range from 1.32 to 1.44, its sensitivity reaches 26000 nm/RIU.

2015, 64 (22): 224301.
doi: 10.7498/aps.64.224301

Abstract +

The mathematical expressions both of displacement and stress fields of circumferential wave propagation in circular tube structure with a weak interface are derived on condition that the interfacial properties between the two circular tubes are characterized by the interfacial spring model. Based on the said displacement and stress expressions derived, the dispersion equation of ultrasonic guided circumferential wave (UGCW) modes is formally presented by using the corresponding mechanical boundary conditions. According to the technique of modal expansion analysis for waveguide excitation, for a given excitation source used to generate circumferential wave in circular tube structure, the corresponding field of circumferential wave propagation can be decomposed into a series of UGCW modes. Using the reciprocity relations and mode orthogonality, the analytical expression of UGCW mode expansion coefficient is derived, which is closely related to the given excitation source for UGCW generation and the interfacial properties between the two tubes. The influences of change in the interfacial property on dispersion and acoustic field of the UGCW propagation are numerically analyzed. In the cases of perfect and sliding interfaces, for a given UGCW mode, the relative change rate of phase velocity is defined, and then its curve versus frequency is calculated, through which the specific frequency can be determined where the UGCW phase velocity appears to be most sensitive to the change in the interfacial property. For a given UGCW mode and driving frequency, it is numerically found that the displacement field on the outside surface of the circular tube structure changes sensitively and monotonically with change in interfacial property between the tubes. Clearly, through choosing the appropriate driving frequency and the mode of UGCW propagation, both the UGCW phase velocity and the displacement field on the outside surface of the circular tube structure will be monotonic and sensitive to change in interfacial property. It is expected that the results obtained in this paper will be of significance for accurately characterizing the interfacial properties of composite circular tube structures by using the UGCW technique.

2015, 64 (22): 224401.
doi: 10.7498/aps.64.224401

Abstract +

Based on the transformation thermodynamics, the thermal conductivity expression for the unit cell of the directional heat transmission structure is derived by the oblique and rotary coordinate transformation. We obtain the two-dimensional plate directional heat transmission structure through periodically arranging the unit cells which are realized by layering copper and thermal insulation materials. The results from the numerical calculation indicate that the heat flux flows from the upper surface of the directional heat transmission structure to the two sides, while the upper and lower surface remain at low temperature. Compared with the temperature of SiO2 aerogel thermal insulation material, the upper surface temperature falls 33.3%, the low surface temperature falls 4.3%, while the temperatures of the two sides rise 40.1%. The decrease of the upper surface temperature indicates that the heat on the upper surface can be guided timely, and then the infrared radiation can be weakened. The decrease of the lower surface temperature indicates that the adiabatic efficiency of the directional heat transmission structure is superior to that of the SiO2 aerogel thermal insulation material. The heat transmission from the upper surface to the sides is conducive to the good use of the heat flux. The directional heat transmission has a potential application in the infrared stealth and heat protection.

2015, 64 (22): 224601.
doi: 10.7498/aps.64.224601

Abstract +

In this paper, we analytically study the spurious lateral mode of the ring (circular) thin-film bulk acoustic resonator (FBAR) by using Tiersten equation. The lateral mode displacement field and frequency dispersion equation are obtained. According to the electromagnetic mode analysis, we find that the mode frequency and spurious electrical responses relate to the ratio of inner radius to outer radius (a/b) of the ring resonator, and its lateral vibration mode can be obtained by coupling other circular FBAR modes. The ring electrode can greatly reduce the number of spurious electrical responses caused by lateral resonances. Suppressing lateral mode and adjusting fundamental frequency can be achieved by controlling a/b. In this paper, the experiments for the same batch of ring and circular FBARs are carried out by using a heterodyne interferometer and a vector network analyzer, including the measurements of acoustic wave fields and eigenmode spectra, which can provide the information about vibration localization and coupling between lateral mode and thickness extensional mode. The data indicate that the lateral vibration mode of ring FBAR can be obtained by coupling the two modes of circular FBARs, whose radii are a and b, respectively, and the lateral mode pattern of n' = 0 is suppressed. When the ring resonator is designed with an a/b ratio of 0.436, the fundamental frequency (～ 1217 MHz) is the same as the (0, 1) mode frequency of the circular FBAR. Based on this observation, the acoustic wave field images and electrical spurious responses can accurately describe the lateral modes, and the obtained results accord well with the analyses of theoretical electromagnetic modes. This phenomenon may be found to have applications in the design and theoretical analysis of the resonators.

