Vol. 56, No. 5 (2007)
The study of symmetries and conserved quantities for one class of linearly coupled multidimensional freedom systems
2007, 56 (5): 2475-2478. doi: 10.7498/aps.56.2475
In this paper, we eliminate the coupled terms in Lagrangian firstly by changing the coordinate scales and rotating the coordinate axes, and obtain the conserved quantities in new coordinates directly. According inverse transform of the coordinates, we can obtain the conserved quantities in original coordinates, the Noether symmetry and Lie symmetry of the infinitesimal transformations of conserve quantities are studied in this paper，and an example is given to illustrate the application of the result.
The purpose of this paper is to study the time-integral theorems for the Birkhoff systems. A time-integral identity of the systems is presented. The power-like equation, the Pfaff-Birkhoff integral variational principle and the Pfaff-Birkhoff-d′Alembert differential variational principle of the systems are deduced by using the time-integral identity.
2007, 56 (5): 2482-2487. doi: 10.7498/aps.56.2482
The spatial distribution of Thomson scattering in an ultrashort laser pulse is investigated. It is found that the spatial distribution of the radiation depends sensitively on the carrier-envelope (CE) phase η0 of the ultrashort driving laser pulse. At η0=0,π, the spatial distribution of the radiation shows fourfold or twofold symmetry, while the symmetry breaks down at other CE phases. The collimation of the radiation also depends sensitively on the CE phase, and it reaches the best at η0＝±π/2. These results suggest that it is possible to control the spatial distribution of Thomson scattering by varying the CE phase of the driving laser pulse, or the CE phase of the ultrashort laser pulse can be measured by utilizing the phase-dependent spatial symmetry of Thomson scattering.
2007, 56 (5): 2488-2492. doi: 10.7498/aps.56.2488
In this paper, sampled-data synchronization of continuous chaotic systems is proposed from the viewpoint of digital measurement of continuous chaotic systems. Based on the results of control theory, a sampled-data synchronization system of Lorenz system families is designed. The synchronization system can synchronize the driven system at sampling points by sampling only one state variable. Numerical simulations illustrated the effectiveness of the synchronization system.
Research on a proportional-integral-derivative neural network decoupling control based on genetic algorithm optimization for unified chaotic system
2007, 56 (5): 2493-2497. doi: 10.7498/aps.56.2493
An improved genetic algorithm (IGA) was proposed. It can optimize the proportional-integral-derivative(PID) neural network decoupling controller's connecting weight value, so that it makes the PID controller's parameser to be optimized and realizes the decoupling control of multivariate nonlinearity systems. The IGA is superior to the elementary genetic algorithm. In the PID controller's parameter optimization, the IGA uses less calculations, is more efficient, and faster in convergence. When the optimized PID controller was applied to unified chaoticsystems, good control results were obtained by simulation experimentation, so t was proved that the PID controller when applied to unified chaotic systems wa effective.
2007, 56 (5): 2498-2503. doi: 10.7498/aps.56.2498
This paper analyses the projective synchronization in autonomous chaotic system. Base on the stability criterion of linear systems, a new approach for constructing chaotically projective synchronization is proposed. The projective synchronizations of Lorenz system, Rssler system and hyperchaotic chen's system are achieved via the linear separation method. Numerical simulations are provided for illustration and verification of the proposed method.
2007, 56 (5): 2504-2516. doi: 10.7498/aps.56.2504
The response of the Maglev system with delayed position feedback control under the sub-harmonic excitation of the flexible guideway is investigated. The dynamical model is linearized at the equilibrium. Employing time delay as its bifurcation parameter, the condition under which the Hopf bifurcation may occur is investigated. Center manifold reduction is applied to get the Poincar normal form of the nonlinear system with guideway disturbance so that we can study the relation between periodic solution and system parameter. The sub-harmonic resonant periodic solution of the normal form is calculated based on the method of multiple scales, and we get the bifurcation equation of the free oscillation. The existence condition of the free oscillation in the solution is analyzed. Relationship between periodic solution and control and excitation parameters is also investigated. Finally numerical method is applied to study how system and excitation parameters affect the system response. It was shown that the critical time delay to keep the response of the system stable is less than that without perturbation. Time delay can not only suppress sub_harmonic resonance, but also control the appearance of the chaos. Control parameter can govern the emergence of the free oscillation and affect the amplitude of the forced oscillation. So carefully selecting the system parameters can restrain the oscillation effectively.
2007, 56 (5): 2517-2522. doi: 10.7498/aps.56.2517
In this context, we introduce anticipation headway to modify the Noise-First NaSch model under condition of periodic boundary. The traffic situation with different parameters can be numerical simulated. It was indicated that the Noise-First model with added anticipation headway is more realistic in reflecting traffic flow. Not only start-stop wave and synchronized flow but also the metastable state can be obtained by numerical simulation. The modified model can reproduce nonlinear phenomena which tallies with the real traffic.
2007, 56 (5): 2523-2528. doi: 10.7498/aps.56.2523
As the conventional matrix converter has the drawback of low voltage transfer ratio, a novel matrix converter referred to as a boost AC-DC-AC matrix converter is proposed. The basic configuration of the new topology and its fundamental principles are firstly introduced, the analytic expression concerning functional relation of voltage transfer ratio to duty cycle is analyzed, and the design method of double-loop control strategy adopted is expounded particularly. Finally the validity and feasibility of the new topology are tested by simulation, and the results indicate that the modulation of voltage transfer ratio and output frequency can be realized optionally through this new converter and an ideal sine output wave with little distortion can be gotten, so that the inherent drawback of conventional matrix converter having low voltage transfer ratio is solved successfully. This work may give some hints on further technological research and engineering application.
2007, 56 (5): 2529-2536. doi: 10.7498/aps.56.2529
The uncooled IR imaging technology has wide applications. Using the optical readout uncooled IR imaging system built in our laboratory, thermal images of human body are obtained successfully by placing a bi-material micro-cantilever array (FPA) without silicon substrate in air. By comparing the IR imaging results obtained by placing FPA under different vacuum pressures, the pressure dependence of the thermal conductance and system noise is modeled, and the influence to the system performance is analyzed. The model of thermal conductance of gas is analyzed and verified experimentally on condition that the mean-free path of the gas molecules is larger than the gas gap between the bi-material cantilever and the wall of vacuum chamber. Experimental analysis indicates that for FPA working in air, the random vibration noise of the cantilever impacted by the gas molecules' thermal motion is the main source of the system noise. When the pressure decreases to lower than 10-2 Pa, the influence of gas conductance can be neglected, the total thermal conductance decreases to the limit of radiative conductance, and the random vibration noise of the cantilever is reduced to a minimum. The experimental results and theoretical analysis agree well.
2007, 56 (5): 2537-2540. doi: 10.7498/aps.56.2537
The colorimetric properties of the conventional arc lamp and LED are calculated and analyzed by using the colorimetry theory. Comparing to the conventional arc lamp used as projection light source, LED illuminator has higher efficiency of the spectral power and better ability of color display. It can reach sRGB standard widely used by digital projectors. The single-color LED driven by temporal pulses will produce high-brightness projection display with saturate colors. Therefore, LED can take place of conventional arc lamp to satisfy the colorimetric requirement of the projection display.
2007, 56 (5): 2541-2546. doi: 10.7498/aps.56.2541
In this article, thermal evaporation technology was employed to deposit erbium fluoride films on germanium(111) substrates. XRD diffraction pattern shows that films turn from amorphous to crystalline with the increase of substrate temperature. Correspondingly, the morphology and infrared optical properties change obviously. The infrared transmission spectrum of the partially crystalline film is homologous to the totally amorphous film but not the crystalline films. Crystal lattice parameters calculation indicates that there is great compression thermal stress in the erbium fluoride film deposited at high deposition temperature. The refractive indices and extinction coefficients of the films were calculated by fitting the infrared transmission spectrum using Lorentz oscillator model. The refractive index and extinction coefficients at 10μm for the amorphous erbium fluoride film are n=1.38 and k=0.01, and for crystalline films are n=1.32 and k=0.006, respectively.
ATOMIC AND MOLECULAR PHYSICS
MRCI potential curves and analytical potential energy functions of the low-lying excited states (1∏，3∏) of ZnHg
2007, 56 (5): 2547-2552. doi: 10.7498/aps.56.2547
The multireference configuration interaction (MRCI) electronic energy calculations with effective core potentials have been carried out for two low-lying excited states (1∏,3∏) of ZnHg dimer. Potential energy curves (PECs) are generated. The PECs are fitted to analytical potential energy functions (APEFs) using the Murrel-Sorbie potential function. The computational equilibrium interatomic distances and dissociation energies are compared with the theoretical values available in the literature. Some important spectroscopic parameters are calculated. Based on the PECs, the vibrational levels of the two states are predicted by solving Schrdinger equation of nuclear motion.
