Vol. 63, No. 10 (2014)
2014, 63 (10): 107802. doi: 10.7498/aps.63.107802
The Ho3+/Tm3+ codoped bismuth germanate glasses containing Ag nanoparticles (NPs) are synthesized by a chemical reduction method based on the conventional melting-quenching technique. The effect of concentration of Ag NPs on the 2 um emission is studied. The absorption band related to the surface plasmon resonance (SPR) of the Ag NPs is located in a range from 500 to 900 nm. Transmission electron microscopic image clearly reveals homogeneously dispersed Ag NPs with the sizes ranging from 5 to 10 nm. The luminescence spectra in a range of 1.7-2.3 um are collected. With the addition mass fraction of the AgCl up to 0.3%, the intensity of emission band of Ho3+ ions, centered at 2.03 um, is increased by 10 folds. The enhancement of 2 um luminescence is attributed to the enhanced local field induced by SPR of Ag NPs. The calculated absorption cross section and emission cross section are 0.491 10-20 cm-2 and 1.0310-20 cm-2, respectively. When the gain coefficient p=0.2, the positive gain would be realised.
2014, 63 (10): 108101. doi: 10.7498/aps.63.108101
-Fe2O3@C core-shell nanorods with average diameter of 20 nm and length of 150 nm are synthesized by transforming FeOOH@PVA nanorods under the condition of high pressure and high temperature (HPHT). The FeOOH@PVA nanorods are prepared via a hydrothermal route. The best synthesis condition for transforming FeOOH@PVA core-shell nanorods into -Fe2O3@C nanorods is 400 ℃ under 1 GPa. Owing to high aspect ratios, the -Fe2O3@C nanorods present a high coercivity of 330 Oe (10 Oe=79.5775 A/m). The possible mechanism for the synthesis of -Fe2O3@C nanorods is also discussed. The HTHP method can provide a new way for preparing of one-dimensional core-shell nanostructures.
Numerical simulation and experimental investigation of the production of multiply charged ions by the ionization of benzene cluster with a moderate intensity laser
2014, 63 (10): 103602. doi: 10.7498/aps.63.103602
The production of multiply charged ions by the interaction of intense femtosecond laser with clusters has been widely reported. Recently, many groups discovered the multiply charged ions when the cluster was irradiated by a 532 nm nanosecond laser with the intensity as low as 1010 W/cm2. Although this interesting phenomenon could be explained by the mechanism of multiphoton ionization triggered-inverse bremsstrahlung heating-electron impact ionization, there is a lack of numerical simulation to explain the generation of multiply charged ions. In this paper, numerical simulation is performed to study the generation process of multiply charged ions in the moderate intensity laser. Firstly, the electron energy is calculated according to ponderomotive potential. Secondly, the cross section of electron impact ionization is calculated on the basis of Lotz formula. Finally, the evolution of multiply charged ions in the cluster is calculated with the kinetic reaction rate equation. The effects of cluster size and electron density on multiply charged ions are investigated in detail. Simulation results show that the ionization process is completed and the balance among C2+, C3+ and C4+ is achieved in 0.7 ns. The relative intensity sequence of multiply charged ions is C2+ C3+ C4+, which is consistent with the experimental results. In addition, numerical simulation results show that the charge state of ions is increased with the increase of cluster size, which is consistent with the experimental results.
2014, 63 (10): 100501. doi: 10.7498/aps.63.100501
Lyapunov characteristic exponent is significant for analyzing nonlinear dynamics. However, most algorithms are not applicable for the switching system. According to the traditional Jacobi method, in this paper we propose a new algorithm which can be used to compute n Lyapunov exponents for an n-dimensional switching system. We first study the geometric dynamics of two adjacent trajectories near the switching manifold, and obtain a compensation Jacobi matrix caused by switching. Then with QR-decomposition of this matrix, we compensate for the diagonal vector of R to realize the Lyapunov exponent expansion. Finally, we use the algorithm in a two-dimensional double-scrolls system, the Glass network and a spacecraft power system, and show its correctness and effectiveness by comparing the results with the Poincaré-map method.
2014, 63 (10): 100502. doi: 10.7498/aps.63.100502
The stochastic resonance is investigated in the generalized Langevin equation with exponential memory kernel subjected to the joint action of internal noise, external noise and external sinusoidal forcing. The system is converted into three-dimensional Markovian Langevin equations. Furthermore, using the Shapiro-Loginov formula and the Laplace transformation technique, the exact expressions of the first moment and the steady response amplitude are obtained. The research results show that with the variations of external sinusoidal force frequency and the parameters of memory kernel and external noise, the system presents bona-fide stochastic resonance, conventional stochastic resonance and stochastic resonance in a broad sense under the condition of Routh-Hurwitz stability. In addition, the stochastic resonance can be weakened as the memory time increases. Moreover, the numerical results of power spectrum of system are in agreement with the analytic results.
2014, 63 (10): 100503. doi: 10.7498/aps.63.100503
Sample entropy or approximate entropy, a complexity measure that quantifies the new information generation rate and is applicable to short time series, has been widely applied to physiological signal analysis since it was proposed. However, on one hand, sample entropy is easily affected by non-stationary sudden noise, because the tolerance during calculation is set to be proportional to standard deviation; on the other hand, it is not independent of the probability distribution, so that it does not purely characterize the new information generation rate. To solve these two problems, a new improved method named equiprobable symbolization sample entropy is proposed in this paper. Through equiprobable symbolization, the effects of both non-stationary sudden noises and probability distribution are eliminated. Besides, since equiprobable symbolization is usually non-uniform, it further breaks through the linear constrains in classic sample entropy. The method is proved to be rational by simulating three typical noises that have different time correlations and new information generation rates. Then the method is applied to electroencephalography (EEG) analysis. Results show that the method can successfully discriminate two different attention levels based on EEG with duration as short as 1.25 s and without removing any artificial artifacts. Therefore, the method is of great significance for EEG biofeedback, in which strong real-time abilities are usually required.
2014, 63 (10): 100504. doi: 10.7498/aps.63.100504
The attitude dynamics equation of free-floating space robots subjected to gravitational gradient effect is investigated. A two-link space robot is employed to analyze nonlinear properties of the perturbed yaw motion of the system in depth, when the manipulator configuration is fixed. Its nonlinear dynamical behavior is described by phase plane plot and Poincaré section. It is shown that the perturbed motion is sensitive to orbital eccentricity. The system takes on periodic motion and tumbling in circular orbit, while additional quasi-periodic motion in elliptic orbit. Furthermore, these nonlinearities are quantitatively studied by means of bifurcation and power spectrum.