2015, 64 (22): 224701.
doi: 10.7498/aps.64.224701

Abstract +

In this paper, a series of experiments are conducted to understand the influence of Soret effect on thermal convection of binary mixture in a cylindrical pool with a free surface. The cylindrical pool is filled with the n-decane/n-hexane mixture with an n-decane initial mass fraction of 50%. The cylindrical pool and the disk on the free surface are kept at constant temperatures of Th and Tc (Th Tc), respectively. Temperature fluctuation pattern on the free surface is obtained by the schlieren method. Various temperature oscillatory patterns on the free surface are observed when the thermal convection of the n-decane/n-hexane mixture destabilizes at different aspect ratios. Results show that the critical thermal capillary Reynolds number of the incipience of the three-dimensional oscillatory flow in the n-decane/n-hexane mixture is smaller than that in the n-hexane fluid, and the variation tendency with the aspect ratio in the n-decane/n-hexane mixture is the same as that in the n-hexane fluid. The solute-capillary force caused by Soret effect plays an important role of the thermal convection in the n-decane/n-hexane mixture. Because the solute-capillary force has the same direction as the thermocapillary force, the thermal convection in the n-decane/n-hexane mixture becomes more instable and the critical thermocapillary Reynolds number is smaller than that in the n-hexane fluid. In the n-decane/n-hexane mixture, when the aspect ratio increases from 0.0217 to 0.0392, the critical thermal capillary Reynolds number decreases from 7.2104 to 5.0104. With the increase of the aspect ratio, the effect of the buoyancy is enhanced, and the critical thermocapillary Reynolds number decreases. When the aspect ratio increases from 0.0392 to 0.0434, the cold plume which facilitates destabilizing the thermal convection cannot be obviously enhanced. There is little effect of the cold plume on the fluid near the bottom. Therefore, the critical thermal capillary Reynolds number increases from 5.0104 to 6.4104 in this range. In the deep pool, the critical thermal capillary Reynolds number is almost a constant value. When the aspect ratio is smaller than 0.0848, the three-dimensional oscillatory flow occurs and the hydrothermal waves are observed. After the three-dimensional oscillatory flow appears, two groups of the hydrothermal waves with opposite propagating directions coexist in the pool. With the increase of the thermal capillary Reynolds number, the honeycomb-like patterns appear on the free surface, which are similar to the Bnard cells. In addition, the non-dimensional fundamental oscillation frequency increases with the thermal capillary Reynolds number. When the aspect ratio is bigger than 0.0848, spoke pattern, rosebud-like pattern and thin-longitudinal stripes will appear sequentially with the increase of thermocapillary Reynolds number. Furthermore, the number of the rosebud-like patterns decreases, while the area on the free surface in the pool occupied by the rosebud-like pattern increases with the increase of the thermal capillary Reynolds number.

2015, 64 (22): 224702.
doi: 10.7498/aps.64.224702

Abstract +

Most of previous studies focused on the boundary-layer receptivity to the convected disturbances in the free stream interacting with localized wall roughness. Whereas the research on the boundary-layer receptivity induced by localized blowing or localized suction is relatively few. In this paper, we investigate two-dimensional boundary-layer receptivity induced by localized blowing/suction within free-stream turbulence through using direct numerical simulation and fast Fourier transformation. High-order compact finite difference schemes in the y-direction, fast Fourier transformation in the x-direction, and a Runge-Kutta scheme in time domain are used to solve the Navier-Stokes equations. The numerical results show that Tollmien-Schlichting (T-S) wave packets are excited by the free-stream turbulence interacting with localized blowing in the two-dimensional boundary layer, which are superposed by a group of stable, neutral and unstable T-S waves. The dispersion relations, growth rates, amplitude distributions and phase distributions of the excited waves accord well with theoretical solutions of the linear stability theory, thus confirming the existence of the boundary-layer receptivity. And the frequencies of the instability waves are between the upper and lower branches of the neutral stability curves. According to the evolutions of the wave packets, the positions of peaks and valleys are tracked over time to calculate the propagation speed by taking the average. The propagation speeds of the wave packets are approximately one-third of the free-stream velocity, which are in accordance with Dietz's measurements. The propagation speeds of wave packets are also close to the phase speeds of the most unstable waves for the numerical results. The relations of the receptivity response to the forcing amplitude, the blowing intensity, and the blowing width are found to be linear, when the forcing amplitude and the blowing intensity are less than 1% free-steam velocity amplitude and 0.01, respectively. And the maximum amplitudes of the T-S waves can be excited while the blowing length is equal to the resonant wavelength /(TS-FS), where TS is the wave-number of the T-S wave, and FS is the wave-number of the forcing disturbance. These results are similar to those given by Dietz [Dietz A J 1999 J. Fluid Mech. 378 291]. Additionally, T-S waves with the same dispersion relations but opposite phases are generated by localized blowing and localized suction respectively, and the amplitudes of the T-S waves excited by localized blowing are nearly 15% greater than those by localized suction under the same condition. According to this theory, an optimal design of localized suction device is able to enhance or delay the laminar-turbulent transition for turbulent control.

