Vol. 62, No. 14 (2013)
2013, 62 (14): 140101. doi: 10.7498/aps.62.140101
In the research of the propagation model of complex network, it is of theoretical and practical significance to detect the most influential nodes. Global metrics such as degree centrality, closeness centrality, betweenness centrality and K-shell centrality can be used to identify the influential spreaders. Each of these approaches is simple but has a low accuracy. We propose K-shell and community centrality (KSC) model. This model considers not only the internal properties of nodes but also the external properties, such as the community which these nodes belong to. The susceptible-infected-recovered model is used to evaluate the performance of KSC model. The experimental result shows that our method is better to detect the most influential nodes. This paper comes up with a new idea and method for the study in this field.
2013, 62 (14): 140201. doi: 10.7498/aps.62.140201
The accurate computation of effective refractive index of optical fiber core and the cladding mode are needed for the fiber structure design, mode dispersion analysis and fiber grating mode coupling research. In this paper, based on three layered optical fiber waveguide model and its accurate eigenvalue equation, by using the cut string algorithm the effective refractive index of core mode is solved. At the same time the computation results are verified by using COMSOL software. By using interval traversal algorithm the effective refractive index of cladding mode is computed. Compared with the existing algorithms, this method can avoid the loss of root during the calculation. It can also avoid the generation of odd point and can keep the normality of each mode. The simulation is based on Mathematica software. The curves relating effective refractive index of core and cladding mode to wavelength are obtained.
2013, 62 (14): 140202. doi: 10.7498/aps.62.140202
A class of sea-air coupled oscillator dynamic model is studied. Using the parameter modification and average method for the singular perturbation, the vibrating approximate solution corresponding to El Nino-Southern Oscillation (ENSO) coupled oscillation dynamic model equations is obtained.
2013, 62 (14): 140203. doi: 10.7498/aps.62.140203
Based on Dini series expansion, p(>0) order quasi discrete Hankel transform (pDQDHT) algorithm is deduced. The application of this algorithm in beam propagation is presented. pDQDHT algorithm is tested with various input functions and used in beam propagation through lens. Experimental results show that the pDQDHT algorithm possesses a high accuracy compared with existing Hankel transformation algorithms. The pDQDHT algorithm can be transformed forwardly and inversely and widely used in beam distribution of transmission. The implementing speed is comparable to that of general fast Hankel transform algorithm.
2013, 62 (14): 140301. doi: 10.7498/aps.62.140301
We investigate the dynamics of a quantized vortex in a trapped superfluid Fermi gas near the unitarity limit. By taking a trial wave function for the order-parameter of a condensate in a rotating axisymmetric trap confinement and using a time-dependent variational analysis we obtain the equations of motion and their solutions for anomalous mode. The results show that the critical rotating frequency of the trap increases when the system ranges from the left to the right side of the unitarity limit, while the period of the vortex decreases in this regime.
2013, 62 (14): 140501. doi: 10.7498/aps.62.140501
The kinetic energy of the circular restricted three-body problem in a rotating frame is no longer a standard positive quadratic function of moment, owing to the additional part in the non-inertial rotating frame, which leads to a difficulty in using force gradient symplectic integrators. To address this problem, we show through the calculation of Lie operators that the force gradient operator on the system is still related to the gradient of the gravitational forces from the two main objects rather than that of the resultant force of both the gravitational forces and the non-inertial force exerted by the rotating frame, just as the force gradient operator on the circular restricted three-body problem in an inertial frame. Therefore, it is reasonable to use the gradient symplectic integrators for integrating the circular restricted three-body problem in the rotating frame from a theoretical point of view. Numerical simulations describe that a fourth-order force gradient symplectic method is always greatly superior to the non-gradient Forest-Ruth algorithm in the numerical accuracy, and its optimized version is best. Because of this, the optimized gradient scheme is recommended for calculating chaos indicators, such as Lyapunov exponents of and fast Lyapunov indicators of two nearby trajectories, which is conductive to obtaining a true description of dynamically qualitative properties.
2013, 62 (14): 140502. doi: 10.7498/aps.62.140502
An integrate-and-discharge model (IDM) is proposed on the basis of an integrate-and-fire model (IFM). Compared with the IFM, the IDM can obtain rich dynamic information including chaos, phase locking, etc., without using varying threshold modulation. The corresponding relation between output symbolic sequences and parameters (i.e., frequency, amplitude, resistance and capacity) of the IDM is established by using symbolic dynamics. Moreover, a method of obtaining symbolic sequence as well as an ordering rule is presented. Simulation and circuit experiment validate the correctness of the method and the rule. The results of circuit experiment show that the frequency resolution can reach up to 0.05 Hz in some frequency ranges and the amplitude resolution can reach up to 1 μA.
2013, 62 (14): 140503. doi: 10.7498/aps.62.140503
Transfer function approximation in frequency domain is not only one of common numerical analysis methods studying portraits of fractional-order chaotic systems, but also a main method to design their chaotic circuits. According to it, in this paper we first investigate the chaotic characteristics of the fractional-order Lorenz system, find some more complex dynamics by analyzing Lyapunov exponents diagrams, bifurcation diagrams and phase portraits, that is, we display the chaotic characteristics as well as periodic characteristics of the system when changing fractional-order from 0.7 to 0.9 in steps of 0.1, and show that the chaotic motion exists in the a lower-dimensional fractional-order Lorenz system. Then, according to transfer function approximation and the approach to designing integer-order chaotic circuits, we also design an analog circuit to implement the fractional-order system. The resistors and capacitors in the circuit are selected according to the system parameters and transfer function approximation in frequency domain. Some phase portraits including chaotic attractors and periodic attractors are observed by oscilloscope, which are coincident well with numerical simulations, and the chaotic characteristics of the fractional-order Lorenz system are further proved by the physical implementation.
2013, 62 (14): 140504. doi: 10.7498/aps.62.140504
Electron transport properties of Si4 cluster-Au (100) electrode nanojunction is investigated with a combination of density functional theory and non-equilibrium Green's function method. We calculate the geometric structures, electronic structures, conductances, transmission spectra and charge transfers of Au-Si4-Au nanoscale junctions under different distances. We also discuss the variations of conductance and current with external voltage when the distance of junction is 12.004 Å.
The sliced trispectrum fluctuation characteristics and flow pattern representation of the nitrogen-water two-phase flow of small channel
2013, 62 (14): 140505. doi: 10.7498/aps.62.140505
A new method of nonlinear analysis is proposed by combining the sliced trispectrum method with the fluctuation characteristics, and the influences of the length of the time sequence and fast Fourier transform (FFT) on the fluctuation characteristic value of sliced trispectrum are discussed. It is found that in the absence of noise, the length of sequence and FFT are proportional to the vector distance value; in the presence of noise, it is approximate inversely related. In order to test the anti-noise ability and characterize complexity ability of the fluctuation characteristics method, the proposed method is applied to fractal sequence (Brown), chaotic sequence (Lorenz) and periodic sequence (sine signal). The results show that compared with other power spectral method, the method of the fluctuation characteristics has good noise immunity of fractal sequence, and the anti-noise ability is relatively weak with periodic sequence; but the fluctuation characteristic theory of sequence internal complexity representation has a good effectiveness. On this basis, the differential pressure signals of nitrogen-water two-phase flow in small rectangular channel (w×h=2 mm×0.81 mm) are studied. By analyzing the differential pressure sliced trispectrum of flow patterns, the secondary coupling phenomenon of main oscillation mode of different flow patterns is established. The fluctuation characteristic values of the sliced trispectrum of different flow patterns are extracted to accurately identify the typical flow patterns of small channel nitrogen-water two-phase flow. At the same time, the fluctuation characteristic theory can be used to provide a useful exploration for the further investigation of flowing mechanism of multi-phase flows.
2013, 62 (14): 140506. doi: 10.7498/aps.62.140506
The local adaptive H∞ consistency is intensively investigated for delayed complex dynamical networks with noise. The network under consideration contains unknown but bounded nonlinear coupling functions and time-varying delays which appear in the coupling term and the node system simultaneously. Based on the Lyapunov stability theory, linear matrix inequality optimization technique and adaptive control, several local adaptive H∞ consistency schemes are established which guarantee robust asymptotically consistency for each node of noise-perturbed network as well as achieving a prescribed robust H∞ performance level. Finally, detailed and satisfactory numerical simulations validate the feasibility and the correctness of the proposed techniques.
2013, 62 (14): 140507. doi: 10.7498/aps.62.140507
Dimensionless dispersion curve equation of one-dimensional periodic structure with three oscillators is deduced. It is proved by singularity theory that the topology of the dispersion curves does not change with mass and stiffness parameter. Then the equations of initial and final frequencies of the bands are presented. The method of band gap design is gained consequently. Two examples i.e., the system with the same stiffness and the system with the same mass, are given to verify it. The method may provide the reference for designing the band gap of the photonic and phononic crystals.