2007, 56 (5): 2553-2558. doi: 10.7498/aps.56.2553
A single-parent genetic algorithm (SPGA) is developed from the traditional genetic algorithm to optimize cluster structures. Two mutation operators are used to find the global minimum on the potential surface. The algorithm for optimizing the structure of molecule clusters is proposed. Combined with the TIP3P potential energy, the structures of water clusters (H2O)n(n≤14) is studied using SPGA. The stable structures obtained are in complete agreement with the reported results obtained theoretically and experimentally. The calculation results show that SPGA is an efficient and reliable method to find the global minimum structure of molecular clusters.
2007, 56 (5): 2559-2569. doi: 10.7498/aps.56.2559
A spectrum-, temporal- and spatial-resolved physical model of LD pumping dynamics is set up, which includes the wavelength chirping and transient behavior of the power output of LD arrays. Based on this model, we investigate in detail the spectrum selective absorption characteristics from LD output by laser materials with different absorption bandwidths. Also the pumping excitation performances for typical three kinds of laser crystals, Yb:YAG, Yb:S-FAP, and Nd:YAG, are investigated specifically for the pulsed-energy-storage operation mode. Finally, we have optimized the initial central wavelength of LD arrays for the above three crystals and compared their sensitivity to the wavelength chirping of LD arrays.
2007, 56 (5): 2570-2576. doi: 10.7498/aps.56.2570
One-photon and two-photon absorption properties of a double-conjugated-segment molecule (BSBAB) composed of two single-conjugated-segment molecules are studied by using density functional theory at ab initio level. The optimized geometrical structure shows that the two single-conjugated-segment parts in molecule BSBAB retain their own coplanarity and are nearly perpendicular to each other, which seems to indicate that the molecule BSBAB contains optical characteristics of both parts. The molecule BSBAB is found to have three two-photon absorption peaks in lowest energy region which are contributed respectively the two single-conjugated-segment parts respectively and the coupling between them. This also demonstrates that the molecule BSBAB is a broadband strong two-photon absorption chromophore. The response theory approach at Hatree-Fock level is used to prove the convergence of results given by the state summation method with truncated approximation. The charge-transfer process between the charge-transfer states is visualized. The theoretical results are in good agreement with the measurements.
2007, 56 (5): 2577-2584. doi: 10.7498/aps.56.2577
An anisotropic intermolecular potential of the He-HF complex has been obtained by utilizing the BFW analytic function to fit the intermolecular energy data, which have been calculated at the theoretical level of the single and double excitation coupled-cluster method with noniteractive perturbation treatment of triple excitation CCSD (T). And the reliability of the potential has been verified by comparing it with other potential models. The differential, partial and total scattering cross sections for collisions between He atoms and HF molecules have been calculated respectively by using the quantum close-coupling method at five different potential models, and the calculated results have been compared and analysed in detail. The research shows that the scattering cross sections are sensitive to the position where potential is zero, the well depth, the strength of the repulsive wall of the spherically averaged and to the anisotropy of the region of the well depth. We hope that the results can provide profitable reference for determining exactly the interaction potential of the atomic-molecular collision systems from the scattering cross sections.
2007, 56 (5): 2585-2589. doi: 10.7498/aps.56.2585
The absolute differential cross sections of e-Ar scattering in laser field are calculated employing the second Born approximation with the static screen potential including polarization potential and the single static screen potential in the special scattering geometry that the incident electron beam is parallel to the polarization direction of laser field. The second Born approximation gives better results than the low-frequency whem formula when compared with the experimental data, and it is found that the electron-atom polarization potential plays an important role in laser-assisted electron-atom scattering.
Taking account of spin-multiplicity states，the ground-state structures of FeBN(N≤6)clusters have been predicted using the generalized gradient approximation(GGA) density functional theory. The results indicate that the spin-multiplicity states of the ground-state clusters are respectively 4,3,2,1,2 and 1，and FeB4 is the most stable of the clusters. Also, we studied magnetic properties of the ground-state clusters systematically and found that the magnetic moment of Fe atom and total magnetic moment are decreasing with the increasing of cluster size except FeB5.
Ground-state structures and magnetisms of GenFe(n=1—8) clusters: The density functional investigations
2007, 56 (5): 2596-2602. doi: 10.7498/aps.56.2596
The geometries, total energies, and frequencies of GenFe(n=1—8) clusters have been systematically investigated using density functional theory with the generalized gradient approximation, and the equilibrium geometries at different spin multiplicities as well as the ground-state structures have been determined. The calculated results indicate that the average binding energy of the GenFe clusters is obviously higher than that of the corresponding pure Gen clusters, which implies that the doping of Fe atom can enhance the stability of the germanium clusters. Except for Ge2 with triplet multiplicity, the ground-state structures of pure germanium clusters are spin singlet; however, the GenFe clusters are all triplet. We also systematically studied magnetic properties of GenFe clusters and found that the total magnetic moment of cluster stabilizes at about 2μB (except for Ge8Fe with total moment of 2.391μB), and the magnetic moment of Fe atom stabilizes around 2.5μB.
CLASSICAL AREA OF PHENOMENOLOGY
2007, 56 (5): 2603-2607. doi: 10.7498/aps.56.2603
A new type of high power microwave device is developed as a quasi-bitron. The device is simple and composes of two parts, the modulation cavity and the extraction cavity, with a coupling between the cavities. The structure has two working modes, one of them is the working mode, and competition between modes can be easily avoided even when the RF field in the device is very strong compared with the BWO (backward wave oscillator). Besides the theoretical analysis, the model has been realized on 2.5 PIC (particle in cell) simulation code. A 9.42GHz bitron oscillator is designed and its theoretical efficiency is 29.4%. The simulation shows the device achieves the efficiency 28% when the beam is 800 kV and 10 kA, and outputs RF power of 2.25GW. The experiment was also carried out on Sinus-700 accelerator. The measured microwave power is 2.44GW at the frequency 9.4GHz.
Investigation of a lossless metamaterial covered conductor cylinder illuminated by electric-line-source
2007, 56 (5): 2608-2615. doi: 10.7498/aps.56.2608
The electromagnetic characteristics of the model for an electric line source radiating parallel to a lossless metamaterial covered infinitely long conductor circular cylinder is investigated. First, the electromagnetic model is presented and exact solution is derived. Second, the exact solution is employed in numerical calculations. The figures of the near field under different geometric and electromagnetic parameters are obtained; the characteristics of the far field properties are also studied through the directivity and normalized radiation resistance. Because of the negative refraction of the metamaterial, the distinct “focus”, which could not be seen in the conventional material, could be obtained in the material layer from the near field pattern. The presented electromagnetic model is compared with the former approach and its validity is proven.
2007, 56 (5): 2616-2622. doi: 10.7498/aps.56.2616
Based on the model of the Maxwell's equations coupled with the rate equations of electronic population, the lasing threshold of optical modes in partially random media is analyzed using the finite difference time domain method. The characteristics of random laser can be described by some factors, such as the strength of randomness, size of medium,etc.,which determine the lasing threshold in random medium. The simulated results show that one or more lasing modes appear when the pumping rate Pr increases beyond the threshold, and the number of modes will increase with random strength and size of medium. In addition, the simulated results demonstrate that the lasing threshold value will attain the minimum value under certain random strength and size of medium, which is at variance with the theoretical result in one-dimensional completely random medium. Such properties of the medium show potential advantage in future application for optical integration.
Coherence vortices in partially coherent beams consisting of a superposition of Laguerre-Gaussian modes
2007, 56 (5): 2623-2628. doi: 10.7498/aps.56.2623
The coherence vortices in a new class of partially coherent beams with separable phase recently proposed by Bogatyryova et al. are studied in detail. The new beams are generated by taking an incoherent superposition of Laguerre-Gaussian (LG) modes with equal azimuthal index. It is shown that the mode indices and weight factors of superposed LG modes, as well as the reference choice, affect the position where the circular edge dislocation takes place, and result in the vortex disappearance or the appearance of more than one coherence vortices. Furthermore, there also exist coherence vortices in partially coherent beams with non-separable phase consisting of an incoherent superposition of LG modes.
2007, 56 (5): 2629-2636. doi: 10.7498/aps.56.2629
Security analysis of optical encryption system based on double random phase encoding indicates that the system can be classified as a linear symmetric block-cipher cryptosystem, which may lead to a great vulnerability. Under the ciphertext-only attack (COA), an opponent can attack such a cryptosystem only on the basis of estimated support of wave function in the object plane with iterative phase retrieval methods, and subsequently deduce the phase keys in the Fourier plane easily. The ciphertext-only attack (COA) requires much less resources than other types of attacks. Estimated support of wave function in the object plane could have some translations relative to the true support, so retrieved wave function could also have translations in both the amplitude and the phase, leading to a translation of retrieved plaintext relative to original plaintext. However, attackers can take this translation as a priori knowledge to traverse estimated support in the object plane until finding the best estimated keys, which bring about the best decryption quality.
