Vol. 62, No. 6 (2013)
2013-03-20
GENERAL
2013, 62 (6): 060201.
doi: 10.7498/aps.62.060201
Abstract +
A class of coupled system of the El Niño-southern oscillation mechanism is studied. Using the singular perturbation theory and method, the outer solution and the initial layer corrective term of the model are solved. And then, the asymptotic expansion of the solution for the problem is obtained and the asymptotic behavior of solution is considered.
2013, 62 (6): 060202.
doi: 10.7498/aps.62.060202
Abstract +
Lack of efficiency in transmitting logging signals has long been one of the crucial problems for the development of logging while drilling. This study aims to address this issue by using the advanced scheme for logging while drilling signal transmission, proposed by NovatekTM. The main points of the study focus on electromagnetic coupling between two adjacent pipes and the signal transmission in coaxial cables imbedded in drilling pipes. According to the axial symmetry of the electromagnetic coupling structure, the numerical mode matching is used to establish the numerical model for it. Through simulation analysis which is based on the numerical modeling of the electromagnetic coupling structure, we analyze how various parameters of the structure influence the coupling, obtain some significant conclusions, and optimize the coupling structure. The conclusions can be used to guide optimization design of coupling structure between the drill-pipe for signal transmission in logging while drilling. In addition, the rectangular transmission line whose characteristic impedance is 50 Ω is designed for the cable imbedded in the drill pipe, and the attenuation is calculated. Finally, simulation and experiment are performed for one unit of the pipeline. The results are in agreement with each other, thereby showing the good transmission performance.
2013, 62 (6): 060203.
doi: 10.7498/aps.62.060203
Abstract +
To carry out spectral image inversion in a static computational spectral imager is a crucial step for accomplishing its theoretical advantages, so the data processing technology for spectral image inversion will determine the final spectral image achieved. Focusing on the spectral image inversion, we have investigated various algorithms such as image reconstruction, image compressed sensing and spectral image inversion theories, and compared them carefully. By taking into account the data transmission link of the system and the error in the engineering development process, a comprehensive simulation is carried out. The key issue of spectral image inversion, and also how to use the inversion algorithms to achieve its optimized routes are pointed out. So a detailed analysis for realizing the theoretical advantages and ensuring instrument technology development is provided.
2013, 62 (6): 060301.
doi: 10.7498/aps.62.060301
Abstract +
We construct an entangled quantum heat engine based on two-coupled-qubit XXZ model with Dzyaloshinski-Mariya interaction. The work done and the heat transfer are discussed according to the definition first given by Kieu, The relations between the entanglement and heat transfer, work output and efficiency are analyzed for different anisotropic parameters. The results show that the second law of thermodynamics holds in entangled systems and the isolines for the efficiency are looped curves. When the anisotropic parameter Δ is small enough, the heat engine can operate in both C1 > C2 and C1C2, however, when Δ is large, the heat engine operates in C1 > C2 only.
2013, 62 (6): 060302.
doi: 10.7498/aps.62.060302
Abstract +
In this paper, we study the dynamical characteristics of magnons generated by the static magnetic dipole-dipole interaction and the external-laser induced dipole-dipole interaction in spin chain within an optical lattice. Specially, we choose a blue-detuned optical lattice and define an effective temperature for the system. We make a comparison between the generation process of magnons and that of photons in an optical vibration cavity. The results show that by suitably choosing the system parameters, the dynamical Casimir effect at finite temperature in the magnon system can be reproduced.
2013, 62 (6): 060401.
doi: 10.7498/aps.62.060401
Abstract +
Using Hamilton-Jacobi method, the Hawking tunneling radiation and temperature are investigated near the event horizon of the Einstein-Yang-Mills-Chern-Simons black hole. The results show that the temperature and tunneling rate depend on the charge and horizon of black holes, and the conclusion is significant for investigating other dynamical black holes. What is more, we also prove that this method can be used to study Hawking radiation in the scalar, vector, Dirac field and gravitational wave cases.
2013, 62 (6): 060501.
doi: 10.7498/aps.62.060501
Abstract +
Based on the analysis of urban road traffic flow affected by emergency vehicle, a muti-lane cellular automaton model is established. Three characteristic variables are introduced to modify the lane change rules, including the give-way state variable, the affected areas of police siren and the safe distance for mandatory lane change. Numerical simulation results indicate that lane number and hybrid vehicle scale factor have a great influence on vehicle speed and lane change number in low-density range. And the parameter setting for affected areas of police siren changes the lane change number within a certain range. Meanwhile, the parameter of safe distance for mandatory lane change mainly affects emergency vehicle speed and lane change number. The study indicates that the appearance of emergency vehicle interferences with traffic flow of lower density obviously, and the proposed parameters make cellular automaton model closer to the actual traffic scenarios under emergency conditions.
2013, 62 (6): 060502.
doi: 10.7498/aps.62.060502
Abstract +
The chaotic forecasting of irregular complex flow oscillation of natural circulation flow instabilities under rolling motion condition based on the largest Lyapunov exponents is performed. The correlation dimension, Kolmogorov entropy and the largest Lyapunov exponent are determined based on the phase space reconstruction theory of experimental data. On the premise that the irregular complex flow oscillation is confirmed to own chaos characteristic, the chaotic forecasting of the irregular complex flow oscillation is carried out by calculating the largest Lyapunov exponent. A comparisons between the prediction results and experimental data indicates that the chaotic forecasting based on the largest Lyapunov exponent is an effective way of producing those two-phase natural circulation flow instabilities. Meanwhile, the maximum predictable scale of chaotic flow instability is determined and a way of dynamic forecasting to monitor flow oscillation is presented. The method employed here provides a new method of studying the complex two-phase flow instabilities.
2013, 62 (6): 060601.
doi: 10.7498/aps.62.060601
Abstract +
In this article, the characteristics of femtosecond laser pulses and the interaction mechanism between them and materials are described, and the characteristics and advantages of femtosecond laser micromachining of energetic materials are discussed. The technology and development of the femtosecond laser machining of energetic materials are reviewed. The experimental and theoretical research of femtosecond laser machining of energetic materials and the corresponding research scheme and key techniques for further development are discussed.
2013, 62 (6): 060701.
doi: 10.7498/aps.62.060701
Abstract +
Nonlinear crystals commonly used in optical rectification for the generation of terahertz (THz) radiation have high refractive indices in the THz frequency range, and thus Fresnel reflection at the crystal-air output surface causes a large part of the generated THz wave to be reflected back into the crystals. Here we report on the design and experimental study of sub-wavelength antireflective micropyramid structures on GaP crystals. Effective medium theory is used to demonstrate the enhancement of THz output by the antireflective micropyramid structures, and further to design the antireflective structures at different frequencies. Several micropyramid structures are fabricated on the output surface of GaP crystals by micromachining, and the correlation between the THz output enhancement and the structure parameters is verified. The agreement between theory and experiment shows that our methodology is applicable to other THz emitters based on optical rectification.
2013, 62 (6): 060702.
doi: 10.7498/aps.62.060702
Abstract +
A new method of polarization modulation based triple-photoelastic-modulator (triple-PEM) is proposed as an key component of photo-elastic modulator-based imaging spectro-polarimeter (PEM-ISP) combined with acousto optic tunable filter. The basic principles of PEM-ISP and triple-PEM-based differential frequency polarization modulation are described, that is, the tandem PEMs are operated as an electro-optic circular retardance modulator in a high-performance reflective imaging system. Operating the PEMs at slightly different resonant frequencies generates a differential signal that modulates the polarized component of the incident light at a much lower heterodyne frequency. Then the basic equations for polarization measurement is derived by analyzing and calculating its Muller matrix. The simulation and experiments verify the feasibility and accuracy of polarization measurement by triple-PEM-based differential frequency polarization modulation. Finally, we analyze the influences of the setting of integral step and sampling interval of the detector polarization measurement, and a preliminary error analyses of field angle, phase retardation amplitude etc are also be carried out. The result shows that the measurement error of DoLP is less than 0.6% when the phase retardation error is 1%. This work provides the necessary theoretical basis for remote sensing of new PEM-ISP and for engineering implementation of Stokes parametric inversion.