2014, 63 (10): 100505. doi: 10.7498/aps.63.100505
According to the phase space reconstruction theory of nonlinear system, we propose a prediction method of support vector machine based on genetic algorithm. Using the improved autocorrelation method and Grassberger-Procaccia algorithm to determine the time delay and embedding dimension of chaotic signal, the phase space reconstruction is implemented. The penalty coefficient and the kernel function parameter of support vector machine are optimized by genetic algorithm. Combined with support vector machine, single-step prediction model of the chaotic sequence is set up, so we can detect the weak signal in chaos from the prediction error (including the transient signal and periodic signal). Lorenz attractor and the data from the McMaster IPIX radar sea clutter database are used in the simulation. The proposed method can effectively detect the weak target from chaotic signal. When the signal-to-noise ratio is -89.7704 dB in the chaotic noise background, by using the new method the root mean square error can be reduced by two orders of magnitude, reaching 0.00049521, while the conventional support vector machine can reach only 0.049 under the condition of -54.60 dB.
2014, 63 (10): 100601. doi: 10.7498/aps.63.100601
High-accuracy ranging system is of significance both in future space-based sciences and in large-scale manufacture. Based on the light intensity, a method of measuring absolute distance in a long range by using the optical frequency comb is demonstrated. The temporal coherence of the pulses emitted from the optical comb is analyzed. The measurement principle is introduced. The peak position of the interference fringe is analyzed, which can contribute to the distance measurement, and numerical simulations are also developed correspondingly. The experimental results show that the absolute distance measurement can be realized. This method can be used to measure a large-scale distance.
Variable frequency range finding technology based on double polarization modulation method and system implementation
2014, 63 (10): 100602. doi: 10.7498/aps.63.100602
Traditional laser range technology has a poor phase measurement accuracy and an additional phase of the system, which restricts the improvement of its accuracy. In this article, by using the technology of variable frequency measurement based on double polarization modulation the phase shift range-finding technology is improved. With the method of double polarization modulation, the demodulation of the phase information is directly implemented on the phase modulator, which can make the system simpler. The variable frequency technology is used to replace the traditional phase discrimination technology; therefore the measurement accuracy of the system will not be persecuted with the phase crimination any more. The theoretical curve of sine (cosine) relation between modulation frequency and output light intensity is proved experimentally. Owing to the fact that the stability of frequency can be better than 10-6, the measurement accuracy can reach ±10.6 u email@example.com m. By using this system to measuring a 200 m-long fiber, the clear curve of modulation frequency versus output of system is obtained.
Design and analysis for the front imaging optical system of the spatiotemporal mixed modulated Fourier transform imaging spectrometer
2014, 63 (10): 100701. doi: 10.7498/aps.63.100701
In order to satisfy the application requirements of the large flux and the high stability for the infrared imaging spectrometer, we propose a new type of static no-slit spatiotemporally mixed modulated Fourier transform imaging spectrometer based on the multi-level micro-mirrors. The working process and the generation manner of optical path difference of the system are analyzed. The front imaging system, as an important component of the imaging spectrometer, determines the distribution of the optical path difference, and its performance directly affects the image quality of target object. According to the generation manner of optical path difference of the system, the front imaging system for the telecentric structure in the image space is analyzed and designed. The athermalization design research is carried out by means of the passive optical elimination thermal difference. The result shows that the modulation transfer function curve of each field reaches the diffraction limit in a temperature range of -20-60 ℃. The front imaging optical system has a good imaging quality in the total step height of the multi-step micro-mirrors. At different temperatures, the maximum incident angle on the image surface is less than 0.02° for the principal ray of each field.
Full three-dimensional simulations of optimized active pixel detector for ionizing particle detection
2014, 63 (10): 100702. doi: 10.7498/aps.63.100702
In this paper, a double epitaxial and high energy implant (DEI) is proposed to improve the performance of charge collection and radiation hardness. Three-dimensional process procedure and physical level simulation are presented. The results show that the internal distributions of electric potential and electric field are improved; the seed point pixel collected charge increases about 70% and reduces the collected time to 64%. In addition, the DEI structure increases the collecting efficiency in a radiation range from 1012 to 1015 cm-2as compared with the standard monolithic active pixel sensors.
ATOMIC AND MOLECULAR PHYSICS
With the quasi-classical trajectory method the stereodynamics of the O+DCl→OD+Cl reaction on the ground potential energy surface is investigated. The characteristic of calculated integral cross-section is consistent with that of the non-energy barrier reaction path on the potential energy surface, which implies that the title reaction is a typical exothermic reaction. The obtained differential reaction cross-section shows that the products tend to both forward and backward scattering, and the forward scattering is stronger than the backward one. So we can infer that the reaction follows the indirect reaction mechanism that has been verified by the randomly abstractive reaction trajectories. The distribution curves of P(θr) and 2(J'· K)> reflect that the degree of rotational alignment of the product OD first increases and then decreases with collision energy increasing. The product rotational angular momentum vector J' is aligned along the y-axis direction but is oriented along the positive direction of y-axis at higher collision energy. With the increase of the collision energy the rotation mechanism of the product molecules transits from the “in-plane” mechanism to the “out-of-plane” mechanism.
Electric field effects on the excited properties of Si2N2 molecule with special configuration：a density-functional study
2014, 63 (10): 103101. doi: 10.7498/aps.63.103101
In order to understand in depth the electroluminescence mechanism, the influences of the external electric field on the geometric and electronic structure in ground state, the molecular vibrational spectra of Si2N2 molecule with Cs special symmetry are studied by density functional theory with B3LYP exchange-correlation prescription at the aug-cc-pVTZ basis set level. Following each optimization, the vibrational frequencies are calculated and all optimized structures are stable. The results show that the molecular vibrational Stark effect, i.e., red-shift for the low-frequency modes and blue-shift for the high-frequency modes are observed with the increase of the applied field strength. The energies of the highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO), the energy gap between HOMO and LUMO of Si2N2 molecule diminish with the increase of external field. A time-dependent density functional theory is used to investigate the excited properties of Si2N2 (Cs) molecule. The calculated absorption spectra of Si2N2 molecule with Cs symmetry are in agreement with the experiment values. The analysis reveals that the absorption spectrum wavelength increases in the visible region with a concomitant increase in the electronic transition oscillator strengths in the course of the increase of the external electric field strength. The results reveal that the excited properties of Si2N2 molecule can be easily tuned by the external electric field, which indicates that the silicon nitride is an interesting optoelectronic functional material. These investigations on the various properties of Si2N2 molecule with Cs symmetry under an external electric field are useful to understand the electroluminescence mechanism for silicon nitride used in molecular electronics.