2007, 56 (5): 2637-2646. doi: 10.7498/aps.56.2637
By using the coupled mode equations, the characteristics of THz wavelength converter based on four-wave mixing (FWM) in λ/4 phase-shifted distributed feedback laser diode (QWS-DFB -LD) are analyzed. The numerical simulation indicates that, larger biasing current and smaller detuning expand the dynamic range of conversion efficiency and extinction ratio, but worsen the frequency chirp. Due to the lasing effect, the peak modulation frequency of extinction ratio and chirp occur at the relaxation oscillation frequency. Good conversion characteristics can be obtained by selecting system parameters reasonably. This configuration would have potentials in optics communications because of its wide-band, high-speed and without pump input.
The atom and field linear entropies of a two-level atom interacting with gray-body field are investigated. The effect of the atomic initial state, detuning, intensity of the incident field, absorptivity of the cavity and temperature of the system on the atom and field linear entropies are dicussed.
2007, 56 (5): 2652-2656. doi: 10.7498/aps.56.2652
A kinetic model is developed to describe the Ne-CuBr UV laser action in a longitudinal pulsed discharge. The influence of main parameters on the laser output characteristics is numerically calculated. The dependence of laser intensity on the buffer gas pressure, the operation temperature and the tube radius is found out and the experimental results are explained quantitatively. The modelling results can provide a reference to improve the laser output characteristics.
2007, 56 (5): 2657-2662. doi: 10.7498/aps.56.2657
A new solid-state laser material——Yb：Y2O3 transparent ceramic based on nano crystalline technology and the vacuum sintering (NTVS) method is reported. The manufacture technology, the physical and chemical properties, the energy level split structure and the spectra characteristics of Yb：Y2O3 ceramic media are discussed and compared with those of Yb:YAG single crystal. Further more, with the compact active mirror laser (CAMIL) design, the laser parameters of Yb:Y2O3 ceramic are demonstrated at room temperature. An optimum output power of 10.5W has been obtained under the max imum pumping power of 35 W. Its corresponding slope efficiency was 37.5%. The laser wavelength red-shift was observed with the increasing of the pump power and the reflectivities while the decrease of the radium of output coupler (ROC). In fact, Yb：Y2O3 ceramic is an ideal laser material, it has excellent physical and chemical properties and spectra characteristics comparable with those of the Yb:YAG single crystal. Further more, its thermal conductivity and emission bandwidths are nearly twice those of Yb:YAG, which shows that it is a promising candidate in high power and femto-second pulsed laser applications.
2007, 56 (5): 2663-2669. doi: 10.7498/aps.56.2663
The laser diode pumped two-frequency solid-state laser with tunable frequencydifference is investigated, the medium is a 1at% doped 2-mm thick Nd:YAG chip. The condition of stable two-frequency oscillation operation is analyzed, the polarization is calculated by Jones matrix in different locations at the resonator. It is proved that the two-frequency electric field vectors are orthogonally polarized everywhere, the frequency difference tuning can be realized by changing the angle between the fast axes of two quarter-wave plates. In the experiment, single-frequency oscillation was obtained with the help of an etalon, then longitudinal mode frequency splitting resulted in the two-frequency oscillating operation by inserting two quarter-wave plates. The obtained maximum frequency difference was 1.3GHz, the minimal was 50MHz because of the strong mode competition. Due to the loss anisotropy of the etalon, the output power is not very stable when tuning the frequency difference, 85mW two-frequency output power was obtained with 100MHz beat note.
Two-photon asymmetric color-locking second-order stochastic correlation of attosecond polarization beats
2007, 56 (5): 2670-2677. doi: 10.7498/aps.56.2670
Based on the phase-conjugation polarization interference between two two-photon processes, we theoretically investigated the attosecond scale asymmetry sum-frequency polarization beat in four-level system (FASPB). The field correlation has weak influence on the FASPB signal when the laser has narrow bandwidth. Conversely, when the laser has broadband linewidth, the FASPB signal shows resonance-nonresonance cross correlation. The two-photon signal exhibits hybrid radiation-matter detuning terahertz damping oscillation, i. e., when the laser frequency is off resonance from the two-photon transition, the signal exhibits damping oscillation and the profile of the two-photon self-correlation signal also exhibits zero time-delay asymmetry of the maxima. We have also investigated the asymmetry of attosecond polarization beat caused by the shift of the two-photon self-correlation zero time-delay phenomenon, in which the maxima of the two two-photon signals are shifted from zero time-delay point to opposite directions. As an attosecond ultrafast modulation process，FASPB can be intrinsically extended to any level-summation systems of two dipolar forbidden excited states.
Study on the rapid numerical difference recurrence algorithm for the simulation of optical pulse propagation and its applications
2007, 56 (5): 2678-2683. doi: 10.7498/aps.56.2678
Maclaurin expansion near zero domain is applied to analysis the difference form of the normalized nonlinear Schrdinger equation in frequency domain, and a rapid numerical difference recurrence formula is deduced in time domain, which considers the nonlinear Kerr effect as well as the effect of chromatic dispersion in optical medium simultaneously. Through some examples, a comparison is made between the results using the algorithm of this paper and the known analytical results, including the results of SSFM method. The calculated results show that this method has not only fairly fast calculation speed, but also very high calculation precision, and is more suitable to the actual situation of light pulse propagation affected by the dispersion and nonlinearity at the same time through optical medium. All the above results show that it is a well founded, fast and effective numerical calculation method to study light pulse propagation in optical medium.
2007, 56 (5): 2684-2688. doi: 10.7498/aps.56.2684
This paper reports the fabrication of the periodically poled 6mol% MgO doped LiNbO3 (PPMgLN) and the demonstration of the PPMgLN-based optical parametric oscillators (OPOs). The PPMgLN wafer was fabricated by high voltage pulse trigged domain reversal technique with periods ranging from 27.8—31.6μm by a step of 0.2μm. The OPO has a wide spectral tuning range from 1.42—1.73μm (for the signal) and 2.76—4.27μm (for the idler) while the wavelength of the pump is 1.064μm. In the experiment, the output power of 4.8W (the sum of the signal and the idler) was achieved when the input pumping power was 10.6W and the slope efficiency was computed to be 47%.
2007, 56 (5): 2689-2692. doi: 10.7498/aps.56.2689
The stimulated Raman scattering (SRS) and luminescence spectra of ZnWO4 crystals were investigated using 532nm laser excitation at room temperature. It was found that the first order (558.7nm) and second order (588.6nm) of SRS have 130 and 77cm－1 line widths and the pump threshold at 558.7nm is 6.8mJ. The luminescence spectrum of ZnWO4 crystals under 532nm laser excitation has the peculiarity of having the band structure composed of three-Gaussian components. The photoluminescence spectrum shows broadband emission from 400nm to 650nm peaked about 472.0nm, which is attributed to the radiative transitions between tungsten and oxygen.
Research on selective optical amplification of Brillouin spectrum of weak scattering signals in water
2007, 56 (5): 2693-2698. doi: 10.7498/aps.56.2693
The principle of selective optical amplification of weak scattering signal in water based on Brillouin amplification is presented. Based on the theoretical model of Brillouin amplification with frequency detuning, the influences of linewidth and frequency detuning of Stokes seed beam on Brillouin amplification are analyzed. The dependence of seed amplification factor on water temperature and seed power density are investigated. The theoretical calculations coincide with experimental results. Researches show that, taking the same frequency detuning into account, amplification factor of a spectrally broadened seed beam is higher than that of a narrow-linewidth signal beam. When 20℃ CS2 is used as amplifier medium, Stokes seed signals produced by different-temperature seawater are efficiently amplified. At the same pump power density, the amplification factor decreases with increasing signal power density. The amplification factor exceeds 102 for signal energy of 5μJ.
2007, 56 (5): 2699-2703. doi: 10.7498/aps.56.2699
The Raman depolarization ratio ρ can be determined accurately by polarization-resolved coherent anti-stokes scattering (CARS) technique (J. Raman Spectrosc., 2000, 31: 725). The polarized CARS spectra are recorded at a series of polarization angles φd, and simulated by a Lorentz bandshape to obtain signal amplitude HR of the resonant part. From the dependence of HR on polarization angle φd, the angle φ0d corresponding to HR=0 can be determined. So the Raman depolarization ratio can be obtained from the relationship ρ＝-1/(tanθtanφ0d), where θ is the polarization angle between Pump beam and Stokes beam. In this report, we introduce another method to process the data from polarized CARS spectrum, namely the method of intersecting point. Compared with the original method, our method needs not any knowledge of frequency distribution of the CARS spectrum, and is simpler.
Least action principle for dissipative solitons and its application in two-dimensional PBG structure
2007, 56 (5): 2704-2708. doi: 10.7498/aps.56.2704
Dissipative and coherent effects of X-point solitons are investigated on the basis of dynamic variational formulation in two-dimensional photonic bandgap (PBG) structure of square lattice. Through the least action principle, the result deduced from analyzing soliton parametric equations indicates that dissipative effect leads to not only amplitude decrease but also section expansion of solitons. In addition, co-soliton interaction for negative potential is decreased by dissipation, and bound sate of solitons in x direction is also weakened. Amplitude and period of fluctuation of relative interval between soliton centers increase in x direction, when x increases to some value, relative interval will increase continuously.
