Vol. 61, No. 5 (2012)
2012, 61 (5): 050201. doi: 10.7498/aps.61.050201
In order to evaluate the node importance in complex network, considering the disadvantages of node deletion method, node contraction method and betweenness method, through defining the node efficiency and the node importance evaluation matrix, a method to find the vital node in complex networks is proposed by using the node importance evaluation matrix. Considered in this method are the node efficiency, node degree and adjacent node importance contributions, and used adjacent node degree and efficiency value to characterize the contribution of their importance. Finally, an optimized algorithm whose time complexity was O(Rn2) is provided. Experiments show that this method is effective and feasible, and it is applicable to large scale complex networks.
2012, 61 (5): 050202. doi: 10.7498/aps.61.050202
The probability models of 2 different quantum cellular automaton (QCA) adders are based on the theory of probabilistic transfer matrix and circuit partition. The effect of individual component on the overall fault-tolerance is fully analyzed at the same level. The simulation shows that the effect of the wire is minor when the success probability is low, while the overall fault-tolerance rises sharply once the success probability is high. And the inverter is considered to be a major factor that affects the overall fault-tolerance in the variation range of parameter. Frobenbius norm of the overall error probabilistic transfer matrix is employed to study the fault-tolerance difference. The result shows that the overall fault-tolerance of QCA adder consisting of 5-input majority is superior to the other. Such fault-tolerance analyses should be used for a better characterization of QCA circuit design and fault-tolerance improvement.
2012, 61 (5): 050203. doi: 10.7498/aps.61.050203
A class of reactive diffusion model for atmospheric plasmas is studied. The asymptotic solution is obtained by using the singular perturbation method, and the validity of the solution is proved.
2012, 61 (5): 050204. doi: 10.7498/aps.61.050204
In this paper, the shape functions are obtained by the moving least-squares method with complex variable (MLSCV). The advantages of MLSCV are that the approximation function of a two-dimensional (2D) problem is formed with one-dimensional (1D) basis function, and the number of the undetermined coefficients is reduced, so it effectively improves the computational efficiency. Based on the MLSCV and meshless local Petrov-Galerkin method, the essential boundary conditions are imposed by the penalty method and the corresponding discrete equations are derived, then a meshless local Petrov-Galerkin method with complex variables is presented for 2D elasticity problems. Some examples given in this paper demonstrate the effictiveness of the present method.
Calculating far field of an arbitrarily oriented electric dipole located above layered half space using reciprocity theorem
2012, 61 (5): 050301. doi: 10.7498/aps.61.050301
Based on the reciprocity theorem, the far field formulation of an arbitrarily oriented electric dipole located at the interface of the layered half space is deduced. Then, considering the optical path difference between directed wave and reflected wave, the formula of the electric dipole located above the interface of the layered half space is discussed. The TE and the TM reflection coefficients of the layered half space can be calculated by the propagation matrix method or the continued fraction method. The presente method, in which the deduction process possesses concise physics concept, is suitable for calculating the radiated field of an electric dipole when the observation point is away from the interface. The numerical results show that using our method the far radiation field of an arbitrarily oriented electric dipole located above the interface of layered half space can be guickly analyzed.
2012, 61 (5): 050302. doi: 10.7498/aps.61.050302
Decoy state has proven to be a very useful method of significantly enhancing the performance of a quantum key distribution (QKD) system with practical light sources. The data-set size in practical QKD protocol is always finite, which will cause statistical fluctuations. The gain and the error rate of the quantum state are analyzed by considering absolutely statistical fluctuation. The relation between key generation rate and the secure communication distance is shown with exchanged quantum signal (N = 106 -1012) by the method of two-decoy-state protocol under the condition that communication wavelength is 1310 nm (or1550 nm). The result indicates that the minimal number of exchanged quantum signals increases obviously with the increase of transmission distance. The secure transmission distance is 135 km under the condition that quantum signal is 1012.
2012, 61 (5): 050401. doi: 10.7498/aps.61.050401
Applying generalized tortoise coordinates with undetermined parameters to a dynamical Vaidya black hole, we find the interesting condition for reducing the wave function into its standard form without divergence. It is shown that the Damour-Ruffini method with tortoise coordinate can be applied only to the event horizon rather than the apparent horizon in a dynamical space time.
2012, 61 (5): 050501. doi: 10.7498/aps.61.050501
The time-delay effect of a genotype selection model driven by noise is investigated. The approximate delay Fokker-Planck equation is obtained based on the small time delay method and the stochastic equivalent rule. The explicit expression of the steady state probability distribution is derived, and the effect of the time delay on the steady sate properties of the genotype selection model driven by noise is discussed. It is found that the time delay can induce the transition from monostable state to bistable state. The time delay is helpful to select one type of genes from another type of mixing genes. The theoretical predictions are found to be in basic agreement with numerical results.
Synchronization for fractional order hyperchaotic Chen system and fractional order hyperchaotic Rssler system with different structure
2012, 61 (5): 050502. doi: 10.7498/aps.61.050502
There is larger key space in fractional order hyperchaotic systems (FOHS), however, the FOHS with different structures are more common in the field of secret communication. Therefore, it is important to study the synchronization method of the system. In this paper, the synchronization between two different FOHS is investigated. i.e., the Chen FOHS and the Rssler FOHs. Based on the fractional order stability theory, two different controllers are designed for synchronizing the drive system and the response system. When the parameters are known in advance, active control synchronization is adopted, the method is simple without constructing any special function. When the parameters are fully unknown, a adaptive control law and a parameter update rule are introduced. Numerical simulations are presented to verify the effectiveness and the feasibility of the synchronization scheme.
2012, 61 (5): 050503. doi: 10.7498/aps.61.050503
The chaos detection method based on Holmes Duffing equation is studied. Since this method is suitable only for the detection of a single signal frequency, the selection of the equation parameters is inconvenient, and the noise may affect the detection results, we propose a new method of detecting weak characteristic signals based on the scale transformation of Duffing oscillator. The numerical simulation shows that the proposed method can be used to detect harmonic signal with any frequency and phase by only a set of determined parameters.
2012, 61 (5): 050504. doi: 10.7498/aps.61.050504
A type of noise-induced synchronization in two-dimensional (2D) complex spatiotemporal system is studied in this paper. First, we employ a 2D complex Ginzburg-Laudau equation (CGL) to present spatiotemporal chaos. Then the synchronization in the CGL equation driven by spatiotemporal noise is studied. Theoretically, the critical control intensity is obtained by linear stability analysis of a constant forced CGL system. Combining with randomness and non-zero mean of the noise, we reveal the mechanism of synchronization and give the required conditions for control parameters and noise intensity resulting in synchronization theoretically and numerically. A complete synchronization in a pair of uncoupled CGL equations is achieved. A good agreement between the theoretical analyses and the numerical results is obtained.
2012, 61 (5): 050601. doi: 10.7498/aps.61.050601
According to the theory of electromagnetism, we establish the equation of resonance frequency for simplified model of cavity of passive hydrogen maser. We find the optimal dimension of the cavity, which makes the product of the Q value and the filling factor reach a maximal value. The experimental results indicate that the so designed cavity can be utilized undoubtedly for the passive hydrogen maser, and achieve the Q value of 9800 and the filling factor of 1.2. Furthermore, the resulting capacity of storage volume is the biggest is ever reported cavities of passive hydrogen maser.
Research of quantitative remote sensing of natural gas pipeline leakage based on laser absorption spectroscopy
2012, 61 (5): 050701. doi: 10.7498/aps.61.050701
Quantitative remote sensing of natural gas pipeline leakage based on laser absorption spectroscopy is reported. Key technical issues of mobile laser remote sensing are discussed through simulating leak detection. For quantitative remote sensing of trace leakage, the signals with counter-phase and isovalue of residual amplitude modulation are introduced. Then the offset is compensated, the influence of harmonic signal is reduced and sensitivity is improved. The improved soft-threshold denoising method is proposed by analyzing the characteristics of echo absorption spectrum. The results show that the signal-to-noise ratio is improved two times as great as that by the traditional soft-threshold denoising method and the shape of second harmonic (2f) signal is well preserved. The minimum remote sensitivity will be 80 ppm/m under the mobile remote sensing.