2013, 62 (6): 060703.
doi: 10.7498/aps.62.060703
Abstract +
Au-Ag alloy films deposited on the glass substrates are used, for the first time, as a wavelength-interrogated near infrared surface plasmon resonance (SPR) sensor. The values of resonance wavelength (λR) of the sensor at different angles of incidence are determined by absorptiometry and its refractive-index (RI) sensitivity is investigated using aqueous glucose solutions as the standard RI samples. As the incident angle increases from 7.5° to 9.5°, the SPR absorption peak shifts from λR = 1215 nm to 767.7 nm, the full width at half magnitude (FWHM) of the peak reduces from 292.8 nm to 131.4 nm, and the RI sensitivity decreases from 35648.3 nm/RIU down to 9363.6 nm/RIU. At the same initial λR, the SPR sensor with the Au-Ag alloy film shows a higher sensitivity than that with the pure Au film (S = 29793.9 nm/RIU at λR=1215 nm with a pure Au film). Adsorption of bovine serum album molecules from the aqueous solution of 1 μmol/L protein results in a redshift of ΔλR = 12.1 nm with the Au-Ag alloy film and ΔλR=9.5 nm with the pure Au film. The experimental data also indicate that the FWHM of the SPR absorption peak with the Au-Ag alloy film is larger than that at the same λR with the pure Au film, leading to a lower spectral resolution than that of the latter.
2013, 62 (6): 060704.
doi: 10.7498/aps.62.060704
Abstract +
It is the research foundation of ship wake detection by laser and new-generation optical homing torpedo to investigate the influence of multiple scattering effect on light scattering intensity and polarization characteristics of the ship wake bubbles. The simulation model of laser back-scattering detection by ship wake bubbles is based on vector Monte Carlo method, and the multiple scattering mechanism is studied. The influences of multiple scattering effect and the bubble density in ship wake on the light scattering intensity and polarization characteristics of echo signal are analyzed. The echo photon polarization contribution reception method and the echo signal polarization statistical method are proposed to solve the problem that the low photon return probability cannot form the echo energy in the system with small receiver field of view. These methods are based on the basic idea of the particle collision importance sampling and the traditional energy receiving method. The polarization detection experimental platform for the simulated wake bubbles is built and the accuracy of the simulation results is verified in experiment. The consistence of the experimental and simulation results shows that the bubble distance and density information can be characterized by echo intensity, polarization information and the echo signal intensity, and the polarization characteristics can be used to detect and distinguish the ship wake bubbles, or even a low density wake bubbles with high precision.
2013, 62 (6): 060705.
doi: 10.7498/aps.62.060705
Abstract +
In this paper, an X-ray communication program, which consists of a sender of grid controlled X-ray source and a receiver of X-ray single-photon detector based on micro-channel plate, is presented. With the detailed information about the signal modulation transmitter, the micro-channel-based X-ray single-photon detector as well as the signal receiving demodulator, a space audio communication system based on X-ray is built. The communication rate of more than 20 kbit/s is realized. According to the preliminary test result analyses of the X-ray space audio communication system test, the X-ray emission success rate restricts the communication speed by the influence of different X-ray intensities, signal shaping time and threshold settings respectively. Therefore, a scheme for further increasing X-ray communication performance is suggested.
ATOMIC AND MOLECULAR PHYSICS
2013, 62 (6): 063101.
doi: 10.7498/aps.62.063101
Abstract +
We use first-principles calculation with pseudo-potential and plane wave method to study the electronic structure of LuI3. Exchange and correlation are treated in the local density approximation based on the density functional theory. The results show that the narrow bands with a width of about 0.2 eV near -4.4 eV are dominated by the 4f bands of Lu. Valence bands are located between -3.55 eV and 0 eV and mainly from p bands of I. Conduction bands are located between 2.44 eV and 12.35 eV and mainly from d bands of Lu, as well as from s bands of Lu. The peaks which appear at -3.46 eV of the s states of Lu, f states of Lu and p states of I show the strong interaction between the Lu and I.
2013, 62 (6): 063102.
doi: 10.7498/aps.62.063102
Abstract +
In this paper, the QCISD(T) method and aug-cc-pVTZ basic set are used to calculate the interactional potential of Ne atom and halogen hydride molecule HF, in which Boys and Bernardi's full counterpoise method is employed to eliminate the basis set superposition error. After obtaining the interactional potential energy data in eleven directions for He-HF, the symmetric potential V0 and the anisotropic potentials V1, V2, V3, etc. of the system are derived, by using Huxley function fitting, so as to describe well the He-HF potential energy surface. Finally, the close-coupling method is used to calculate the total collision excitation cross section, elastic partial wave cross section and inelastic elastic partial wave cross section.
2013, 62 (6): 063103.
doi: 10.7498/aps.62.063103
Abstract +
The tensile and compressive mechanical properties of gold nanotubes in different crystal orientations as well as the tensile mechanical properties of the same thinkness of gold nanotubes at different radius. are investigated using the molecular dynamics simulation method. In the simulation, we select embedded atom method as the interatomic potential function. The result shows that mechanical properties in the tensile and compressive process in different crystallographic orientations are dramatically different from each other, where the yield strength of the direction is the highest and the yield strength and the Young's modulus in the direction are less than in the and crystal orientation. The yield strength has no major changes when the radius is less than 3.0 nm, but it obviously decreases with the increase of the radius when the radius is larger than 3.0 nm.
EDITOR'S SUGGESTION
2013, 62 (6): 063201.
doi: 10.7498/aps.62.063201
Abstract +
To study the relationship of atomic beam flow with cooling intensity, laser detuning, and magnetic field gradient, the numerical simulation is performed and a two-dimensional magneto-optical trap setup is built. A low-velocity atomic beam flow is generated with a total flux of 2.1 109/s. Theoretical analysis and experimental results are in good consistence. Optimal detuning and magnetic field gradient can produce the largest atomic beam flow.
2013, 62 (6): 063202.
doi: 10.7498/aps.62.063202
Abstract +
The characteristic X-ray spectra produced by the impact of highly charged ions of 152Eu20+ with energies from 2.0 to 6.0 MeV on Au surface are measured. It is found that highly charged ions could excite both the characteristic X-ray spectra of Mζ, Mα and Mδ of Au and the characteristic X-ray spectra of Mα of Eu. The total X-ray yield increases with the ion kinetic energy increasing. The total production cross section of Au induced by Eu20+ is measured and compared with those obtained from the binary encounter approximation, plane-wave-Born approximation, and the energy-loss Coulomb deflection perturbed stationary state relativistic theoretical models.
2013, 62 (6): 063301.
doi: 10.7498/aps.62.063301
Abstract +
The high level quantum chemistry ab initio multi-reference configuration interaction method with reasonably large aug-cc-pVQZ basis sets is used to calculate the potential energy curves of 14 -S states of BCl+ radical correlated to the dissociation limit B+(1Sg)+Cl(2Pu) and B(2Pu) +Cl+(3Pg). In order to get the better potential energy curves, the Davidson correction and scalar relativistic effect are taken into consideration. The spin-orbit interaction is first considered, which makes the lowest 4 -S states split to 7 states. The calculational results show that the avoided crossing rule exists between the states of the same symmetry. The analyses of the electronic structures of -S states determine the electronic transition of each state and demonstrates that the -S electronic states are multi-configurational in nature. Then the spectroscopic constants of the bound -S and states are obtained by solving the radial Schrdinger equation. By comparison with experimental results, the spectroscopic constants of ground states are in good agreement with the observed values. The remaining computational results are reported for the first time.
2013, 62 (6): 063601.
doi: 10.7498/aps.62.063601
Abstract +
This paper focuses on the piezoelectric properties of zigzag graphene nanoribbons with hydrogen selective modifications by first-principles calculations. The structures of hydrogen modified graphene nanoribbons are optimized and the calculated hydrogen binding energies indicate that these structures are very stable. Owing to the hydrogen atom selective adsorption, the adjacent carbon atoms have different charge states and breaking inversion symmetries of nonpiezoelectric graphene. So, the positive charge centers and the negative charge centers of the hexatomic carbon ring in these structures separate from each other under uniaxial tensile strain, inducing the macroscopical electric polarization. Furthermore, the gradient of strain induced dipole moment density is related to ribbon width, i.e., the wider the ribbon, the better the piezoelectric property is. Besides, the dipole moment density of hydrogen selective modified graphene nanoribbons without strain could be controlled by changing the edge modification configuration of hydrogen atoms effectually.