2015, 64 (22): 224703.
doi: 10.7498/aps.64.224703

Abstract +

How to solve hypersonic aerothermodynamics and complex flow mechanism covering various flow regimes from high rarefied free-molecular flow of outer-layer space to continuum flow of near-ground is one of the frontier basic problems in the field of fluid physics. In this work, the unified Boltzmann model equation based on the molecular velocity distribution function is presented for describing complex hypersonic flow transport phenomena covering all flow regimes by physics analysis and model processing of the collision integral to the Boltzmann equation. The discrete velocity ordinate method is developed to simulate complex flows from low Mach numbers to hypersonic flight, and the gas-kinetic coupling-iteration numerical scheme is constructed directly to solve the evolution and updating of the molecular velocity distribution function by employing the unsteady time-splitting method and the NND finite-difference technique. Then, the gas-kinetic unified algorithm (GKUA) is presented to~simulate the three-dimensional hypersonic aerothermodynamics and flow problems around space vehicles covering various flow regimes from free-molecule to continuum. To verify the accuracy and reliability of the present GKUA and simulate gas thermodynamic transport phenomena covering various flow regimes, firstly, the two-dimensional supersonic flows around a circular cylinder are simulated in the continuum regime of Kn= 0.0001 and in the high rarefied regime of Kn= 0.3 through the comparison between the Navier-Stokes (N-S) solution and the direct simulation Monte Carlo (DSMC) result, respectively. It is indicated that the GKUA can exactly converge to the N-S solution in the continuum flow regime, and the computed results of the GKUA are consistent with the DSMC simulation with a small deviation of 0.45% in the high rarefied flow regime. Then, the three-dimensional complex hypersonic flows around reusable satellite shape are studied as one of the engineering applications of the GKUA with a wide range 0.002 Kn 1.618 of the free-stream Knudsen numbers and different Mach numbers during re-entering Earth atmosphere with the flying altitudes of 110-70~km. The computed results are found to be in high resolution of the flow fields and in good agreement in a deviation range of 0.27%-8.56% by comparison among the relevant reference data, DSMC and theoretical predictions. The complex flow mechanism, flow phenomena and changing laws of hypersonic aerothermodynamics are revealed for spacecraft re-entry into the atmosphere, and the effects of rarefied gas and wall temperature on the aerothermodynamics characteristics of re-entry satellite shape are compared and analysed with different Knudsen numbers and wall temperature ratios of Tw/T = 1.6, 10 and 15.6. It is validated that the non-dimensional heat flux coefficient in the rarefied transitional flow regime is higher than that of the continuum and near-continuum flow regimes, the high wall temperature results in the enlarging amplitude of temperature variation on the stagnation line and the serious effect on the heat flux of the stagnation point, and wall temperature becomes lower, the heat flux coefficient of wall surface becomes bigger, and the friction force and pressure coefficients decrease. The non-equilibrium level of flow velocity slip and temperature jump on the surface of space vehicle becomes severer, and the stronger heat transfer effect between the space vehicle and the gas flow is produced as the Mach number or Knudsen number of the free-stream flow increases. It can be realized from this study that the gas-kinetic unified algorithm directly solving the Boltzmann model velocity distribution function equation may provide an important and feasible way that complex hypersonic aerothermodynamic problems and flow mechanisms from high rarefied free-molecule to continuum flow regimes can be solved effectively and reliably.

2015, 64 (22): 224704.
doi: 10.7498/aps.64.224704

Abstract +

A numerical model is developed using the coupled level set and volume of fluid method including heat transfer and contact resistance to simulate air entrapment during a droplet impacting on a wetted surface. The dynamic characteristics of the phase interface are analysed. The mechanisms of deformation of the phase interface and formation of entrapped air are explored. The effects of impacting velocity and thickness of liquid film on characteristics of entrapped air are studied. The mechanism of heat transfer is also obtained in this article. The obtained results are as follows. The pressure difference between liquid and gas before the droplet impacting is the main factor determining the deformation of phases interface and the formation of air entrapment. The larger the impacting velocity, the larger the pressure inside the compressed air film is. When the droplet contacts the liquid film, the velocities of the droplet and liquid film increase to their maximum values, and at the impacting axis, they are approximately the same, nearly half the impacting velocity. The velocity distributions of phase interface of the droplet and liquid film are nearly the same in the area of impacting center. The impacting velocity has important effects on the dimensionless arc from bottom to breaking point and the dimensionless diameter of the air. The dimensionless arc and dimensionless diameter decrease with increasing impacting velocity. The dimensionless deforming heights of the droplet and liquid film are closely related to Stokes number: the larger the Stokes number, the larger the dimensionless deforming heights are, and they can be expressed as a power function with Stokes number. The initial thickness of liquid film also affects dimensionless deforming heights of the droplet and liquid film and dimensionless diameter of the entrapped air: the larger the dimensionless thickness of the liquid film, the larger the dimensionless deforming heights are, and the dimensionless diameter decreases with increasing dimensionless thickness of the liquid film. At the very initial stage of the impact, the entrapped air is important for surface heat flux distribution. The entrapped air presents contraction, breakup and detachment. The surface heat flux distribution changes closely with evolution of the entrapped air and tends to be uniform. The effect of the entrapped air on the surface heat flux distribution decreases gradually.