2012, 61 (5): 050702. doi: 10.7498/aps.61.050702
The influence of channel length on total ionizing dose effect in a 180 nm complementary metal-oxide semiconductor technology is studied. When other conditions such as radiation bias, device structure are the same, the overall radiation response is determined by the charges trapped in the oxide. The off-state leakage due to the charges trapped in the shallow trench isolation oxide inverting the parasitic sidewall channel has correlation with the channel length. A shorter channel leads to a larger leakage current. For the first time, we report that the leakage current also exhibits the radiation enhanced channel-length modulation effect, which further degrades the device performance.
2012, 61 (5): 052901. doi: 10.7498/aps.61.052901
One-dimensional ZnO nanostructure is especially attractive because of its unique properties such as high surface-to-volume ratio and a large exciton binding energy, but how to put it into a device is still a challenge. In this article, we show that a novel lateral metal-semiconductor-metal ultraviolet detector composed of ZnO nanowires is fabricated on glass substrate by a single-step hydrothermal approach. The fabricated photodetector demonstrates that the response to UV illumination in air is fast, the rise time is about 4 s, and the fall time is about 5 s, which could be attributed to the fact that the adsorption and the desorption of water molecules in the air onto oxygen vacancy of the nanowire significantly influence the photoresponse.
ATOMIC AND MOLECULAR PHYSICS
2012, 61 (5): 053101. doi: 10.7498/aps.61.053101
The plane-wave ultrasoft pesudopotentials based on the density functional theory is used to study the formation energy, the electronic structure and the optical properties of molybdenum and different nonmetallic elements codoped anatase TiO2, where the nonmetallic elements are boron, carbon, nitrogen, oxygen and fluorine. Firstly, we calculate the total densities of states and the band structures of different kinds of codoping TiO2, and analyse the codoping modulation effect on band gap by using the energy band theory. Furthermore, we analyse the synergistic effects on stability and catalysis of TiO2 caused by codoping. And then the state of each atom's bond effect is obtained by analysing the total density map. Finally, we come to the conclusion that molybdenum-carbon codoping structure is superior to others on modulating the photocatalysis of TiO2 in visible light. Our theoretical research will be instructive and meaningful for photocatalysis area of TiO2 in the future.
2012, 61 (5): 053102. doi: 10.7498/aps.61.053102
The dynamical potentials of highly excited vibrational states of HOCl in the bending and OCl stretching coordinates with anharmonicity and Fermi coupling are studied. The result shows that HO stretching vibration mode has significantly different effects on OCl stretching mode and HOCl bending mode under different polyad numbers. The dynamical potentials and the corresponding phase space trajectories are studied in the case that the polyad number is 24 and it is found that the quantal vibrational energy levels could be classified by the fixed points of dynamical potentials and the quantum environments could be identified by the numerical values of action integrals.
ELECTROMAGENTISM, OPTICS, ACOUSTICS, HEAT TRANSFER, CLASSICAL MECHANICS, AND FLUID DYNAMICS
2012, 61 (5): 054101. doi: 10.7498/aps.61.054101
An ultra-wide band metamaterial may be achieved via the design of some structures. A metamaterial unit supporting two-dimensional (2D) incident electromagnetic (EM) wave is proposed based on the mushroom type-structures, which has an ultra-wide band with seamlessly combined band of right-handed and left-handed pass-bands. This unit is designed by setting two reverse symmetrical mushroom-shaped strips on each side of the dielectric substrate respectively, and the electric resonance and the magnetic resonance could be excited simultaneously. With CST software, the right-handed and left-handed properties are analyzed and verified by means of spectrum analysis, effective parameters of permittivity, permeability and index of refraction extracted from S parameters, and equivalent magnetic resonance circuits. The results show that the structure can present left-handed properties with 1 GHz left-handed pass-band in X waveband, either EM wave is incident in the direction perpendicular or parallel to the plane of the substrate. When the EM wave is incident in the direction perpendicular to the substrate, the right-handed and the left-handed pass-bands appear at 7.2 GHz9.3 GHz and 9.3 GHz11 GHz respectively; while when the EM wave is incident in the direction parallel to the substrate, the right-handed and the left-handed pass-bands appear at 7.0 GH9.0 GHz and 9.0 GHz10 GHz respectively. It also shows that the zero indexes of refraction occur at 9.3 GHz and 9.0 GHz in the tow instances above. So that a plus-zero-negative metamaterial is constructed and a 2D incident balanced-structure with an ultra-wide band of 3 GHz is achieved.
2012, 61 (5): 054102. doi: 10.7498/aps.61.054102
Based on the influence of nonuniform temperature distribution on interconnect delay, an analytical model to estimate the optimal size of repeaters inserting RC interconnect is presented in this paper. In the proposed analytical model the temperature distribution is taken into account and a temperature correction factor is introduced to modify the repeater size and obtain the optimal interconnect delay. Adopting parameters of 90 nm and 65 nm process technology, the proposed model is compared with the model without considering the temperature distribution. Results show that the new model is more accurate and saves the repeater insertion area with maximum values of 15.6% and 36.7% in 90 nm and 65 nm technology, respectively.
2012, 61 (5): 054103. doi: 10.7498/aps.61.054103
In this paper, a design optimization method for left-handed materials with specific frequency bands is proposed. A microstructure of the left-handed material is constructed based on the transmission line theory, and the properties of this material are simulated numerically with the S-parameter retrieval method and analyzed with equivalent transmission line model. The effects of the size parameters of the microstructure on the left-handed characters are analyzed. The analysis results verify that this structure does exhibit left-handed characteristics with appropriately sized microstructure. Then the mathematical model of the microstructure with maximized frequency band-width and lower loss at given frequencies is proposed, and examples are given. The design result confirms that the optimized model is applicable and accurate.
Dynamic evolution of composite coherence vortices by superimpositions of partially coherent hyperbolic-sine-Gaussian vortex beams in non-Kolmogorov atmospheric turbulence
2012, 61 (5): 054201. doi: 10.7498/aps.61.054201
Some experiments show that the practical atmosphere deviates from ideal Kolmogorov model. In this paper, based on the extended Huygens-Fresnel principle and the non-Kolmogorov turbulence model proposed by Toselli et al., the analytical expression for the propagation of partially coherent hyperbolic-sine-Gaussian vortex beams through non-Kolmogorov atmospheric turbulence is derived and used to study the dynamic evolutions of composite coherence vortices formed by coherent and incoherent superpositions of two partially coherent hyperbolic-sine-Gaussian vortex beams in non-Kolmogorov atmospheric turbulence. It is shown that the evolution process of the average intensity of the superimposed beam depends on the general exponent of the non-Kolmogorov turbulence, the sign of the topological charge of the superimposed vortex beam in the source plane, and superposition scheme. The motion, the creation and the annihilation of composite coherence vortices may take place upon propagation through non-Kolmogorov turbulence, and the general exponent , sign of the topological charge and superposition scheme affect the evolution behavior. Finally, the results are compared with those of the previous work.
Study on properties of the H1 photonic crystal slab cavity using the effective index perturbation method
2012, 61 (5): 054202. doi: 10.7498/aps.61.054202
In this paper the effective index perturbation technique in combination with two-dimensional (2D)/three-dimensional (3D) plane wave expansion methods is used to predict resonant mode frequencies of donor-like and acceptor-like H1 photonic crystal slab cavity, and their results are very close to the ones obtained by three-dimensional finite difference time domain method. For donor-like H1 photonic cavity, when the perturbed effective index by matching dielectric band edge is used, there is a relatively small frequency difference; however, for acceptor-like H1 photonic cavity, the matching criterion should be set at middle band position. The effective index perturbation method can not only save computation time and computer memory with reducing dimensionality (from 3D to 2D), but also ensure the accuracy of the simulation results, which is substantially important for the extensive application of photonic crystal slab cavity.
2012, 61 (5): 054203. doi: 10.7498/aps.61.054203
The effects of oxide aperture shape whose size is small, on the mode characteristics of external cavity oxide-confined photonic crystal vertical cavity surface emitting laser (VCSEL) is studied extensively based on the finite-difference time-domain algorithm. A new three-dimensional model of optical field in photonic crystal VCSEL is built and the influence of the variation in aperture shape on the characteristics of far-field and spectrum is pointed out. It is found that the modal characteristics of PhC-VCSEL are influenced by the shape of oxide aperture, the influence on spectrum feature is more apparent. It can be explained from the distribution of modal field by the fact that the symmetrical characteristic of prismatic oxide aperture is not identical to that of high-order mode. But the influence is less apparent while the depth of photonic crystal hole is deeper. The reason for this matter is illustrated. The results can be used to guide the fabrication of external cavity oxide-confined photonic crystal VCSELs.