ELECTROMAGENTISM, OPTICS, ACOUSTICS, HEAT TRANSFER, CLASSICAL MECHANICS, AND FLUID DYNAMICS
2013, 62 (6): 064101.
doi: 10.7498/aps.62.064101
Abstract +
Split ring resonators (SRRs) can be used as a negative permeability medium near its magnetic resonance frequency. In this paper, a new type of THz-band magnetic resonance structure is proposed by introducing metal wires into traditional SRRs, and the effect of metal wires on the transmission characteristics of SRRs is numerically investigated. The results show that the resonant frequency of SRRs significantly decreases with the number of metal wires increasing. The parameters of metal wires, such as length Lx, width Wx and distance Gx also have influence on the resonant frequency of SRRs. Meanwhile, the results verify that the introduction of metal wires play an important role in reducing the size of the device, and cannot be affected by the presence of dielectric substrate. The new magnetic resonance structure proposed in this paper provides a reference for the design and practical applications of metamaterials in the future.
2013, 62 (6): 064102.
doi: 10.7498/aps.62.064102
Abstract +
The reflection and transmission characteristics of electromagnetic waves by the uniaxially anisotropic chiral slab with the optical axis parallel to the interface are investigated. Formulas of the reflection and transmission coefficients (power) are derived. The curves of powers of the reflected and transmitted electromagnetic waves are presented for four cases of dielectric constants according to their signs. The effects of chirality parameter on the reflection and transmission are discussed. Especially, the dependences of pseudo-Brewster angles on the chirality parameter are plotted.
2013, 62 (6): 064103.
doi: 10.7498/aps.62.064103
Abstract +
A metamaterial absorber with high absorptivity, wide incident angle and no surface ullage layer is designed and applied to microstrip antenna to reduce its radar cross section (RCS). The results show that the absorber can exhibit an absorption of 99.9% with a thickness of 0.3 mm. Compared with the conventional microstrip antenna, the proposed antenna has an RCS reduction of more than 3 dB in the boresight direction in the working frequency band, and the largest reduction can reach 16.7 dB, the monostatic and bistatic RCS reduction are over 3 dB from -30° to +30° and -90° to +90° respectively, while the radiation performance is kept, which proves that the absorber has an excellent absorptivity and could be applied to microstrip antennas to achieve in-band stealth.
2013, 62 (6): 064201.
doi: 10.7498/aps.62.064201
Abstract +
Image edge detection is an important tool of image processing, in which edge representation and extraction with uncertainty is one of key issues. Based on the physics-like methods for image edge representation and extraction, a novel cognitive physics-based method with uncertainty is proposed. The method uses data field to discover the global information from the image and then to map it from grayscale space to the appropriate potential space. From the point of view of the field theory, the method establishes an extensible theoretical framework and unifies the existing physics-like methods. On the other hand, the method defines the ascending half-cloud to construct the internal relationship between the range of cloud uncertainty degree and the edge representation and extraction. Finally, the method achieves image edge representation and extraction with uncertainty using the cognitive physics. The time complexity of the proposed algorithm is approximately linear in the size of the original image. It is indicated by the quantitative and qualitative experiments that the proposed method yields accurate and robust result, and is reasonable and effective.
2013, 62 (6): 064202.
doi: 10.7498/aps.62.064202
Abstract +
Two resonant radio frequency fields are added to lambda three-level system in this paper. By discussing the behaviors of probing field absorption profiles under the effect of different Rabi frequencies of two radio frequency fields, the splitting of electromagnetically induced transparency (EIT) can be seen and the overlapping between EIT and gain can be obtained. The results show that the two radio frequency fields have different control functions on the system. The radio frequency field which interacts with hyperfine levels of ground state plays a role in the splitting of EIT, but the radio frequency field which interacts with hyperfine levels of excited state does not work on it. In addition only when the Rabi frequency of radio frequency field interacting with hyperfine levels of ground state is greater than with hyperfine levels of excited state, can the new features about the overlapping between EIT and gain be obtained.
2013, 62 (6): 064203.
doi: 10.7498/aps.62.064203
Abstract +
High power Bessel pulses directly output from a fiber-based amplifier system are demonstrated. A compact solution based on the inverse micro-axicon (IMAX) on fiber end is proposed for the conventional ultrashort pulse fiber laser system to enable the direct generation of high power Bessel pulses from lasers without any additional exhausting alignments. The IMAX is fabricated on one facet of a ytterbium-doped large mode area fiber by focusing ion beam technique and constitutes an integrated beam shaper in combination with an inherent collimating lens in the fiber laser system. The experimental results accord qualitatively with the simulations. The system can directly generate chirped Bessel pulses with diffraction-free propagation in meter-scaled free space. The highest average power of such a wavepacket can reach 10.1 W, correspongding to 178 nJ, and the pulse duration can be dechirped to 140 fs.
2013, 62 (6): 064204.
doi: 10.7498/aps.62.064204
Abstract +
A novel design of reflective color filters based on a two-dimensional subwavelength grating structure is proposed, which exhibits an incident angle independent property with unpolarized incident light in the visible range. By using rigorous coupled-wave analysis method, the effects of the grating period, the groove depth and the size of the structure on the reflectance spectrum are investigated in detail. The structural parameters of the gratings are optimized, and a color filter with high angular tolerance is achieved. Simulation result shows that the maximal reflectance is 56% at 424 nm with a bandwidth of 45 nm, and that the grating can almost keep its reflectance, bandwidth and the peak position at the incident angle up to about 60° under unpolarized incident light. The peak position of the color filter can be tuned from 400 nm to 520 nm by changing structural parameters of the gratings, and keep its incident angle-independent property.
2013, 62 (6): 064205.
doi: 10.7498/aps.62.064205
Abstract +
A method of multi-level authentication based on two-beam interference is proposed. By verifying the "password" and "phase key" of one user simultaneously, the system can thus achieve the two-factor authentication on the user's identity. This scheme can not only check the legality of one user, but also verify his identity level as an authorized user and then grant the user the corresponding permissions to access the system resources. While operating the authentication process, which largely depends on an optical setup based on interference, a "phase key" and a password-controlled "phase lock" are firstly loaded on two spatial light modulators (SLMs), separately. Then two coherent beams are respectively, modulated by the two SLMs and then interfere with each other, leading to an interference pattern in the output plane. It is recorded and transmitted to the computer to finish the last step of the authentication process: comparing the interference pattern with the standard verification images in the database of the system to verify whether it is an authorized user. When it turns to the system designing process for a user, which involves an iterative algorithm to acquire an estimated solution of an inverse problem, we need to determine the "phase key" according to a modified phase retrieval iterative algorithm under the condition of an arbitrarily given "phase lock" and a previously determined identity level (corresponding to a certain standard verification image). The theoretical analysis and simulation experiments both validate the feasibility and effectiveness of the proposed scheme.
2013, 62 (6): 064206.
doi: 10.7498/aps.62.064206
Abstract +
Some key parameters of illuminating beam and the influence on imaging quality are investigated via ptychography in visible light band. The influences of overlap ratio, size and shape of illuminating beam on imaging quality and their relationship are studied using ptychographical iterative engine algorithm. The simulation results show that the overlap ratio of illuminating beam is a main factor influencing imaging quality. Shape of illuminating beam mainly influences the convergence of ptychography. And the size of illuminating beam less influences directly the imaging quality and convergence. Therefore, the simulation results play an important theoretic guiding role in optimizing the beam parameters in visible light, the X-ray and electronic band and other bands.
2013, 62 (6): 064207.
doi: 10.7498/aps.62.064207
Abstract +
In this paper, we discuss the generation and evolution of continuous-variable entanglement in a two-mode single-atom laser, where the atomic coherence is induced by two classical microwave fields, which drive the corresponding fine atomic transitions. The results show that the intensity of the microwave field can influence effectively the entanglement properties of the cavity field. In addition, our numerical results also show that the intensity and the period of entanglement between the two cavity modes as well as the total mean photon number of the cavity field can be increased synchronously by adjusting the corresponding frequency detuning.