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

2015, 64 (22): 227101.
doi: 10.7498/aps.64.227101

Abstract +

By using first-principles with pseudopotentials method based on the density functional perturbation theory, in this paper we calculate the electronic properties of wurtzite 2H-SiC crystal under the strong laser irradiation and analyze the band structure and the density of state. Calculations are performed by using the ABINIT code in the generalized gradient approximation for the exchange-correlation energy. And the input variable tphysel, which is a variable in the ABINIT code and relates to the laser intensity, is used to define a physical temperature of electrons Te. The size of Te is set to simulate the corresponding electron temperature of the crystal when intensive laser irradiates it in an ultrafast time. The high symmetry points selected in the Brillouin zone are along -A-H-K--M-L-H in the energy band calculations. After testing, we can always obtain a good convergence of the total energy when choosing a 20 Hartree cut-off energy and a 442 k-points grid. Then, optimizing the structure, and the structural parameters and the corresponding electronic properties of 2H-SiC in the different electron-temperature conditions are studied using the optimized equilibrium lattice constant. The calculation results indicate that the equilibrium lattice parameters a and c of 2H-SiC gradually increase as the electronic temperature Te goes up. With the electronic temperature going up, the top of valence band is still at , while the bottom of conduction band shifts from the K point with increasing electronic temperature, resulting in the fact that 2H-SiC is still an indirect band-gap semiconductor in a range of 0-2.25 eV and when the electronic temperature reaches 2.25 eV and even more than 2.5 eV, the crystal turns into a direct band-gap semiconductor. With Te rising constantly, the bottom of the conduction band and the top of valence band both move in the direction of high energy or low energy. When Te exceeds 3.5 eV, the top of valence band crosses the Fermi level. When Te varies in a range of 0-2.0 eV, the forbidden bandwidth increases with temperature rising, and when Te varies in a range of 2-3.5 eV, the forbidden bandwidth quickly decreases. This variation shows that the metallic character of 2H-SiC crystals increases with electronic temperature Te rising. The total density of states (DOS) and partial density of states are calculated at Te=0 eV and 5 eV. The DOS figures indicate that 2H-SiC is a semiconductors and its energy gap equals 2.3 eV. At Te =5 eV, the gap disappears, exhibiting metallic properties. This result shows that the crystal covalent bonds weaken and metallic bonds strengthen with temperature rising and the crystal experiences the process of melting, shifting to metallic state.

2015, 64 (22): 227301.
doi: 10.7498/aps.64.227301

Abstract +

Optical properties of two-dimensional periodic annular cavity arrays in hexagonal packing are investigated by finite difference time domain simulation method in this paper. According to simulated reflectance/transmission spectra, electric field distribution and charge distribution, we confirm that multiple cylindrical surface plasmon resonances, which result in reflectance dips, can be excited in annular cavities by linearly polarized light. Mechanism of the cylindrical surface plasmons is investigated. A coaxial waveguide mode TE11 is excited in the annular cavities and a Fabry-Perot resonance is fulfilled along the depth direction of the annular cavities at the resonance wavelengths. While the number of reflectance dips and wavelengths of these dips in reflectance spectra are dependent on the geometric sizes of the annular cavities, the periodicity and polarization of incident light do not affect their reflectance spectra dramatically. Incident light beams with resonant wavelengths are localized in annular cavities with large electric field increasing and dissipate gradually due to metal loss. Reflectance dips can be tuned from 350 to 2000 nm by adjusting geometric size parameters of the annular cavities, such as outer and inner radii of the annular gaps, gap sizes and metal film thickness values. Reflectance dips shift toward longer wavelength with increasing inner and outer radii of the annular gaps, metal film thickness and with reducing the gap distance. In addition, infiltrate liquids in the annular gaps will result in a shift of the resonance wavelengths, which makes the annular cavities good refractive index sensors. A refractive index sensitivity up to 1850 nm/RIU is demonstrated. The refractive index sensitivities of annular cavities can also be tuned by their geometric sizes. Annular cavities with large electric field enhancement and tunable cylindrical surface plasmons can be used as surface enhanced Raman spectra substrates, refractive index sensors, nano-lasers and optical trappers.

2015, 64 (22): 227302.
doi: 10.7498/aps.64.227302

Abstract +

Blocked impurity band (BIB) detectors, developed from extrinsic detectors, have long been employed for ground-based and airborne astronomical imaging and photon detections. They are the state-of-the-art choice for highly sensitive detection from mid-infrared to far-infrared radiation. In this work, we demonstrate the existence of an interfacial barrier in blocked impurity band structures by evidence of temperature-dependent dark currents, bias-dependent photocurrent spectra and corresponding theoretical calculations. The origin of the build-in field is studied. The temperature-dependent characteristics of space charge effects are also investigated in detail. It is found that at higher temperature (T 14 K), the space charge influence is negligible, and the interfacial barrier is mainly caused by bandgap narrowing effects. Based on interfacial barrier effects, a dual-excitation model is proposed to clarify the band structure of BIB detectors. The photocurrent spectra related to the two excitation processes, i.e., the direct excitation over the interfacial barrier and excitation to the band edge with subquent tunneling into blocking layer, are successfully extracted and agree reasonably well with the calculated band structure results. The effects of interfacial barrier on the photocurrent spectrum, peak responsivity and internal quantum efficiency of the devices are investigated. With the consideration of interfacial barrier effects, the calculated peak responsivity shows good agreement with the experimental result. It is suggested that interfacial barrier effects should be considered for successfully designing the BIB detectors. Additionally, the build-in field is found to equivalently lower the critical field for impact ionization. This study provides a better understanding of the working mechanism in BIB detectors and also a better device optimization.