Experimental research of target uniform illumination using smoothing by spectral dispersion and continuous phase plate
2012, 61 (5): 054204. doi: 10.7498/aps.61.054204
To improve laser irradiation uniformity, experimental research on smoothing by spectral dispersion (SSD) and continuous phase plate (CPP) is carried out on the technical integration line (TIL). A bulk phase modulator with 9.2 GHz modulation frequency is adopted. The output spectrum of the phase modulator is stable and the residual amplitude modulation is quite small. The experimental results indicate that when the number of color cycles (Nc) is adopted to be 1, imposing of SSD in this divergence does not lead to pinhole closure in the spatial filters of the preamplifier and main amplifier. The contrast of the focal spot with 95% energy included with SSD and CPP drops to 0.47 compared with 1.71 without SSD and CPP. When the pulse width of the third harmonic wave is 1 ns and its energy is 1115 J, no damage is found in CPP and other final optics. The experiment solves some key techniques by using SSD and CPP on high-power laser facilities, and provides sound basis for the upcoming physics experiment.
2012, 61 (5): 054205. doi: 10.7498/aps.61.054205
It is difficult to obtain low-dose three-dimensional images of a long sample with high spatial resolution in a single scan using conventional X-ray micro-CT technique because the synchrotron X-ray beam is prolate. Spiral scanning is induced into synchrotron X-ray micro-CT (SR -CT) to resolve this problem. We investigate the effects of some factors on imaging quality and scanning speed by simulation and experiment, such as pitch, vertical beam width and projections per 180. When the pitch is not greater than 2, it is found that the relative error and the CT scanning speed decrease while the projections per 180 increases. The scanning speed increases with vertical beam width. Furthermore, as pitch becomes greater, the scanning speed becomes faster. The relative error is minimized by choosing the appropriate parameters to make the ratio between rotation and vertical transfer be an integer. The simulated results are confirmed by the synchrotron-based spiral micro-CT (SRS -CT) experiments performed at the Shanghai Synchrotron Radiation Facility and parameters are optimized. The results presented herein provide a theoretical and experimental basis for future applications of SRS -CT.
2012, 61 (5): 054206. doi: 10.7498/aps.61.054206
A one-meter-long high nonlinear photonic crystal fiber is pumped by laser pulse train at room temperature, and the generated correlated idler and signal photons are centred at 830 nm and 1411 nm, respectively. The full widths at half maximum of the broad band filters of the two channel are 15 nm and 35 nm, respectively. The fitting results of single channel photon counts reveal that almost all the photons originate from spontaneous four wave mixing and the influence of Raman scattering is eliminated. When the production rate is 0.0085 photon per pulse, the ratio between coincidence rate and accidental coincidence rate is measured to be 102, which not only confirms the low noise property of the photon pair, but also shows that the photon pair are naturally narrow band, and the collecting efficiency in our experiment is high. Moreover, these high purity photon pairs connect different optical bands, and have potential applications in quantum information technologies.
Effect of Doppler broadening on the vacuum induced coherence-dependent probe field absorption in Y-type four-level system
2012, 61 (5): 054207. doi: 10.7498/aps.61.054207
Using the numerical result, the influence of the Doppler broadening on the vacuum induced coherence (VIC) related probe field absorption is discussed. It is shown that if there exists no Doppler broadening, only when VIC is absent can the electromagnetically induced transparency (EIT) phenomenon occur; VIC leads the probe field absorption to vary obviously and the gain to appear; in both cases with and without VIC, the absorption curve has a double-peak structure which is symmetrical about the probe detuning. If there exists the Doppler broadening, in both cases with and without VIC, EIT phenomenon can always occur; VIC still leads the probe field absorption to vary obviously and the gain to appear; no matter whether the VIC is present. The probe field absorption has the following characteristics: the absorption curve no longer has symmetry about the probe field detuning and changes gradually from the double-peak structure to the single-peak structure with Doppler broadening width (D) increasing; the probe field absorption does not monotonically increase or decrease with D increasing; the probe field absorption, when the probe and driving fields propagate along the opposite directions, is smaller than that when the probe and driving fields propagate along the same directions.
Generation of microwave subcarrier phase modulation signal based on optical injection into a semiconductor laser
2012, 61 (5): 054208. doi: 10.7498/aps.61.054208
Radio-over-fiber technology has become an important solution for ultra wide band wireless communication, and the convergence of signal processing between optics and microwave/millimeter wave is more crucial. In this paper, microwave subcarrier phase modulation signal generation based on optical injection into a semiconductor Fabry-Perot laser is proposed. According to the period-one(P1) oscillation effect of laser output optical field, one modulation component of the optical phase modulation signal is amplified by sideband of P1 oscillation. The amplified component beats with injection optical carrier to generate microwave subcarrier. The phase shifts lead to the phase shift of subcarrier, thus the phase information is converted into phase information about microwave subcarrier. The optical phase-shift-keying signals at 1.3 Gb/s, 2.7 Gb/s, 2 Gb/s are converted into microwave subcarrier phase modulation signal, and the single sideband phase noise is measured. By logically comparing the demodulated signal with original signal, the feasibility of data information conversion is proved experimentally.
2012, 61 (5): 054209. doi: 10.7498/aps.61.054209
A simple model of calculating the linewidth enhancement factor ( factor) is presented by introducing the correlative theory and its conversion formula of the factor in detail. The contributions of interband transition, free carrier absorption and band gap narrowing to the factor are taken into account. Carrier concentration and differential gain dependence of photon energy are obtained from the gain curves for different carrier concentrations. The gain curves and the factor of InGaAs/GaAs quantum well are simulated, separately, and the results accord well with those reported in the literature. Subsequently discussed are two important parameters of InGaAs/GaAs quantum well laser containing quantum well width and In mole fraction. The results show that the increase of two parameters leads the factor to increase.
Coherent beam combination of two high power double clad fiber lasers by using an all-fiber Michelson cavity
2012, 61 (5): 054210. doi: 10.7498/aps.61.054210
Michelson cavity technique provides an effective solution for coherent beam combination. In this paper, coherent beam combination of two fiber lasers by using an all-fiber Michelson cavity technique is demonstrated, and 11.75 W coherent output power with a coherent combination efficiency of 95.76% is obtained. The influences of the relation of coherent combination efficiency to laser cavity length difference, and the pump symmetry on coherent combination efficiency are studied experimentally. Experimental results show that the coherent combination efficiency is highly sensitive to the influence of laser cavity length difference, that the difference in combinination efficiency, induced by laser cavity length difference, can be as large as 18%, and that there is an optimal cavity length difference. The maximal influence of pump symmetry on coherent combination efficiency is within 2%, which can be neglected.
Study on wave plate angles of polarization controller in nonlinear polarization rotation mode-locked fiber laser
2012, 61 (5): 054211. doi: 10.7498/aps.61.054211
Based on Jones matrices, a numerical model is proposed for the simulation of nonlinear polarization rotation (NPR) mode-locked fiber lasers with the advantage of a clear physical meaning. Ultra short pulses are derived with symmetric shape, stable amplitude and a square width of 0.28 ps. A self-starting NPR mode-locked fiber laser experimental setup is established and the fundamental repetition of the pulses is 67.35 MHz. Both the numerical simulation and the experimental results indicate that the angle of the half wave plate has a period of 180 while the quarter wave plate has a period of 90. Due to the influence of random birefringence, the tuning range of the angle of the wave plates measured in experiment is smaller than that shown in numerical simulation, which is verified by adding the effect of random birefringence in numerical simulation.
2012, 61 (5): 054212. doi: 10.7498/aps.61.054212
Two kinds of gold nanoparticles with different surface-capping groups (Au-DT and Au-DT-CTAB), 1-dodecanthiol and cetyltrimethylammonium bromide (CTAB) are prepared by a two-phase method. Structures and linear optical properties of the materials are characterized using transmission electron microscopy and ultraviolet--visible absorption spectroscopy. The effects of surface modification on nonlinear optical (NLO) performance and optical limiting behavior of gold nanoparticles are investigated using the openZ-scan technique at a laser wavelength of 532 nm. The results show that Au-DT-CTAB shows an enhanced NLO effect in comparison with Au-DT. The secondary modification with CTAB can not only improve local field enhancement effect but also make the contribution of thermoelectrons to NLO performance of gold nanoparticles on laser irradiation.