2013, 62 (14): 140601. doi: 10.7498/aps.62.140601
A nonlinear pre-shaper which optimizes initial pulses for self-similar evolution in a following short fiber amplifier is demonstrated. It consists of a pair of transmission gratings and a segment of single mode fiber, by which pulses are shaped temporally and spectrally before amplification. To confirm the benefit of nonlinear pre-shaping for the self-similar evolution, pulse amplifications with and without the nonlinear pre-shaper are simulated. From comparison, pulses optimized by nonlinear pre-shaper show a shorter pulse duration, less pedestal and broader spectrum after amplification and compensation. With this optimization, the self-similar amplification can be realized in a 2.2-meter Yb3+-doped fiber in a large range of pump power, generating 60 fs transform-limited pulses after compression. This nonlinear pre-shaping method can efficiently shorten the fiber length and release the seed quality required for self-similar amplification. An all-normal dispersion mode-locked fiber laser is employed as the seed of a self-similar amplifier for the first time, thus facilitating an all-fiber system.
2013, 62 (14): 140701. doi: 10.7498/aps.62.140701
Measurements of residual stress in beryllium thin film under standard Bragg-Brentano geometry are always problematic. In this article, a new experimental method using grazing- incidence X-ray diffraction is presented according to the convential sin2Ψ method, which effectively increases the signal-to-noise ratio. Analysis shows that the assumption (isotropic material) is logical, because the values of stress results from the three families of planes are camparable. The stress gradient can be measured at diffrenent grazing incidence angles. The results indicate the uniformity of the residual stress of the thin film along various Φ directions.
2013, 62 (14): 140702. doi: 10.7498/aps.62.140702
Electrical resistivity and Hall-effect in InSe under high pressure are accurately measured in situ. The measurement results of electrical resistivity and the temperature dependence of electrical resistivity show that InSe undergoes semiconductor-to-metal transition at 5-6 GPa and transforms from rhombohedral layered phase P1 (InSe-I) to metallic rocksalt cubic phase P3 (InSe-III) at 12 GPa. Certainly, the pressure-induced metallization of InSe results from the pressure-induced structural phase transition. In addition, Hall-effect measurements display the carrier transport behavior of InSe under pressure, which indicates that InSe undergoes a carrier-type inversion around 6.6 GPa and the increases of the carrier concentration is the dominant factor producing the decrease of the resistivity after 9.9 GPa.
2013, 62 (14): 140703. doi: 10.7498/aps.62.140703
The electrical model that ionizing radiation reduces the effective power output of GaN-based blue light-emitting diode is proposed by investigating the light/dark current generation mechanism in active region of GaN-based blue light emitting diode device under ionizing irradiation. The model that the ionizing radiation increases the 1/f noise of GaN-based blue light-emitting diode device is proposed by studying the 1/f noise mechanism of the active region of GaN-based blue light-emitting diode device under exposure to ionizing radiation. In the small injection region (I1 A)， the space charge region and the recombination current increase with irradiation dose increasing. Meanwhile, with the increase of the ionizing-irradiation-generated defects, the 1/f noise amplitude increases. In the large injection region (I1 mA), due to the dominant influence of the series resistance, the surface recombination velocity and current increases with irradiation dose increasing. Meanwhile, with the increase of ionizing-irradiation- generated defects, the 1/f noise amplitude increases. The I-V and 1/f noise test results before and after irradiation are in good agreement with theoretical results. In the middle injection region (1 A I 510-5 A), due to the competition between mobility fluctuation caused by energetic carrier scattering and the carrier number fluctuation caused by the newly irradiation-generated defects, as the radiation dose increases, 1/f noise has no significant changes in the frequency domain. However, through the 1/f noise time domain multiscale entropy complexity analysis, a conclusion can be drawn that with the increase of radiation dose, the 1/f noise domain multi-scale entropy becomes more complex. 1/f noise amplitude ultimately proves to be sensitive to reflect the reliability of GaN-based blue light-emitting diode ionizing irradiation in the case of small injection and large injection. The greater the noise amplitude, the higher the irradiation induction trap is, and the greater the generation-recombination current related to the dark current, the smaller the photocurrent related to the diffusion current is, so that the luminous efficiency of the device, the optical output power, and other performance parameters decrease, thus affecting the reliability of the device and resulting in the more failure devices. 1/f noise time domain multiscale entropy complexity can reflecte ionizing irradiation reliability of GaN-based blue light emitting diodes sensitively in the middle injection region.The more the multiscale entropy complexity, the bigger the irradiation induction generation-recombination current is, and the worse the reliability of the device is. The present study provides a method of characterizing the GaN-based blue light-emitting diode ionizing irradiation reliability according to 1/f noise.
Characterization of elastic properties of a sample by atomic force microscope higher harmonic amplitude
2013, 62 (14): 140704. doi: 10.7498/aps.62.140704
When the atomic force microscope cantilever in tapping-mode is vibrated at a frequency close to its fundamental resonance frequency, the tip on its free end will be close to and away from the sample periodically. The higher harmonic signals produced by non-linear interactions between the tip and sample surface contain more nanomechanical information. We study the influence on the contact time by different elastic modulus and the relationship between higher harmonic amplitude and contact time. By theoretical analysis and calculation, we obtain the law of characterizing the sample surface elastic difference with the higher harmonic amplitude. Moreover, we obtain the experimental result consistent with the theory, on our homemade higher harmonic system.
Real-time data processing of remote measurement of air pollution by infrared passive scanning imaging system
2013, 62 (14): 140705. doi: 10.7498/aps.62.140705
Infrared passive remote sensing allows remote monitoring and early warning of air pollution. The key to the quantitative resolution is to obtain the background radiation in application process. The existing methods (such as measuring the background and atmosphere radiance spectrum in the same condition or uptake) need in advance or at the same time to measure the background spectrum without target. So, they are difficult to satisfy the demand for rapid response in the practical application. According to the infrared radiation transmission model analysis and by studying the principle of measured spectrum in the 800-1200 cm-1 band containing the target, background and environmental radiation, we propose a real-time spectrum extraction algorithm that does not require to measure the background target feature. The method is applied to the developed polluting gas scanning imaging passive FTIR system. The remote sensing of SF6 is done. Concentration data and the data obtained by the method at the same condition are compared, and the results show that the correlation coefficient squared value reaches 0.99, indicating that the method is feasible, which can effectively improve the response speed of the system and the scope of application.
ATOMIC AND MOLECULAR PHYSICS
2013, 62 (14): 143101. doi: 10.7498/aps.62.143101
According to the plane wave pseudo-potential method of the first-principles based on density functional theory, we systematically investigate the cell parameters, the formation heat and the reaction enthalpy in the Li-N-H hydrogen storage process. The calculated lattice parameters of these compounds are in good agreement with previous theoretical and experimental results. The values of formation heat at 298 K for (lithium nitride) Li3N, (lithium hydride) LiH, (lithium amide) LiNH2 and (lithium imide) Li2NH are estimated to be -168.7, -81.0, -173.0 and -190.8 kJ/mol, respectively. The reaction enthalpy of this system is calculated to be 78.5 kJ/mol H2 at T=298 K and this value accords with the experimental value of 75.67 kJ/mol H2 at T=300 K. As a result, the reaction enthalpies for the two-step reactions are very close to their calculation results respectively.
2013, 62 (14): 143601. doi: 10.7498/aps.62.143601
Using density functional theory, the structures, stabilities and magnetic properties of (FeCr)n (n≤ 6) alloying clusters are systematically investigated. For smaller clusters with n≤3, the results show that the ground-state system possesses collinear antiferromagnetic order. For n≥4 cases, however, the ground-state cluster has noncollinear magnetic order. Therefore, there is a collinear-to-noncollinear magnetic transition at n=4 in (FeCr)n systems. In addition, although the spin-orbit coupling effect of 3d transition metal atom is often weak, the results indicate that the orbital magnetic moments of some certain clusters are significant and important. Finally, the chemical bond of noncollinear magnetic clusters and the physical origin of the magnetic transition are analyzed.
2013, 62 (14): 143602. doi: 10.7498/aps.62.143602
The low-lying candidates of Ge65, Ge70 and Ge75 are extensively investigated through combining the tight-binding potential of germanium with two kinds of global minima search strategies: compressing liquid and genetic algorithm. Then, we perform accurate ab initio calculations to optimize the atomic structures of these low-lying candidates and identify the ground state structures. Our calculations predict that there exist two kinds of stable isomers characterizing the sphere- and ellipsoid-like structural features for each sized cluster, and the two kinds of isomers are energetically competitive. This is consistent with the observation for the large sized clusters Gen (65 ≤ n ≤ 80) in experiment. Meanwhile, we also briefly analyze the electronic properties of these three kinds of clusters.