2014, 63 (10): 103601. doi: 10.7498/aps.63.103601
The average mixed cluster sizes in different mixing proportions of Ar-CH4 mixed cluster and Ar-H2 mixed cluster in supersonic gas jet are studied by Rayleigh scattering method. It is found that Ar-CH4 mixed cluster could form easily when the mixed Ar and CH4 gas are used in gas jet, and the maximum cluster size is achieved when the content of Ar is 50%. The maximum cluster size of Ar-CH4 mixed cluster is larger than that of either Ar cluster or CH4 cluster. Being different from pure hydrogen cluster which only forms at liquid nitrogen temperature, Ar-H2 mixed cluster can form at room temperature. So this is the first time we have obtained hydrogen cluster at room temperature. Ar-H2 mixed cluster starts to form at H2 content value higher than 40% and it reaches maximum size when the content of H2 is 60%. Hydrogen (deuterium) mixed clusters introduce heavier Ar element on the basis of hydrogen (deuterium) clusters. It will further accelerate the deuterium ions to higher energy in deuterium cluster laser fusion experiments, so we can obtain higher neutron yield and fusion efficiency.
ELECTROMAGENTISM, OPTICS, ACOUSTICS, HEAT TRANSFER, CLASSICAL MECHANICS, AND FLUID DYNAMICS
Newmark method for finite-difference time-domain modeling of wave propagation in frequency-dispersive medium
2014, 63 (10): 104101. doi: 10.7498/aps.63.104101
The complex polarizations of three kinds of general dispersive medium models, i.e. Debye model, Lorentz model, Drude model, are described by rational polynomial fraction in jω. The relationship between the polarization vector P and the intensity of electric field E in time domain is obtained by utilizing the transformation relationship from frequency domain to time domain jω→∂/∂t. Then, the time domain second order equation is solved by using the Newmark β and γ method, which has higher accuracy than the traditional center difference method. Once the recursive formulations for E and P are obtained, the recursive formulations for D and E in time domain can be also obtained based on the constitutive relation. Therefore for a dispersive medium the iterative electromagnetic field calculation is conducted by finite-difference time-domain (FDTD) method. The present numerical results demonstrate that the proposed method is a general algorithm for three kinds of general dispersive medium models, and has higher accuracy than the shift operator-FDTD, which is based on the central difference discrete scheme.
Effect of source parameters on polarization characteristics of electromagnetic beam propagating in atmospheric turbulence
2014, 63 (10): 104201. doi: 10.7498/aps.63.104201
According to the theory of beam spreading and taking electromagnetic Gaussian Shell-model beam as an object of research, the change in its spectral degree of polarization is studied by numerical analysis. Based on the numerical results, the mechanism that governs the change in polarization of an electromagnetic beam on propagation is discussed. The results show that the beam spreading of two components of an electromagnetic beam results in the change in polarization of beam directly, and the beam spreading is determined by source parameters and atmospheric turbulence. The difference between beam spreading of two components induced by coherence leads to the change in polarization in free space. The change in degree of polarization is influenced by source parameters and turbulence on propagation in atmosphere. The result is similar to that in free space over relatively short distance, which is mainly governed by source parameters. With the enhancement of turbulence over a sufficiently long distance, the change in polarization in atmosphere is different from that in free space.
2014, 63 (10): 104202. doi: 10.7498/aps.63.104202
Since the third generation synchrotron radiation source came into service, the X-ray techniques which relate to coherent property have quickly developed and been widely used. Typically, X-ray phase contrast imaging has become a conventional imaging method. The X-ray techniques, such as coherence scattering, coherent diffraction imaging, and photon correlation spectroscopy, have received more attention and shown unique superiority in the field of high spatial and time resolution. So quantifying the coherent property of X-ray source is meaningful for those novel X-ray techniques. In this article, based on the Talbot self-imaging phenomenon, the spatial coherent property and the scale of X-ray source of X-ray imaging and biomedical application beam line in Shanghai synchrotron radiation facility are measured. The results show that when the photon energy is 33.2 keV, the spatial coherence length is 8.84 μm and source size is 23 μm in the vertical direction, and the test result is in agreement with the theoretical value.
2014, 63 (10): 104203. doi: 10.7498/aps.63.104203
The theoretical basis of coherent field imaging technique (also called the Fourier telescopy) is reconsidered. The limitations of the technique principle are analyzed by defining the measured object and deriving rigorously mathematical expressions. The reconstructed image of the technique ineluctably contains the gradient information about the object; as a result the technique cannot acquire the reflectivity of the object exactly. The computer simulation verifies the conclusion. Based on the conclusion, the reconstructed image of the technique can be evaluated and three-dimensional coherent field imaging technique may be developed.
2014, 63 (10): 104205. doi: 10.7498/aps.63.104205
We apply the ABCD formalism to a gradient negative index medium (NIM) and investigate the propagation and transformation properties of Gaussian beams in this medium. First, we derive the ABCD formalism in a positive gradient NIM and obtain the propagation model. Spatial soliton and the spatial breather propagation in this medium are revealed. Our research suggests that the gradient coefficient has a significant effect on the focusing ability of slab. When the gradient coefficient increases, the quasi-lense effect becomes more prominent and notable. As a result, the focusing ability improves and the beam waist in the focal point shrinks. Second, when Gaussian beams propagate in the negative gradient NIM, the beam waist enlarges as the distance increases. There is neither spatial soliton phenomenon nor breather transmission phenomenon, which is completely different from the propagation characteristics in the positive gradient NIM.
2014, 63 (10): 104206. doi: 10.7498/aps.63.104206
With different strategies collecting images, the extraction results of refraction information using the diffraction-enhanced imaging (DEI), generalized diffraction-enhanced imaging (G-DEI) or multiple-image radiography (MIR) algorithm of analyzer-based imaging technique are greatly different from each other. Compared with the conventional strategy in which the images are acquired at the waist positions of rocking curve (RC), the one with the positions closer to the central axis of RC can obtain more accurate refraction information for the DEI algorithm. For the G-DEI algorithm taking three images, if the positions, where images are obtained, are symmetrical with respect to the central axis of RC, the extraction result of refraction information is better when the positions are asymmetrical. Besides, by comparison with the strategy in which the images are acquired at the waist or toe positions, the one with the positions closer to central axis of RC can obtain better result. If the angle interval of neighbor positions is larger than the full-width at half-maximum (FWHM) of RC, the extraction result of the MIR algorithm is bad. When the angle interval is smaller than the FWHM, and the value of angle range of all positions collecting images is close to the maximum theoretical value of sample, the extraction result is very good. This study will be helpful for reasonably choosing the image-collecting strategies in experiment, and further understanding the extraction algorithms of refraction information.