2007, 56 (5): 2709-2713. doi: 10.7498/aps.56.2709
The gain curve in periodically poled LiTaO3 crystal (PPLT) was calculated for type 0 (e+e→e) quasi-phase matching (QPM). A high-gain amplifier centered at 1064nm based on degenerated optical parametric chirped-pulse amplification was built. The gain of ～106 and ～10.3% conversion efficiency were achieved with low pump energy of several hundreds of μJ. The experimental results are consistent with the simulation.
2007, 56 (5): 2714-2718. doi: 10.7498/aps.56.2714
The upconversion luminescence spectra of Tm3+/Yb3+-codoped oxyhalide tellurite glasses have been investigated, and the action mechanisms of Tm2O3 content on upconversion luminescence of Tm3+/Yb3+-codoped oxyhalide tellurite glasses were analyzed. The results showed that there exists concentration quenching of Tm3+ in oxyhalide tellurite glasses. With increasing Tm2O3 content, upconversion blue and red emission intensities of Tm3+ first increase, reach its maximum at Tm2O3%=0.1mol%, and then decrease. The obtained results are conducive to increase upconversion luminescence efficiency of Tm3+.
2007, 56 (5): 2719-2724. doi: 10.7498/aps.56.2719
The breaking of universal symmetry of electromagnetic field distribution in an anisotropic magnetoelectric material will give rise to nonzero vacuum momentum. This may lead to the transfer of momentum between the anisotropic quantum vacuum and the magnetoelectric material. Very recently, Feigel considered the quantum vacuum contribution to the momentum transfer effect ［Phys. Rev. Lett. 92 (2004) 020404］. An alternative approach is proposed based on the eigenvector equation of electromagnetic field to calculate the total mechanical contribution of all anisotropic quantum-vacuum modes to the material momentum. It is suggested that the said macroscopic mechanical effect of quantum vacuum on the anisotropic material can be detected by current technology (e.g. fiber optical sensor), which can measure nanoscale velocity. Physical mechanism of such quantum vacuum effects and potential applications are discussed.
2007, 56 (5): 2725-2730. doi: 10.7498/aps.56.2725
Glasses with compositions (99.5-χ) GeO2-χBaO-0.5Bi2O3 (χ=3, 6, 9 mol%)and (99.5-φ)GeO2-φWO3-0.5Bi2O3 (φ=3，6，9 mol%)were prepared by conventional melting method. Their emission spectra (using 800nm LD excitation), fluorescence decay curves, absorption spectra and DTA curves were measured. Concomitant with the increase of WO3 concentration in the glasses system GeO2-WO3-Bi2O3, were the enhancement of glass thermal stability and emission intensity (centered at 1260nm) as well as the increase of FWHM and lifetime, and along with the increase of BaO concentration in the glass system GeO2-BaO-Bi2O3, were the enhancement of emission intensity (centered at 1290nm), the increase of FWHM and the prolongation of lifetime. In addition, in both groups, the absorption edges showed a red-shift with the increasing amount of WO3 or BaO, indicating that the Bi5+ ions may be responsible for the super broadband emission. In particular, the values of σp×τ and σp×Δλ parameters deduced from the emission properties suggested that the prepared glasses are a kind of promising material for optical amplifier covering the O to S communication bands.
2007, 56 (5): 2731-2735. doi: 10.7498/aps.56.2731
In this paper, a kinetic model for the radiation-induced production of the intrinsic point defect E′ center is proposed. The results show that E′ center concentration increases linearly with dose in low dose regions, and it tends to saturate in high dose regions. Further, it is found that E′ center is dual exponential function of dose, and the saturate state collapses if the second reaction is considered. The theoretical result is in good agreement with the experiment.
2007, 56 (5): 2736-2741. doi: 10.7498/aps.56.2736
This paper reports on the spectroscopic properties of ultraviolet laser photosensitive Er3+-doped bismuth-silicate glasses. Two series of the glasses Na2O-Bi2O3-SiO2-Er2O3 and Na2O-B2O3-Bi2O3-SiO2-Er2O3 have been prepared by conventional melting and quenching technique. The absorption spectra, emission spectra and lifetime of Er3+ ion have been measured. The optical parameters have been analysed by Judd-Ofelt theory. It is found that the full width at half maximums (FWHMs) increase with increase of the value of Bi2O3/SiO2 and B2O3 content. Meanwhile, the lifetimes of 4I13/2 level of Er3+-doped glasses decrease with increasing Bi2O3 and B2O3 content. When the content of B2O3 is 40%, the measured stimulatied emission cross section, lifetime and FWHM of 4I13/2→4I15/2 transition of Er3+-doped Na2O-B2O3-Bi2O3-SiO2 glass are 8.49×10－21cm2, 0.52ms and 78nm respectively.
2007, 56 (5): 2742-2746. doi: 10.7498/aps.56.2742
The resonant frequency splitting caused by photonic crystal (PC) defect coupling can be well described by the tight-binding (TB) approximation in which only two coupling parameters are used. However, the description of the resonant frequency shift that represents the difference between the center resonant frequency of the coupled structure containing odd number of defects and the resonant frequency of a single defect, requires the employment of the rigorous TB that involves three coupling parameters. By using the relation between the coupling parameters and the resonant frequencies, we extract the values of the three coupling parameters from the simulation of a three-coupled-defect structure and use them to predict the resonant frequency shift and splitting of the coupled structures consisting of arbitrary number og PC defects. They are in good agreement with the simulation results based on the finite-difference time-domain method.
Calculation of photonic band gap and first Brillouin zone for two-dimensional monoclinic lattice photonic crystal
2007, 56 (5): 2747-2750. doi: 10.7498/aps.56.2747
Two-dimensional (2D) monoclinic lattice photonic crystal has potential application for light focusing device and photonic crystal waveguides. In this article, the first Brillouin zone (BZ) and the method of calculating the photonic band gaps of two-dimensional monoclinic lattice photonic crystal are discussed. The photonic band gap (PBG) structure of triangular lattice photonic crystal calculated by the method described in this article agrees well with that from the conventional method. Furthermore, the calculated PBG structure of 2D monoclinic lattice in critical conditions are consistent with either hexagon or rectangle BZ, which shows that the method described here is valid.
Improving the optical profile of electroabsorption modulator using single-sided large optical cavity structure
2007, 56 (5): 2751-2755. doi: 10.7498/aps.56.2751
An electroabsorption modulator with large optical cavity was designed and fabricated successfully. Both the simulated and experimental results show that, the larger optical cavity structure introduced could obviously improve the optical profile of EA modulator, the traditional elliptical near-field spot becomes more rounded, so it will match better with the optical fiber and is beneficial for raising the coupling efficiency.
Measurement of carrier-envelope phase of few cycles Ti:sapphire laser by difference frequency technique
2007, 56 (5): 2756-2759. doi: 10.7498/aps.56.2756
The self-reference technique is widely used to measure the carrier-envelope phase offset (CEO) of femtosecond laser pulses in the normal femtosecond laser frequency comb system. However, the instability of the frequency comb system is increased because the photonic crystal fiber is introduced into the system to broaden the spectrum of femtosecond laser pulses outside the cavity. In this paper, we presented another method to measure specifically the CEO for the broad spectrum Ti:sapphire oscillator with the pulse duration of 7fs, namely the difference frequency technique, with which we have obtained the beat frequency signal with a S/N ratio greater than 31dB. This provided a good precondition for the next generation of the femtosecond laser frequency comb system without the photonic crystal fiber.
2007, 56 (5): 2760-2764. doi: 10.7498/aps.56.2760
Phase controlled Femtosecond Ti:sapphire laser frequency comb have been widely used in measurement of absolute optical frequency, which is the revolutionary progress in the optical metrology. In this paper, based on the home-made 90MHz femtosecond Ti:sapphire laser, we firstly broadened the spectrum output directly from the laser to more than one octave with a piece of photonic crystal fiber，then locked precisely both the repetition rate and the carrier-envelope phase offset frequency to the same Cs clock with the stability of 6×10－14 by use of two sets of different phase-locked loops. As a result, we obtained the phase controlled Femtosecond laser frequency comb with the stability of the Cs clock.
2007, 56 (5): 2765-2768. doi: 10.7498/aps.56.2765
Spectrum evolution in the filament was experimentally demonstrated with a 420μm diameter, 18cm length hollow fiber. The experimental results show that the spectrum evolution in filamentation result from the different pulse duration along the filamentation. In our experiment, at a particular position of the filament, the pulse can be compressed to 24.8fs from 50fs of the incident pulse.
The theoretical study and numerical simulation of self-similar transmission of high energy wave-breaking free ultra-short pulse
2007, 56 (5): 2769-2775. doi: 10.7498/aps.56.2769
The approximation of the phase introduced by self-phase modulation (SPM) is reasonably calculated and the spectrum broadening factor is given basing on nonlinear Schrdinger equation. The spectrum of propagating pulses is broadened seriously by SPM in the fiber with normal group velocity dispersion (GVD) when gain (G) exists. And the spectra broadening of the central part of pulses is quite different from that of the wings of the pulses. The self-similar evolution of Gaussian pulses is described. The temporal shape of the wings of the pulses being covered by the central part gradually is discovered. A theoretical explanation of self-similar pulses propagation is given which verified by the numerical simulation for the self-similar evolution of three kinds of pulses.