2013, 62 (6): 064208.
doi: 10.7498/aps.62.064208
Abstract +
Physical model and caculation method are established to describe the laser pulse superpositon in multi-pass amplification process. In this model, the inversion pupulation density is consumed by the pulse leading edge and tailing edge simultaneously. It is demonstrated that this model can not solve the problem of laser superposition amplification in the time-delay coordination. The superposition amplification is solved by building a new time-space coorination. Base on the physical model and caculation method, computer simulation is performed and the pulse shape distortion is discussed at different cavity mirror positions in two-pass amplification process.
EDITOR'S SUGGESTION
2013, 62 (6): 064209.
doi: 10.7498/aps.62.064209
Abstract +
The chaotic ultra-wideband (UWB) pulse signals are generated by directly modulating semiconductor laser subjected to optical feedback. We simulate that the -10 dB bandwidth and the central frequency of the RF spectrum of the chaotic UWB signals are influenced by the bias current and feedback strength. The research results demonstrate that the -10 dB bandwidth of the RF spectrum of the UWB signals increases with the increases of the bias current of the semiconductor laser and the feedback, the central frequency also increases with the increases of the bias current and the feedback. In our experiments, chaotic UWB signals with steerable and flatted power spectrum are generated by directly modulating DFB-LD subjected to optical feedback. The power spectrum of UWB signals is fully compliant with the FCC indoor mask, while a large fractional bandwidth of 133% and a central frequency of 6.6 GHz are achieved. The central frequency and -10 dB bandwidth of the chaotic UWB signals are on a large scale tunable by adjusting the bias current and feedback power. In addition, the chaotic UWB signals transmit through a 34.08 km single mode fiber and the power spectrum does not have any discrete spectrum line.
2013, 62 (6): 064210.
doi: 10.7498/aps.62.064210
Abstract +
The effects of codopants and the deposition parameters such as gas flowrate and pressure in tube during the fabrication of fiber performs are studied. It is found that the fluorescence spectrum of Yb3+ can shift when codoping other elements, according to which, a double cladding Yb3+-doped fiber that is beneficial for 1018 nm laser is fabricated for the first time. When the fiber length is 7 m, an output of 22.8 W at 1018 nm is obtained. The optical-optical conversion efficiency is approximately 70%, and there are neither spontaneous radiation nor saturation.
EDITOR'S SUGGESTION
2013, 62 (6): 064211.
doi: 10.7498/aps.62.064211
Abstract +
A new method of characterizing the femtosecond pulse is proposed based on the self-diffraction process in a thin transparent bulk medium and the self-reference spectral interferometry. A simple device is designed based on this technique and is successful in characterizing a ～40 fs pulse at 800 nm centeral wavelength. The result is in accordance with that measured by a commercial self-reference spectral phase interferometry for direct electric reconstruction. Pulses in a spectral range from deep UV to middle IR are expected to be measured by this new method and corresponding simple device.
2013, 62 (6): 064212.
doi: 10.7498/aps.62.064212
Abstract +
The use of a waveguide perpendicular to boundaries of the microdisk cavity is a newly developed coupling technique. We make a detailed analysis about the adaptation of the cavity size, the adaptation of the wavelength and the expansibility of the ports. The results confirm that the perpendicular coupler is valid in large cavity or multiple bands. It is shown that multiple perpendicular couplers can exchange energy with a microdisk cavity and work as filter, beam splitter, and crossroads of optical path. The usage of perpendicular coupler in integrated optical circuit with microcavity components will make the selection of materials and arrangement of the optical path more flexible.
2013, 62 (6): 064213.
doi: 10.7498/aps.62.064213
Abstract +
According to the nonlinear Schrödinger equation and Poisson equation of thermal diffusion, we investigate the interaction of double beams in (1+2)-dimension thermal nonlocal medium, using the slip-step Fourier algorithm and multi-grid method. The results show that the two beams intertwine with each other during propagation. If the power and the tilt parameter are appropriate, the projections of the trajectories of the beams in (X, Y) plane are approximately circle, even if the incident distance between the beams is changed. Because of the strongly nonlocal property of thermal medium, the influences of boundaries and initial transverse momentum can be felt when beams are far from the boundaries; there will be an oscillatory propagation when the mass center of the input field deviates from sample center or the initial transverse momentum is unequal to zero.
2013, 62 (6): 064214.
doi: 10.7498/aps.62.064214
Abstract +
A novel multi-core large-mode-area few-mode fiber (MC-LMA-FMF) is proposed in this paper. The special structure of air holes makes it operate in few modes (HE11 and HE21 mode only). Numerical analysis shows that the 7-core-LMA-FMF can maintain a stable dual-mode operation and the effective area of the fundamental mode can reach 866.54 μm2. The regular pattern that fiber structure parameters affect mode characteristics and the effective area is investigated, and the similarities and differences brought in by increasing the number of cores is also analyzed. The advanced 19-core-LMA-FMF inherits the few-mode characteristic, meanwhile, the effective area of the fundamental mode can be as high as 3617.55 μm2. Compared with the reported few-mode fibers, MC-LMA-FMF obtains a large effective area and good bending characteristics. These advantages enable this new type of fiber to be a potential candidate for high-speed large-capacity optical fiber transmission systems or high power fiber amplifiers and lasers.
2013, 62 (6): 064215.
doi: 10.7498/aps.62.064215
Abstract +
The optical properties of photonic crystal fiber cladding knot among the three air holes are analyzed. The mode area, nonlinear coefficient and dispersion characteristics of the core and cladding knot are contrasted. Cladding knot of photonic crystal fiber has a small core and highly nonlinear characteristics. For larger cladding air holes, double zero dispersion curves are obtained. According to the dispersion curve, phase-matching features are analyzed for dispersive wave generation. Variation rules of the central wavelength of the dispersive wave with pump power and wavelength are achieved. The photonic crystal fiber designed is fabricated. The visible and infrared broadband dispersive waves above 300 nm are obtained in experiment. Experimental and theoretical results are completely consistent with each other. These are foundation for wavelength conversion and supercontinuum broadband light source.
2013, 62 (6): 064216.
doi: 10.7498/aps.62.064216
Abstract +
The influence of the monitoring point wavelength on the axial strain sensitivity of high-birefringence fiber loop mirror is investigated. The theoretical expression for the axial strain sensitivity of high-birefringence fiber loop mirror is developed. The results show that the sensitivity increases with the wavelength of the monitoring point increasing when the high-birefringence fiber material is certain, that the sensitivity is constant and the wavelength shift is linear versus strain for the certain monitoring point. The axial strain sensitivities of the different wave peaks are monitored in experiment. The experimental results of data fitting are in good agreement with the theoretical ones. The research results help to improve the strain sensitivity, the temperature sensitivity, etc. of high-birefringence fiber loop mirror.
2013, 62 (6): 064217.
doi: 10.7498/aps.62.064217
Abstract +
We numerically study the propagations of Gaussian beams in four types of separation modulated photonic lattices. The results shows that the potential wells between double positive hyperbolic secant and rectangular potential barriers and between the potential barriers in the forms of double negative hyperbolic secant and rectangular functions can both support localized linear modes. Moreover, the coupling effects between two linear modes in the potential barriers can be used to realize all-optical switch. Furthermore, the nonlinear localization can also be observed in high power. Our results supply new ideas for all optical switch, light controlling and manipulation in photonic lattices.
2013, 62 (6): 064301.
doi: 10.7498/aps.62.064301
Abstract +
Underwater sound absorption coating is significant to the stealth of a submarine, so it attracts a lot of attention. Underwater sound absorption of visco-elastic composites coating containing micro-spherical glass shell was investigated theoretically. The mechanical and acoustic properties of the composites in response to the volume of the micro-spherical glass shell were analyzed by the effective parameters method. Sound absorption of a single layer composites coating containing different volume of micro-spherical glass shell was calculated by the one-dimensional model, in which sound propagates in multi-layer media. The calculated results show that the sound absorption at low frequencies can be promoted by increasing the volume of micro-spherical glass shell, but the sound absorption at high frequencies is depressed. The volume distribution of the micro-spherical glass shells across the thickness of the coating was optimized by the genetic algorithm. The optimal multi-layer structure can promote the sound absorption at low frequencies, and keep the sound absorption coefficients above a limited value (0.7) at high frequencies. The optimal multi-layer composite coating can work at high pressure since it does not contain hollow macro-structure. Its structure is simple, so the technique of its fabrication should not be complicated. The theoretical method achieved in this paper can be applied in the design of underwater sound absorption coating.