2014, 63 (10): 104207. doi: 10.7498/aps.63.104207
We propose a digital holographic microscopy (DHM) setup employing a configuration with two Lloyd's mirrors, which is based on self-referencing and dual-wavelength optical phase unwrapping. We use two Lloyd's mirrors to fold the beam which does not exhibit sample structure and acts as the reference beam, returning onto itself to form a dual-wavelength hologram. Two wrapped phase images for every wavelength are reconstructed by angular spectrum method. Then the wrapped phase image and the three-dimensional profile image are acquired by dual-wavelength optical unwrapping method. In the experiment, we use two lasers of different wavelengths of 532 and 632 nm to record a hologram. Numerical methods are subsequently applied to reconstruct the hologram to enable direct access to both phase and amplitude information. The quantitative experimental results with dual-wavelength DHM involve a deviation less than 5% from the calibration values. The validity of this method is demonstrated.
2014, 63 (10): 104208. doi: 10.7498/aps.63.104208
Microchip laser can output two orthogonal frequency splitting modes due to its own internal residual stress. In this paper it is found that the frequency difference of microchip laser is modulated by laser feedback. The modulation is a sinusoid-like curve whose center is the original frequency difference and period is half-wavelength. The amplitude of the modulation curve is proportional to feedback level. But when the feedback is too strong, so that the polarization switching occurs and only one polarization exists sometimes. In a range of external cavity, the amplitude of the modulation curve is also proportional to original frequency difference. The theoretical analysis and simulation based on the composite laser cavity theory and self-consistent theory are in good agreement with the experimental results. The potential applications of this phenomenon in precision measurement are discussed.
Terahertz bandpass filter using double-layer reformative complementary frequency selective surface structures
2014, 63 (10): 104209. doi: 10.7498/aps.63.104209
The simulation and experimental study of a bandpass frequency selective surface filter in terahertz (THz) range using double-layer modified complementary structures are conducted in this paper. The modified four-split complementary electric inductive capacitive (CELC) structure is introduced as the resonant cell of the filter. The primary design objective is to improve the filtering performances of double-layer complementary metamaterial structures built on intensified thickening quartz substrate. The bandpass filter centered at 0.28 THz is simulated, fabricated and measured. Experimental results from 0.1 to 0.6 THz measured by THz time-domain spectroscopy are in excellent agreement with simulation. The reformative CELC bandpass filter has the advantages of a low cost, low loss, steepness of skirts, out-of-band rejection, and etalon resonance rejection.
Gyrator transform of generalized sinh-Gaussian beam and generation of dark hollow light beam with vortex
2014, 63 (10): 104210. doi: 10.7498/aps.63.104210
Starting from the Gyrator trnasform formula, the closed-form filed distribution of generalized sinh-Gaussian beam passing through such a transform system is derived, and the intensity distribution and the corresponding phase distribution associated with the transforming generalized sinh-Gaussian beams are analyzed. Based on the numerical method, the distributions are graphically drawn and it is found that, for appropriate beam parameters and by carefully adjusting the transform angle of Gyrator transform, the dark hollow beam with topological charge index being unity can be realized with sinh-Gaussian beam carrying an edge dislocation. And the influences of the beam parameters and the transform angle on the formation of perfect dark hollow beams are analyzed. However, it is impossible to obtain dark hollow beam with Gyrator transforming cosh-Gaussian beam.
2014, 63 (10): 104211. doi: 10.7498/aps.63.104211
To meet the application requirements for high energy kHz repetition rate femtosecond laser, a high-efficiency femtosecond Ti:sapphire linear regenerative amplifier is designed. By optimizing the parameters of the cavity, 5.8 mJ chirped pulses at 800 nm are obtained, under pump energy of 20 mJ with wavelength of 527 nm at 1 kHz repetition rate, corresponding to a slope efficiency of 30.7%. By compensating for the dispersion, 4 mJ laser pulses at 800 nm with pulse duration of 45.7 fs are achieved. The energy fluctuation is 0.18% (RMS) in 5 h.
2014, 63 (10): 104212. doi: 10.7498/aps.63.104212
In order to understand in depth plasma behavior during ultra-high power fiber laser deep penetration welding, the plasma inside and outside the keyhole is observed, and the spectrum of fiber laser-induced plasma is measured and analyzed. Based on the measured data of plasma, the electron temperature and electron density, ionization degree and pressure are calculated, and the characteristics of plasma parameters at different values of keyhole depth and outside the keyhole are investigated. The results indicate that the distribution of plasma inside the keyhole is uneven, and the vapor plume is much bigger outside the keyhole. The spectrum of plasma show that the fiber laser-induced plasma is weakly ionized and radiates a few spectral lines. The further calculation results also confirm that the plasma induced by fiber laser is in a weakly ionized state. However, the electron density of plasma still stays in a high level, and the transient pressure of plasma is up to hundreds of times as large as atmospheric pressure.
Quantitative analysis of laser induced breakdown spectroscopy of Pb in navel orange based on multivariate calibration
2014, 63 (10): 104213. doi: 10.7498/aps.63.104213
The detection accuracy of laser induced breakdown spectroscopy (LIBS) is affected by system parameters, ambient gas, matrix effect, sample morphology, calibration methods etc. Heavy metals in Gannan navel orange are determined by LIBS in our laboratory. The experimental parameters are optimized. In this work, multivariate linear regression model is used to predict the concentration of Pb element in navel oranges. The real concentration of Pb is quantitatively determined by atomic absorb spectroscopy (AAS). The concentration is set as dependent variable, while the intensity of Pb I 405.78 nm, the intensity sum of Ca Ⅱ 393.37 nm and Ca Ⅱ 396.84 nm, and the integrated intensity in a range of 405.03-405.96 nm are taken as independent variable. The calibration results indicate that the maximum relative error between the predicted Pb concentration from the multiple linear regression model and the measured one by the AAS is 12.99%, and the average relative error of the samples is 4.87%. And the fitting degree of the results of two methods is 0.995. The result shows that the multivariate calibration method can utilize the information about the spectra and reduce the influence of the matrix effect. The multivariate linear regression model is proved to be feasible in improving the prediction accuracy of LIBS.
2014, 63 (10): 104214. doi: 10.7498/aps.63.104214
An intensity modulation hard-target differential laser absorption lidar system for CO2 sensing is demonstrated. On and off wavelength lasers are modulated with 10 kHz and 12 kHz sinusoidal waves. The echo is extracted using coherent detection technique. The system is fiber based, which makes it compact and removable. We obtain three day variation of horizontal column averaged CO2 concentration of Shanghai district. We also propose an accuracy evaluation method based on electronic noise analysis combined with laser frequency modulation. The result shows that the measurement precision for the column corresponds to 3.39×10-6 (rms) with 1s integral time and 450 m path.