ELECTROMAGENTISM, OPTICS, ACOUSTICS, HEAT TRANSFER, CLASSICAL MECHANICS, AND FLUID DYNAMICS
Analysis and measurments of path loss effects for ultra high frequency radio-frequency identification in real environments
2013, 62 (14): 144101. doi: 10.7498/aps.62.144101
Based on the principles of radio-frequency identification (RFID) technology and Friis propagation equation, the path loss expression of ultra high frequency (UHF) RFID in free space is provided. The Fresnel clearance and horizontal interval between reader antenna and tag are employed as dependent variables, and then the obstructing effect of the first Fresnel zone on path loss is discussed. By the methods of linear regression and minimum mean-square error, a dual-slope Logarithm distance path losses model is proposed. The path losses of UHF RFID under different parameters are measured in open indoor environment. The measurement results indicate that RFID system experiences less fading when the Fresnel clearance is 1.5 times higher than the first Fresnel radius. The standard deviation of the proposed model with two slopes reduces ten percent or more compared with that of traditional logarithm distance path loss model.
2013, 62 (14): 144102. doi: 10.7498/aps.62.144102
The Green's function of three-dimensional combined-layer soil model is firstly derived using simulated image method. Based on the specified soil model, the influence of soil model on the earth surface potential (ESP) is analyzed for different soil structure configurations. It is found that difference in soil model only changes the absolute value of ESP, but the influence of ESP difference can be ignored under certain conditions. And the physical mechanism for the effect of soil parameter change on ESP is explained theoretically. One of the challenges that have not been solved effectively is how to establish the proper soil model to calculate DC current flowing through the neutral point of transformer accurately. For this challenge, the criteria and threshold value for selecting the soil model reasonably are introduced. Therefore, the arbitrariness and blindness of selecting the soil model can be avoided. The results are useful for calculating accurate results, which can further provide theoretical references for selecting substations near the HVDC ground electrode.
2013, 62 (14): 144103. doi: 10.7498/aps.62.144103
Transport channel of the electron beam in a short-wavelength free-electron laser is quite narrow and long, which is needed for the electrons to have excellent motion feature so as to prevent the transverse divergence. In this paper the characteristics of the ultra-relativistic electron motion in a planar wiggler filed with transverse distribution are studied by analytical derivation, nonlinear simulations and Kolmogorov entropy calculations. Results show that the wiggler not only forces the electron to periodically wiggle on the transverse plane, but also superpose a transverse shift motion leaving away from the wiggler axis, which leads to the transverse divergence of the electron motion in the absence of an external magnetic focusing system. However, it is shown that the proper choice of the initial value of the electron transverse velocity could suppress the transverse motion divergence, where the electron can smoothly pass through a narrow and long channel with a length of 10 m and a width of 0.09 mm and the electron motion is stable even if there is no external magnetic focusing system.
Super-diffraction imaging in three-dimensional localization precision of the double-helix point spread function
2013, 62 (14): 144201. doi: 10.7498/aps.62.144201
The combination of switch effect and single molecule positioning can realize the sample super diffraction resolution imaging, and double helix point spread function will be a single molecular orientation nanometer resolution extending from two dimensions to three dimensions. In this paper, three-dimensional (3D) positioning accuracy of the double helix point spread function is discussed. First, the theoretical localization accuracy for an unbiased estimator of the double-helix point spread function is compared with that for 3D localization using Fisher information and the influences of localization accuracy including photon number, varying levels of background and pixel size are analyzed. Second, based on the localization algorithm of gaussian fitting, which is usually adopted in data analysis of single molecular orientation in the experimental process, the axial localization accuracy of double helix point spread function is obtained by the error transfer function law. Computer simulation results show that under the condition of more than 1000 photons in number, the positioning accuracy of gaussian fitting localization algorithm and theory localization accuracy of Fisher information are in good agreement. The discussion of the paper provides not only the theoretical basis for 3D positioning accuracy of double helix point spread function, but also the theoretical guidance for the experiment at the same time.
2013, 62 (14): 144202. doi: 10.7498/aps.62.144202
Atom movement in light-induced dipole lattices is investigated in this paper. Momentum-dependence of the dipole potential is particularly considered. Momentum squeezed variance and position amplified variance feature are revealed, from which we predict that a single particle state of boson atoms confined in optical lattice will approach to a momentum squeezed line state. The results show also that under red detune, the atom movement experiences three phases. In the first phase, the atom is slowed down gradually in a long period with atom momentum damply oscillating. In the second phase, atom momentum will quickly reduce to hk(Ω/γ)2 with hk momentum of one photon, Ω Rabi frequency, and γ decay constant of atom wave function, as the momentum reduces to the momentum of one photon. Finally, atoms will be confined in the vicinity of wave nodes.
We derive a useful formula for the normal ordering of two-mode operator by the technique of integration within an ordered product (IWOP) of operators. Furthermore, we also calculate the Radon transformations for the coherent state, special squeezed coherent state, intermediary entangled state in the view of the quantum mechanics. Based on the results above, the Mexican hat's mother wavelet function is selected to analyse Ridgelet transformations of these three quantum optical states.
Plasma characterization studies of laser dressing for bronze-bonded diamond wheel by a pulsed fiber laser
2013, 62 (14): 144204. doi: 10.7498/aps.62.144204
In this paper, we present the optical emission studies of the spatial evolution of plasma during pulsed fiber laser dressing of bronze-bonded diamond grinding wheel and especially investigate the plasma light emission, which is measured through a high sensible optical spectrometer. Space-resolved spectra in a wavelength range of 500-600 nm are measured at different laser average powers and pulse repetition frequencies, and the intensity of spectral lines achieves a maximum intensity at about 2.4 mm away from the surface of the grinding wheel. The electron temperature is determined by employing the Boltzmann plot method under the assumption of local thermodynamic equilibrium using six Cu (I) lines, and the highest electronic temperature is calculated to be 4380 K at about 3 mm away from the surface of wheel. Finally the effect of the laser parameters on the electron temperature of the plasma is studied, and the results show that there are different variation laws in the electron temperature of the plasma with laser average power and pulse repetition frequency.
2013, 62 (14): 144205. doi: 10.7498/aps.62.144205
A corner-pumped type is a new pumping type in the diode-pumped solid-state lasers, which has the advantages of high pump efficiency and favorable pump uniformity. Using the single corner-pumped type, an efficient corner-pumped Nd:YAG/YAG composite slab continuous-wave 946 nm laser is demonstrated in this paper. The compact plane-concave linear cavity configuration is adopted and the cavity length is only 20 mm. When the pumping power is 50 W, the maximal output power can reach as high as 5.29 W with an optical-to-optical conversion efficiency of 10.6% and a slope efficiency of 12%. This laser has a compact structure, simple tuning and low cost, which will have a broad application prospect.
Frequency indistinguishibility investigation of generated coincident-frequency entanglement via ultra-fast pulsed sources
2013, 62 (14): 144206. doi: 10.7498/aps.62.144206
The frequency entangled biphoton source generated via spontaneous parametric down-conversion process (SPDC) has found numerous applications in quantum information processing and relevant fields. We report on an experimental generation of coincident-frequency entanglement from periodically poled potassium titanyl phosphate, pumped by an ultra-short pulsed optical source with duration less than 20 fs. Based on the Hong-Ou-Mandel interferometric coincidence measurement setup, a visibility of about 42% is demonstrated, which indicates degraded frequency indistinguishibility of the down-converted biphotons. Through theoretical investigation, such a degradation can be perfectly explained by the nonnegligible second-order dispersion terms in the Taylor-expanded phase mismatching function for the case of ultra-broadband spectrum of the pulsed pump. The fitting to the experimental results is further used and perfect agreement is achieved. The results imply that the spectral bandwidth of the pump can affect the generated coincident-frequency entanglement.
2013, 62 (14): 144207. doi: 10.7498/aps.62.144207
Chaotic laser characteristics of an optoelectronic oscillator are investigated theoretically and numerically, and the influences of the offset phase and the feedback strength on the time-delay signature of chaos which is generated by an optoelectronic oscillator are studied, based on the method of autocorrelation function. Numerical analyses show that the increase of the feedback strength can further suppress or even conceal the time-delay signature. The closer to the extreme point of the transmission curve the operating point corresponding to the DC offset phase, the weaker the time-delay signature is. The time-delay signature can be suppressed obviously as the offset phase is set to be 0. We also find that the sign of the correlation coefficient corresponding to the time-delay signature is changed when the offset phase and the phase shift caused by the delay feedback produce a phase-shift value of π/2.