2012, 61 (5): 054213. doi: 10.7498/aps.61.054213
Resonant cavities are used widely in the field of integrated optics due to their excellent frequency-selective properties. Based on the characteristics of resonant coupling between two photonic crystal ring resonators, four cavities with different sizes and waveguides, the demultiplexing of four wavelengths, i.e. 1310 nm, 1550 nm, 1600 nm and 1650 nm is realized. The numerical results obtained by the finite-difference time-domain method show that the output efficiency of the four wavelengths can be higher than 90% only by modulating the radii of the border rods of output waveguides. The presented device that has not only a compact size with 12 μ m× 17μm but also a high output efficiency, may have potential application in the future optical communication field.
2012, 61 (5): 054214. doi: 10.7498/aps.61.054214
In its application of gas sensing, the transmission band of the hollow-core Bragg fiber should match the main absorption peak of the target gas. In this paper, we introduce the design method of the hollow-core Bragg fiber transmission band and develop a fabrication process supporting its transmission band control. Fiber samples with fundamental transmission bands at 10.6 m and 3.3 m are fabricated, whose transmission losses are 5.9 dB/m and 8.8 dB/m, respectively, measured by the cut-back method. Utilizing the fiber sample with a transmission band of 3.3 m, the injection and the expulsion of CH4/N2 gas are realized and observed by the change of fiber transmission spectrum. The detection limit of the experimental system is measured to be 26 ppm for CH4 by exponential dilution method, demonstrating the feasibility of hollow-core Bragg fiber in its application of gas sensing.
2012, 61 (5): 054215. doi: 10.7498/aps.61.054215
Based on the structure health monitoring of civil engineering structure such as bridges, dams, high-rise building, the fiber Bragg grating sensor is used to achieve the real-time monitoring and diagnosis of structural performance, in order to discovery the damage and to evaluate the safety of structure. Fiber-optical grating sensing based on flextensional member is proposed. The measuring of pressure, strain and acceleration can be realized. The strain of camber beam belly is analysed theoretically and verified experimentally. The strain of the camber beam belly is proportional to the pressure and acceleration in the vertical direction. Its biggest advantage is small size under the same sensitivity. The transverse dimensions is limited in well and the longitudinal acceleration needs to be measured, so beam based sensor structure are not likely to be used. Under the condition of measuring longitudinal acceleration sensor structure, beam pumping are not likely to be used. This new type of flextensional member elastic sensitive structure can serve as elastic element for the design of fiber bragg grating strain, weighing and pressure, and acceleration sensor.
2012, 61 (5): 054216. doi: 10.7498/aps.61.054216
Preliminary research on large-size optical polymer image transmitting fiber faceplate is carried out by the stacking-heated fusing method with commercial low-loss optical PMMA fiber which is arranged in a home-made mold by hexagonal packing. A fiber faceplate sample with 2 mm in thickness, 500 μm in pixel diameter and 59052 in pixel number, is manufactured by cutting and stitching the preforms The fiber faceplate sample has a good image transmission capacity. The resulting large-size optical polymer fiber mosaic faceplate screen has the role that cannot be replaced for large-screen LCD protection and elimination of display seam.
2012, 61 (5): 054301. doi: 10.7498/aps.61.054301
It is suitable to solve the acoustic problems by using the fast multipole boundary element method (FMBEM), since the FMBEM can accelerate the matrix-vector multiplication dramatically by reducing the CPU time and memory of conventional boundary element method to O(N log2N) and O(N) respectively. We propose a 3D acoustic FMBEM based on Burton-Miller formulation in this paper. A new adaptive algorithm is applied to the diagonal form FMBEM, and a new proposed analytical moment formulation is used in the moment computation. Both of them further improve the efficiency of FMBEM. Acoustic scattering of soft sphere at resonant frequency is investigated to validate the accuracy of solution using Burton-Miller formulation. Comparisons of solution to the multi-radiating spheres problem with the one solved by Bapat's program demonstrate the accuracy and the efficiency of our algorithm in solving large-scale acoustic problems. In the end, we use our algorithm to analyze the inner sound filed of a car and dolphin acoustic scattering.
2012, 61 (5): 054302. doi: 10.7498/aps.61.054302
In this work, the axisymmetric seismoelectric logging response excited by a point-pressure source is formulated as an integral in the frequency-wavenumber domain. To analyze the properties of each constituent wavepacket in the seismoelectric logs, the poles and the branch points of the integrands are examined by the complex analysis. The electromagnetic wavenumber in the borehole fluid proves to be a removable branch point. The argument principle is used to search for the complex poles. After the excitation intensity as well as the dispersion curves has been obtained for each component wave for a typical sandstone formation, the ratio of the electric excitation intensity to the acoustic excitation intensity is evaluated. Especially it is found that the argument of this ratio of the Stoneley wave is sensitive to the permeability. The excitation curves corresponding to the branch points show that the compressional wave has a higher seismoelectric conversion efficiency than the shear wave and the mode waves.
Researches on the interference structure in low-frequency acoustic field based on space-time filter theory
2012, 61 (5): 054303. doi: 10.7498/aps.61.054303
Compared with the waveguide invariant, the interference structures in low-frequency acoustic field are more comprehensively described by four system functions: coherent function, power response function, spread function and dual-frequency function, from space-time filter theory. A stable interference structure in the low-frequency acoustic field is testified based on theoretical analysis, simulation and sea trial data processing. When the target moves in a uniform rectilinear motion from far to near then from near to far, the interference structure of coherent function (LOFARgram) appears to be a family of quasi hyperbolas; the interference structure obtained by power response function represents the group delay difference and the dispersive characteristic of the normal mode; and the phase slowness difference can be exhibited by dual-frequency function. Each system function owns special characteristics, which can give prominence to certain features of the interference structure.
Orthogonal wavelet transform weighted multi-modulus blind equalization algorithm based on quantum particle swarm optimization
2012, 61 (5): 054304. doi: 10.7498/aps.61.054304
When constant modulus blind equalization algorithm (CMA) is used to equalize high-order QAM signals, there occur the defects of the slow convergence rate and big steady mean square error. In order to overcome these disadvantages, orthogonal wavelet transform weighted multi-modulus blind equalization algorithm based on the quantum particle swarm optimization (QPSO-WTWMMA) is proposed. In this proposed algorithm, quantum particle swarm optimization algorithm and orthogonal wavelet transform are combined into weighted multi-modulus blind equalization algorithm (WMMA) according to the feature of higher-order QAM signal constellations. Accordingly, the equalizer weight vector can be optimized by QPSO algorithm, the autocorrelation of the input signals can be reduced via using orthogonal wavelet transform, and WMMA is used to choose appropriate error models to match QAM constellations. The theoretical analyses and the computer simulations in underwater acoustic channels indicate that the proposed algorithm can obtain the fastest convergence rate and the smallest steady mean square error in equalizing high-order QAM signals. So, the proposed algorithm has important reference value for the underwater acoustic communications.
2012, 61 (5): 054305. doi: 10.7498/aps.61.054305
Bi-static long-range bottom reverberation in shallow water is investigated, and the impulse response function is derived using the linear theory of sound channel. For the case of almost layered medium, the adiabatic mode solution is introduced for the derivation of transfer function. The reverberation intensity is calculated for the range-dependent waveguide, and then both the intensities and the decaying rules are compared with their corresponding counterparts in different wedged sea areas. It is shown that the propagation effect on intensity of distant bottom reverberation can not be neglected for the bi-static active sonar in shallow water waveguide.
2012, 61 (5): 054306. doi: 10.7498/aps.61.054306
In order to improve the identification of moving sound sources and solve the ghost source problem, the short-time wave superposition relation for moving sound source is established according to the Doppler frequency shift in short time, the equivalent sound source is pre-estimated by beam-forming, based on the pre-estimation result, the wave superposition equation is developed and the sound source is calculated by solving the equation. Then the dynamic wave superposition method for moving sound source is obtained. This method extends the wave superposition method to the case of the moving sound sources and solves the ghost source problem in moving sound source identification without adding extra microphones or changing the layout of microphone array. This method is used in the experiment to measure the moving sound sources, and satisfactory results prove the effectiveness of the method.