Algorithm and experimental investigation of patch nearfield acoustic holography based on wave superposition approach
2007, 56 (5): 2776-2783. doi: 10.7498/aps.56.2776
A new patch nearfield acoustic holography (PNAH) based on wave superposition approach is proposed in this paper. In the method, the sound field is extrapolated by superposing wave fields produced by a number of fictitious sources placed inside the radiator, and then the FFT is used for reconstructing the normal velocity or the field on and near the radiator. This method has higher computational accuracy and efficiency than other PNAH techniques, and is very easy for applications. In order to prove its validity, the method was used in the experiment to reconstruct the normal velocity of a clamped point-driven steel plate, and satisfactory results were obtained.
2007, 56 (5): 2784-2789. doi: 10.7498/aps.56.2784
Using FDTD method, the characteristics of elastic/acoustic wave propagating in two dimensional phononic crystals (PCs) consisting of Fe square arrayed cylinders in water was investigated. The absolute band gap was found in high frequency range for Fe/water structure, and the resonance gap was found in low frequency range for coated Fe/water structure, where the Fe cylinders were softly coated by rubber. These two gaps correspond to different mechanisms. Constructed with these two kinds of PCs, a combined wide gap from high to low frequency can be achieved, so the elastic wave propagating can be prohibited in a wide frequency range. Furthermore, the combined wide band gap can be adjusted effectively through changing the structure parameters, such as the size of Fe core, thickness of coating layer, filling ratio etc.
2007, 56 (5): 2790-2795. doi: 10.7498/aps.56.2790
To realize high deposition rate is an important problem in low-cost industrialization of microcrystalline silicon solar cells. Deposition of μc-Si:H films at a high rate was investigated using very-high-frequency plasma-enhanced chemical vapor deposition (VHF-PECVD) in this paper. The influence of gas residence time on de position rate and the photoelectric and micro structural properties in the CVD process were studied by changing total gas flow. As a result, the efficiency of the microcrystalline silicon solar cell prepared at deposition rate 12?/s reached 5.3%.
2007, 56 (5): 2796-2801. doi: 10.7498/aps.56.2796
Based on percolation theory of porous medium, the fracture as a very important permeability channel is taken into account in the theoretical research on percolation of medium. The research method on percolation of more general porous and fractured double-medium is put forward. Through the numerical calculation on the pore and fracture in donc in 2D, the relation between percolation probability and important parameters such as porosity, fracture fractal dimension and initial value of fracture distribution are obtained. The mathematical formula on percolation threshold of porous and fractured double-medium is also obtained. The percolation law of pore and fracture is further shown.
PHYSICS OF GASES, PLASMAS, AND ELECTRIC DISCHARGES
2007, 56 (5): 2802-2807. doi: 10.7498/aps.56.2802
Considering the disturbance of the plasma electrons at the beam-ion channel boundary, the eigen-equation of TM mode has been derived. On the basis of comparing the electromagnetic characteristics of the beam-ion channel with a traditional medium waveguide for the condition of a step-boundary defined in this paper, the numerical simulation results show that by changing the plasma frequency one can control the electromagnetic working mode in ion channel. Analyzing the influence of the disturbed charge boundary and the step-boundary on electromagnetic mode in the beam-ion channel, one can find that the cut-off frequency of the beam-ion channel have increased in the low frequency range effectively (ωωp， ωp is the plasma frequency) and some new electromagnetic modes appear in the high frequency range (ω>ωp) with the disturbed charge boundary. These findings can provide an important theoretical basis for the design of the ion channel electronics cyclotron maser (ICECM) and the ion channel laser (ICL).
CONDENSED MATTER: STRUCTURAL, MECHANICAL, AND THERMAL PROPERTIES
2007, 56 (5): 2808-2812. doi: 10.7498/aps.56.2808
In a stainless steel autoclave of 25ml capacity, pure hexagonal aluminum nitride (h-AlN) nanowire has been successfully synthesized by direct reaction of AlCl3 with NaN3 in non-solvent system at low temperature. The obtained grayish-white powder is characterized by high-resolution transmission election microscopy, which shows that the grayish-white products consist of long straight-wires with diameter from 40nm to 60nm and the longest ones were up to several micrometers. The electron diffraction and XRD analysis indicat that the AlN manowire has hexagonal/cubicl monocrystal structure. A possible growth mechanism for h-AlN nanowire is disscussed.
Structural stability and electronic properties of ordered identical Nb4 clusters on the Cu(100) surface
2007, 56 (5): 2813-2820. doi: 10.7498/aps.56.2813
First-principles density-functional theory and supercell models are employed to study the structural stability and electronic properties of the periodic two-dimensional arrays of identical Nb4 clusters on the Cu(100) surface. The total-energy calculations show that Nb4 clusters with both tetrahedron and quadrangle configurations can be stably absorbed on the Cu(100) surface, which might have important applications. The adsorption of quadrangular Nb4 clusters is shown to be more stable than that of tetrahedral Nb4. The energy barrier for the transition of tetrahedral Nb4 adsorption to the quadrangular one is around 0.94eV/cluster. Electronic structure calculations suggest that adsorption of Nb4 on Cu(100) surface causes significant charge redistributions between the surface Cu layer and the Nb4 adsorbate leading to remarkable changes in the electronic structure of the copper surface.
Generation of background-free pulses at 1064nm accurately synchronized with femtosecond laser pulses at 794nm
2007, 56 (5): 2821-2827. doi: 10.7498/aps.56.2821
This paper reports on generation of background-free pulses at 1064nm accurately synchronized with femtosecond pulses at 794 nm, by the use of a continuous-wave He-Ne laser seeded non-collinear optical parametric amplification (NOPA) pumped by frequency-doubled Ti:sapphire femtosecond laser at 396nm. Experimental results showed that the 1064 nm light pulses can be used as the pump seed for optical parametric chirped pulse amplification to realize an all-optical synchronization between pump and signal beams. Background-free pulses at 1064 nm could also be generated by using a NOPA with continuous-wave seed from the intracavity 632.8 nm light of an appropriate He-Ne cavity. The spectral and spatial chirp properties of the idler pulses were quite similar in both cases according to our measurements. The generated idler pulses in the intracavity seed case have single pulse energy 10 times that of the external cavity case.
2007, 56 (5): 2828-2832. doi: 10.7498/aps.56.2828
The pressure effects on the elastic properties of lithium doped by hydrogen are studied using first-principles methods based on density functional theory. A supercell including 16 Li atoms in hcp lattice is constructed with a hydrogen atom interstitially doped in. The supercell is fully relaxed at 0K. The elastic properties of the stable lattice are studied. The result shows that the bulk modulus increases after hydrogen atom is added. The elastic constants C11, C33, C66 and C12 are larger than those of pure Li，while the shear modulus C44 is smaller. The elastic constants increase with external pressure, with C11, C33 and C66 larger than those of pure Li while C12 being smaller. C13 is abnormally lower than that of pure Li within the pressure range from 2GPa to 4GPa, while C44 and C13 are greater at higher pressure area. The discrepancy of shear modulus between the doped and the pure system is enhanced by external pressure. The analysis of anisotropy ΔW shows that the doped system maintains isotropic under external pressure as does pure Li.
2007, 56 (5): 2833-2837. doi: 10.7498/aps.56.2833
We present first-principles calculations combined with mean-field potential model to study the thermodynamic properties of hexagonal-close-packed (hcp) Be for pressures up to 150GPa and temperatures up to 1500K, including the properties of Be under ambient conditions, the isothermal equation of state up to high pressure, the temperature dependence of equilibrium volumes and bulk modulus under ambient pressure, and Hugoniot curve in the P-V plane. The equationof state at zero temperature is computed based on density-functional theory within the generalized-gradient approximation. The vibrational contributions are calculated by the mean-field potential model. Due to high Debye temperature of Be, we consider the zero-point energy correction to the free energy. The calculated properties are in good agreement with available static and shock-wave experimental measurements.
CONDENSED MATTER: ELECTRONIC STRUCTURE, ELECTRICAL, MAGNETIC, AND OPTICAL PROPERTIES
2007, 56 (5): 2838-2844. doi: 10.7498/aps.56.2838
The site substitution behavior of the 4d transition elements and the alloying effect of Nb and Mo have been studied using the first-principles discrete variational (DV) and DMol method within the framework of the density-functional theory. The transfer energy calculation indicates that Y, Zr, Nb, and Mo show the tendency to occupy Ti sites while Tc, Ru, Rh, and Pb have the tendency to occupy Al sites in γ-TiAl. In addition, the alloying effect of Nb and Mo on the electronic structure of γ-TiAl is also studied. It was found that Nb and Mo give rise to strong interaction and charge transfer with the neighboring host atoms of TiAl, resulting in a strong solution strengthening effect.