2013, 62 (14): 144208. doi: 10.7498/aps.62.144208
In this paper, we theoretically and experimentally investigate the controllable nonlinear focusing of the weak laser beam on the basis of the induced nonlinear effects by another strong pump laser beam. When couple laser beams co-propagate in the Kerr medium, it is possible to change the refractive index by the pump beam, and the locally changing medium in turn modifies the weaker probe beam. Depending on cross-phase modulation theory, we obtain an exact analytical solution to the model under the approximate condition and qualitatively verify it in experiment. The results show that the weaker probe beam can be induced to focus at any position due to induced nonlinear effects from the stronger pump beam under certain conditions.
2013, 62 (14): 144209. doi: 10.7498/aps.62.144209
The analytical solution for the shape-variant astigmatic elliptical breathers in strongly nonlocal nonlinear medium is obtained, and the propagation properties of this type of breather is investigated according to the solution. During propagation, the beam in x- and y-direction keeps Gaussian, the beam width and the curvature of the cophasal line vary periodically but asynchronously. For the two-dimensional off-waist input case, the initial convergence (divergence) makes the width narrowed (broaden) first near the entrance plane, no matter what the input power is; then varies periodically but asynchronously in x- and y-direction. For the one-dimensional on-waist input case, the beam might breathe only in one direction. The breather of the beam width induces the periodical variation of the curvature for the elliptically cophasal surface and the ellipticity of the pattern. If the location of the waist in x-direction and that in y-direction are identical with each other, the product of the maximum and the minimum of the ellipticity keeps equal to unify. In this case, the position of the entrance plane does not affects the maxima and the minima of the ellipticity, but affects the uniformity for the variation velocity of the ellipticity in a period.
2013, 62 (14): 144210. doi: 10.7498/aps.62.144210
Passive ranging has received much attention because of its concealment. The distance between sensor and the object is measured by the infrared radiation attenuation in atmosphere. A mono-station single-band ranging method based on spectra transmission characteristics of oxygen is developed. The concept of light of sight (LOS) is introduced. The ray tracing model is built. With the method of discrete transfer and the discretization of physical properties distribution field of oxygen, the radiation strength in oxygen band of the target is calculated by the LOS along the seeker detectors. The radiation strengths in resonant frequency band and non-resonant frequency band are analyzed. The distance is achieved by their relation. High-resolution spectrometer is adopted to build experiment platform. Real-time dynamic ranging can be implemented by field calibration. The present experimental range is 75-200 m. The maximal relative error is 7.56%.
Propagation of polychromatic partially coherent decentred laser beams propagating in non-Kolmogorov turbulence
2013, 62 (14): 144211. doi: 10.7498/aps.62.144211
In this paper, the analytical expressions for the total intensity, the on-axis spectrum and the degree of coherence of polychromatic partially coherent decentred laser beams propagating in non-Kolmogorov turbulence are derived. The influences of the beam decentred parameter β, the fractal constant α of the atmospheric power spectrum, and the bandwidth Ω of spectrum on propagation property are studied. It is shown that the larger the value of β, the bigger the deviation of centre of beam gravity from the propagation axiis, and the more unsymmetrical the coherence degree is. However, the on-axis spectrum is nearly independent of β. The influence of α on total intensity, on-axis spectrum and coherence degree is non-monotonic. When α=3.1, the propagation properties are most affected by turbulence. It is mentioned that at certain propagation distances, the shifts of on-axis spectrum are the same for different values of α. Furthermore, the on-axis spectral shift disappears at other propagation distances which are independent of Ω, and these propagation distances decrease due to turbulence. The results obtained in this paper will be useful for the applications of polychromatic partially coherent decentred laser beams propagating in non-Kolmogorov turbulence.
Influence of local zero-plane displacement on effective aerodynamic parameters over heterogeneous terrain
2013, 62 (14): 144212. doi: 10.7498/aps.62.144212
In this paper, we investigate the statistical features of the influences of local zero-plane displacement on effective roughness length and effective zero-plane displacement for three surface categories, with a so-called joint calculation scheme for effective aerodynamic parameters over heterogeneous terrain proposed by Zhong et al. [Zhong Z, Lu W, Song S and Zhang Y 2011 J. Hydrometeor. 12 1610]. The local zero-plane displacement is found to magnify the effective roughness length. However, such a magnification effect tends to decrease as the roughness step increases. On the other hand, the effective zero-plane displacement increases as local zero-plane displacement increasing, though it is always less than the area-weighted linear average of the local zero-plane displacement. However, the effective zero-plane displacement tends to decrease with the roughness step increasing.
2013, 62 (14): 144213. doi: 10.7498/aps.62.144213
Secondary electromagnetic polarimetric scattering from a barchan dune is studied. The secondary polarimetric scattering field from the secondary scattering facet is deduced in the Kirchhoff approximation with the waves reflecting from the first scattering facet based on the Ray-tracing theory. The results show that secondary scattering plays an important role in specific range of angle. The co-polarized backscattering coefficients of the lee slope have a local peak value when the incident angle equals the repose angle. Cross-polarized bistatic coefficient of the lee slope near the backscattering region is enhanced when the incident angle is close to the repose angle. The secondary polarimetric scattering between two dunes in line is especially noticeable. The information about wind field in desert region can be retrieved and analyzed by the results in this paper.
2013, 62 (14): 144214. doi: 10.7498/aps.62.144214
Nanowire-grid polarizer is of a periodic sub-wave structure of metallic nanowire on the substrate, fabricated by nanoimprint technology. Different from traditional polarization prism and dichroic polarizer, the nanowire-grid polarizer has many advantages such as compact size, easy integration and high polarization performance. However, in the ultraviolet and visible regions, it is infeasible to improve the performance of single layer nanowire structure by reducing the character size of nanowire because of the bottleneck of lithographic process. The double-layer nanowire-grid structure could improve the polarization characteristics at some special wavelengths but not full-wave band of ultraviolet and visible regions. In this paper, we propose a tunable double-layer structure to enhance the extinction ratio and transmission at each wavelength by tuning the distance between two nanowire-grid polarizers through adjusting the voltage applied to PZT. To calculate the transmittance and transmission extinction ratio of tunable structure, software VirtualLab is employed and the Fourier model method is used. The numerical simulation results show that the tunable structure have a higher polarization characteristic in ultraviolet and visible regions than single layer structure and double layers structure.
Demodulation and multi-input multi-output equalization for mode division multiplexing system using a novel few-mode fiber
2013, 62 (14): 144215. doi: 10.7498/aps.62.144215
In order to solve the mode crosstalk in a mode division multiplexing (MDM) transmission system, we design and fabricate a novel few-mode fiber (FMF) with high mode differential group delay (MDGD) that ensure that all supporting modes can transmit through the FMF independently. Then, we propose a cascaded multi-input multi-output (MIMO) delay equalization method, in order to further suppress the mode crosstalk from the mode division multiplexer/demultiplexer (MUX/DeMUX) and improve the transmission distance and spectrum efficiency of MDM system. Our simulation results verify that the cascaded MIMO delay equalization method together with the novel FMF can effectively mitigate the mode crosstalk of MDM transmission. Compared with the traditional MIMO equalization method, our proposed equalization method can be successfully applied to the QPSK signal transmission through the FMF with large MDGD, while no obvious increase of computation complexity is observed. Our results show that for a 40 Gbps two-mode division multiplexing system, the Q factor can be improved by 1.7 dB after 40 km FMF transmission using the proposed cascaded MIMO equalization method. Therefore, all those results are very conducive to the next-generation high capacity fiber-optic transmission.
2013, 62 (14): 144216. doi: 10.7498/aps.62.144216
The end-face reflection property of micro-nano fiber (MNF) is one of its important transmission features, which plays remarkable roles in a variety of practical applications of MNF. In this paper, we propose a novel method to study the MNF end-face reflection property using an optical circulator, which overcomes the deficiencies of the direct measurement relying on a 3 dB coupler. On the basis of this method, we further introduce the aerogel fixing and power compensation techniques to effectively eliminate the errors caused by MNF pigtail floating, the light source output instability and internal loss. Therefore, the whole experimental design improves the measurement accuracy. The MNF end-face reflections with respect to the diameter of MNF and the wavelength of the guided light are investigated. The experimental results are in good agreement with simulation results, showing that this new approach is reliable for measuring the reflectivity of specific MNF. This detailed investigation on the end-face reflection versus various factors will be conducive to the constructions of versatile micro-nano scale devices based on the MNF, such as the MNF lasers, MNF amplifiers, MNF couplers and MNF filters.
