Vol. 64, No. 15 (2015)
2015, 64 (15): 150301. doi: 10.7498/aps.64.150301
In recent years, the PM2.5 air pollution has been increasingly serious, which not only affects the air quality and visibility, but also has effects on free space optical signal transmission. However, the research about the relationship between the PM2.5 air pollution and the free space quantum communication has not yet been started. To investigate this relationship, the PM2.5 distribution function and its chemical extinction should be analyzed first. According to the degree of PM2.5 atmospheric pollution and the humidity of the atmosphere, the relationships among the PM2.5 index, the humidity of the atmosphere and the channel attenuation of the free space quantum communication can then be established. According to the amplitude damping channel and the depolarizing channel, the effects of the degree of PM2.5 air pollution on channel capacity, channel average fidelity, channel bit error rate are put out and simulated finally. Simulation results show that, if the air humidity is 30% and the PM2.5 index is 50, the channel capacity, channel average fidelity and the channel bit error rate of free space quantum communication will be 0.83, 0.91 and 0.0048 respectively. While the air humidity is 30% and the PM2.5 index is 300, the above channel parameters will be respectively 0.21, 0.56 and 0.0192. Further more, the channel average fidelity has an obvious difference between the two kinds of channel, and it is also related to the probability of the value of the source characters. Thus, the degree of PM2.5 air pollution has a significant effect on the performance of free space quantum communication. And, in order to improve the reliability of quantum communication in free space, the parameters should be adjusted adaptively based on the status of PM2.5 air pollution.
2015, 64 (15): 150501. doi: 10.7498/aps.64.150501
Under the effect of external driving force and noise, a directed transport model for coupled particles in a two-dimensional potential is established. Here, a one-dimensional potential is taken as the periodic piecewise ratchet potential, and the other one is taken as the periodic symmetric non-ratchet potential to which the external periodic driving force and noise are applied. According to the nonequilibrium statistical theory and the nonlinear dynamics, the transport characters of the coupled system in the overdamped case are researched and discussed. Numerical results show that an obvious directed transport can appear both in the ratchet potential and in the non-ratchet potential case. But, the average velocities of the coupled system in the two potentials have completely different dependence on the system parameters. In the case of ratchet potential, the average velocity is strongly dependent on the coupling intensity, noise intensity, the driving strength, and the particle population; the average velocity can reach the maximum at appropriate coupling intensity, noise intensity, the driving strength or the particle population. Otherwise, in the case of non-ratchet potential, the average velocity is strongly dependent on the barrier height for the non-ratchet potential, but fluctuates as the coupling intensity, the driving strength, the driving initial phase difference or the particle population varies. This shows that the average velocity of the coupled system in the non-ratchet potential has weak dependence on system parameters, including the coupling intensity, the driving strength, the driving initial phase difference and the particle population.
2015, 64 (15): 150502. doi: 10.7498/aps.64.150502
In a passive target tracking system, the position and velocity of a target can be estimated based on time difference of arrival (TDOA) and frequency difference of arrival (FDOA) received by different stations. But TDOA and FDOA equations are nonlinear, which make the target tracking become a nonlinear estimation problem. To solve the nonlinear estimation problem, the most extensive research algorithms are those of extended Kalman filter (EKF), particle filter (PF), unscented Kalman filter (UKF), quadrature Kalman filter (QKF), and cubature Kalman filter (CKF). But the existing algorithms all come up with shortcoming in some way. EKF only retains the first order of the nonlinear function by Taylor series expansion, which will bring large error. PF has to face the degeneracy phenomenon and the problem of large computational complexity. The standard UKF is easy to become divergence in a high dimensional state estimation. QKF is sensitive to the dimension of state, and the calculation is of exponential growth with the growth of dimension. Although CKF can effectively improve the shortcomings, the discarded error is proportional to the state dimension, which may be large in high dimensional state. In view of the above problems, this paper presents an orthogonal cubature Kalman filter (OCKF) algorithm. This algorithm reduces the sampling error by introducing special orthogonal matrix to change the method of cubature sampling based on CKF. It eliminates the dimension impact on the sampling error. In the absence of additional computation, it effectively improves the tracking precision. Simulation results show that, based on the TDOA and FDOA, compared with the EKF and CKF algorithms, OCKF algorithm can improve the tracking performance significantly.