2015, 64 (22): 227303.
doi: 10.7498/aps.64.227303

Abstract +

Using first-principles calculations based on density functional theory and projector augmented wave method, we investigate the electronic structures of one-dimensional wurtzite (WZ) and zinc-blende (ZB) GaSb nanowires with different diameters along the [0001] and [111] directions, respectively. The results show that the band gap of the GaSb nanowire increases as the size of the nanowire decreases due to the quantum confinement, and the band structures of the GaSb nanowires display an indirect band structures feature when the diameter of the nanowire is smaller than 3.0 nm, whereas bulk GaSb has a direct gap. Owing to the different responses of the valence band maximum/conduction band minimum energies to strain, the band structures of GaSb nanowires experiences a noticeable indirect-to-direct transition when the nanowires are under the uniaxial strain. For example, an indirect-to-direct band gap transition in the band structure of [111] ZB GaSb nanowires can be realized by applying a uniaxial tensile strain, and this transition in the band structure of [0001] WZ GaSb nanowires can take place by applying both uniaxial tensile and compression strain when the diameter of the nanowire is about 2.0 nm. In addition, it is found that carrier effective mass is dependent on the diameter of the GaSb nanowire, therefore both the electron and hole effective mass values decrease as diameter increases. It is also found that the hole effective mass is smaller than the electron effective mass for GaSb nanowires with the same directions and sizes, indicating that the hole transportation is more prominent than the electron transportation.

2015, 64 (22): 227801.
doi: 10.7498/aps.64.227801

Abstract +

The surface polarized reflectance is able to fully reflect the physical characteristics of surface, such as vegetation classification, plant biomass estimation, leaf angle distribution and surface water content. The bidirectional polarization distribution function is a useful tool for quantitatively describing the surface polarized reflectance. The multi-angle polarized remote sensing reveals a significant advantage in capturing the radiation and polarization information. The polarization and directionality of the earth reflectance (POLDER) instrument is the only sensor which has provided a long-term trend of polarized measurements. Its combination of multi-spectral, polarization and multi-angle observations has considerable capability for retrieving ocean, land, cloud and aerosol properties. Until now, data obtained from POLDER has been widely used to study various surface bidirectional polarized reflectance models, especially Nadal model. However, parameters of Nadal model reveal a low accuracy in China region. In this study, the parameters of Nadal model suitable for China region are obtained and analyzed based on the POLDER-2 polarized reflectance data. Based on the modified parameters of Nadal model, polarized reflectance under different surface types is further analyzed. Our results show that the polarized reflectance retrieved from modified parameters of Nadal model reveals better correlation with the POLDER-2 products than the polarized reflectance from Nadal official parameters under different surface types. The polarization properties of three typical surfaces (forest, grassland and desert) are further investigated and reveal that 1) different surface polarized reflectances decrease with the increase of the scattering angle, and the polarized reflectance of the same object decreases as the normalized difference vegetation index increases; 2) significant discrepancies exist between the polarized reflectances of different surfaces, the polarized reflectance of forest is the lowest in the three surface types, then that of grass is the second lowest, and desert reveal the largest value (about twice that of forest), 3) the discrepancies of polarized reflectance between different surfaces have an increasing trend as satellite view zenith angle increases. This study will provide a priori knowledge for the detection of surface polarization properties and aerosol parameters based on multi-angle polarization remote sensing data, and also establish a good foundation for the quantitative applications of GF-5 satellite multi-angle polarization imager to be launched soon in China.