2012, 61 (5): 054701. doi: 10.7498/aps.61.054701
In this paper, we present a modified smoothed particle hydrodynamics (SPH) method. The kernel approximation and the particle approximation are corrected to ensure that polynomial functions are exactly interpolated up to a given degree. Riemann solver is adopted to solve equations of fluid motion. A surface tension calculation program is used for considering the surface tension effects in droplets splashing. The process of droplet impact on liquid surface is numerically simulated by the modified smoothed particle hydrodynamics method. The numerical results clearly demonstrate that the modified SPH method can effectively describe the dynamics of droplet splashing and the variation of the free surface, and that the accuracy of the results can be stable.
Study on static pressure of fiber cable spool based on distributed fiber Bragg grating sensing technology
2012, 61 (5): 054702. doi: 10.7498/aps.61.054702
Based on the theoretical analysis of the static pressure distribution of fiber cable layer in an optical fiber cable spool, we establish a theoretical model of the force in the cable winding system. Simulations indicate that with the increase of the twining layers, the pressure of every layer in the spool increases very quickly at the beginning and mildly then. The static pressure of fiber cable layers in the spool on cable winding device is sensed by use of distributed fiber Bragg gratings (FBG). The quantitative relationship between the variation of FBG center wavelength and the system pressure is given theoretically. It is shown experimentally that with the increase of the twining layers, the variation pace of FBG center wavelength in every layer is very quick at the beginning and gentle then. Theoretical simulations coincide with experimental results very well. This technology provides us a real-time method to monitor the pressure of the fiber cable layer in the cable winding process.
Supercontinuum has wide and important applications in a range of fields, so it is one of the hot research spots in recent years. In this paper the mechanism and the latest achievements in high power supercontinuum generation under continuous wave laser pump and pulsed laser pump regimes during the past several years are reviewed. The challenges and the problems that need to overcome in scaling the average output power of supercontinuum generation are analyzed. An all-fiber supercontinuum source constructed at the national university of defense technology is demonstrated with 177.6 W average output power which overcomes the problems of low splicing loss, facet damage of the output fiber, the heat treatment and the effective control of the nonlinear effects through the optimization of the whole system. To the best of the author's knowledge, the 177.6 W average output power is the highest value in ever reported ones.
PHYSICS OF GASES, PLASMAS, AND ELECTRIC DISCHARGES
Molecular dynamics simulation of energetic hydrogen isotopes bombarding the crystalline graphite(001)
2012, 61 (5): 055201. doi: 10.7498/aps.61.055201
Molecular dynamics simulation is applied to the investigation of the isotopic effects during a hydrogen isotope atom bombarding the crystalline graphite containing four graphene sheets. Both Brenner's reactive empirical bond order potential and Ito's interlayer intermolecular potential are adopted to represent `àBAB" stacking of graphite. The simulation results reveal that the mass of the incident species has a big influence on the absorption on and the reflection from the upside graphite surface, the peaks of which shift toward higher end side of incident energy as the mass increases. The absorption coefficient of the incident tritium is large, compared with that of the incident either hydrogen or deuterium. To penetrate the four- sheet graphite at some striking locations, deuterium and tritium need more kinetic energy. It is found that both the mass and the incident energy of the incident species affect the energy transfer to background substrate. These results would be important for understanding the tritium retention occurring in fusion devices.
CONDENSED MATTER: STRUCTURAL, MECHANICAL, AND THERMAL PROPERTIES
Simulation study on the dynamic mechanisms of nucleation during the initial stage of supercooled liquid potassium
2012, 61 (5): 056101. doi: 10.7498/aps.61.056101
The solidification process of liquid metal potassium is simulated by using the molecular dynamics method. According to the evolution properties of average atomic energy in system, bonding type and clustering type among atoms, and the dynamic parameters of mean-square displacement and non-Gaussian parameter, the dynamic mechanisms in initial nucleation of supercooled liquid potassium are deeply studied. It is found that the supercooled liquid region can be divided into two different stages according to the evolutions of thermodynamic, dynamic and structural properties of supercooled liquid. And the potential crystallization nuclei appear in the lower temperature region of supercooled liquid. It is also found that the potential crystallization nuclei are formed with the disaggregations of icosahedron clusters during the -relaxation regime, and the critical size of nucleus is about 300 atoms.
2012, 61 (5): 056102. doi: 10.7498/aps.61.056102
The Gd-based amorphous/nanocrystal composite is prepared by controlling the cooling rate and the element ratio. The X-ray diffraction, differential scanning calorimeter and atomic force microscope/magnetic force microscope are used to confirm the composite microstructures from different perspectives. The magnetic test shows the great enhancement of magnetocaloric effect in the metallic glassy composite. The large magnetic refrigerant capacity (RC) up to 103 J. kg-1 is more than double the RC values of the Gd-based bulk metallic glass and pure Gd. The full width at half maximum of the magnetic entropy change (Sm) peak almost spreads over the whole low-temperature range, which is five times wider than that of the pure Gd. The maximum Sm approaches a nearly constant value in a wide temperature span (over 80 K). The super paramagnetic nanoclusters of the composite increase the magnetic refrigerant capacity greatly. In combination with the low magnetic hysteresis and large resistance, the metallic glass composite may be a potential candidate for the ideal Ericsson-cycle magnetic refrigeration.
Preparation and properties of graphite-like carbon films fabricated by unbalanced magnetron sputtering
2012, 61 (5): 056103. doi: 10.7498/aps.61.056103
A series of graphite-like carbon films is fabricated by the middle frequency magnetron sputtering technique. The microstructures and the morphologies of the resulting films are investigated by Raman spectroscopy, high resolution transmission electron microscopy and atomic force microscopy, respectively. The mechanical and the tribological properties of the films are studied by nanoindentation and CSM tribometer. The results show that the deposited carbon film is dominated by sp2 sites, and has an amorphous structure, a moderate hardness, low internal stress, high surface roughness and superior tribological properties. With the increase of the duty ratio, the intensity ratio between D and G peaks first decreases and then increases, while the film hardness first increases and then decreases. Tribological testing in humid atmosphere demonstrates that the present carbon film has a superior wear resistance (～10-11 cm3/N-1.m-1) and high load bearing capacity (～2.5 GPa). Although the duty ratio has no obvious influence on friction coefficient, the wear rate decreases obviously and then increases slightly with the increase of duty ratio. The superior tribological properties of the graphite-like carbon film are attributed mainly to its unique structure, low internal stress and high structure stability.
2012, 61 (5): 056104. doi: 10.7498/aps.61.056104
(BEDT-TTF)[FeBr4] single crystal is grown with electrochemical crystallization method, and its crystal structure is determined by four-circle single crystal diffraction. From 300 K to 185 K, the crystal resistivity is related to the efficiency of electronic thermal excitation and the strength of the electron scattering by ionized impurities, while from 185 K to 10 K, the conductivity depends mainly on the ionization of the impurities and defects in crystal. Because of electron scattering increased by the effect of Lorentz force, the magnetic reluctance has positive value. The magnetism of the crystal is determined mainly by magnetic ferric ions, and neither ferrromagnetic coupling nor antiferromagnetic coupling between the ferric ions in crystalis induced. The resonance signals of the electrons and the d electrons are found in the electron paramagnetic resonance spectra, and the -d coupling is enhanced as the temperature is decreased. When the crystal rotates along different axes, the resonance signal changed with the angle of rotation, is greatly different.
Surface microstructure and stress characteristics in pure titanium after high-current pulsed electron beam irradiation
2012, 61 (5): 056105. doi: 10.7498/aps.61.056105
The specimens of polycrystalline pure titanium are irradiated by high-current pulsed electron beam (HCPEB). Surface microstructures and defects induced by HCPEB irradiation are investigated by using X-ray diffraction (XRD), scanning electron microscopy and transmission electron microscopy technique. The XRD results show that the high value of stress (GPa order) is introduced into the irradiated surface layer, and the characteristics of preferential orientations (100), (102) and (103) are present after HCPEB treatment. The surface microstructure observations indicate that martensitic transformation occurs in the irradiated surface and a large number of plate martensite structures are formed in the irradiated surface. Moreover, strong plastic deformation is triggered by HCPEB treatment. After one pulse, (100) type slip bands are formed in the interior of grain, which leads to the increase of dislocation density. After multi- pulses, deformation microstructures change significantly, and the number of deformation twins increases evidently. The formation of these deformation structures produces a significant effect both on the evolution of surface textures and on grain refinement, which improves the mechanical performance of irradiated surface. It is suggested that HCPEB technique is an effective approach to surface hardening for pure titanium.