2013, 62 (6): 064302.
doi: 10.7498/aps.62.064302
Abstract +
Code shift keying (CSK) is generally used to overcome the spreading gain versus data rate limitation in underwater acoustic (UWA) communication as generalized M-ary spread spectrum technology. In addition, the concept of orthogonal CSK is introduced into the UWA communication to achieve higher rate, mitigate crosstalk from the other thoroughfare and utilize the redundant information of CSK adequately. In this paper, we propose a new scheme employing orthogonal double thoroughfare CSK spread spectrum UWA communication with utilizing code phase information combined. First, each symbol integration output form of the proposed method is deduced. Furthermore, the property of orthogonal CSK is analyzed and its bit error rate is investigated as compared with conventional CSK and double thoroughfare CSK via simulation. Finally, the validity of simulation comparison is verified in experiment. 580.6 bps data rate of the proposed communication scheme is realized in 104 bit volume and 4 kHz bandwidth efficiently. It is shown that the proposed method provides significantly improved communication performance through formula, simulation and test.
2013, 62 (6): 064303.
doi: 10.7498/aps.62.064303
Abstract +
Existing detection methods have mismatch problem when applyed to the real uncertain ocean, which will lead to the detection performance degradation. However, there has been little work on defining the practical quantitative measures of environmental sensitivity. In this article we define a measure of environmental sensitivity for target detection performance loss in an uncertain ocean for realistic uncertainties in various environmental parameters (water-column sound speed profile and seabed geoacoustic properties). The Monte Carlo approach is used to transfer the environment uncertainty through the forward problem and quantify the resulting variability in the detection performance loss. The computer simulation is based on the Malta Plateau, a well-studied shallow-water region of the Mediterranean Sea. The simulation result shows that 1) the sensitivity is range and depth dependent and in the sound channel the sensitivity is much smaller than in other regions of the ocean; 2) the sound speed profile and the upper seabed layer are most sensitive parameters for the detection performance loss; 3) the sensitivity is frequency dependent. The seabed layer properties such as sediment thickness, density and attenuation coefficient have less influence on the detection as the frequency increases.
2013, 62 (6): 064304.
doi: 10.7498/aps.62.064304
Abstract +
In order to get the factor of influence of bubbly liquid on the acoustic wave propagation, the linear wave propagation in bubbly liquid is studied. The influence of bubbles is taken into account when the acoustic model of bubbly liquid is established, and we can get the corrected oscillation equation of the bubble when the interaction of bubbles is taken into the Keller's model. One can get the acoustic attenuation coefficient and the sound speed of the bubbly liquid through solving the linearized equation of wave propagation of bubbly liquids and the oscillation equation of bubbles when (ωR0)/c << 1. After the numerical analysis, we find that the acoustic attenuation coefficient increases and the sound speed will turn smaller as the numbers of bubbles increases and the bubbles gets smaller when the driving frequency of sound field keeps constant; when the driving frequency is far bellow the resonance frequency of bubble and both the volume fraction and the size of bubbles are kept constant, the sound speed will changes in a way contrary to the case of driving frequency of sound field; it is not evident that the bubble interaction influences the acoustic attenuation coefficient and the sound speed. Finally, we deem that the volume concentration, the size of bubble and the driving frequency of sound field are the important parameters which determine the deviations of the sound speed and the attenuation from those of bubble-free water.
2013, 62 (6): 064501.
doi: 10.7498/aps.62.064501
Abstract +
By introducing fundamental integrals of one-dimensional linear damped oscillators the other first integrals can be constructed, including time-irrelevant integrals. The above method is extended to multidimensional systems, in order to construct different integrals of two-dimensional and n-dimensional linear damped oscillators. It is proved that there are three independent time-irrelevant integrals for all kinds of two-dimensional linear damped oscillators, and 2n-1 independent time-irrelevant integrals for n-dimensional linear damped oscillators. Using the transformation of variables the first integrals of linear damped oscillator transform into ones of harmonic oscillator.
2013, 62 (6): 064502.
doi: 10.7498/aps.62.064502
Abstract +
A concept of fundamental integrals of one-dimensional harmonic oscillator is presented, and other integrals can be constructed by use of fundamental integrals. The above concept and method are extended to multidimensional harmonic oscillators. By directly constructing other integrals from the fundamental integrals, it is proved that there are three independent time-independent integrals for all kinds of two-dimensional harmonic oscillators and there are 2n-1 independent time-independent integrals for n-dimensional harmonic oscillators. The characteristics of the anisotropic two-dimensional harmonic oscillator is discussed when the ratio between two frequencies is rational or irrational number.
2013, 62 (6): 064701.
doi: 10.7498/aps.62.064701
Abstract +
Hydrodynamic characteristics of a near-wall circular cylinder oscillating in direction perpendicular to steady current are experimentally investigated at a Reynolds number of 2× 105. Forces in both in-line and cross-flow are measured by the three-dimensional force transducers. The effects of gap ratio, oscillating frequency and amplitude on the hydrodynamic charactersistic of the cylinder are studied. Experimental results indicate that 1) mean drag reduces rapidly when the gap ratio decreases from 0.7 to 0.3; 2) for an oscillating cylinder, the critical gap ratio of vortex shedding suppression is smaller than that for a still cylinder; 3) the existence of near-wall influences the energy transfer between the structure and fluid significantly, which means that hydrodynamic coefficient based on free-wall cylinder may not be suitble for predicting vortex induced vibration of pipelines; 4) for an oscillating cylinder, added mass is not a constant except for in a certain range of oscillating frequency, and the absolute value increases with the decrease of gap ratio in low frequency range; 5) mean drag coefficient, oscillating drag coefficient and oscillating lift coefficient all increase with oscillating amplitude increasing.
2013, 62 (6): 064702.
doi: 10.7498/aps.62.064702
Abstract +
In this paper, we present a numerical simulation of a single liquid drop impacting onto solid surface with smoothed particle hydrodynamics (SPH). SPH is a Lagrangian, meshfree particle method, and it is attractive in dealing with free surfaces, moving interfaces and deformable boundaries. The SPH model includes an improved approximation scheme with corrections to kernel gradient and density to improve computational accuracy. Riemann solver is adopted to solve equations of fluid motion. An new inter-particle interaction force is used for modeling the surface tension effects, and the modified SPH method is used to investigate liquid drop impacting onto solid surfaces. It is demonstrated that the inter-particle interaction force can effectively simulate the effect of surface tension. It can well describe the dynamic process of morphology evolution and the pressure field evolution with accurate and stable results. The spread factor increases with the increase of the initial Weber number. The numerical results are in good agreement with the theoretical and experimental results in the literature.
2013, 62 (6): 064703.
doi: 10.7498/aps.62.064703
Abstract +
Traditional studies on bubble dynamics near solid boundaries mainly focus on its pulsation and jet features near a full plate. A hole will be formed when a warship is attacked by an underwater weapon and it may be subjected to a subsequent attack generated by charge explosion. And the hole on the plate would affect the blow effect of the underwater explosion bubble nearby. To study the bubble pulsation and jet features near a plate with a hole in the middle, a series of experiments is carried out using a spark bubble generator and high-speed camera. We find that when a bubble is generated homocentricly near the hole, cavity-attraction effect of the bubble will be formed due to the effect of the hole, and the opposite-jets can then be formed. Then the influences of dimensionless standoff distance and hole size are analyzed. Finally, the dynamic behavior of a bubble which is generated decenteredly near the hole is studied to show that the blow effect of a bubble increases with decentered position increasing.
2013, 62 (6): 064704.
doi: 10.7498/aps.62.064704
Abstract +
In a stratified fluid tank, experiments on the propagating, shoaling and breaking of the internal solitary waves over a gentle slope similar to the topography in the northeast of the South China Sea are conducted. The qualitative analysis on the evolving characteristics of the internal solitary waves is accomplished by use of the dye-tracing technique, and their quantitative measurement is carried out by using the multi-channel conductivity-probe arrays. It is shown that due to the shoaling effect the internal solitary waves with large amplitude are restrained, but the waves with small amplitude are magnified. The shoaling effect will also lead to the decrease of the propagation velocity of the internal solitary waves. Further, the shoaling effect will bring about strong shear flow instability, and then makes the internal solitary wave broken. The breaking wave will result in the fission from one large amplitude wave into several small amplitude waves with the same polarity. By means of the Mile's stability theory, the instable happening-location of the internal solitary wave over the gentle slope can be described through the Richardson number. The experimental results accord well with the theoretical analyses.