The mechanism of the interference structure in shallow water vector acoustic field and experimental investigation
2013, 62 (14): 144301. doi: 10.7498/aps.62.144301
The microscopic structure characteristic of low frequency sound propagation in shallow water is that a stable space-frequency interference structure represented by waveguide invariant exists. Both the scalar field and the vector field determine all the features of the acoustic field. The vector field characteristics of space-frequency interference structure in shallow water acoustic field are investigated. The theoretical analyses of the mechanism of interference structure in the sound pressure spectrum, the kinetic energy density spectrum, and the acoustic intensity flow spectrum in shallow water vector acoustic field are conducted, the numerical simulation studies of the interference of the Pekeris waveguide in energy and energy flow density characteristics are performed, and the sea trials for broadband sound source radiation vector sound field interference properties and characterization are also carried out. The experimental results accord with the theoretical analyses and simulation results very well. The research results show that stable space-frequency interference structure in acoustic vector field exists in many forms for modal coherent in short-distance and long-distance. In addition to various forms of energy and energy flux density spectra, in coherence-coefficient spectrum there appear the interference characteristics. These forms of space-frequency interference structure can be described by waveguide invariant theory effectively.
2013, 62 (14): 144302. doi: 10.7498/aps.62.144302
Based on high speed moving target robust high-resolution direction of arrival (DOA) estimation problem under low signal-to-noise ratio and a small number of snapshots in the underwater acoustic vector signal processing framework, a novel spatial spectrum model combined with compressive sensing method is proposed. By studying the acoustic vector sensor structure, a generalized temporal filtering method based on sound pressure and particle velocity combined treatment is presented. According to the matrix spatial prefiltering theory, a new spatial filter with stopband constraint and mean square error min-max principle in passband is proposed which is used as vector sonar spatial prefiltering algorithm. Based on the methods above, a novel time-space domain jointly filtering high-resolution DOA estimation algorithm based on compressive sensing is proposed. The mathematical model, physical interpretation, and specific implement are explained in detail. Theoretical analysis and computer simulation results show that the new method has a lower dual-target distinguishing threshold and a higher estimation accuracy in solving the vector sonar high speed moving target robust DOA estimation problem under a small number of snapshots (single snapshot) condition. The higher robustness and better results of the proposed method are verified in the lake experiment.
2013, 62 (14): 144303. doi: 10.7498/aps.62.144303
An underwater acoustic communication scheme using intrinsic dolphin sounds is proposed to overcome the sound exposure derived from traditional fixed carrier modulation for covert communication in this paper. The communication signal can be detected but it could easily be excluded in the process of recognition/classification by an adversary. This is because the signal resembles dolphin sounds. Properties of dolphin sounds are investigated in this paper. Synchronization in this scheme is achieved by using dolphin whistles while dolphin clicks are used as information carrier. The time interval between dolphin clicks conveys information. Channel equalization is performed by passive time reversal mirror technique, whereas channel estimation is done through matching pursuit method under the theory of compressive sensing. The efficiency and feasibility of the proposed method are verified by lake trial. The received signal and emission mimic bio-signal are very similar. Bit error rate less than 10-4 is achieved above 29 bps to a distance of about 2 km.
2013, 62 (14): 144701. doi: 10.7498/aps.62.144701
Based on the large eddy simulation, combined with the 5th order weighted essentially non-oscillatory scheme and the immersed boundary method, the shock wave interacting with an R22 cylinder is numerically simulated. Our numerical results present clearly the deformation of cylinder induced by the Richtmyer-Meshkov instability due to the interaction of shock wave with R22 cylinder, which accords well with previous experimental results of Haas and Sturtevant [Haas J F and Sturtevant B 1987 J. Fluid Mech. 181 41]. In addition, the numerical results reveal the generation process of a jet induced by the refracted shock focusing near the right interface of the inner cylinder, as well as the roll-up of the secondary vortexes along the slip layer of two main vortexes. The mixing mechanism of R22 gas and air is also expatiated. Furthermore, the evolution of R22 cylinder under reshock condition is numerically simulated with two different end wall distances. For the long distance case, the reflected wave interacts with severely distorted R22 volume, making it further compressed on the x-axis. While for the small case, two Mach reflections occur between the reflected shocks during their propagating upstream within the cylinder. The two high pressure areas behind two triple points can accelerate the boundary of the R22 cylinder while they are passing through it and induce two jets.
2013, 62 (14): 144702. doi: 10.7498/aps.62.144702
In this study, analytical solutions are presented for the unsteady electroosmotic flow of linear viscoelastic fluid between micro-parallel plates. The linear viscoelastic fluid used here is described by the general Jeffrey model. Using Laplace transform method, the solution involves analytically solving the linearized Poisson-Boltzmann equation, together with the Cauchy momentum equation and the general Jeffrey constitutive equation. By numerical computations, the influences of the dimensionless relaxation time λ1 and retardation time λ2 on velocity profile are presented. In addition, we find that when the retardation time is zero, the smaller the relaxation time, the more close to the Newtonian fluid velocity profile the velocity profile is. With the increases of the relaxation time and the retardation time, the velocity amplitude also becomes bigger and bigger. As time goes by, the velocity tends to be stable gradually.
2013, 62 (14): 144703. doi: 10.7498/aps.62.144703
In this work the bubble dynamics near a plate with circular hole is investigated. Numerical model for bubble dynamics is established based on incompressible potential theory. To overcome the numerical limitation of traditional boundary element method for the case with small initial distance between bubble and the edge of hole, the fluid domain is divided into two semi-infinite domains which are solved separately by fusing the bubble wall and the plate wall together, by which numerical simulation in various parameter ranges is implemented. The numerical results match the experimental ones well. Through the analysis of bubble dynamics near circular hole, we find that the influence of hole is opposite to that of solid wall. During the expansion phase, cavity-attraction effect is exerted on the bubble, while the bubble is pushed away during the collapsing phase. In some specific cases, opposite-jets are formed under the conjunction of solid plate and circular hole. Finally, the case where bubble wall and solid wall are fused together is analyzed to study the bubble dynamics after the separation of fluid domain and the influences of no-dimensional parameters.
2013, 62 (14): 144704. doi: 10.7498/aps.62.144704
The lattice Boltzmann method is used to simulate the thermal field and flow field of nanofluid Raleigh-Benard convection in a rectangular cavity. The heat transfer characteristics of nanofluid are compared under different Raleigh numbers, volume fractions of nanoparticles and particle sizes. The results show that under the same Raleigh number and volume fraction, the convection heat transfer of nanofluid becomes weakened by increasing the particle size. Under the same Raleigh number and particle size, the convection heat transfer of nanofluid becomes strengthened by increasing the volume fraction of nanoparticles.
Bubble velocities in the nonlinear Rayleigh-Taylor and Richtmyer-Meshkov instabilities in non-ideal fluids
2013, 62 (14): 144705. doi: 10.7498/aps.62.144705
In a reference system moving with the bubble vertex we investigate the effects of fluid viscosity and surface tension on the bubble velocity in the nonlinear Rayleigh-Taylor (RT) and Richtmyer-Meshkov (RM) instabilities, by extending the ideal fluid model [Goncharov V N, Phys. Rev. Lett. 88 134502 (2002)] to the non-ideal fluid case. First of all, the governing equation (i.e. self-consistent differential equations) describing the dynamic of the bubble front in RT and RM instabilities is obtained. Then, the numerical and asymptotic solutions of the bubble velocity in two-dimensional planar geometry and three-dimensional cylindrical geometry are obtained. Moreover, we quantitatively study the effects of fluid viscosity and surface tension on the RT and RM bubble velocities. It is found that in the fully nonlinear evolutions of RT and RM instabilities, the bubble velocity and amplitude in the non-ideal fluid are both less than those in its ideal fluid counterpart. That is to say, the effects of fluid viscosity and surface tension tend to stabilize the RT and RM instabilities.
The identification and forecasting of chaos for natural circulation flow instabilities under rolling motion
2013, 62 (14): 144706. doi: 10.7498/aps.62.144706
Chaos identification and forecasting of the irregular complex flow oscillations in a two-phase natural circulation system under the rolling motion are performed. The irregular complex flow oscillation has chaotic characteristics by calculating the geometric invariants such as the correlation dimension, Kolmogorov entropy and the largest Lyapunov exponent. But the reliability of calculation result is liable to be influenced by data length and the interference of measurement noise, false judgment results may exist in the direct method. To avoid misjudgment for chaos flow oscillation, both the geometric invariants and chaos identification need to be calculated by surrogate-data method. The chaos is identified by the iterated-amplitude adjusted Fourier transform method. Chaotic forecasting for the irregular complex flow oscillation is carried out by adding weight one-rank local region method. By surrogate-data method, we can confirm that the irregular complex flow oscillation is chaotic oscillation from the deterministic system. Comparisons between the prediction results and experimental data indicate that the chaos forecasting based on adding weight one-rank local region method is an effective way for two-phase natural circulation flow instabilities, and a way of dynamical forecast to monitor flow oscillation is presented.