2014, 63 (10): 104215. doi: 10.7498/aps.63.104215
A breather soliton solution of the higher-order Hirota equation is given under the integrable condition, and the rogue solution of Hirota equation is obtained on the basis of the breather soliton solutions, which is helpful to understand the characteristics and the physical reason of rogue wave. The excitation of rogue wave is studied by a cw and periodic perturbation or a Gaussian type perturbation. As an example, by distribution Fourier method, the transmission characteristics of rogue wave is studied with considering the frequency shift and the Raman gain, and the effects of the frequency shift and Raman gain on the interaction between rogue waves are also analyzed.
2014, 63 (10): 104216. doi: 10.7498/aps.63.104216
A gated operation dynamic bias control strategy of InGaAs single-photon avalanche diode (SPAD) and circuit implementation are proposed based on the research of the SPAD performances. By the gated operation active quenching method the quenching time can be lowered, also dark count and afterpulsing effect are inhibited. The circuit fabricated by standard complementary metal oxide semiconductor (CMOS) technology and SPAD fabricated by non-standard CMOS technology are interconnected through the indium column interconnection hybrid packaging process. In the low temperature (-30 ℃) test conditions, the avalanche current signal triggered by light is extracted and avalanche phenomenon is quickly quenched. Studies in this paper are the sensing resistance and critical sensing voltage effect on electrical performance of the detector and the implementation method of the detection circuit. The recovery time and transfer delay of the SPAD are 575 and 563 ps, respectively and the quenching time is 1.88 ns. These characteristics meet the requirements for the nanosecond precision sensor detection application.
2014, 63 (10): 104217. doi: 10.7498/aps.63.104217
Based on power spectral density method, the uniform sampling leading to the leakage of low frequency in atmosphere phase screens is analyzed. A new method - non-uniform sampling is proposed. The non-uniform sampling is modeled. The covered sampling frequency regions and the powers of single sampling region by the two sampling methods are discussed and compared. The new method proves to be effective and feasible. For the Kolmogorov spectrum of atmospheric turbulence, the numerical simulation phase screens are generated by the two sampling methods. The simulation results show that the random phase screens generated by the non-uniform sampling method under the condition of increasing neither sampling number nor computation burden, possesses rich high and low frequency information.
Underwater acoustic synchronization telemetry research based on binary offset carrier modulated signal with zero correlation window
2014, 63 (10): 104301. doi: 10.7498/aps.63.104301
Aiming at the problem that synchronized correlation of linear frequency synchronization of m sequence possesses self noise and there are strong side lobes in a code range besides main lobe, in this paper we propose a new underwater acoustic synchronization scheme based on binary offset carrier (BOC) modulated signal with no interference windows which employs the property that the sum of aperiodic auto correlation function is 0 to realize no interference window between the range of one code and zero inference window and utilizes BOC (1, 1) to modulate signal with sub-carrier to mitigate side lobes in one code range besides main lobe. The schemes are designed for both single and couple channel signal, and the validity of underwater acoustic synchronization, channel measurement and estimation are testified through simulation and experiments.
2014, 63 (10): 104302. doi: 10.7498/aps.63.104302
Matched field processing techniques have been studied extensively in recent decades, and a lot of detail algorithms have been put forward for practical use. When the underwater target is obscured by the strong surface interferences, the performance of matched field processing localization will degrade severely. The now existing spatial filter technique can be used to suppress the surface interferences, but the burden of calculation is heavy and the memory usage is large. In this paper, a scheme of optimizing spatial filter design based on the compressed replica vectors is presented, and the broadband data are processed incoherently. In contrary to the existing spatial filter, the optimized spatial filter can effectively reduce the computational complexity and memory usage when the number of array elements N is greater than the number of the effective modes Q, meanwhile, it also retains the original performance of interference suppression. Numerical simulations in a littoral shallow water environment are performed to validate the performance of the spatial filter and the promotion of computation speed. Then, data processing results obtained from an experiment conducted in the littoral shallow water environment are presented. It follows from the results that the weak underwater target can be distinguished from the strong surface interference clearly by use of the incoherent matched field processing with the application of the spatial filter based on compressed replica vectors.
2014, 63 (10): 104303. doi: 10.7498/aps.63.104303
To use a small number of acoustic pressure measurement data to reconstruct the radiated acoustic field of the complicated structure, a theory of source strength density acoustic radiation modes is proposed and a formula of acoustic field reconstruction is developed. In the space defined on the surface of the structure, functional form of the acoustic radiation power expression in which parameter is source strength density is constructed. In terms of the functional a linear self-adjoint and positive radiation operator is defined whose eigenfunctions are source strength density acoustic radiation modes. And then it is proved that source strength density acoustic radiation modes possess space filter characteristic through analyzing the source strength density radiation modes of rectangular plate and cylinder with hemisphere ends. The formula of acoustic field reconstruction with the space filter nature is obtained. The sphere simulations and plate experiment validate the feasibility and robustness of the proposed acoustic field reconstruction method. The acoustic field reconstruction method based on the proposed radiation modes is simple, has high accuracy that can be obtained by using only a few measurement data, so this method is especially applicable for low frequency acoustic field reconstruction of the complicated structure.
2014, 63 (10): 104304. doi: 10.7498/aps.63.104304
Doublet mechanics (DM) is applied to materials with dispersing property. The wave equation in multi-layered skin tissue is obtained through the method of DM in this article. By changing the Poisson ratio in melanoma skin tissue, the growth and spread of melanoma (Breslow thickness), the internode distance, the reflection coefficients of the longitudinal waves incident vertically in the multi-layered skin model are calculated. And the acoustic velocity and acoustic attenuation coefficient changing with tissue parameters are also calculated. The results show that we can synthesize the change of the number of the minimal reflection coefficient, acoustic velocity and attenuation coefficient to discriminate cancerous skin from normal skin within a certain frequency.
Perturbation to Noether symmetries and adiabatic invariants for Birkhoffian systems based on El-Nabulsi dynamical models
2014, 63 (10): 104501. doi: 10.7498/aps.63.104501
In this paper, we study the problem of perturbation to Noether symmetries and adiabatic invariants for a Birkhoffian system under small disturbance based on the El-Nabulsi dynamical model. First, the dynamical model presented by El-Nabulsi, which is based on the Riemann-Liouville fractional integral under the framework of the fractional calculus, is extended to the Birkhoffian system, and El-Nabulsi-Birkhoff equations for the Birkhoffian system are established. Then, by using the invariance of the El-Nabulsi-Pfaff action under the infinitesimal transformations, the definition and criterion of the Noether quasi-symmetric transformation are given, and the exact invariant caused directly by the Noether symmetry is obtained. Furthermore, by introducing the concept of high-order adiabatic invariant of a mechanical system, the relationship between the perturbation to the Noether symmetry and the adiabatic invariant after the action of small disturbance is studied, the condition that the perturbation of symmetry leads to the adiabatic invariant and its formulation are presented. As a special case, the perturbation to Noether symmetries and corresponding adiabatic invariants mechanics of non-conservative systems in phase space under El-Nabulsi models and classical Birkhoffian systems are discussed. At the end of the paper, taking the well-known Hojman-Urrutia problem for example, its Noether symmetries under the El-Nabulsi dynamical model is investigated and corresponding exact invariants and adiabatic invariants are presented.