2015, 64 (22): 227802.
doi: 10.7498/aps.64.227802

Abstract +

The equations of state and phase transition of ZnTe in zinc blende (ZB) and cinnabar (CB) structures under high pressure are investigated by the projected augmented wave method in the scheme of density functional theory. The primitive cell volumes, electronic structures and optical properties are also predicted before and after phase transition. The variations of the calculated total energy with volume, for the structures of ZB and CB, yield the information about the static equation of state and phase stability. The results show that the ZB phase of ZnTe has lower energy, and is more stable than its CB phase. The pressure-induced transition occurs along the common tangent line connecting the tangential points on the two enthalpy-volume curves. The calculations show that the phase transition pressure is 8.6 GPa from the ZB structure to the CB structure. The value is also compatible with those of other available theoretical and experimental results. Just before the ZB phase is transferred to the CB phase at about 8.6 GPa, the volume is reduced by 13.0% relative to the former volume at the ambient pressure condition. The calculated critical volumes and volume compressibilities by using two methods agree well with other results in the literature. The lattice parameters and equations of state of the two structures are also obtained. Metallization case of other similar materials such as ZnS caused by high pressure does not occur here. The CB phase has the behavior of indirect band gap with 0.98 eV along the symmetry of GK. After phase transition, the distributions of density of states of Zn and Te atoms of the CB structure shift towards lower energy, especially in the conduction band bottom, and the band gap decreases. Energy level overlapping is more obvious in the CB structure, and orbital hybridizations still exist, that is the reason why it is the stable phase under high pressure condition. Stronger orbital hybridization helps the transitions between Te 5p and Zn 3d electrons. The main peak of imaginary part of dielectric constant is enhanced apparently with abnormal red shift, while other two peaks disappear at the same time. Macroscopic dielectric constant of ZB structure decreases as pressure increases. For CB structure, the macroscopic dielectric constant with 13.60 eV is not affected by pressure. The results provide a theoretical basis for the polarization research of ZnTe material in static electric field under high pressure.

2015, 64 (22): 227901.
doi: 10.7498/aps.64.227901

Abstract +

Charging characteristics of an insulator specimen due to non-penetrated electron irradiation have been attracting a great deal of attention in the fields such as scanning electron microscopy, electron probe analysis, and space irradiation. In this paper, we use a numerical simulation model based on Monte Carlo method for investigating the electron scattering. The elastic scattering is calculated with the Mott cross-section, and the inelastic scattering is simulated with Penn model and the fast secondary electron model according to the primary energy. The charge transport caused by the build-in electric field and charge density gradient is calculated with finite-difference time-domain method. Multi-combined effect of correlative parameters on charging characteristics is investigated by efficient multithreading parallel computing. During the irradiation, the landing energy of primary electrons decreases due to the negative surface potential, which makes the secondary electron yield increase. Variations of secondary electron current and sample current are presented to verify the validity of the simulation model by comparing with existing experimental results. Evolutions of leakage current, surface potential and internal space charge quantity are calculated under different conditions of incident electron current, primary energy and sample thickness. The results are presented in contour maps with different multi-parameter combinations, primary energy and sample mobility, primary energy and sample thickness, and primary energy and incident current. The balance state of charging will be determined by leakage current under conditions of a larger primary energy, sample mobility, incident current, or a less sample thickness, which is shown as the leakage current dominated mode. While in the cases of a lower primary energy, sample mobility, incident current, or a larger sample thickness, the balance state of charging is mainly dominated by secondary electron current, as the secondary electron current dominated mode. In other cases except the above two, the balance state will be determined by both leakage and secondary currents as the mixture mode. In the same mode, variations of charging characteristics with parameters are monotonic. When the change of a parameter makes the negative surface potential increase, the effect of this parameter on negative surface potential will be weakened, while the effects of other parameters on the negative potential will be enhanced. With the change of current dominated mode, the total charge quantity exhibits the local maximum with respect to the sample thickness, and the value of this maximum increases with primary energy. Moreover, the leakage current increases with incident current linearly. The presented results can be helpful for understanding regularities and mechanisms of charging due to electron irradiation, and estimating the charging intensity under different conditions of irradiation and sample material.

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

## EDITOR'S SUGGESTION

2015, 64 (22): 228101.
doi: 10.7498/aps.64.228101

Abstract +

As is well known, most natural diamonds usually contain not only aggregated nitrogen up to thousands of ppm but also hydrogen. Therefore, the studies of nitrogen and hydrogen impurities in a diamond are of interest for improving the physical properties of a diamond and solving the problems about natural diamond genesis. From this point of view, in this paper, we choose C3N6H6 powders as a nitrogen and hydrogen source and select high-quality seed crystals with {100} facets as the growth facets. The effects of nitrogen and hydrogen co-doped on {100}-oriented single diamond in the NiMnCo-C system at pressures ranging from 5.5 GPa to 6.2 GPa and temperatures of 1280-1450 ℃ are investigated. Experimental results show that both pressure and temperature, which are the synthesis conditions, increase with the increases of nitrogen and hydrogen content in diamond-growth environment, and the V-shape region of diamond-forming moves up. From the obtained Fourier transform infrared spectra, we notice that there is a significant change of the nitrogen concentration in the synthesized diamond with increasing the nitrogen and hydrogen content in the diamond-growth environment. We calculate the nitrogen concentrations in those diamonds and the results indicate that the highest concentration of nitrogen is up to 2000 ppm. Meanwhile, we notice that the hydrogen associated infrared peaks of 2850 and 2920 cm-1 are gradually enhanced, which shows that both nitrogen and hydrogen are successfully co-doped into the diamond. Scanning electron microscope micrographs show that the {111} face is elongated and has triangulated textures appearing on the surface with nitrogen and hydrogen co-doped into the diamond. This result indicates that the synergistic doping of nitrogen and hydrogen has a great influence on the morphology of {100}-oriented single diamond. From the obtained Raman spectra, we find a shift towards higher frequency of the Raman peak from 1330.23 cm-1 to 1330.40 cm-1 and the full width at half maximum increases from 3.12 cm-1 to 4.66 cm-1 with increasing the concentrations of nitrogen and hydrogen in diamond-growth environment. This is the first report about nitrogen and hydrogen co-doped on 100-oriented single diamond by far. This work can provide a new method to study the influences of nitrogen and hydrogen impurities on diamond synthesis and it will help us to further understand the genesis of natural diamond in the future.