PHYSICS OF GASES, PLASMAS, AND ELECTRIC DISCHARGES
A new method of simultaneous focal spot shaping and polarization smoothing using crystal phase plate
2013, 62 (14): 145201. doi: 10.7498/aps.62.145201
A new method is proposed in which uniaxial crystal is used to fabricate the phase plate, with which shaping and polarization smoothing of the target focal spot can be achieved simultaneously in the laser driver of inertial confinement fusion. The principle of crystal phase plate (PS-CPP) is analyzed using the scalar diffraction theory. The solution of crystal surface shape is obtained from the distribution characteristics of the focal spot. Numerical simulation is developed to analyze the application of PS-CPP. The results show that under the conditions of particular crystal cutting angle and surface shape, PS-CPP can be obviously effective for focal spot shaping and smoothing, and the effectiveness is as good as that of the combination of continuous phase plate and the polarizing smoothing crystal.
CONDENSED MATTER: STRUCTURAL, MECHANICAL, AND THERMAL PROPERTIES
A comparative study of different potentials for molecular dynamics simulations of melting process of silicon
2013, 62 (14): 146401. doi: 10.7498/aps.62.146401
Molecular dynamic simulations of bulk melting and surface melting of Si are carried out. The atomic interactions in Si are calculated by stillinger-weber (SW), modified embedded-atom method (MEAM), Tersoff and highly optimized empirical potential (HOEP) potentials respectively. The results show that the four potentials could exhibit the fundamental laws of Si, such as thermal expansion, melting at high temperature, endothermic and volume shrinkage in melting process. However, the detailed analyses demonstrate that the Tersoff and MEAM potentials are best to describe the melting and crystal growth of Si, SW potential is the next. The HOEP potential is even unsuitable for describing the melting and crystal growth of Si.
2013, 62 (14): 146402. doi: 10.7498/aps.62.146402
In situ X-ray diffraction patterns of the powder titania polymorphs are recorded in a temperature range from room temperature (RT) to 1200℃ in static air and vacuum. The results show that the temperature converting anatase into rutile is at 850℃ in static air and at 855℃ in vacuum. Lattice parameters for anatase (RT-850℃) and rutile (RT, 900-1200℃) in static air and those for anatase (27-850℃) and rutile (950-1200℃) in vacuum are refined. The variations of lattice parameters of anatase and rutile with temperature (℃) are therefore well described. Linear () and volume () thermal expansion coefficients of anatase (RT-850℃) and rutile (RT, 900-1200℃) are calculated. The change laws of and with temperature for anatase and rutile in static air and vacuum are summarized. At RT, the thermal expansion coefficients for anatase are a=4.5506310-6/℃, c=7.754310-6/℃, and =16.8583610-6/℃ in static air and a=4.6942910-6/℃, c=9.0285010-6/℃, and =18.6968810-6/℃ in vacuum while those for rutile are a=6.8124310-6/℃, c=8.7164410-6/℃, and =22.2217810-6/℃ in static air and a=6.0583410-6/℃, c=8.3928010-6/℃, and =20.5236210-6/℃ in vacuum, respectively.
2013, 62 (14): 146801. doi: 10.7498/aps.62.146801
Controlling the wettability of solid surfaces is an important issue that has aroused the increasing interest from both fundamental and practical perspective by tailoring surface morphology and surface chemical compositions. The underlying theories for interpreting wetting phenomena still mainly focus on the Young's equation, the Wenzel equation, and the Cassie-Baxter equation, despite the fact that the wetting phenomena have been studied over the past decades. While there are a lot of experimental studies on wettability of surface roughness, there is still a lack of a thorough analysis of the contributions of micro and nano-scale roughness to wettability behavior despite interesting features these surfaces have. In this article, the basic theories and their applicabilities are addressed in detail, and the mutual transition between Wenzel state and Cassie-Baxter state is described from different viewpoints in general, and from single-scale and dual-scale point of view in particular. The design concept of geometrical model with stable superhydrophobicity is also described, which is based on the typical theories about wettability. Finally, some promising breakthroughs in the theoretical progress are proposed.
CONDENSED MATTER: ELECTRONIC STRUCTURE, ELECTRICAL, MAGNETIC, AND OPTICAL PROPERTIES
Lu2Si2O7: Ce (LPS: Ce) is a well-known scintillator due to its excellent scintillation properties such as high light yield and fast decay time. However, the oxygen vacancies would degrade its scintillation properties. In this paper, we perform first-principles investigation on the stabilities of Li, Na, Mg and Ca in LPS and their influences on the oxygen vacancy. We focus on the defect formation energies and densities of states. It is found that the dopants occupy interstitial sites under oxygen-poor limit. They can suppress oxygen vacancies due to the requirement of charge balance. We discuss the influence of dopants on the density of states of perfect LPS.
2013, 62 (14): 147102. doi: 10.7498/aps.62.147102
In this paper, compression test is carried on electrodeposited nano-crystalline (NC) Ni at 77 K. The results show that the NC Ni has extremely high compressive strengths: around 2.5 GPa at room temperature (RT), 3.5 GPa at 77 K, and moderate plasticity with 0.1 fracture strain at RT, 0.05 at 77 K. The strain rate sensitivity m and activation volume v are calculated, and scanning election microscope and high-resolution transmission election microscope are used to verify the deformation mechanism. Analysis indicates that the dislocation interaction with grain boundary (GB) dominates the deformation of NC Ni both at 77 K and RT. Based on the calculated values of m and v, the deformation process is described by considering the intrinsic dislocation of GB bowing out and expanding towards opposite GB in the inner grain dislocation-free zone. And the mechanism is studied in the residual dislocation parts affecting the increase of strain compatibility and the decrease of stress concentration. It is indicated that the difference in compression property between at RT and 77 K is relatated to grain boundary-dislocation coordination mechanism and relation between residual dislocation motion and temperature.
2013, 62 (14): 147301. doi: 10.7498/aps.62.147301
A method of preparing film silicon with variable optical gap is introduced, and the relevant results are given. An easy way to determine the film optical gap by transmittivity curve tangent is shown, and the preparation craftwork and conditions are given. The gap state distribution maps of various materials are presented. In experiment, it is found that the optical gap width of material is related not only to the quantum size effect, but also to the height and width of the barrier formed by lacuna, and to the length of short range order (atom cluster length) as well. A relationship between optical gap of silicon film and atom cluster length is given. Computations show that electron liquid level lifts in potential well with atom cluster length increasing, and the scattering of carrier is weakened by defect barrier. When the atom cluster length is shorter, the electron liquid level does not always lift with atom cluster length increasing, but fluctuates heavily and forms an indention wave. The computations also show that in the case of a constant ratio of the barrier width to the length of the atomic string, the electronic liquid level is also related to the barrier height.
2013, 62 (14): 147302. doi: 10.7498/aps.62.147302
ZnO nanobelts are synthesized in high yield by a simple chemical vapor deposition method, at low temperature. And then Au nanoparticles are sputtered on the ZnO nanobelts. The effect on the photoluminescence of Au-ZnO composite nanobelts by surface plasmon is systematically investigated by the scanning electron microscopy, transmission electron microscopy and photoluminescence spectrum. The enhancement ratio by surface plasmon resonance and the almost completely suppressed defect emission for Au-ZnO composite nanobelts are observed, and the emission enhancement ratio η of Au-ZnO composite nanobelts reaches a maximal value of 85-fold. Additionally, the photoluminescence mechanism is proposed in terms of the scattering and absorption by Au nanoparticles, the Purcell enhancement factor, and the Ostwald ripening.
2013, 62 (14): 147303. doi: 10.7498/aps.62.147303
The transmittance property and steady-state magnetic field distribution of the sliver circular gap embedded with a sliver nanorod are investigated using the finite-difference time-domain method. Since the titled nanorod breaks the symmetric steady-state magnetic field distribution, Fano resonance occurs in the composite system. In addition, the transmittance spectrum depends strongly on the tilting angle and topologic shape of the nanorod. These results would be useful for designing filter for specific usage.
2013, 62 (14): 147304. doi: 10.7498/aps.62.147304
Radar and optical dual band-pass film material can be fabricated by the method of loading frequency selective surface into metallic mesh substrate. In this paper its electromagnetic characteristics are analyzed and calculated by the vector mode matching method. Using substrate, metallic mesh and frequency selective surface integration design method, its electromagnetic characteristics are optimized. The calculated and measured results show that optically transparent frequency selective surface which is designed by the integration design method can not only quickly obtain the electric field basis functions but also accurately predict the resonant size, therefore, the computational efficiency is improved and the occurence of the modal interaction null is avoided. With integration design method, optically transparent band-pass frequency selective surface having a stable filter characteristic is obtained in this paper.