2013, 62 (6): 064705.
doi: 10.7498/aps.62.064705
Abstract +
A new conservative interaction potential with short-range repulsion and long-distance attraction is constructed by a quartic equation function. The multiphase flow through a cross-shape mesoscopic channel is simulated by dissipative particle dynamics with this new potential function. The results show that the new method is capable of simulating the flow process and flow pattern.
PHYSICS OF GASES, PLASMAS, AND ELECTRIC DISCHARGES
2013, 62 (6): 065201.
doi: 10.7498/aps.62.065201
Abstract +
An inactivation mechanism study on Hela cancer cells by means of an atmospheric pressure cold plasma jet is presented. Cell morphology is observed under an inverted microscope after plasma treatment. The neutral red uptake assay provides quantitative evaluations of cell viability under different conditions. The effect of the inactivation efficiency of Hela cancer cells in the argon (900 mL/min) with addition of different amount of oxygen (1%, 2%, 4%, 8%) into atmospheric pressure cold plasma jet is discussed under the fixed power 18 W. Results show that 2% O2 addition provides the best inactivation efficiency, and the survival rate can be reduced to 7% after 180 s treatment. When the oxygen addition exceeds 2%, the inactivation efficiency gradually weakens. The effect is not so good as that in pure argon plasma when the oxygen addition arrives at 8%. According to the emission spectrum of the plasmum, it is concluded that the reactive oxygen species in the plasma play a key role in cancer cell inactivation process.
CONDENSED MATTER: STRUCTURAL, MECHANICAL, AND THERMAL PROPERTIES
2013, 62 (6): 066201.
doi: 10.7498/aps.62.066201
Abstract +
Mechanical properties of titanium and iron co-doped Sapphire crystal are first studied at room temperature Large (ø180 × 280 mm3 in dimension 30 kg in weight) titanium and iron codoped sapphire single crystal is grown by the Kyropoulos technique It is shown that the fracture strength and surface hardness and fracture toughness of as-grown crystals are significantly improved and the visible-infrared optical property is not adversely affected by titanium and iron codoping and certain heat treatment. The Fe3+ in the doped Fe2O3 palys a role of substituting Al3+, leading to an in creased internal stress in the crystal. And the Ti4+ in the doped TiO2 crystallizes the second phase needle crystal and brings in a toughening effect through certain heat treatment. As a consequence, the mechanical properties of as-grown sapphire are improved at room temperature. The present work has the realistic significance for developing the sapphires of excellent mechanical properties.
2013, 62 (6): 066801.
doi: 10.7498/aps.62.066801
Abstract +
In the present paper, we synthesize CdSe quantum dots (QDs) that are stabilized by thioglycolic acid according to the water-phase synthesis. The X-ray diffraction and HRTEM results confirm that the samples prepared each possess a sphalerite structure. The EDS and FT-IR spectra of the samples show that a core-shell structure is formed between the CdSe nanoparticles and the ligand. The fine band structures and the characteristics of the surface states in a connection with the structures are identified by the surface photovoltage (SPV) spectrum of the samples. Two SPV response peaks, located at 475 nm (2.61 eV) and 400 nm (3.1 eV), are closely related to the band-band transitions of the core-CdSe and the shell-CdS, respectively; the SPV response at 370 nm (3.35 eV) is correlated with the n → π* transition between the hydroxyl and sulfydryl (or hydroxyl). It is because of an obvious quantum size effect of the samples that both PL line broadens and SPV response intensity increases with the decrease of the grain size of the sample. The change trend of the surface photoacoustic signal intensity is contrary to that of the SPV response intensity of the samples synthesized at varying pH. Moreover, the fine band structures at surfaces and grain boundaries of CdSe QDs prepared are probed by the SPV spectra of the samples at varying pH values. The relationship between the grain size and the photo-generated carrier transport behavior is discussed according to the detected EFISPV results of the QDs.
2013, 62 (6): 066802.
doi: 10.7498/aps.62.066802
Abstract +
As a lead-free piezoelectric material with potential application, 0.5Ba(Ti0.8Zr0.2)O3-0.5(Ba0.7Ca0.3)TiO3 (BZT-0.5BCT) ceramics, which has a morphotropic phase boundary composition, deserves much attention due to its excellent ferroelectric and piezoelectric properties. BZT-0.5BCT lead-free piezoelectric film has been synthesized on a Si (100) substrate by Sol-Gel process. The topography of the film measured using an atomic force microscope and a scanning electron microscope shows that the surface of the prepared film is smooth, and the grain is in the shape of hemisphere with a diameter of 80-100 nm. The film is 1.7 μm in thickness, with pores inside. Friction experiments show that the friction between the tip and the piezoelectric film is much larger than that between the tip and the SiO2 substrate, because of the existence of electrostatic force between the film and the silicon tip. However, the friction coefficients obtained are approximately equal. Nano-scratch experiments show that the BZT-0.5BCT film has a high normal carrying capacity, but a poor tangential wear resistance. The average elastic modulus of the film is 23.64 GPa ± 5 GPa, and its hardness is 2.7-4 GPa, both being slightly lower than those of the bulk value in PZT ceramics.
CONDENSED MATTER: ELECTRONIC STRUCTURE, ELECTRICAL, MAGNETIC, AND OPTICAL PROPERTIES
2013, 62 (6): 067301.
doi: 10.7498/aps.62.067301
Abstract +
Based on the tight-binding model, the electronic state and band of zigzag graphene nanoribbons are given analytically by a new method. The results show that there are only two kinds electronic states, i.e., the standing wave state and edge state. For the standing wave state, the wave function is sine function and the vector is real; for the edge state, the wave function is hyperbolic sine function and the vector is complex, whose real part is 0 or π/2. The energy band is composed of the energy of standing wave state and the energy of edge state. The accurate ranges of infinite direction wave vector and energy of the edge state are deduced. Then we discuss the transition point between the edge state and the standing wave state and find that the two kinds of electronic states tend to the linear relationship regarding the site of carbon lattice in different ways at the phase transition point. When the width of two restricted boundary goes to infinity, the result of the limited graphene tends to the infinite case.
2013, 62 (6): 067701.
doi: 10.7498/aps.62.067701
Abstract +
Combining nonequilibrium Green's functions and first-principles quantum transport calculations in density-functional theory, we investigate the effect of biaxial strain on the leakage current of BaTiO3 ferroelectric thin film. The results show that the compressive strain can effectively reduce the leakage current of ferroelectric thin film. Especially when the compressive strain is 4%, the leakage current will be reduced by nearly 10 times that of strain-free case. By calculating the transmission coefficient and the density of states, we find that the transmission probability of ferroelectric tunnel junction with compressive strain is smaller than that with tensile strain. Moreover, we find that the valence band shifts toward the lower energy zone while the conduction band moves toward the high energy zone, which leads to the enlarged energy band gap, thereby reducing the leakage current. Our study suggestes a suitable way to reduce the ferroelectric thin film leakage current and improve the performance of ferroelectric thin film and its relevant ferroelectric memory.
2013, 62 (6): 067702.
doi: 10.7498/aps.62.067702
Abstract +
The accuracy of geomagnetic reference map, geomagnetic sensor, and inertial navigation system is the key factor which affects the reliability of magnetic contour matching (MAGCOM) system. In order to improve the reliability of MAGCOM, the influence of the sensor's accuracy on the matching success rate has been studied. For different geomagnetic reference maps, the factors and their physical mechanism affecting matching success rate have been analyzed, the factors are geomagnetic sensor error, velocity error, and heading angle error. The error range of sensors was determined by the simulation computing which was based on a practical system and a simulation experimentation was implemented by using the geomagnetic reference map. The simulation results show that MAGCOM can allow the velocity error 0.14 m/s, the heading angle error 0.6 and the standard deviation of geomagnetic sensor noise 11 nT, when the matching success rate is 90%.