2015, 64 (22): 228401.
doi: 10.7498/aps.64.228401

Abstract +

The operation principle of digital voltage-mode controlled buck converter with dual-edge modulation is analyzed in this paper. Based on the state equation of buck converter and six possible evolutions in one switching cycle, the discrete iterative-map model of digital voltage-mode controlled buck converter with dual-edge modulation is established. Ignoring the quantization error of analog-digital converter and on the basis of its discrete iterative-map model, the nonlinear dynamical behavior of digital voltage-mode controlled buck converter with dual-edge modulation is investigated in detail. Taking the input voltage and the load resistance as bifurcation parameters, the output voltage bifurcation diagram and the inductor current bifurcation diagram are plotted. Through analyzing the bifurcation diagrams, it is indicated that there are two kinds of similar but different Hopf bifurcation phenomena. By use of Poincar section, time-domain simulation waveforms and phase portraits, two different Hopf bifurcations and low-frequency oscillation phenomena are compared and studied. Observing the inductor current and capacitor voltage waveforms respectively, it is obviously found that their oscillation frequencies and amplitudes are different, the shapes of two Poincar$ sections and phase portraits are also different. In order to verify the correctness of the simulation and theoretical analysis, the eigenvalues of Jacobian matrix of the discrete iterative map model are introduced and solved in two kinds of stable evolutions. Through analyzing variation of eigenvalues of Jacobi matrix with input voltage, the existence and difference of two kinds of Hopf bifurcation phenomena are proved theoretically. Moreover, it is observed in this paper that the odd period-doubling bifurcation phenomenon exists in digital voltage-mode controlled buck converter with dual-edge modulation for the first time, where the operation state of the buck converter turns from period-one into period-three. Its authenticity is verified by using the time-domain simulation waveforms and phase portraits. In order to approach to the actual circuit, the equivalent series resistances of capacitor and inductor are considered. The actual circuit is simulated by using the software Psim. A comparison shows that there are little differences between the theoretical simulation and the actual circuit simulation. So the theoretical simulation can be used to analyze the performances of the actual circuit. The research results in this paper have guiding significance and practical value for designing the digital voltage-mode controlled buck converter with dual-edge modulation.

2015, 64 (22): 228502.
doi: 10.7498/aps.64.228502

Abstract +

Organic thin-film transistor (OTFT) based on pentacene semiconductor with an embedded copper oxide (CuO) thin layer is investigated. With the 3 nm-thick CuO layer embedded in the pentacene semiconductor, the drain current of the OTFT increases more than 3 times compared with that of pentacene organic field-effect transistor without CuO layer, and the absolute threshold voltage reduces from -21 V to -7.9 V. The hole mobility and current on/off ratio are much improved. It is interpreted by the mechanism based on the analysis of the interface charge transfer between pentacene layer and CuO layer. Results of X-ray photoelectron reveal electron transfer from pentacene to high work function CuO and the formation of charge transfer (CT) complexes based on electron transfer near the contact of CuO and pentacene. The CT complexes between pentacene layer and CuO layer could reduce the exponential density of state near the band edge of pentacene and the pentacene bulk hole trap density, and enhance the pentacene bulk hole carriers injection, which leads to the improvement of the field-effect mobility of OTFT with CuO layer. Electrons are transfered from the highest occupied molecular orbital of pentacene to the thin CuO layer which can generate holes in pentacene. The generated hole has the same effect as that with applying negative gate voltage which influences the threshold voltage. The drain current of the device increases and the threshold voltage shifts from -21 V to -7.9 V. Therefore, the thin CuO layer that is directly embedded in the organic semiconductor layer, serves as the hole-injection layer, which is responsible for reducing the contact barrier of OTFT with CuO layer.