2013, 62 (14): 147305. doi: 10.7498/aps.62.147305
A good rectifying behavior is observed in a temperature range from 80 K to 300 K in the BaTiO3/p-Si heterostructure, which is fabricated by a pulse laser deposition. The diffusion voltage (VD) decreases with the increase of temperature. A significant photocarrier injection effect is also observed with light irradiation. The photocarrier injection effect increases with the energy of photon increasing. Meanwhile, R-T curve of the BaTiO3 film indicates that the oxygen-deficient BaTiO3 is an n-type semiconductor.
Investigation on the relationship between the properties of atomic layer deposition ZnO film and the dose of precursor
2013, 62 (14): 147306. doi: 10.7498/aps.62.147306
In this paper, we present the properties of new type of material ZnO and the ZnO films prepared on sapphire substrate through atomic layer deposition (ALD). In experiment, we use N2 as the carrier, DEZn and DI-water as the precursors. The deposition temperature is 180℃. The value of Zn/O could be modified through changing the dose of DEZn. Furthermore, we investigate the influences of Zn/O value on the thickness, growth rate, crystalline property, surface morphology, three-dimensional structure and roughness of the ZnO film prepared by the ALD method.
Characteristics of polarization separation of frequency selective surface by lumped inductors and capacitors
2013, 62 (14): 147307. doi: 10.7498/aps.62.147307
Based on the circuit resonance theory of the electromagnetic local resonance, a novel frequency selective surface structure with the property of polarization separation is designed by loading the lumped inductors and capacitors to the periodic array with “I” elements. The operation principle of the structure at all polarizations is analyzed by using the equivalent circuit method. And the influences of the polarization, incident angle and parameters of the lumped device on the transmission characteristics of the FSS are simulated and discussed by using the full-wave numerical analysis method. The results indicate that the designed structure has a good property of polarization separation at about 6.37 GHz, the transmission characteristics are stable with the scan angle increasing from 0° to 40° at TE and TM polarization, and by controlling the lumped inductors and capacitors, the transmission characteristics for TM polarization can be regulated independently while the transmission characteristics for TE polarization remain constant. The proposed structure is conductive to the designs of the polarization separator and the polarized wave generator.
Improvement on the electron transport efficiency of the carbon nanotube field effect transistor device by introducing heterogeneous-dual-metal-gate structure
2013, 62 (14): 147308. doi: 10.7498/aps.62.147308
To improve the carbon nanotube field effect transistor (CNTFET) device performance and enhance the electron transport efficiency of the device, a heterogeneous-dual-metal-gate (HDMG)-CNTFET is proposed. By appropriately modifying the transport model for single-metal-gate (SMG)-CNTFET, the electron transport properties of the HDMG-CNTFET device are investigated. The results indicate that the work function WGS of the metal gate near the source (S-gate) is fixed such that it is equal to that of the intrinsic CNT, and the work function WGd of the metal gate near the drain (D-gate) is selected to be smaller than WGS within a certain range, the electric field distribution can be optimised and the average electron velocity in the CNTFET channel can be significantly increased; at the same time, due to the electric potential modulation by the D-gate, the device has a lower threshold voltage. When the same operating voltage is applied, HDMG-CNTFET has a larger on-state current than SMG-CNTFT; and due to the shielding effect of the drain voltage variation by D-gate, the HDMG-CNTFET device exhibits good gate-control ability and can suppress the drain-induced barrier lower effect, hot electron effect and ambipolar conduction behavior compared with SMG-CNTFET. This study, by reasonably selecting the gate electrode work function of the HDMG-CNTFET, can effectively overcome the deficiency of existing research on improving the CNTFET performance at the expense of reducing the on-current, more importantly, can improve the electron transport efficiency, thereby improving the characteristic frequency and reducing the delay time of the device, which will be of benefit to CNTFET application in high-speed/high-frequency circuit.
Magnetic field-induced martensitic transformation, superspin glass and exchange bias in Heusler alloys NiCoMnSn
2013, 62 (14): 147501. doi: 10.7498/aps.62.147501
The crystal structures and magnetic properties of Ni50-xCoxMn39Sn11 (8≤x≤10) Heusler alloys are investigated. As a result, we achieve the magnetic field induced martensitic transformation in Ni42Co8Mn39Sn11. It is found that the saturation magnetic moments of alloys increase with Co content increasing. Moreover, a superspin glass behavior and a large exchange bias effect are also found in samples with Co content being higher than 8. We confirm the coexistence of superspin glass of the martensite and ferromagnetic parent phase, which is the physical origin of the exchange bias effect. On the other hand, we propose that the origin of superspin glass in our NiMnCoSn system is due to the occurence of Mn-Mn cluster as reported by Ma et al. in Heusler Mn2Ni1.6Sn0.4 alloys [Ma L, Wang W H, Liu J B, Li J Q, Zhen M, Hou D L and Wu G H 2011 Appl. Phys. Lett. 99 182507].
The study of Raman spectrum, distortion of lattice and spin reorientation phase transition on Pr doped DyFeO3 system
2013, 62 (14): 147601. doi: 10.7498/aps.62.147601
Pr doped rare-earth orthoferrites DyFeO3 is synthesized by solid state reaction. X-ray diffraction shows that the lattice parameters of Dy1-xPrxFeO3increase and distortion of lattice decreases with Pr content x increasing. Raman spectroscopy reveals that the change of effective mass (meff) [meff=xmPr+(1-x)mDy] together with lattice structure change results in the shift of vibration modes. With the increase of Pr content, Raman modes of wave number less than 200 cm-1 remain constant, but the modes of wave number greater than 200 cm-1 decrease with Pr content increasing (except the mode B3u near 420 cm-1). The weak ferromagnetic ordering, created by Dzyaloshinsky-Moriya interaction, is reduced with doping level increasing. The interaction of rare earth ions with Fe3+ ion, together with the change of lattice distortion, results in the increasing of spin reorientation phase transition temperature when xx increasing to over 0.3.
Optimization of nuclear magnetic resonance refocusing pulses to enhance signal intensity in gradient B0 field
2013, 62 (14): 147602. doi: 10.7498/aps.62.147602
It is an efficient protocol to use the refocusing flip angle pulse optimization technique to solve special engineering technical problems in nuclear magnetic resonance (NMR) measurements. By reducing RF pulse duration, the low refocusing flip angle pulses can consume lower power, satisfy specific absorption rate of samples, and improve signal-to-noise ratio as well. To further analyze the function mechanism of pulse angles, the dependence of signal intensity on RF pulse is studied in homogenous magnetic field and constant gradient magnetic field respectively. Afterwards, echo amplitudes with various tip angles and flip angles ranging from 0° to 180° are compared with conventional sequence of 90° pulse followed by 180° pulses theoretically and experimentally. For the constant gradient field, the refocusing pulse of flip angle can be as low as 140°, defined as the optimum herein, to obtain the strongest signal intensity, enhanced by 13% compared with that of 180°. Moreover, T1 distributions measured by the conventional and optimal sequences for distilled water at room temperature are compared, and good conformances of T1 between the two pulse sequences are obtained, which demonstrates the optimal refocusing pulse can be directly applied to T1 measurement. The results provide constructive suggestion for designing pulse sequences for signal intensity enhancement in NMR logging while drilling and NMR online quick analysis.
2013, 62 (14): 147801. doi: 10.7498/aps.62.147801
Considering the exciton-biexciton coherence, we analytically study the linear absorption and nonlinear propagation properties of the probe and signal field in a semiconductor quantum dot. It is found that in the linear case, in the system takes place an electromagnetically induced transparency phenomenon. Furthermore, we obtain that in the system there occurs one- or two-transparency window, or optical gain, which is controlled by adjusting the coupling control field. For the nonlinear case, the weak probe field, which is induced by a signal field, splits into two parts. When their excited self-Kerr and cross-Kerr nonlinear effects balance the refractive effect of the beam, the system exhibits bright-bright, bright-dark, dark-bright, dark-dark soliton pairs.
2013, 62 (14): 147802. doi: 10.7498/aps.62.147802
Positron emission tomography (PET) is the most state-of-the-art clinical examination method in the field of nuclear medicine imaging technology. The PET is one of the best tools for tumor diagnosis and guidance in the clinical treatment. The source data that are obtained by PET, need to be processed into a sinogram form data which can be used for image reconstruction. Parallel beam tomographic reconstruction and fan-beam tomographic reconstruction are two methods of imagetomographic reconstruction, respectively, corresponding to parallel beam data processing and fan beam data processing. The processing of the original data is inevitable to destruct the integrity of original data. Nowadays, the PET is a commonly used method of parallel beam image reconstruction. Parallel beam data processing will change original data by data interpolation operation, and the fan beam data processing avoids data interpolation operation. In this paper, we study the influence of data interpolation on PET image reconstruction by comparing the results obtained by the parallel-beam image reconstruction with those obtained by fan-beam image reconstruction.