Conformal invariance and conserved quantity of relative motion holonomic dynamical system in phase space
2014, 63 (10): 104502. doi: 10.7498/aps.63.104502
Conformal invariance and conserved quantity of relative motion holonomic dynamical system in phase space are studied. The definition of conformal invariance of relative motion holonomic dynamical system in phase space is provided. The necessary and sufficient conditions that conformal invariance of the system would be Lie symmetry are deduced. By use of a structure equation that the gauge function satisfies, the corresponding conserved quantity of the system is derived. Finally an illustrative example is given to verify the results.
2014, 63 (10): 104503. doi: 10.7498/aps.63.104503
Variations of pressure along the vertical direction in a cylindrical silo are measured, of which the side wall is comprised of two separated up and down parts. The deviation of the measured results from the so-called Janssen model of silo stress is discussed. The measurements show that certain deviations from the model are always observed, no matter whether the slowly downward motion of silo bottom by “fully mobilized friction” as suggested by Vanel et al is carried on. Moreover without mobilizing the friction, the deviation may increase considerably if a tiny sink of the upper side-wall caused by weight of filled grains is present. The effect can be however eliminated by the friction mobilized. These indicate that distributions of grain weight at the bottom and lateral wall are complex and variant, and dependent on preparation manner and deformation of experiment setup. So stress boundary conditions need to be taken into account in the nonlinear elastic theory for analyzing static stress distribution of granular matter.
2014, 63 (10): 104701. doi: 10.7498/aps.63.104701
Based on the wettability alteration caused by the modified hydrophobic solid surface, the phenomenon of wettability alteration is simulated numerically in terms of linear and instantaneous modification by using the lattice Boltzmann method which can properly reflect the interaction of solid-liquid molecules, combined with the volume of fluid method to dispose the quality of interface layer. Results show that the wettability changes smoothly in the process of linear modification, the time needed for wetting significantly decreases, and the relationship between the contact angle and attractive coefficient of solid-liquid accord well with literature data. The more greatly the amplitude of instantaneous modification changes, the stronger the force of solid acting on droplet is, which is reflected by the obvious change of wettability. It is also found that the contact angle changes exponentially with time after instantaneous modification, which is in good agreement with the existing conclusions. Further investigation shows that the liquid oscillation exists in the whole spreading process. The vibration peak is associated with the modified amplitude of linear modification. And liquid film velocity increases suddenly at sometime after instantaneous modification, which is associated with entrained air.
2014, 63 (10): 104702. doi: 10.7498/aps.63.104702
In this paper, the graphic processing unit (GPU) parallel computing of dissipative particle dynamics (DPD) based on compute unified device architecture is carried out. Some issues involved, such as thread mapping, parallel cell-list array updating, generating pseudo-random number on GPU, memory access optimization and loading balancing are discussed in detail. Furthermore, Poiseuille flow and suddenly contracting and expanding flow are simulated to verify the correctness of GPU parallel computing. The results of GPU parallel computing of DPD show that the speedup ratio is about 20 times compared with central processing unit serial computing.
2014, 63 (10): 104401. doi: 10.7498/aps.63.104401
The Rayleigh-Bénard convection in a binary fluid mixture is one of typical models for studying the nonlinear dynamics of nonequilibrium convection. In this paper, using the numerical simulations of the two-dimensional full equations of hydrodynamics, we study the bifurcation and evolution of patterns in the traveling wave convection in binary fluid mixtures with strong Soret effect (separation ratio Ψ=-0.60) in a rectangular cell. The system exhibits 5 types of traveling wave convection solutions with the increasing of reduced Rayleigh number r along the upper branch of the bifurcation curve. They are localized traveling wave convection, traveling wave convection with defects, traveling wave convection, undulation traveling wave convection, and stationary overturning convection. Second, the influence of separation ratio on convection solutions is investigated. By comparing the convection solutions with strong Soret effect (Ψ=-0.60) with those of weakly Soret effect (Ψ=-0.11), we find that those with strong Soret effect are richer. Because of the complexity in convection with strong Soret effect, the convection solutions at Ψ=-0.60 are different from those at Ψ=-0.20, -0.4.
2014, 63 (10): 104204. doi: 10.7498/aps.63.104204
In this paper, the linear polarization light satisfied nonlinear coupled differential equations containing the Raman effect are utilized in a low birefringence fiber. The coupling model equation satisfied by the Stokes parameters is derived by introducing the Stokes parameters. Poincaré sphere is used to analyze the influence of Raman scattering effect on the state of polarization evolution in the low-birefringence fiber. The results show that the state of polarization evolution can be changed and the polarization ellipticity can also be changed due to Raman scattering effect in the low birefringence fiber when between the input power and motion constants satisfy a certain relation.
2014, 63 (10): 104218. doi: 10.7498/aps.63.104218
The research progress of the silicon-based photonic integration in the world and the results of our group in recent years are reported in this paper, including exploration of design and improvement on process of the optical transceiver modules, Ⅲ-V/silicon lasers and other integrated photonics. The silicon photonics is predicted to have a great prospect in many fields due to its good compatibility with complementary metal oxide semiconductor technology. It is concluded that the trend of photonic integration on silicon substrate is toward high data rate, low power consumption and large integration density, and the silicon based photonic integration will become a main research subject of silicon photonics in the future.
CONDENSED MATTER: STRUCTURAL, MECHANICAL, AND THERMAL PROPERTIES
Opto-electronic properties of the single-walled carbon nanotube film and melamine formaldehyde resin composite
2014, 63 (10): 106101. doi: 10.7498/aps.63.106101
The single-walled carbon nanotube (SWCNT) can strongly absorb light, particularly in the near infrared region, and can convert it into thermal energy, while the SWCNT also has a great capacity of converting thermal energy into electrical energy. In this paper, by means of vacuum filtering, SWCNT film is prepared by SWCNT arrays generated by the chemical vapor deposition. A simple experimental device of SWCNT is designed and the conversion from the infrared light to voltage output is successfully achieved. The SWCNT/melamine formaldehyde resin composite film is prepared by the functional steps. The experimental results show that the composite material can produce an output voltage of opposite sign, which indicates that the SWCNT and melamine formaldehyde resin composite material have good application prospect in the field of photo-electrical applications.