The influence of electric field introduced polymer molecular orientation on the formation cross-section of singlet and triplet excitons in PLED
2007, 56 (5): 2845-2851. doi: 10.7498/aps.56.2845
Ir (PPY)3 and DCJTB doped PVK films were fabricated in the presence of electric field which induced the orientation of polymer molecules, and their photoluminescence and electroluminescence were measured. The influence of electric field-induced orientation on the formation cross-sections of singlet and triplet excitons is investigated. The results showed that the electric field-induced orientation of polymer does not cause significant change in PL spectrum. However, the orientation of polymer results in obvious change in EL spectrum. For the devices based on polarized PVK, the EL from triplet states is reduced, which shows that polarization of PVK leads to the increase in the formation cross-section of singlet exciton and the decrease in the formation of triplet exciton.
Characteristics of hopping conductivity in one-dimensional binary disordered system with off-diagonal correlations
2007, 56 (5): 2852-2857. doi: 10.7498/aps.56.2852
Based on a tight-binding disordered model describing a single electron band, we establish a model of electronic transport in one-dimensional binary disordered systems with off-diagonal correlations, and derive the dc conductance formula. By calculating the dc conductivity, the function of disorder and off-diagonal correlations in electronic transport are analyzed, and the relationships between electric field and conductivity and between temperature and conductivity are studied. The results indicate that the conductivity of the systems decreases with the increase of the degree of lattices energy disorder, and off-diagonal correlations lead to delocalization and enhance the electrical conductivity of system. The model also quantitatively explains the temperature and electric field dependence of the conductivity of systems, that is, the dc conductivity of the systems increases with the increasing of temperature and electric field.
2007, 56 (5): 2858-2862. doi: 10.7498/aps.56.2858
p-type Bi2Te3-based thermoelectric materials were fabricated by various methods, such as zone-melting, mechanical alloying, spark plasma sintering (SPS) and hot-pressing. Electrical conductivity (σ), Seebeck coefficient (α), and thermal conductivity (κ) were measured in the temperature range of 300—500K. The influence of the preparation methods on thermoelectric properties was studied. Compared with that of the zone-melted ingot, the figure of merit ZT (ZT=α2σT/κ) of those materials fabricated by the other methods mentioned were all increased to some extent. The obtained maximum ZT value was about 1.15 for that fabricated by the combination of zone-melting and spark plasma sintering technique.
2007, 56 (5): 2863-2867. doi: 10.7498/aps.56.2863
A macroscopic phenomenological model based on the magnetic dynamic equation was proposed to investigate spin-polarized current induced magnetization switching in a nano-scale pseudo-spin-valve structure. The movement and even reversal of the free layer's magnetization resulted from two factors, the net spin flux flowing into the free layer and the spin relaxation within it. The dynamic equation incorporated the spin-dependent scattering at the ferromagnetics/nonmagnetics interfaces, and the relaxation of spin accumulation in the ferromagnetic layer. Conditions of magnetization reversal and the corresponding critical currents were found by solving the dynamic equation analytically. The qualitative and quantitative results agree with most of experiments reported. According to the model, the influencing factors on the critical current were analyzed and the methods to improve device performances were pointed out.
2007, 56 (5): 2868-2872. doi: 10.7498/aps.56.2868
With the optical interference and exciton diffusion principle, the mechanism and process in a typical organic donor-acceptor solar cell are analyzed and simulated in virtue of a model established by MATLAB software. The effect of the organic film's thickness on the characteristics of organic solar cell is analyzed quantitatively, photo-absorptivity and exciton diffusivity are optimized by confining the thickness of organic active layers. The reliability of optimized model is validated by experiment.
Determination of chemical composition and average crystal lattice constants of InGaN/GaN multiple quantum wells
2007, 56 (5): 2873-2877. doi: 10.7498/aps.56.2873
The samples of InGaN/GaN multiple quantum wells (MQWs) have been grown on (0001) sapphire substrate with n-GaN buffer layer by metal-organic chemical vapor deposition (MOCVD). According to the results of Rutherford backscattering (RBS)/channeling along 〈0001〉 axis, the conventional θ—2θ scans normal to GaN (0004) and (1014) plane at 0° and 180° azimuth angles and the photoluminescence (PL) properties at room temperature, we concluded that In atoms in the InGaN/GaN MQWs are highly substituted, with the substitution rate over 99%, and the average crystal lattice constants of InGaN/GaN MQWs were calculated accurately (aepi=0.3195nm，cepi=0.5198nm), which are almost equal to theoretical data. Using HRXRD and RBS,the atomic content of In was determined to be respectively 0.023 and 0.026, the result was consistent with actual growth conditions of InGaN/GaN MQWs. However,there was a great difference compared with In chemical composition obtained by PL properties, which shows that PL properties are not suitable for measuring the In chemical composition in InGaN/GaN MQWs.
Temperature-dependent absorption spectra investigation of shallow levels in HgCdTe grown by liquid phase epitaxy
2007, 56 (5): 2878-2881. doi: 10.7498/aps.56.2878
Temperature-dependent absorption spectra in temperature range of 11—300K are recorded for a series of unintentionally doped HgCdTe grown by liquid phase epitaxy. The abnormal energy shift of about 7—20meV of absorption edge in the low temperature range (<70K) has been analyzed. The results suggest this the phenomenon to be caused by the Hg vacancies and the abnormal red-shift is related to the composition and the carrier density of the materials. The Hg vacancy level is estimated to be at 20meV above the valence band, which is well consistent with the results of Hg vacancy acceptor level calculated by the empirical expression. The results may provide a preliminary explanation that the bandgap obtained by the conventional transmission spectroscopy is slightly higher than the cutoff energy of the photocurrent response in practical device applications.
2007, 56 (5): 2882-2889. doi: 10.7498/aps.56.2882
The performance of two-color middle wavelength photovoltaic HgCdTe detector is simulated numerically for design optimization. Three recombination mechanisms (radiation, Auger, and Shockley-Read-Hall(SRH)recombination), trap-assisted tunneling, and band-to-band tunneling are considered in two-dimensional model. The tunneling through barrier layer is calculated by transfer matrix method. The n-p-p-p-n structure is designed in simultaneous mode. The effect of SRH recombination electron lifetime in p-region on spectral response is examined, and the dependence of crosstalk on composition gradient of barrier layer is analyzed. Simulation results show that the quantum efficiency decreases rapidly with decrease of SRH electron lifetime in p-region,and at least about 10ns of SRH electron lifetime is essential for good performance of the detector. Crosstalk decreases to the steady value determined by optical crosstalk as the composition gradient of barrier layer increases to about 0.03, so critical composition gradient of about 0.03 is necessary for suppressing the electrical crosstalk.
Investigation of the impact of band offset perturbations on the performance of abrupt HBT with heavily doped base
2007, 56 (5): 2890-2894. doi: 10.7498/aps.56.2890
Heavy impurity doping leads to bandgap narrowing (BGN) and this causes perturbations to the value of the band offsets at the hetero-interface. As a result, the form and height of energy barriers in abrupt HBT is disturbed, which changes the value of the current flowing through its interfaces. The analysis is based on the thermionic field-diffusion model which combines the drift-diffusion transport in the bulk of the transistor with the thermionic emission and tunneling at the base-emitter interface. The calculations reveal a more important role to the transport of carriers played by the modification of the built-in potential than that of the range of barrier energies available for tunneling because the impact of the built-in potential on the current is exponential. Therefore, it is important for a better description of the currents to use an accurate dopant-dependent BGN distribution model between bands.
Fabrication and the characteristics at room temperature of FP-HEMT are reported, followed by a comparison of the actual characteristics with the conventional HEMT. With the incorporation of field plate, the breakdown voltage was enhanced from 52 to 142V.Comparison between AlGaN/GaN FP-HEMT and the conventional HEMT are also made, using Silvaco, as the simulation tool. The effect of enhancing the breakdown voltage is also investigated.
2007, 56 (5): 2900-2904. doi: 10.7498/aps.56.2900
Passivation is generally used to suppress the current collapse, however, it also cause the decrease in breakdown voltage. By incorporating a field plate, the breakdown voltage was enhances from 46 to 148V as tested using the drain current inject technology (DCIT), which indicates that the field plate can increase the breakdown voltage remarkablely. Then, comparisons between the degrees of current collapse in the conventional HEMTs, the HEMTs after passivation and FP-HEMTs indicate that the field plate performs better in suppressing current collapse than passivation. It is conclusded that adopting the field plate structure is a good solution to the problem of incresing the breakdown voltage and at the same time suppressing the current collapse in GaN HEMTs.