2013, 62 (6): 067801.
doi: 10.7498/aps.62.067801
Abstract +
In this work we explore the influence of the exciton recombination zone (RZ) on magnetic-field effect in tris-(8-hydroxyquinolinato) aluminum (Alq3) based organic light-emitting diodes by changing the thickness of Alq3. The magneto-electroluminescence and magneto-conductance (MC) in these devices are investigated at various temperatures and bias voltages. It is found that the sign of MC changes from positive to negative, and then back to positive with the reduction of the thickness of Alq3 at 50 K. The phenomenon observed is ascribed to the change of the exciton density in the exciton RZ. Based on the mechanisms including the hyperfine mixing, the triplet-charge interaction and interfacial dissociation or quenching of excitons, the observed results are explained qualitatively.
2013, 62 (6): 067802.
doi: 10.7498/aps.62.067802
Abstract +
The single isolate vacancy in diamond exists in three charged states, neutral, negative and positive; and many complicated defects such as di-vacancies, impurities-vacancy complexes could also be formed in diamond. In this paper, we investigate the optical properties of the irradiation-induced neutral vacancy in diamond by low-temperature micro-photoluminescence technology, which will play a guiding significant role in the further studies of the complex defects in diamond.
INTERDISCIPLINARY PHYSICS AND RELATED AREAS OF SCIENCE AND TECHNOLOGY
2013, 62 (6): 068101.
doi: 10.7498/aps.62.068101
Abstract +
Sub-diffraction-limit fabrication of 6H-SiC is investigated with femtosecond laser direct-write setup. Micro/nano-fabrication on 6H-SiC is studied with a home-made micro/nano-fabrication platform, which is integrated with a fluorescence microscope and a Ti:sapphire laser with a central wavelength of 800 nm and pulse duration of 130 fs. Micro/nano-structures are characterized with scanning electron microscope. It is found that the spatial resolution is improved with the decrease of laser power and the increase of scanning velocity. The smallest resolution achieved is 125 nm and line array with a line width of 240 nm and a period of 1 μm is fabricated. This work paves the new way for integrated micro electro-mechanical systems devices.
2013, 62 (6): 068102.
doi: 10.7498/aps.62.068102
Abstract +
Using nonequilibrium Green's function method, the thermal transport properties of zigzag graphene nanoribbons (ZGNR) embedding a finite (semi-infinite or infinite) long linear defect are investigated in this paper. The results show that defect type and defect length have significant influence on the thermal conductance of ZGNR. When the embedded linear defects have the same lengths, thermal conductance of ZGNR embedding t5t7 defect is lower than that of ZGNR embedding Stone-Wales defect. As for the ZGNR embedding finite and the same type defects, their thermal conductance reduce with the increase of the defect length. However, as the linear defect is long enough, the thermal conductance is insensitive to the change of length. By comparing the ZGNRs embedding finite, semi-infinite and infinite long defects, we find that the thermal conductance of ZGNR embedding an infinite long defect is higher than that of ZGNR embedding a semi-infinite defect, while the thermal conductance of the latter is higher than that of ZGNR embedding a finite long defect. This is due to the fact that different structures possess different numbers of scattering interfaces in the phonon transmission direction. The more the scattering interfaces, the lower the thermal conductance is. These thermal transport phenomena are explained by analyzing transmission coefficient and local density of states. These results indicate that linear defects can tune thermal transport property of ZGNR efficiently.
2013, 62 (6): 068103.
doi: 10.7498/aps.62.068103
Abstract +
Nanoimprint lithography has the advantages of low-cost, high-throughput, ultrahigh resolution, which could make it one of the next generation lithography technologies. However, the bubble-defect is always a problem which may damage the duplicate patterns, so it is an urgent issue to propose effective solutions. A novel methods, which is suitable for compressional gas cushion press nanoimprint lithography in gas atmosphere and could prevent gas from entering the gap between mold and substrate, is presented here. The annular plate capillary gap formed between the smooth substrate and the prominent O-ring processed by etching the original mold would be filled with the fluid medium. The capillary liquid bridge between the O-ring and substrate produces a closed cavity. The stiction induced by adhesion force and the capillary force induced by air-liquid surface tension could resist the compressed gas and avoid the bubble defect. The effective widths of the prominent O-ring, which are different for various fluids with different surface properties, are deduced by theory analysis. The analysis results provide theoretical basis for the preparation of the mold.
2013, 62 (6): 068104.
doi: 10.7498/aps.62.068104
Abstract +
Locked-to-sliding phase transition is studied based on the two-dimensional Frenkel-Kontorova model in this paper. The method of molecular dynamics simulation is used. The effect of the static friction force on system parameter is investigated numerically when the upper layer atoms are of the hexagon symmetric structure.
First-principles studies of the structural and thermodynamic properties of TiAl3 under high pressure
2013, 62 (6): 068105.
doi: 10.7498/aps.62.068105
Abstract +
In this paper, the structural properties of TiAl3 intermetallics are investigated by the plane-wave pseudopotential density functional theory method. The calculated results are consistent with experimental and other theoretical ones. Through the quasi-harmonic Debye model we calculate the thermodynamic properties and obtain the dependences of relative volume V/V0 on pressure P and temperture T, as well as the thermal expansion and specific heat coefficients under different temperatures and pressures. For the calculated results of TiAl, we find that the increase rate of thermal expansion coefficient of TiAl under the increase of temperature is higher than that of TiAl3, and further, the effect of temperature weakens with the increase of pressure. The specific heat of TiAl3 is nearly twice that of TiAl.
2013, 62 (6): 068201.
doi: 10.7498/aps.62.068201
Abstract +
We study the control of electron localization in the dissociation of H2+ using three laser pulses by numerically simulating the time-dependent Schrödinger equation. First, we use an attosecond pulse to excite the wave packet of H2+ from 1sσg to 2pσu. Then, two-color femtosecond pulses (800 nm+400 nm) are used to control the dissociation of H2+. By manipulating the phases of two femtosecond pulses, the electron localization can be controlled effectively. For the proper laser parameters, the maximal probability that the electron is located on the selective nucleus is up to 90%. This theoretical scheme can be realized by the state-of-art laser technology.
2013, 62 (6): 068401.
doi: 10.7498/aps.62.068401
Abstract +
Wind direction retrieval depending on other background sources, e.g., the visible wind-induced streaks, numerical weather prediction model data, scatterometer data and buoy data is the key problem existing in the ocean wind field retrieval using airborne synthetic aperture radar (SAR) data based on geophysical model function which influences the wind speed and direction retrieval accuracies. To solve this problem, a new ocean wind field retrieval method is proposed, with which the wind speed and direction are estimated simultaneously through using the normalized radar cross sections corresponding to different incidence angles and geophysical model function according to the sounding characteristics of airborne SAR. To evaluate the ocean wind field retrieval errors and effects, the simulated data and C band airborne SAR data are used to obtain the wind speed and direction by the proposed method. The verification results show that the wind field retrieval method is suited to retrieve highly accurate wind speed and direction from airborne SAR sounding data without other background sources. The major error can be explained by the insufficient accuracy in calibration of the NRCS for wind speed and wind direction retrieval. The wind speed error increases with the value of speed increasing and at high wind speeds exceeding 18 m/s the error increases distinctly. The value of wind speed has no obvious influence on wind direction retrieval accuracy.
2013, 62 (6): 068501.
doi: 10.7498/aps.62.068501
Abstract +
A two-dimensional electrothermal model of the bipolar transistor (BJT) is established, and the transient behaviors of the BJT originally in the forward-active region are simulated with the injection of electromagnetic pulse from the base. The results show that the damage location of the BJT shifts with the amplitude of the pulse. With a low pulse amplitude, the burnout of the BJT is caused by the avalanche breakdown of the emitter-base junction, and the damage location lies in the cylindrical region of this junction. With a high pulse amplitude, the damage first occurs at the edge of the base closer to the emitter due to the second breakdown of the p-n-n+ structure composed of the base, the epitaxial layer and the substrate. The burnout time increases with pulse amplitude increasing, while the damage energy changes in a decrease-increase-decrease order with it, thus generating both a minimum value and a maximum value of the damage energy. A comparison between simulation results and experimental ones shows that the transistor model presented in the paper can not only predict the damage location in the BJT under intense electromagnetic pulses, but also obtain the damage energy.