Spectroscopic ellipsometry study of the Ag2O film deposited by radio-frequency reactive magnetron sputtering
2012, 61 (5): 056106. doi: 10.7498/aps.61.056106
Ag2O film has a potential application in high-density optical and magneto-optical disks. In this paper, a series of Ag2O films is deposited by radio-frequency reactive magnetron sputtering at different substrate temperatures, a deposition pressure of 0.2 Pa and an oxygen flow ratio of 2:3. The spectroscopic ellipsometry spectra of the films are fitted by using a general oscillator model (including one Tauc-Lorentz oscillator and two Lorentz oscillators). In an energy range between 1.5 eV and 3.5 eV, the refractive index and extinctive coefficient of the film are in ranges between 2.2 and 2.7, and between 0.3 and 0.9, respectively. The film indicates a clear abnormal dispersion in an energy range of 3.5 eV and 4.5 eV, meaning that the plasma oscillator frequency of the film is in this energy range . A redshift of the absorption edge of the film occurs with substrate temperature increasing, which can be attributed to the increased lattice strain. The optical constants of the film clearly show the dielectric properties.
2012, 61 (5): 056801. doi: 10.7498/aps.61.056801
Several methods of fabricating chalcogenide thin films are introduced. In this paper, thermal evaporation and radio frequency methods are used to fabricate Ge-Sb-Se thin films. The thicknesses and roughnesses of the films are measured by surface profile-meter. The film growth rates are calculated. The component difference between film and target material is tested by X-ray photoelectron spectroscopy. The third-order optical nonlinearity and the transmission spectra of films fabricated by thermal evaporation are investigated using femto-second Z-scan method and spectrophotometer, to obtain the values of nonlinear refraction, nonlinear absorption and thickness of films. The results show that the films fabricated by thermal evaporation have excellent physical structures and optical properties, and possess promising potential applications in integrated optical devices.
CONDENSED MATTER: ELECTRONIC STRUCTURE, ELECTRICAL, MAGNETIC, AND OPTICAL PROPERTIES
2012, 61 (5): 057101. doi: 10.7498/aps.61.057101
According to the full-potential linearized augmented plane wave method, we calculate the optical properties of In-doped superlattices ZnO. The results reveal that the Fermi energy level enters into the conduction band after doping In. Dielectric function, absorption, refractive index, reflectivity are found to each have a new transition peak near the Fermi energy level. With the increase of the concentration, this transition peak shifts toward the low energy and the spectrum of the peak is the biggest for the two-layer doping case. The optical absorption edge decreases with the increase of doping concentration. Compared with In-doped ZnO supercell, ZnO superlattice has a high transmissivity in the visible light range.
2012, 61 (5): 057102. doi: 10.7498/aps.61.057102
We employ first-principles to calculate the adsorption energy, the band structure, the density of states, the charge populations, the work function and the optical properties of 1/4ML Cs adsorption on (2 2) GaN(0001) surface using the density-functional theory within a plane-wave ultrasoft pseudopotential scheme. The results show that the most stable position of Cs adatom on GaN(0001) surface is at the bridge site of N atoms for 1/4 coverage. The surface of GaN(0001) shows still metallic character after adsorption. Cs adatom affects mainly Ga atoms at surface. The transfer of Cs6s state electrons to Ga atoms at outmost layer leads to the decrease of work function. By analysis of optical properties, we can see imaginary part of dielectric function, absorption spectrum and reflected spectrum shift toward low energy after Cs adsorption.
2012, 61 (5): 057201. doi: 10.7498/aps.61.057201
High-density In-Al codoped ZnO (In, Al, ZnO) nanobunches are synthesized by using chemical vapor deposition method, which can be used to fabricate In, Al, ZnO nanobunches photodetectors. The ZnO nanobunches each have a hexagonal wurtzite structure. It is found that the average length of the nanobunches is ～5 m. The photoconduction mechanism and a series of photoelectric characteristics are studied including I-V characteristic measured in dark and UV illumination, responsivity and response time. The results indicate the presence of an internal gain mechanism. The response time is less than 0.5 s and decay time is about 23 s, so the fabricated device can indeed be used for light detection.
2012, 61 (5): 057202. doi: 10.7498/aps.61.057202
AlGaN/GaN high electron mobility transistors (HEMT) are exposed to 3 MeV protons irradiation. The drain saturation current decreases 20% and the maximum transconductance decreases 5% at a fluence of 1× 1015 protons/cm2. As fluence increases, the thread voltage is shifted toward more positive values. After proton irradiation, the gate leakage current increases. The degradation caused by 1.8 MeV proton is significantly higher than by 3 MeV proton irradiation at the same fluence. The radiation damage area and the density of vacancies at a given depth are obtained from software SRIM. As the energy of the incident proton increases, the non-ionizing energy transferred to the crystal lattice decreases. It is concluded that vacancies introduced by proton irradiation may be the primary reason for the degradations of electrical characteristics of AlGaN/GaN HEMT.
Room temperature NO2-sensing properties and mechanism of the sensors based on tungsten oxide nanowires/single-wall carbon nanotubes composites
2012, 61 (5): 057301. doi: 10.7498/aps.61.057301
One-dimensional tungsten oxide nanowires are synthesized by the solvothermal method. The sensors based on tungsten oxide nanowires/single-wall carbon nanotubes (SWNTs) composites are fabricated by introducing SWNT, and their NO2 sensing properties are evaluated at room temperature. X-ray diffraction and field emission scanning electron microscope characterizations indicate that the as-synthesized nanowires are monoclinic W18O49, and SWNTs are embedded within the nanowire matrix in the prepared tungsten oxide nanowires/SWNT composites. The tungsten oxide nanowires/SWNT composites-based sensors show high sensitivity, good selectivity and super fast response to NO2 gas at room temperature. The NO2 sensing properties of the sensors increase with the decrease of SWNT content. The sensing mechanism of the composites-based sensor is discussed and it is thought that the introduction of SWNT induces the formation of a large number of p-n hetero junctions and cross-linked diffusion channels in the structure of the composites, which are responsible for the good NO2 sensing properties at room temperature.
2012, 61 (5): 057302. doi: 10.7498/aps.61.057302
In this paper, the piezoresistive effect of the multiwalled carbon nanotube (MWCNT) film is studied. Carbon nanotubes are synthesized by hot filament chemical vapor deposition. The piezoresistive effect in the MWCNT film is studied by a three-point bending test. The gauge factor of the MWCNT film under 500 microstrain is found to be at most 120 at room temperature, exceeding that of polycrystalline silicon (30) at 35℃. The origin of the piezoresistivity in MWCNT film is also discussed.
Non-classical eigen state and the persistent current in one-dimensional mesoscopic ring with the electron-two-phonon interaction
2012, 61 (5): 057303. doi: 10.7498/aps.61.057303
For the one-dimensional mesoscopic ring with the ferromagnetic texture, to restrain the quantum fluctuations caused by the electron-one-phonon interaction, the non-classical correlation effects are used in our research to solve this puzzling problem, i.e. 1) the hopping electron-displaced phonon state correlation; 2) the process correlation between the phonon squeezed state, and the one-phonon coherent state, originating from the squeezed coherent state of phonon; 3) the renormalization of the phonon displacement. It is found that due to the electron-two phonon interaction, the squeezing effect of phonon is enhanced significantly. Because of the effect of the electron-displaced phonon correlation the non-classical eigen state energy declines significantly and the amplitude of the persistent current increases substantially. Particularly the process correlation between the squeezed phonon state and the one-phonon coherent state is by far the most important contribution to these non-classical effects. First of all, this effect more greatly increases the squeezing effect of phonon field in contrast to the ideal squeezed state. As a result, it will restrain effectively the Debye-Waller effect (factor wph) with wph wph(0). Furthermore, when we combine the effective renormalization of the phonon displacement with the effect of process correlation between the phonon squeezed state and the one-phonon coherent state, the phonon squeezing effect will increases substantially, at the same time, the D-W effect decreased more substantially (wph wph(0), thereby weakening the quantum fluctuation to a bigger degree. With these results, the non-classical eigen energy (En) is much lowered (En En(0)), while the amplitude of eigen persistent current is increased most significantly (In In(0)).