2007, 56 (5): 2905-2909. doi: 10.7498/aps.56.2905
A novel AlGaInP thin-film light emitting diode (LED) with omni directional reflector structure was proposed, the corresponding fabrication process was developed. This reflector is realized by the combination of a low-refractive-index dielectric layer and a high reflectivity metal layer. The AlGaInP LED layers with dielectric-metal reflector is invertedly bonded to the GaAs submount by using 80Au-20Sn (wt%) alloy as a solder (Reflector-Submount, RS-LED), and then GaAs substrate is removed. The light that would otherwise be absorbed by the GaAs substrate is reflected by the high reflectivity dielectric-metal reflector. The optical and electrical characteristics of the RS-LED are presented and compared with the conventional AlGaInP absorbing substrate (AS) LED and AlGaInP absorbing substrate LED with distributed Bragg reflectors (DBR). A great improvement in the brightness and efficiency is observed. It is shown that the light output and lumen efficiency from the RS-LED at forward current 20mA exceed those of AS-LED by about a factor of 2.2 and 1.2, respectively, and ～2× the light output of AS-LED (DBR) and ～1.5× the lumen efficiency of AS-LED (DBR) were achieved. 194.3mcd luminous intensity from the RS-LED (at 20mA, peak wavelength 627nm) could be obtained under 20mA injection, which is 2.8 and 1.6 times higher in luminous intensity than the AS-LED (at 20mA, peak wavelength 624nm) and AS-LED(DBR) (at 20mA, peak wavelength 623nm), respectively.
2007, 56 (5): 2910-2914. doi: 10.7498/aps.56.2910
Single layer Alq3, double layer PBD/Alq3 and quantum well PBD/Alq3/PBD structures have been grown by organic molecular beam deposition. Energy levels and optical property of these samples are determined by cyclic voltammetry, optical absorption and photoluminescence (PL). Cyclic voltammetry and optical absorption results indicate that PBD/Alq3/PBD structures can be classified as type I quantum well. The PL experimental results indicate that PL peak, of single layer Alq3 structures do not shift to higher energy with decreasing layer thickness. However, for double layer PBD/Alq3 and quantum well PBD/Alq3/PBD structures, PL peaks shift to higher energy with decreasing layer thickness. The reason for blue shift of PL in organic quantum well structures is discussed in light of the theoretical and experimental results published in the literature.
Deposition of p-type nc-SiC：H thin films with subtle carbon incorporation for applications in p-i-n solar cells
2007, 56 (5): 2915-2919. doi: 10.7498/aps.56.2915
This paper presents a detailed study on the effects of carbon incorporation and substrate temperature on structural, optical, and electrical properties of p-type nanocrystalline amorphous silicon films. A p-nc-SiC：H thin film with optical gap of 1.92eV and activation energy of 0.06eV is obtained through optimizing the plasma parameters. By using this p-type window layer, single junction diphasic nc-SiC：H /a-Si：H solar cells have been successfully prepared with a Voc of 0.94eV.
The structural stability and electronic structures of Nb planar atomic sheets are studied by employing first-principles plane wave pseudopotential method based on the density functional theory. The calculations for all the 2-dimensional lattice structures show that, as a result of Jahn-Teller effect, the high-symmetry square and hexagonal two-dimensional structures are metastable. The most stable structures are the low-symmetry oblique and centered rectangular structures, which are the distorted structures of the hexagonal 2-D structure due to Jahn-Teller effect. The 2-D rectangular structure of Nb atomic sheet can not be formed. The relative structural stabilities, the electronic energy bands and the density of states are discussed based on the ab initio calculations and the Jahn-Teller effect.
Steady-state and nanosecond time-resolved photoluminescence spectroscopies of aqueous CdTe quantum dots
2007, 56 (5): 2926-2930. doi: 10.7498/aps.56.2926
We have used femtosecond laser pulse to investigate the steady-state and nanosecond time-resolved photoluminescence of aqueous CdTe quantum dots (QDs). Up-conversion luminescences of the CdTe QDs induced by two-photon excitation were observed. Compared with 400nm excitation, the peak wavelength of photoluminescence induced by 1208nm excitation was red-shifted about 28nm (88meV). The decay kinetics showed a fast and a slow decay component which can be assigned to band-edge excitonic state (3—5ns) and surface trapping state (30—50ns), respectively. Luminescence and up-conversion luminescence at room temperature showed similar decay kinetics. The relative change of photoluminescence intensity between excitonic and trapping state is responsible for the red shift of emission peak. It was found that the proportion of excitonic emission in whole luminescence at 400nm excitation is bigger than that of excitonic emission at 800nm excitation, so with increasing the wavelength of laser excitation steady state spectra have the red shift of emission peaks.
2007, 56 (5): 2931-2936. doi: 10.7498/aps.56.2931
The thickness dependence of the spontaneous polarization and the hysteresis loop of epitaxial PbZr0.4Ti0.6O3 thin films deposited on (001) SrTiO3 substrate was investigated via the Landau-Devonshire's phenomenological theory considering the coupling of the stress field of the edge dislocation and the polarization. The results show that the critical thickness for the formation of misfit dislocation is ～1.27nm, and there is a drastic variation in the polarization near the dislocation in films with thickness greater than the critical value, which results in the formation of the dead layer that severely degrades ferroelectric properties. With decreasing the film thickness, both the dislocation spacing and the ratio of the dead layer to film total thickness increase. The thickness dependence of the hysteresis loop indicates that the remnant polarization decreases as the thickness decreases.
2007, 56 (5): 2937-2944. doi: 10.7498/aps.56.2937
GaP IR thin films were deposited through RF magnetron sputtering with a GaP disk as the target. The intensity of Ar I 750nm optical emission line was kept the same in different experiments, with which various deposition parameters were got, and the deposition processes were studied with computer simulation. The results showed that under low sputtering power and high working gas pressure, the sputtering yields and transporting efficiencies of Ga and P atoms as well as their energies at arriving the substrate's surface are small, the sputtering yield and transporting efficiency of Ga atom are both larger than those of P atom, hence the depositing rate of GaP film is low, the content of Ga in the film is larger than that of P, which makes the film have high absorbance in IR waveband. But under high sputtering power and low working gas pressure, the atoms' sputtering yields, transporting efficiencies and energies at arriving the substrate's surface all increase, the sputtering yield of Ga atom is larger than that of P atom, whereas its transporting efficiency is smaller than that of P atom, hence the depositing rate of GaP film is high and stoichiometric film with low absorbance is deposited, which is advantageous to depositing large-thickness GaP film.
2007, 56 (5): 2945-2950. doi: 10.7498/aps.56.2945
Using the transfer matrix method, we investigated the influence of permittivity error on the properties of magneto-optical multi layer isolator with single-defect, and presented a new type of multi layer structure with multi defect. Compared with single-defect structure, the bandwidth of Faraday rotation frequency response of multi defect structure was increased greatly, and the tolerance of permittivity was increased by one order. Furthermore, the multi defect structure has greater tolerance of film thickness and incidence angle.
Spectroscopic properties and energy transfer of Tm3+/Ho3+-codoped Ga2O3-Bi2O3-GeO2-PbO-PbF2 glasses for 1.47μm luminecence
2007, 56 (5): 2951-2957. doi: 10.7498/aps.56.2951
This paper reports on the spectroscopic properties and energy transfer of Tm3+/Ho3+-codoped 14Ga2O3-25Bi2O3-20GeO2-31PbO-10PbF2 glasses. The Judd-Ofelt intensity parameters Ωt(t=2，4，6), spontaneous-emission probability, branch ratio and radiative lifetime of each level of Tm3+ have been calculated by Judd-Ofelt theory. The effective bandwidth and peak emission cross-section of the 3H4→3F4 transition (1.47μm) of Tm3+ have also been obtained. Codoping of Ho3+ significantly decreased the population of 3F4 level of Tm3+, while the population of 3H4 level of Tm3+ reduced slightly. Our results indicate that GGBPF glass is a promising candidate for 1.47μm optical amplifiers, and Ho3+ is an effective codopant to enhance the 1.47μm luminescence.
2007, 56 (5): 2958-2961. doi: 10.7498/aps.56.2958
By using time-resolved photoluminescence and time-resolved Kerr rotation, we have studied the unique electron spin dynamics in InAs monolayer (ML) and submonolayer (SML), which were sandwiched in GaAs matrix. Under non-resonant excitation, the spin relaxation lifetimes of 3.4ns and 0.48ns were observed for 1/3ML and 1ML InAs samples, respectively. More interestingly, the spin lifetime of the 1/3ML InAs decreased dramatically under resonant excitation, down to 70ps, while the spin lifetime of the 1ML sample did not vary much, changing only from 400 to 340ps. These interesting results come from the different electron-hole interactions caused by different spatial electron-hole correlation, and they provide a direct evidence of the dominant spin relaxation process, i.e. the BAP mechanism. Furthermore, these new results may provide a valuable enlightenment in controlling the spin relaxation and in seeking new material systems for spintronics application.
2007, 56 (5): 2962-2967. doi: 10.7498/aps.56.2962
The fluorescence spectra of the ethanol-water mixture excited by 236nm UV-light and the polarization of the fluorescence are investigated respectively. With different polarization degree of an incident light, the fluorescence was collected between 300nm and 400nm both in horizontal and vertical polarization direction. The experimental result shows that the fluorescence, emitted from the ethanol-water solution when excited by linearly polarized light, is a polarized light and its polarization degree is calculated too. By analyzing theoretically, the Stokes vector of the fluorescence polarization condition has been obtained, and the polarization condition is testified. By calculating Mueller matrix of the solution, the orientation characteristic of ethanol-water cluster molecules has been analyzed. The research can contribute to the study of the new molecule clusters of ethanol-water.