2013, 62 (6): 068502.
doi: 10.7498/aps.62.068502
Abstract +
For avoiding the invalid acceleration experiments caused by the changes of the failure mechanism, the relationship of the failure mechanism consistency and the parameters of degradation data distribution in the early stage under different accelerated stress levels has been derived. Conditions for judging the failure mechanism consistency are also given as follows: firstly, the shape parameters of failure distribution has a uniform distribution mi=m, i=1,2,3,···; secondly, the dimension parameter ηi follows the equation ηi=AFi·η. A method to rapidly discriminate the consistency of failure mechanism under different experimental stresses in the early stage was obtained, and the invalid acceleration experiments caused by the changes of the failure mechanism could be avoided. Finally, theoretical degenerate data in the early stage of the accelerated test and the initial degenerate data of the MCM thick-film resistor were used for estimating, Weibull distribution parameter, and the consistency of failure mechanism degradation was also judged.
2013, 62 (6): 068701.
doi: 10.7498/aps.62.068701
Abstract +
The inhomogeneous tissue equivalent water thickness correction method is an important part of research in proton radiotherapy. In this paper, we simulate the transport processes of a high-energy proton beam being injected into the water and other materials using Monte-Carlo multi-particle transport code Fluka, and according to the energy deposition distribution we obtain the depth of the Bragg peak when the protons are injected into different materials. Then we fit an analytic formula (R = αE0p) to the relationship between initial proton energy and the depth of the proton Bragg peak in different materials. It is found that for the different energies of proton beam being injected into non-uniform organization, the difference between the Bragg peak depth from fitting and the depth of the proton beam Bragg peaks from Fluka program is less than 1 mm. If we can establish a database about the relationship of Bragg peak ratio between medium and water, with electron density, then the equivalent water thickness correction method will be able to applied to the dose calculation of for homogeneous medium in proton therapy.
2013, 62 (6): 068702.
doi: 10.7498/aps.62.068702
Abstract +
To satisfy the need of low-dose and high-speed in practical application of hard X-ray differential phase contrast imaging, according to the theoretical analysis and the optimal design of parameters for the experimental system, we propose a two-stepping phase shift algorithm to retrieve the object phase information. The method can effectively reduce the radiation dose and substantially improve the speed of retrieving phase information, which lays the foundation for the X-ray phase contrast imaging in medical and industrial applications.
2013, 62 (6): 068703.
doi: 10.7498/aps.62.068703
Abstract +
An all-fiber Fourier domain mode locking (FDML) swept laser source at 1300 nm for swept source optical coherence tomography is reported. The swept laser source is realized with power amplification and laser resonator which includes gain medium, tunable filter and dispersion managed delay line. FDML swept laser can realize high-speed tuning, and phase is stable since its highly stable mode locking operation. The turning range of fiber Fabry-Perot tunable filter (FFP-TF) based FDML swept laser is 130 nm, and the 3 dB bandwidth is 70 nm with an average output power of 11 mW. The tunable speed of FDML laser is 48.12 kHz compared with 8 kHz of short-cavity FFP-TF based swept laser. The axial resolution in OCT imaging of FDML swept laser is 7.8 μm (in tissue), which is improved by 1.9 μm compared with that of short-cavity swept laser.
2013, 62 (6): 068704.
doi: 10.7498/aps.62.068704
Abstract +
Symbolic partial mutual information is proposed in this paper, which is based on partial mutual information. This algorithm can be used to analyse the coupling between multivariate time series. We use this method to treat and analyse the sleeping multivariate bioelectricity signal (MBS) and wake one, it turns out that the coupling of wake MBS is obviously bigger than that of sleeping MBS. Finally hypothesis testing is done to prove that this method works and the average energy dissipation can be used as a parameter to detect nonequilibrium.
GEOPHYSICS, ASTRONOMY, AND ASTROPHYSICS
2013, 62 (6): 069201.
doi: 10.7498/aps.62.069201
Abstract +
To expand the torrential rain which is a meso-and-micro scale weather process to a meso-and-long scale weather process, in this paper we choose the middle and lower reaches of the Yangtze River (MLRYZ) as a sample region, and propose the conception of "Cumulative Effect" of torrential rain (CETR) by using the daily precipitation observational data from 740 stations in China. On the statistical analysis of observations, we define CETR as the cumulation or superposition of many torrential rain processes, and three indexes, which are continuous time (Ld), control area (Ar) and precipitation contribution rate (Qs), which are used for explaining the conception of CETR. Then taking these three indexes into consideration, we establish the intensity index of CETR (BQDI) and study the relationship between the BQDI and the summer precipitation in MLRYZ. Results show that the interannual and interdecadal variations of BQDI are similar to those of summer precipitation in MLRYZ. The distribution of correlation coefficient between the BQDI and the summer precipitation in Eastern China and the composite analysis of representative years in BQDI show a large positive relation area in MLRYZ (significance test at the 95% level) and two large negative relation areas in North and South China (significance test at the 95% level), which reveals that the variations of BQDI not only correspond to the variations of summer precipitation in MLRYZ but also correlate with the distribution of summer precipitation in Eastern China to some extent. Besides, an empirical orthogonal analysis is performed on the frequency of torrential rain in MLRYZ, we find that the four major spatial modes of torrential rain are also similar to those of summer precipitation in MLRYZ. In conclusion, the precipitation caused by CETR greatly influences even determines the amount and distribution of summer rainfall, which is worth further investigating.
2013, 62 (6): 069202.
doi: 10.7498/aps.62.069202
Abstract +
There are always high false alarm ratios when warning against the severe hail with the severe hail index (SHI) which is supplied by digital weather radar system. To solve this problem, the extraction algorithm with several novel features, such as "overhang", is designed and realized, and these features can describe the severe hail conceptual model from different aspects. Then we take short-time heavy rainfall cells which are easy to be confused with severe hail cells as counter examples to perform statistic analysis for these features and the SHI. Test results show that they have more significant difference between two kinds of samples and hence each of them can reflect one aspect characteristic of severe hail cells. Then a severe hail recognition model that is the Support Vector Machine with radial primary kernel function is learned. Finally, the normalized distance between the sample to be recognized and the optimal separating hyper-plane is determined as a new SHI for warning against the severe hail. Experimental results show that the method proposed in this paper makes severe hail hit ratio higher than the SHI being used and the false alarm ratio is reduced substantially.
Summer precipitation response to the length of the preceding winter over yangtze-huaihe river valley
2013, 62 (6): 069203.
doi: 10.7498/aps.62.069203
Abstract +
By using NCEP/NCAR reanalysis datasets, the length of preceding winter (LPW) in the Yangtze-Huaihe River valley (YHRV) from 1961 to 2011 is derived. We investigate the variation of LPW and the relationship between LPW and following summer precipitation, and the results indicate that LPW clearly displays interannual and decadal changes in the period of 1961-2011. The variation of LPW is closely related to temperature, pressure and meridional wind speed, statistical analysis indicates that a longer LPW corresponds to a lower temperature, a higher pressure and a stronger meridional wind, which shows that temperature, pressure, meridional wind are probably the key factors of adjusting the LPW. These characteristics also vary from region to region. There is significantly positive correlation between the summer precipitation and LPW. The statistical analysis also indicates that the longer (shorter) the LPW, the more (less) the summer precipitation in YHRV is. The comprehensive analysis of the circulation field indicates that when LPW is significantly longer than climatic status, a blocking situation is formed easily in the region of Ural Mountains and the Sea of Okhotsk in the summer, which will affect the summer rainfall in YHRV. By using singular value decomposition method, it is found that the relationship between summer precipitation and LPW is also very significant.
2013, 62 (6): 069701.
doi: 10.7498/aps.62.069701
Abstract +
In order to improve the denoising quality of the pulsar signal, an empirical mode decomposing method (EMD) of pulsar signal denoising based on mode cell proportion shrinking is proposed. Firstly, the pulsar signal is decomposed into a series of intrinsic mode functions (IMF), and the part between the two adjacent zero-crossing within IMF is defined as a mode cell. Then, the optimal proportional shrinking factor is constructed by treating mode cell as the basic unit of analysis. Finally, the all mode cells within IMF are denoised by proportion shrinking, and the mode cell proportion shrinking denoising model is established. The experimental results show that compared with the two EMD denoising algorithms based on coefficient threshold and mode cell threshold, the proposed method can more effectively remove the pulsar signal noise, with better preserving the useful detail information in the original signal.