2012, 61 (5): 057304. doi: 10.7498/aps.61.057304
Based on Fermi's golden rule and the theory of Boltzmann collision term approximation, hole scattering mechanism in strained Si/(001)Si1-xGex, namely, tetragonal strained Si is studied, including ionized impurity, acoustic phonon, non-polar optical phonon and total scattering rates. It is found that the total scattering rate of hole in strained Si/(001)Si1-xGex decreases obviously with the increase of stress when Ge fraction (x) is less than 0.2 and the values continue to show a constant tendency. The total hole scattering rate of strained Si/(001)Si1-xGex decreases about 66% at most in comparison with one of unstrained Si. The hole mobility enhancement in strained Si material is due to the decrease of hole scattering rate. The result can provide valuable references for the research of hole mobility of strained Si materials and the design of PMOS devices.
2012, 61 (5): 057501. doi: 10.7498/aps.61.057501
The magnetic properties of Eu1-xSrxMnO3 (x=0.4, 0.5, 0.6, 0.7) are systematically studied. Nuclear magnetic resonance, magnetization and electrical transport measurements are carried out, and the results indicate that the substitution of Sr for Eu could produce ferromagnetic phase in antiferromagnetic EuSrMnO3 matrix. The competition between ferromagnetic and antiferromagnetic phases induces the spin-glass behavior. In addition, the magnetic properties of Eu0.4Sr0.6MnO3 and Eu0.3Sr0.7MnO3 show more complex features at low temperatures, resulting mainly from the appearance of ferromagnetic clusters and the existence of disordered structure.
2012, 61 (5): 057502. doi: 10.7498/aps.61.057502
The pure phase BiFeO3 nanoparticles with different sizes are prepared by an ethylenediaminetetraacetic acid complexing sol-gel process. The structure, the morphology and the magnetic properties of BiFeO3 nanoparticles are characterized by X-ray diffractometry, scanning electron microscopy, superconducting quantum interference device magnetometer and Mossbauer spectra analysis. The results show that the BiFeO3 nanoparticle possesses an weak ferromagnetism at room temperature with a strongly size-dependent property. It reflects that the weak ferromagnetism present in the BiFeO3 nanoparticle should be attributed to the grain size-confinement effect, rather than the existence of the magnetic impurity or Fe2+.
Experimental and theoretical studies on the influence of unintentionally doped carbon on magnetic properties in ZnMnO:N
2012, 61 (5): 057503. doi: 10.7498/aps.61.057503
Mn-N co-doped ZnO film on sapphire substrate is fabricated by metal-organic chemical vapor deposition method with changing the acceptor-doped source and importing the hydrogen and increasing the pressure to suppress carbon (C) approach gradually. X-ray diffraction displays the strong C-axis orientation. Raman sepectrum is employed to analyze vibration modes related to C elements. Hall measurements on the samples by van der Pauw method reveal the transition from n-type to p-type after suppression of C, which is possible due to the complex of (CN)O acting as a shallow donor. The first principles simulation calculation for Mn and N codoped ZnO crystals has been perfermed, and the total density of states reveals the strong p-d interaction and magnetic moment existing in the Mn and N codoped ZnO. The introduction of the complex of (CN)O, causes the p-d interaction to disappear and the magnetic moment to reduce even disappear. Therefore, the formation of magnetic bound polaron of Mn 3d electronics and N 2p local bound electronic determines the magnetic interaction effect, which can be explained from the theoretical predication on the Mn 3d and N 2p ferromagnetic (hole) coupling on the ferromagnetism.
Intercalation assembly, characterization and luminescent properties of novel composite material, Tb(III) complex-montmorillonite
2012, 61 (5): 057801. doi: 10.7498/aps.61.057801
Tetrapodal ligand L (2, 2, 2', 2'-tetra[N-benzyl-N-phenyl (acetamide)-2-oxymethyl] n-butyl ether) and complex [TbL] (ClO4)3 are synthesized. The complex ion with tetrapodal ligand L, [TbL]3+ is intercalated into the interlayer space of montmorillonite (MMT) by ion exchanging. The obtained luminescent supramolecular composite material, [TbL]3+-MMT is characterized by elemental analysis, X-ray diffraction, Fourier transform infrared spectroscopy, and ultraviolet-visible spectroseopy. At the same time, the luminescent properties of the material are also studied. The results show that the intercalated material with regular layered structure and the interlayer spacing (d001) approximate to the diameter of the complex ion arranged in the interlayer space of the MMT in the formation of a monolayer emit characteristic green emission of Tb3+ strongly under the excitation of ultraviolet light. It is also found that the relative luminescence intensity, the luminescence monochromatic index and the light stability of the intercalated material are distinctly improved after the [TbL]3+ has been intercalated into the MMT. And the excitation wavelength of the complex is modulated till the red shift occurs in the composite material.
Influence of polarization effects on photoelectric response of AlGaN/GaN heterojunction p-i-n photodetectors
2012, 61 (5): 057802. doi: 10.7498/aps.61.057802
Based on the simulation of the polarization effect by the sheet charge layer approximately, the energy band structures and electric field distributions of AlGaN/GaN heterostructures with different doping concentrations of p-AlGaN and polarization effects are calculated by self-consistenly solving the Poisson-Schrödinger equations. The corresponding photoelectric response is calculated and discussed by solving the carriers continuity equation. The results show the interaction between the doping concentration and the polarization effect has an important influence on the performance of the p-i-n detector. Specially, under the condition of complete polarization, a high doping concentration in the p-AlGaN layer will result in a narrow depletion region in p-AlGaN layer and the i-GaN layer will be depleted easily, which corresponds to a low photocurrent. Similarly, a strong polarization will result in a wide depletion region in p-AlGaN and high photocurrent for the same doping concentration in p-AlGaN layer. Finally, the effect of temperature on the performance of the detector is calculated and analyzed. It is concluded that AlGaN/GaN heterostructure p-i-n ultraviolet detector can be used in the high temperature environment.
2012, 61 (5): 057803. doi: 10.7498/aps.61.057803
Excitonic dynamic equations, which are derived from the quasi-Boson approach, are useful tools in investigating the ultrafast optical responses of semiconductor nanostructures. To apply these equations to the exciton dynamics in semiconductor quantum wells, we need exciton wavefunctions and their representations in momentum space to obtain the coefficients in the excitonic dynamic equations. By discussing in detail the exciton wavefunctions and their momentum-space representations, we present a method of obtaining the essential coefficients in the excitonic dynamic equations. We finally use these coefficients to understand the nonlinear effects in the terahertz-pulse-induced intraexcitonic transitions caused by high exciton densities. The obtained theoretical results are in good agreement with recent experimental results.
INTERDISCIPLINARY PHYSICS AND RELATED AREAS OF SCIENCE AND TECHNOLOGY
2012, 61 (5): 058101. doi: 10.7498/aps.61.058101
We report on the design, the fabrication and the measurement of a microwave dual band absorber in microwave regime. The single unit cell consists of two closed ring resonators and a metallic ground plane separated by an FR4 layer. Experimental results show that the absorber has two distinctive absorption peaks at frequencies 4.06 GHz and 6.66 GHz with absorptions 99.60% and 95.83%, respectively, and the absorptivity drops in a wide range of incident angle. Fourfold rotational symmetry of the geometry makes the absorber insensitive to transverse electric and transverse magnetic polarization. The dual band absorber is a promising candidate as absorbing elements in multi-frequency imaging and thermal detection, due to its multi-band absorption performance, polarization-insensitivity, and wide angle of incidence.
Study on the thermal conductivity of diamond-like carbon functionally graded material on copper substrate
2012, 61 (5): 058102. doi: 10.7498/aps.61.058102
In recent years, with the rapid development of electronic technology and digital network information, copper has increasing applications in micro-electronics, micro-electro-mechanical systems and Hi-tech materials. However copper has its material limitations. In particular, it has relatively low hardness, high oxidation and wear rate, which have severely restricted its widespread applications. In this paper, aiming at difficulties of copper applications, Ti/TiC/DLC has been proposed as functionally graded material to deposit on the copper substrate with plasma depositing method, which intensifies the adhesion between DLC film and copper substrate and improves the properties of copper. The maximal value of thermal conductivity of DLC film with optimized parameter of graded intermediate layer is 3.63 Wm-1.K-1, which enhances the heat transfer effect of copper substrate.