2007, 56 (5): 2968-2973. doi: 10.7498/aps.56.2968
The structure of Tm3+-doped langasite(La3Ga5SiO14，abbreviated as LGS) crystal has been measured by XRD. The crystal lattice parameters were calculated with DICVOL91 program. The absorption spectra and emission spectra (excitated under 470nm) of Tm：LGS single crystal was measured at room temperature. And the optical parameters, such as transition-line intensity, oscillator strength and absorption section were calculated by using Judd-Ofelt theory. The three intensity parameters Ωt(t=2，4，6) were fitted to be 2.694×10-20cm2，1.842×10-20cm2，0.030×10-20cm2，respectively. The lifetime, branching ratio and integrated stimulated-emission cross section were obtained and the results were discussed.
2007, 56 (5): 2974-2978. doi: 10.7498/aps.56.2974
The carrier injection process of polymer light-emitting diodes (PLEDs) with MEH-PPV(poly［2-methoxy，5-(2-ethylhexoxy)-1，4-phenylene vinylene］) as the light-emitting layer, was investigated using impedance spectroscopy, capacitance-voltage (C-V) and capacitance-frequency (C-F) techniques. The diodes have the configuration of ITO/PEDOT/MEH-PPV/Ba/Al. The experiments showed that there is an ohmic contact between electrodes and MEH-PPV, and holes and electrons injected from the cathodes are unbalanced. Charged traps in the MEH-PPV result in space-charge under positive bias, and the trap concentration was estimated at about 3.75×1016cm-3.
2007, 56 (5): 2979-2985. doi: 10.7498/aps.56.2979
A conventional organic light emitting diode, which has a structure of ITO/NPD/Alq3/LiF/Al, was fabricated and the magnetic field effects(MFEs) on its electroluminescence(EL) were measured. We found that the MFEs of the organic EL is strongly dependent on the applied bias on the device while the photoluminescence from the Alq3 film within the device is independent of the magnetic field. At low bias, the MFEs first rapidly increase with the magnetic field and then saturate at about 40kA/m, however at high bias the MFEs will decrease after this saturation, and the larger the bias, the stronger the decrease. The mutual annihilation of triplet-triplet pairs producing excited singlet excitons and the interaction between triplet excitons and excess carriers of Alq3 anion or cation are the physical mechanisms of the phenomena observed in this study.
2007, 56 (5): 2986-2991. doi: 10.7498/aps.56.2986
The electronic structure of MnS was studied using resonant inelastic soft X-ray scattering (RIXS) from 2p63d5→2p53d6→2p63d5 channel. The d-d excitation and charge-transfer (CT) excitation were obtained which were resonant at excitation energies corresponding to L absorption edge and the satellites, respectively. RIXS and XAS spectra were simulated by Hartree-Fock method under atomic SO3 and cubic Oh symmetries. The crystal field 10Dq value was evaluated to 0.80eV—0.85eV. The stronger CT excitation of MnS than that of MnO originates from its narrower bandwidth.
2007, 56 (5): 2992-2997. doi: 10.7498/aps.56.2992
Exponential-doping photocathodes, in which from the GaAs bulk to the surface the doping concentration is distributed exponentially from high to low, can form a stable built-in electric field in the active layer, and the electric field facilitates the excited photoelectron emission. The quantum yield formulas of both reflection-mode and transmission-mode of exponential-doped photocathodes have been solved from the 1-diemnsion continuity equations, in which the build-in electric field is considered. According to these formulas, we calculate the theoretical quantum yield of the exponential-doped photocathodes. The calculated results show that the exponential-doping structure can increase the quantum yield of photocathodes significantly. To compare with the uniformly-doped photocathodes, the integral sensitivity of the reflection-mode exponential-doped photocathodes increases by nearly 20%, and for transmission-mode photocathodes the increase is more than 30%. The performance improvements of exponential-doping photocathodes are mainly attributed to the built-in electric field, the photoelectrons driven by the field move towards the cathode surface by way of diffusion and drift, accordingly, decrease the influence of the back-interface recombination velocity on photoemission and increase the equivalent electron diffusion length of cathodes.
INTERDISCIPLINARY PHYSICS AND RELATED AREAS OF SCIENCE AND TECHNOLOGY
2007, 56 (5): 2998-3003. doi: 10.7498/aps.56.2998
Nitrogen doping of fluorinated diamond-like carbon (FN-DLC) films were deposited on p-type (100) silicon wafers at different deposition condition. The contact angle of water on the FN-DLC films surfaces was measured by a sessile-drop method. The surface morphology of films was characterized by atomic force microscopy (AFM). The Raman spectrum, infrared spectrum and X-ray photoelectron spectrum (XPS) demonstrate that the hydrophobic nature of FN-DLC films lies on chemistry structure, polarization intensity and roughness of films surface. As the ratio of sp3/sp2 decreases, the quantity of CF2 and roughness of film surface increases, thus contact angle increases. Otherwise, contact angle decreases. The contact angle is correlated with the deposition condition, which low temperature, low radio frequency power and high flux ratio r(r=CF4/［CF4+CH4］) brings the contact angle to ascend.
2007, 56 (5): 3004-3009. doi: 10.7498/aps.56.3004
Nitrogen doped fluorinated diamond-like carbon (FN-DLC) films were deposited on p-type silicon wafers under different deposition conditions. Fourier transform infrared absorption spectrometry (FTIR) shows that the number of C—H bonds decreases with increasing r(r=N2/［N2+CF4+CH4］), but those of C=N, C≡N bonds increase gradually. Gaussian fit results of C1s and N1s in X-ray photoelectron spectra (XPS) show that the β-C3N4 and a-CNx(x=1,2,3) structures have formed in the films. The G band widening and the peak shift to the low wave-number in Raman spectra show that doping of N2 increases the fraction of sp2. Atomic force microscopy (AFM) reveals that the surface morphology of the films becomes smooth due to doping of nitrogen.
Femtosecond laser pulse control of multidimensional vibrational dynamics: (Ⅰ) the three vibrational mode two electronic state system
2007, 56 (5): 3010-3016. doi: 10.7498/aps.56.3010
With the combination of optimal control theory and MCTDH method, given a certain target state, the quantum dynamics of pyrazine molecule system, which has been modeled as two electronic states and three vibrational modes, have been simulated. With the electronic excited state as the target state, what we get of the optimal pulse has a form of cuniform, which excites the wave-packet to the electronic excited state. In the tuning mode coordinate, the reduced probabilities oscillate strongly to obtain a high yield of target state. Given different target states, the evolution of probability density also demonstrates different behaviors. With the target state at the vibrational ground state of each coordinate in electronic excited state, the wave-packet is directly excited to their equilibrium positions. With the target state at the shifted vibrational state, the wave-packet oscillates strongly to reach its target state.
GEOPHYSICS, ASTRONOMY, AND ASTROPHYSICS
2007, 56 (5): 3017-3021. doi: 10.7498/aps.56.3017
The mechanism of polystyrene submicrosphere assembly in aqueous suspension was studied systemically. During the evaporation of solvent, the PS spheres were transported to the water-air surface, where these spheres self-assembled into colloidal crystal through the capillary force between the particles at water-air interface. The wettability of the PS spheres at the water-air interface could change from complete wetting to partial wetting and finally to non-wetting with the evaporation of water, so that the contact angle between the PS spheres and water increased gradually. The researches showed that only after the contact angle reached a critical value θ′critical, could the PS sphere-assembly occur. The critical value θ′critical which was obtained from the Wenzel equation,was greate than the measured value θcritical＝22°, because the surface of PS colloidal crystal was “regularly” coarse. The change of the PS spheres at water-air interface from hydrophilic to hydrophobic is essential for the PS spheres assembling into colloidal crystals.
The present paper is concerned with characteristics of flow structures of typhoons, and the double variational method is applied. First, the typhoon flow field is decomposed into two parts: the maximum asymmetric vortex and irrotational environmental flow field. Then the maximum asymmetric vortex is also decomposed into two parts: the maximum symmetric vortex and β-gyres. Finally, typhoon Pearl(2006) is simulated by WRF model(weather research and forcasting model), and flow fields at various times at 500hPa are decomposed using the above method. It turns out that the decomposed flow fields are useful for researches on motion and track prediction of typhoons.
2007, 56 (5): 3028-3030. doi: 10.7498/aps.56.3028
Sea surface salinity is a key physical parameter in ocean science. It is important in the ocean remote sensing to retrieve sea surface salinity by the microwave probe technology. Based on the in situ measurement data and remote sensing data of the Yellow Sea, we have built a new empirical model in this paper, which can be used to retrieve sea surface salinity of the Yellow Sea by means of the brightness temperature of the sea water at L-band. In this model, the influence of the roughness of the sea surface is considered, and the retrieved result is in good agreement with the in situ measurement data, where the mean absolute error of the retrieved sea surface salinity is about 0.288 psu. This result shows that our model has greater retrieval precision compared with similar models.