Effect of raster scan number on damage resistance of KDP crystal irradiated by ultraviolet pulse laser
2013, 62 (14): 147803. doi: 10.7498/aps.62.147803
The raster scanning experiments for KDP crystal are carried out in R-on-1 mode at energy density below zero probability damage threshold to investigate the damage resistance capability as a function of pulse number after ultraviolet (UV) laser irradiation by using a tripled Nd:YAG laser. The1-on-1 damage measurements after laser irradiation indicate that the damage resistance of KDP is obviously enhanced and the increment is dependent on scanning number. The optical absorption and photoluminescence spectra show that the evolution of oxygen-vacancy centers in KDP crystal is important to the damage resistance capability. The Raman and infrared spectra show that the polarization distortions of PO4, P–OH and P＝O groups induced by UV laser irradiation also contribute to the changes of damage resistance.
INTERDISCIPLINARY PHYSICS AND RELATED AREAS OF SCIENCE AND TECHNOLOGY
2013, 62 (14): 148101. doi: 10.7498/aps.62.148101
Graphene/silver nanocomposites are synthesized in the presence of sodium hydroxide, with graphene oxide and AgNO3 used as the raw materials. X-ray diffraction, Fourier transform infrared spectroscopy (FT-IR), and transmission electronic electron microscope, UV-vis spectrophotometer are used to characterize the obtained composites. Results indicate that the graphene oxide is partically reduced to graphene sheets, silver ions are reduced to silver nanoparticles and distributed on the graphene sheets uniformly. The action temperature, quantity of silver nitrate, adding order of NaOH and the way of mixing precusors have an influence on the silver size and particulate size distribution. The size distribution of Ag nanoparticles is centred at 12 nm under a suitable number of silver ions.
Reducing tag collision in radio frequency identification systems by using a grouped dynamic frame slotted ALOHA algorithm
2013, 62 (14): 148401. doi: 10.7498/aps.62.148401
In a radio frequency identification system, designing a tag anti-collision algorithm with good performance is the key to improving the system throughput. In this paper, we propose a principle of dynamic frame size regulation and a method of tag grouping, and design a novel algorithm called grouped dynamic framed slotted ALOHA (GDFSA) through combining the above, which first estimates the number of unread tags and divides them into groups, then uses dynamic frame slotted ALOHA in each group. The simulation results show that the proposed algorithm can stabilize the throughput rate between 34.6%-36.8% and the time slots increase linearly in the identification procedure. When the tag number is beyond 2000 compared with conventional algorithms, the proposed algorithm can enhance the slot efficiency by 30%, and optimize the throughput rate obviously and save the system resource.
2013, 62 (14): 148701. doi: 10.7498/aps.62.148701
DNA sequencing technology has markedly advanced the development of biological and medicinal sciences. High-throughput pyrosequencing instruments that combine the pyrosequencing with microfabricated high-density picoliter reactors have been proved to be suitable for de novo sequencing and metagenome sequencing. In the present work, we report on an alternative sequencing chip consisting of hundreds of thousands of picoliter sized honeycombed SU8 reaction vessels on a fiber-optic slide by lithography technique for high-throughput pyrosequencing instruments. Highly reproducible fabrication process of SU8 sequencing chip is achieved through the improvement on SU8 film thickness uniformity and relaxation of SU8 residual stress during fabrication. To achieve the optical isolation required for SU8 reaction wells, metal film is selectively deposited on the side walls of the reaction vessels by reformating vacuum coating. With the metal coating, the average value of optical cross talking between SU8 reaction vessels is reduced from 25% to 1%. The SU8 sequencing chip demonstrates an excellent light transmission characteristic and meets the need of pyrosequencing application.
2013, 62 (14): 148702. doi: 10.7498/aps.62.148702
In order to solve the problem of power line noise and environmental magnetic noise to magnetocardiogram, a filtering method based on singular value decomposition (SVD) and generalized S transform is proposed. On the basis of discrete condition, S transform and its inverse transform in matrix form are derived. The signal is first preprocessed using generalized S transform in order to get better time-frequency resolution, then the time-frequency domain is decomposed by SVD, and the valid time-frequency region is determined. Therefore, the adaptive filtering can be realized. Numerical simulation studies indicate that the method presented can achieve better performance than other methods with less singular values. Illustrative examples verify the effectiveness of the proposed method.
2013, 62 (14): 148703. doi: 10.7498/aps.62.148703
Reconstructing cardiac current source by magnetocardiogram signal is a noninvasive method of revealing cardiac electrical activity. The visualization of spatial and temporal information about cardiac electrical activity will be conducive to the analysis of the heart function and the diagnosis of heart disease. In this paper, the capability of reconstructing distribution of current sources, whose distribution is changed over time, is studied by a reduced time window (RTW) beamformer, and the relationship between source reconstruction results and ventricular excitation propagation is analyzed in simulation. The cellular automaton is used to simulate ventricular excitation propagation and to generate the current sources. Moreover, a heart-torso model is constructed by the boundary element method to simulate the effect of the volume conductor. The simulation results show that the RTW beamformer can provide a good accuracy of reconstructing the distribution of current sources. The RTW beamformer is also used to analyze a normal subject with the lead field of infinite homogeneous medium. The results demonstrate that they reflect the basic characteristic of ventricular excitation propagation.
2013, 62 (14): 148901. doi: 10.7498/aps.62.148901
An improved Moran's I method is proposed to describe the spatio temporal features of road traffic flow based on the traditional Moran's I. The spatio temporal distribution classification of road traffic status including homogenous congested and uncongested traffic, heterogeneous congested and uncongested traffic is then given through Moran plotting graph. To verify the applicability of the proposed method, the traffic flow data along the second ring-shape urban expressway in Beijing is introduced for analysing the spatio temporal distribution and evolution of road traffic status from global and local autocorrelation through using the proposed method.
GEOPHYSICS, ASTRONOMY, AND ASTROPHYSICS
Retrieval of liquid cloud microphysical properties from spaceborne active and passive sensor data based on optimal estimation theory
2013, 62 (14): 149201. doi: 10.7498/aps.62.149201
In order to meet the urgent requirement for accurate retrieval of liquid cloud microphysical properties, integrating the detecting advantages of active and passive sensors and combining radar reflectivity and optical depth information from CloudSat and Aqua, a new retrieval algorithm of liquid cloud microphysical parameters is proposed according to the optimal estimation theory. By assuming the lognormal size distribution of cloud droplets and establishing functional relationships between measurement and retrieval variables based on forward physical model, with the prior information about spectral distribution parameters, the optimal solutions of spectral parameters are obtained after iteratively calculating, then the microphysical parameters of liquid cloud could be retrieved based on forward physical model, and the uncertainty can be calculated according to error propagation theory. By designing retrieval scheme and using measured case data, the retrieval results are compared with the data published by CloudSat official institutions and those retrieved using empirical algorithms, showing that retrievals of liqiud cloud microphysical parameters based on optimal estimation theory by using combined active and passive sensor data are well consistent with official released data, which makes up for the disadvantages of empirical algorithms that have large error and poor expansibility and gives some important references for retrieval research of liquid cloud microphysical parameters based on domestic spaceborne and airborne W-band millimeter-wave radar data.
2013, 62 (14): 149401. doi: 10.7498/aps.62.149401
Spacecraft surface charging and discharging in a plasma environment are affected by many factors, and the charging time is an important factor to influence the discharging frequency. In this paper, considering microstructure and material parameters of the plasma characteristics, appling the principles of mechanics to each particle, and using statistical methods, the expression of isolated conductor ball surface charging potential time-domain is deduced. Using the general expression of voltage, we deduce the expression of the time domain of electrostatic charging quantity of isolated conducting sphere and the expression of the time domain of the electrostatic field energy. Taking low earth orbit and geosynchronous orbit for example, we discuss the potential of isolated conducting sphere, static load and the characteristics of the electrostatic field energy. The influences of space environment parameters and the size of the conducting sphere radius on surface charging are analyzed. The laws of the time domain of isolated conductor surface charging in plasma environment are summarized.
2013, 62 (14): 149402. doi: 10.7498/aps.62.149402
The negative effect of spacecraft inside isolated conductor surface charging is more unobvious, causing more serious direct and potential harm. Considering the electrons and ions, the secondary electron emission, according to the kinetic theory of gases and combing the theory of particle in Maxwell velocity distribution function, calculate general expression in a plasma environment unbiased solid surface charging. Using the voltage general expression, we deduce the expression of energy and electric quantity, discuss the relationship between energy of isolated conductor's electrostatic field and surface as well as space environment in special environment, compare the energies of electrostatic field in different environments and different surfaces with electromagnetic pulse discharge damage of ground electronic components, and finally summarize the rules of area effect of isolated conductor's surface charging.