2015, 64 (22): 228801.
doi: 10.7498/aps.64.228801

Abstract +

Reducing production cost to accelerate the industrialization process of thin film solar cells (TFSCs) makes it urgently demanded to elevate the deposition rate and reduce the needed thickness of absorbers in addition to the prerequisite performance improvement. Based on very high frequency plasma enhanced chemical vapor deposition process with a low bombardment energy and large ion flux, ultra-thin, high-deposition-rate, and high-performing hydrogenated microcrystalline silicon (c-Si:H) single- and related hydrogenated amorphous silicon (a-Si:H)/c-Si:H double-junction TFSCs are developed in this study. By tuning various process parameters (silane concentration, power, and pressure), the deposition rates and electrical properties of c-Si:H materials are studied in detail. Device-level c-Si:H intrinsic materials with a deposition rate of 10.57 /s and photosensitivity of 7.54102 can be obtained when depositing with a silane concentration of 9%, a power of 70 W, and a pressure of 2.5 Torr. By further applying device-level high-deposition-rate c-Si:H intrinsic materials in c-Si:H single-junction TFSCs on magnetron-sputtered and wet-etched aluminum-doped zinc oxide (ZnO:Al) substrates with optimized surface morphologies and photoelectrical properties, and by combining advanced device designs, an initial conversion efficiency of 7.49% can be achieved for pin-type ultra-thin and high-deposition-rate c-Si:H single-junction TFSCs (the thickness values of intrinsic layers are 1.1~m). To further improve the conversion efficiency of TFSCs, pin-type a-Si:H/c-Si:H tandem TFSCs are fabricated by using n-a-Si/n-c-Si/n-nc-SiOx:H/p-nc-SiOx:H as the tunnel recombination junctions (TRJs), which, however, have unaddressed issues that the wide band-gap nc-SiOx:H materials with a low conductivity strongly reduce the recombination rate of carriers, thereby resulting in the photo-generated carriers accumulating near the TRJs, weakening the built-in electric field in the top sub-cells and leading to an open circuit voltage (Voc) loss in a-Si:H/c-Si:H tandem TFSCs up to 115~mV far above average values. By simultaneously inserting the p- and n-type narrow-gap c-Si:H materials, which are highly defective and narrower than the band gap of nc-SiOx:H materials, into the TRJs to implement the electrically lossless interconnection between the a-Si:H top and c-Si:H bottom sub-cells, the Voc loss is successfully reduced to 43~mV and an initial efficiency of 12.03% (Voc=1.48~eV, Jsc=11.67~mA/cm2, FF=69.59%) is achieved for ultra-thin pin-type a-Si:H/c-Si:H tandem TFSCs with a total thickness of 1.48~m, thus paving the way for the low-cost production of TFSCs.

###### GEOPHYSICS, ASTRONOMY, AND ASTROPHYSICS

2015, 64 (22): 229301.
doi: 10.7498/aps.64.229301

Abstract +

Signal collected from magnetic resonance sounding (MRS) instrument is only a few tens of nano-volt and susceptible to environmental noise, leading to a low signal-to-noise ratio. In addition, the accuracy of characteristic parameter extraction from MRS signal is seriously affected, and the resulting error of the subsequent inversion interpretation increases. In this paper, a fast fixed-point algorithm for independent component analysis (FastICA) is utilized to enhance the performance in the high noisy environment. First, the applicability of FastICA algorithm to noise cancellation of MRS signal is analyzed. Whether the mixed signal can be separated completely depends on the appropriate choice of nonlinear function in FastICA algorithm, moreover, the choice of nonlinear function is closely related to the Gaussian type of signal. Thus, in this process, the kurtoses of noise and full-wave MRS signal are calculated, and then the Gaussian type of signal is determined. Therefore based on the Gaussian type of signal, we can choose the corresponding nonlinear function applied to the FastICA algorithm in order to realize the effective separation of the mixed signals. Secondly, undetermined blind source separation is one of common problems of ICA. To cope with this tough situation, a digital orthogonal method is adopted to construct some extra observed signals combined with the existing observed one as the input signal of this algorithm. Hence, the digital orthogonal method can satisfy the application condition of ICA, i.e., the number of observed signal must be greater than or equal to that of source signal. This means that it is able to remove the application limitation of ICA when there is only one observed signal. Owing to the problem of variable amplitude of separated signals after ICA, it is crucial to recover the initial amplitude of the separated MRS signal, because it represents the amount of water content in the aquifer. Aiming at this problem, a spectrum correcting method is proposed. In frequency domain, the spectrum of separated MRS signal is restored into the original value that is the spectrum of observed signal at Larmor frequency, then transformed into time domain by inversing fast Fourier transform to obtain the desired MRS signal.
In the validation of the proposed algorithm, two tests are considered: simulation and field data processing. In the simulation case, the observed signal constructed by full-wave MRS signal and two power-line harmonics with different frequencies is the main processing object, and the proposed algorithm is utilized to realize the observed signal separated into ideal MRS signal and noise effectively. To verify the applicability of this proposed algorithm further, under the condition of different initial amplitudes and relaxation times, the characteristic parameters of separated MRS signal are extracted by this proposed algorithm and the corresponding relative fitting error is determined. The simulation results show that adopting this algorithm can effectively realize the separation of the noisy full-wave MRS signal. In addition, the relative errors of initial amplitude and relaxation time after data fitting are both within 5.00%. When compared with the denoising ability of some other classical algorithms, the performance of this proposed algorithm is superior. Finally, this algorithm is applied to the processing of the field data. The results indicate that power-line harmonics and other single-frequency interference contained in the MRS signal can be removed effectively.