Inhibition effect of low-temperature pre-irradiation of helium ions on the growth of helium bubble in stainless steel：a Monte Carlo simulation
2014, 63 (10): 106102. doi: 10.7498/aps.63.106102
Zhang Chong-Hong group has found that pre-irradiation of helium ions at low temperature with very low dose could effectively suppress helium bubble growth in materials. This experimental result lacks theoretical explanation. In this paper, we use Monte Carlo simulation to study the effects of different temperature modes on helium bubbles based on a three-dimensional lattice gas model under continued particle injection, and find that the phenomenon that pre-irradiation of helium atoms at low temperature can suppress the helium bubble growth occurs in simulation results as it does in the experiments. The reason is that small helium bubbles with high number density are produced by pre-irradiation at low temperature, and the high number density of bubbles controls the increase of sizes of helium bubbles.
Preliminary experimental research of electron emission characteristics on surface area of two kinds of electrodes
2014, 63 (10): 106801. doi: 10.7498/aps.63.106801
The principle of measuring electron emission threshold of surface area metal electrode is presented. The measurement system of electron emission thresholds of two electrodes is developed. The electron emission characteristics of materials with different roughness degrees or Cr2O3 coating-deposit 40 um are measured in “Chenguang” accelerator. The results show that polishing can suppress electron emission. The higher the roughness degree of material, the lower the electron emission threshold is. Material coated with Cr2O3 has higher electron emission threshold than polished material.
CONDENSED MATTER: ELECTRONIC STRUCTURE, ELECTRICAL, MAGNETIC, AND OPTICAL PROPERTIES
First principles study of the effect of high V doping on the optical band gap and absorption spectrum of ZnO
2014, 63 (10): 107101. doi: 10.7498/aps.63.107101
Nowadays, the studies on optical band gap and absorption spectrum of V doped ZnO have presented two distinctly different experimental results, that is, the blue shift increases and decreases when the mole fraction of impurity increases in a range from 0.0417 to 0.0625. To solve this contradiction, according to the first-principles plane-wave ultrasoft pseudopotential of the density functional theory, we set up models for a pure ZnO cell and two supercells of Zn1-xVxO (x=0.0417, 0.0625) to calculate the total density of state, partial density of state, magnetism and absorption spectrum through using the method of GGA+U. The calculation results indicate that with the doping amount increasing from 2.083 at% to 3.125 at%, the magnetic moment of doping system increases and magnetism augments, too. Moreover, the volume of doping system increases, the total energy decreases and the formation energy becomes lower, thereby making the system more stable. Meanwhile, its optical band gap becomes wider, and the absorption spectrum shifts toward low energy. The calculation results are consistent with the experimental data.
2014, 63 (10): 107102. doi: 10.7498/aps.63.107102
Based on the first principles, we investigate the structures and electronic properties of fluorinated BC3, BC5, and BC7. Through the fluorination of BC structure, boron-carbon sheets are more stable than the hydrogenation. The results show that the system becomes semiconductor only on condition that the boron atoms can be bonded with the carbon atoms, whereas, the whole system will become the conductor when all atoms participate in the bonding. With the variation of fluorination degrees, semiconductor-metal transitions appear in the BC3 compounds and metal-semiconductor-metal transitions appear in the BC5 and BC7 sheet. Theoretical analyses find that pz orbital of boron atoms plays an important role in the electronic transition. Because of the rich electronic properties, this kind of fluorinated boron-carbon compound will become potential nanoelectronic materials and our results can play a role in guiding experiments.
Electronic structures, band-gap origins and magnetisms of Ti2Cr-based alloys with CuHg2Ti-type structure
2014, 63 (10): 107103. doi: 10.7498/aps.63.107103
The electronic structures, band-gap origins and magnetisms of Ti2Cr-based alloys with CuHg2Ti-type structure are studied using the first principles calculations. It is found that Ti2CrK (K=Si, Ge) alloys are semiconductors Ti2CrK (K=Sb, Bi) alloys are predicted to be half-metallic ferrimagnets and their half-metallic band gaps are affected directly by the S states of Sb and Bi atoms. Ti2CrSn alloy is a completely-compensated ferrimagnetic semiconductor. Due to the different band-gap origins of Ti2CrSn alloy in two spin directions, we can adjust the width of band gap by doping engineering. The ferrimagnetic spin-gapless materials are achieved by substituting Si or Ge for Sn. Substituting Fe or Mn for Cr, we gain a series of half-metallic materials. Ti2Cr1-xFexSn and Ti2Cr1-xMnxSn alloys are in ferrimagnetic states. All the half-metallic Ti2Cr-based alloys follow Mtotal=Zt-18 rule (Mtotal is the total magnetic moment and Zt is the valence concentration).
Surface and interface phonon-polaritons in four layer systems consisting of polar ternary mixed crystals
2014, 63 (10): 107105. doi: 10.7498/aps.63.107105
Surface and interface phonon-polaritons in a four-layer (vacuum/polar binary crystal slab/polar ternary mixed crystal slab/polar binary crystal substrate) system are investigated with the modified random-element-isodisplacement model and the Born-Huang approximation, based on the Maxwell's equations with the usual boundary conditions. The numerical results of the surface and interface phonon-polariton frequencies as functions of the wave-vector, composition x, and thickness of slab in the two four-layer systems, i.e., AlxGa1-xAs/GaAs and ZnxCd1-xSe/ZnSe, are obtained and discussed. It is shown that there are seven branches of surface and interface phonon-polariton modes in the heterostructure systems, and that the frequencies of the surface and interface modes vary non-linearly with the composition and thickness of slab. The “one mode” and “two mode” behaviors of the ternary mixed crystals are also shown in the dispersion curves.
Size and ternary mixed crystal effects on interband absorption in wurtzite ZnO/MgxZn1-xO quantum wells
2014, 63 (10): 107301. doi: 10.7498/aps.63.107301
Adopting a numerical method of solving self-consistently the Schrödinger equation and Poisson equation, the eigenstates and eigenenergies of electrons (holes) in a two-dimensional electron-hole gas are obtained for wurtzite asymmetric ZnO/MgxZn1-xO single quantum wells (QWs). In our computation, a realistic heterostructure potential is used, in which the influences from energy band bending, material doping and the built-in electric field induced by spontaneous and piezoelectric polarizations are taken into account. Furthermore, based on the Fermi's golden rule, the optical absorptions of electronic interband transitions in QWs, and their size and ternary mixed crystal effects are discussed. The results indicate that the increase of the Mg component in MgxZn1-xO enhances the build-in electric field, which forces electrons (holes) to approach to the left (right) barrier. This causes the interband transition absorption peak to decrease exponentially and to be blue-shifted. For different widths of QWs, the calculated results show that absorption peak decreases and transition energy shows a red shift with the increase of well width. The above conclusions are expected to give a theoretical guidance for improving the opto-electronic properties of materials and devices made of heterostructures with suitable optical absorption spectra and wave lengths.