2012, 61 (5): 058401. doi: 10.7498/aps.61.058401
The voltage mode controlled boost converters contain plenty of bifurcation phenomena. Tangent bifurcation is one of these phenomena. The investigation of the tangent bifurcation in voltage mode controlled boost converters operating in discontinuous conduction mode (DCM) is performed. The third iterative map is derived according to the first iterative map. Based on the tangent bifurcation theorem, the conditions of producing the tangential bifurcation in the DCM boost converters are deduced mathematically. The computer simulations are performed to capture the effects of some chosen parameters on the tangent bifurcation behavior of the system. The results show that the variation of the feedback factor leads to tangent bifurcation and intermittent chaos phenomena. Experimental results show that the system does exhibit tangent bifurcation under the particular operating conditions, thus validating the theoretical analysis and the simulation results.
2012, 61 (5): 058402. doi: 10.7498/aps.61.058402
Based on the super-resolution property of time reversal electromagnetic wave far-field focusing, a subwavelength antenna array is proposed. The elements of the array are etched into those that have microstructures, placed at an interval of λ/20 apart from one another, and have independent channels. Compared with the subwavelength antenna arrays, each of which is composed of other microstructure elements, the array, whose radiation property of individual antennae is similar to others, shows better super-resolution characteristic of far-field focusing. Therefore, an effective wireless terminal antenna array is provided for the high-speed and high-quality multiple-antenna communication using the time reversal technology.
Key issues for high-efficiency silicon thin film solar cells prepared by RF-PECVD under high-pressure-depletion conditions
2012, 61 (5): 058403. doi: 10.7498/aps.61.058403
Our recent work on deposition and characterization of hydrogenated microcrystalline silicon (μ c-Si:H) thin films and silicon thin film solar cells prepared by RF-PECVD under high-pressure-depletion conditions is summarized in this paper. Several key issues are studied in detail: 1) process windows for device-quality μ c-Si:H thin films, 2) formation mechanism of amorphous silicon incubation layer and the effective methods to reduce the incubation layer thickness, 3) modification of crystalline fraction volume of intrinsic μ c-Si:H layers and its influence on the device performance of μ c-Si:H solar cells, 4) deposition of high conductive p-type μ c-Si:H window layers with high crystalline fraction volume, and the influence of p-layer on the device performance. After solving the above key issues, a high efficiency of 8.16% is obtained for μ c-Si:H sing-junction solar cell with intrinsic layer prepared by RF-PECVD under high-pressure-depletion conditions. When it is used as bottom cell in a-Si:H/μ c-Si:H tandem solar cell, the efficiency of tandem cell reaches 11.61%.
2012, 61 (5): 058501. doi: 10.7498/aps.61.058501
Due to short of pre-charge, a high power semiconductor switch reversely switching dynistor (RSD) suffers from switching-on locally. Based on the direct pre-charge circuit, a new two-step switching method is introduced. The conditions for the normal turn-on of RSD are analyzed and the model of RSD in a cell structure is built. The model simulation shows that the RSD has better performances: shorter width pre-charge current, and reduced recombination of plasma during the pre-charge. Two-step switching experiment indicates that a proper preliminary turn-on current is important for uniformly switching-on the RSD, and the reverse pre-charge current works mainly for switching preliminary forward current. Simulation and experimental results both show that two-step switching increases RSD uniform turn-on characteristic compared with the direct pre-charge circuit, because the stored charges inside are increased obviously by two-step switching.
2012, 61 (5): 058701. doi: 10.7498/aps.61.058701
In this paper we use an off-lattice bead-spring model to study the behaviors of polymer chain confined in an infinite cylinder by Monte Carlo simulations. Our simulation results show that when the bending energy b increases in the presence of the adsorption energy of the inner-surface of cylinder, the polymer chain is first randomly attracted on the inner surface of cylinder, and then gradually takes on helix structure, and finally is stretched into the rod-like structure. In order to understand the process of structure transition of polymer chain, we calculate the average of number of helical turns per chain Nt, the average percentage Ph of beads of helical structure per chain and the energy fluctuations with different values of bending energy b. The confined semirigid chain can form helical structure with the appropriate bending energy and the proper radius of cylinder. This study can help us understand the conformational behaviors of biopolymers in confined space.
2012, 61 (5): 058801. doi: 10.7498/aps.61.058801
Reasonable setting of engine parameters can improve the performance of the spectrum use. By analyzing engine parameter adjustment of cognitive wireless network, the mathematical model is given, and then it is converted into a multi-objective optimization problem. A chaos multi-objective immune algorithm is proposed to solve the problem. Logistic mapping is used to initialize population and search for the best solutions in every generation. The operators of cloning and antibodies updating are designed, which ensures that the distribution of Pateto optimal solutions is more diverse and uniform. Finally, the simulation experiments are done to test the algorithm under multi-carrier system. The results show that the algorithm can adjust transmission power and modulation mode according to the change of channel and user demands. More solutions with preferences are obtained, so it meets the demands for parameter optimization of cognitive engine.
GEOPHYSICS, ASTRONOMY, AND ASTROPHYSICS
2012, 61 (5): 059101. doi: 10.7498/aps.61.059101
Study of propagation and evolution of nonlinear seismic wave in earth crust is of important practical significance. In this paper, evolutions of nonlinear seismic longitudinal wave and transverse wave in microstructured earth crust are simulated detailedly by using the finite difference method. The results indicate that nonlinear seismic longitudinal wave and transverse wave in microstructured earth crust may evolve into one solitary wave or two solitary waves or solitary wave train, or disappearance. The initial amplitude of seismic wave, dispersion coefficient and body force factor significantly influence the evolution process. Our results reveal the evolution law of nonlinear seismic wave in a microstructured earth crust, which will help to explain some special seismic wave phenomena in theory.
New parameterization of wave-current interaction used in a two-way coupled model under typhoon conditions
2012, 61 (5): 059201. doi: 10.7498/aps.61.059201
The two important upper ocean physical processes, i.e., radiation stress and wave-induced mixing effects, have not been simultaneously taken into consideration in the wave-current coupled parallel run models. Under the current formulations, neither the effect of stratification nor the error during computation is fully suggested. A new simplified parameterization scheme combining wave radiation stress with wave-induced mixing is introduced into a coupled model which consists of wave (WAVEWATCH Ⅲ) and current (POM) model, and typhoon KAEMI (2006) is taken as the example to carry out four sets of experiments. The numerical experiments show that the turbulence enhanced by the wave-induced mixing in mixing layer transfers momentum and heat into down water layers, but in general, does not regularly change surface currents. The wave radiation stress transfers momentum horizontally, and thus it changes horizontal currents regularly. When both physical processes are included in this coupled model, two effects are superposed in the simulation. As an important dynamic factor, the current affects the significant wave height and its distribution, which transfers horizontal momentum and makes significant wave height much higher in the present study. Furthermore, the current vector rotates clockwise at the time of the 48th hour due to the inertial oscillation probably being the signal of wave-current interaction amplified by the typhoon approaching.
Evaluation of noninductive charging mechanisms and simulation of charge characteristic structure in the early thunderstorm based on RAMS V6.0
2012, 61 (5): 059202. doi: 10.7498/aps.61.059202
A new regional thunderstorm model, coupled with two primary non-inductive electrification mechanisms, namely Takahashi (1978) and Saunders (1991) schemes, is developed based on the newest version of RAMS V6.0. Based on the simulation on a thunderstorm in its early stage which occurred in Beijing, the results show that the total charge distribution is tripolar in both schemes when the electric field reaches breakdown value. The results are in agreement with some observations and results of other model. However, the evolution processes and shapes of cloud charge distribution in the two schemes are different. The result of Takahashi's (1978) scheme produces a tripolar charge distribution in the cloud before the first lightning. The Saunders' (1991) scheme produces a transform from inverted dipole distribution to tripolar charge distribution. This is because they used different experimental conditions: different factors lead to different results. The results from both schemes show that the positive charge carrier at a low level of thunderstorm is rain droplet, the aggregates and graupels are located in high level of thunderstorm, and the charge center distribution of graupels is similar to the distribution of total charge in thunderstorm cloud.