A low on-resistance silicon on insulator lateral double diffused metal oxide semiconductor device with a vertical drain field plate
2014, 63 (10): 107302. doi: 10.7498/aps.63.107302
To reduce the on-resistance and enhance the breakdown voltage of silicon on insulator (SOI) lateral double diffused metal oxide semiconductor (LDMOS) device at the same time, a low on-resistance SOI-LDMOS device with a vertical drain field plate and trench gate and trench drain (VFP-TGTD-SOI-LDMOS) is proposed. The device has the features as follows: first, a trench gate and a trench drain are adopted, which can widen the vertical current conduction area, shorten the lateral current conduction path, and lower the on-resistance. Secondly, a vertical field plate is used, which modulates the electric field around it, reduces the high electric field at the end of the drain electrode, and increases the breakdown voltage. The VFP-TGTD-SOI device is compared with a conventional SOI device, a trench gate SOI device, a trench gate and trench drain SOI device with the same dimensional device parameters using the two-dimensional semiconductor simulator MEDICI. The results show that under the condition of their own highest figure of merit (FOM), the specific on-resistance value of the VFP-TGTD-SOI device is reduced by 53%, 23%, and increased by 87%, respectively and the breakdown voltage is increased by 4% and reduced by 9% and increased by 45%, respectively. By comparing the FOMs of the four structures, it can be seen that the VFP-TGTD-SOI device has the highest FOM, which indicates that among the four structures, it maintains the lower on-resistance and holds the higher breakdown voltage at the same time.
2014, 63 (10): 107701. doi: 10.7498/aps.63.107701
By using a traditional temperature reduction method, a large scale potassium dihydrogen phosphate (KDP) crystal is prepared. The refractive indices of different parts of this as-grown crystal are measured at 12 wavelengths between 0.253 and 1.530 um by using the new generation measuring instrument with an accuracy of 10-6. The measurement results show that refractive indices are inhomogenous in different parts of the large scale KDP crystal. The refractive index of the selected part close to the recovery area of crystal is smaller than that of selected part far from the recovery area. The deviation is on the order of 10-5-10-4. The refractive index inhomogeneity is related to the difference in crystalline quality among different parts in large KDP crystal. Furthermore, our measured data are compared with those in the literature published previously and the difference between them is analyzed. Meanwhile, by using root mean square method, numerical fitting to Sellmeier equation is conducted.
Surface-enhanced Raman scattering of gold/graphene oxide composite materials fabricated by interface self-assembling
2014, 63 (10): 107801. doi: 10.7498/aps.63.107801
The colloidal gold nanoparticles (AuNP) are synthesized by the classic Frens' method, and the sandwich-structured AuNP/graphene oxide/AuNP (AuNP/GO/AuNP) composite materials are constructed on the phosphorus doped diamond-like carbon film by the interface self-assembling. The surface enhanced Raman scattering behaviors of the AuNP/GO/AuNP composites are investigated by using the rhodamine B (RhB) as the probe molecules. Our results indicate that the Raman intensity of RhB obtained from the AuNP/GO/AuNP composites shows a 16.5-fold increase over that from the AuNP monolayer due to the coupled effect of chemical enhancement of GO and localized electromagnetic field enhancement of plasmonic gold. The designed composite materials with metal/GO/metal sandwich configuration exhibit great potential applications in biochemical analysis, environmental monitoring, disease controlling, and food safety.
2014, 63 (10): 107803. doi: 10.7498/aps.63.107803
In this paper, we calculate the extinction spectra and the distribution of electric near-field of the nanoparticles which are embedded a disk in a hollow square structure through using the discrete dipole approximation method, and compare our results with extinction spectra and the distribution of electric near-field of the single hollow square nanostructure. The research results show that a new resonant mode, which is located in the traditional excitation wavelength range of surface enhanced raman scattering, can be produced due to the coupling interaction. Then, we can use this mode to meet the shortage of the hollow square nanostructure fabricated by nanoskiving. In addition, the surface plasmon resonance peak can be tuned by changing the shape parameters of the silver nanoparticle. These results can be used in Raman scattering, molecular biological, and chemical detection.
2014, 63 (10): 107901. doi: 10.7498/aps.63.107901
Polyoxymethylene (POM) is a good absorber of CO2 laser, so it is important to study the ablation mechanisms of polymer materials. Because the laser impact phenomena are terribly complex, there is no general understanding of the mechanism of laser induced ablation of POM. An explicit thermal-chemical coupling model is presented in this paper, which takes account of laser heating, phase transition, thermal degradation, and plume emission. Random thermal degradation is adopted to describe the chemical reaction process when POM is heated up, and consequently, the components of the degradation products under different degradation rates are acquired. The group contribution method is used to evaluate the thermodynamic properties of the degradation products, and the normal boiling point and critical temperature of the product mixture are obtained by the mixing law. If the product temperature is lower than the critical temperature, POM is ablated in the manner of liquid evaporation; otherwise the ablation mechanism is gas-dynamics emission. As for the former, Knudsen layer relationship is employed to calculate the ablation mass; and for the latter, the conservation laws associated with the Jouguet condition are used. Based on the model, the quantitative results of ablation mass, ablation temperature, product component and mass rate of different ablation mechanisms vs. laser fluence are achieved and analyzed, which are consistent with the experimental data quite well.
Abnormal variation of optical properties of vanadium oxide thin film at semiconductor-metal transition
2014, 63 (10): 107104. doi: 10.7498/aps.63.107104
The optical properties of vanadium oxide thin film are measured at semiconductor-metal transition, including reflectance and transmittance results at different wavelengths which show different trends during the phase transition. With a multi-level reflection-transmission model of incoherent light, we calculate the values of refractive index n and extinction coefficient k at different wavelengths, and show that the abnormal optical properties result not only from the dependences of n and k on the wavelength, but also from multiple reflections in the absorbing film.
INTERDISCIPLINARY PHYSICS AND RELATED AREAS OF SCIENCE AND TECHNOLOGY
We modify the deterministic Eigen model of species evolution into a randomized model in order to render the Eigen model more realistic for the description of species evolution. In the framework of the Eigen model, we regard the locus mutation rate of a genetic sequence as a Gaussian distributed random variable. Thus the Eigen model turns into a random model. In this randomized model, we can see that when the fluctuation strength of the mutation rate is small, the error threshold of the quasispecies changes slightly and still serves as a phase transition point. However, when the fluctuation strength becomes large, the error threshold shifts from a phase transition point into a crossover region. Since the error threshold in the real species evolution is a crossover region, we should consider the upper limit of the crossover region when dealing with practical problems.