2015, 64 (15): 150701. doi: 10.7498/aps.64.150701
In order to build a robust background model and improve the accuracy of the foreground object detection, we give a comprehensive consideration on the same location pixels of the relevance of time and the correlation of space with its adjacent pixels; and based on the classic ViBe of random algorithm ideas, a kind of complex background model and foreground detection method is proposed. Using the first n series of images to initialize the background model with the sample consistency principle, we can avoid the appearance of the “Ghost” phenomenon; and get the difference between each pixel and its multiple sample value in the background model, and then compute the sum and the average. The average shows the dynamic degree of the background point which is the corresponding pixel background of dynamic feedback information. We get the adaptive clustering threshold and adaptive updating threshold with the dynamic feedback to make random clusters realize the adaptability to dynamic background and combine the global disturbance threshold with the local pixel level judgment threshold to implement the immunity of illumination with slow changes, fast changes or sudden changes, so that we can segment the prospect target accurately. By selecting neighborhood pixels to update the neighborhood background randomly in terms of spatial information dissemination mechanism, a good detection effect is obtained in the case of camera shake. Through multiple sets of test data, experimental results show that this algorithm can significantly improve the adaptability and robustness of the background model such as dynamic backgrounds, illumination changes, and camera shake. The algorithm can well apply to the occasion of moving targets in infrared image detection, and expand its application range. Without any image preprocessing and morphological post-processing, the original detection accuracy of foreground is superior to other algorithms.
2015, 64 (15): 150702. doi: 10.7498/aps.64.150702
As the working frequency of a vacuum electron device reaches the terahertz frequency band, the cross section of the surface wave oscillators (SWO) becomes very small, and the micro-fabrication precision of the device cannot be guaranteed, at the same time, because the electromagnetic field of SWO concentrates on the inner surface in slow wave structure, when the working voltage of surface wave oscillator is very high, the explosive emission probability of the slow wave structure increases greatly, and the explosive emission can influence the working characteristic of the device. This paper analyses the distributing property of the electrical field in the slow wave structure of 0.14 THz SWO. Parameters of the SWO under study are as follows: working voltage is 312 kV, explosive emitted current is 1.67 kA, periodic length of the slow wave structure is 0.7 mm, width of the slot is 0.4 mm, and the height of it is 0.3 mm; cold-test results indicate that the amplitude of the electrical field in the slow wave structure varies sinusoidally; the amplitude of the electrical field reaches a maximum value in the middle of the slow wave structure near its inner surface, and the explosive electron emission can occur most possibly in this position, because the electrical field in the slow wave structure varies with very high working frequency. The explosive emitted electron may bombard back the slow wave structure, and the secondary electrons will be emitted at a certain probability, for which the formula proposed by Vaughan is used to compute the secondary emission yield, and this formula is implemented in the self-developed particle-in-cell code UNIPIC; while the code is used to simulate 0.14 THz SWO with explosive emission in the slow wave structure. In the simulation, the slow wave structure multipactor discharge induced by electrons is also considered; the phase space of the electrons emitted from the slow wave structure shows that the energy of secondary electrons is below 5 keV, so the validity for secondary electron yield is affirmed. Numerical simulation results indicate that because the emitted electrons from the slow wave structure change the distribution character of the electrical field in the slow wave structure, especially the amplitude of the electrical field in the middle of the slow wave structure, the beam-wave interaction is weakened, and as a result, output power decreases from about 22.6 megawatts to only 1.89 megawatts.
ATOMIC AND MOLECULAR PHYSICS
Theoretical study on spectroscopic properties and predissociation mechanisms of the electronic states of carbon monofluoride
2015, 64 (15): 153101. doi: 10.7498/aps.64.153101
The potential energy curves of twenty-five Ω states generated from the eleven Λ-S states (X2Π, a4Σ-, A2Σ+, B2Δ, 14Π, 12Σ-, 24Π, 14Δ, 14Σ+, 22Σ- and 24Σ-) of the carbon monofluoride are calculated using the internally contracted multireference configuration interaction approach with the Davidson modification (icMRCI+Q) in the correlation-consistent aug-cc-pV5Z and aug-cc-pV6Z basis sets, for the first time so far as we know. The spin-orbit coupling, core-valence correlation, and relativistic corrections are taken into account, and all the potential energy curves are extrapolated to the complete basis set limit by separately extrapolating the Hartree-Fock and correlation energies scheme. Based on the calculated potential energy curves, the spectroscopic parameters of the bound and quasibound Λ-S and Ω states are obtained, and a very good agreement with experiment is achieved. It demonstrates that the spectroscopic parameters of A2Σ+(1st well), 24Π Λ-S and the eleven Ω states reported here for the first time can be expected to be reliably predicted results. The 24Π quasibound state caused by avoiding crossings are found, and the important electronic configurations of the bound and quasibound Λ-S states near the equilibrium positions Re are given. Various crossings in curves of Λ-S states are revealed, and with the help of our computed spin-orbit coupling matrix elements, the predissociation mechanisms of the a4Σ- and B2Δ states are analyzed. Dissociation relationships and dissociation channels of the twenty-five Ω states also are given. The transition properties of the A2Σ+-X2Π transitions are finally predicted, and our computed Franck-Condon factors and radiative lifetimes match the available experimental results very well.
2015, 64 (15): 153102. doi: 10.7498/aps.64.153102
Density functional theoretical (B3PW91) method with LANL2 DZ basis sets has been used to study the equilibrium structure, total energy, the highest occupied molecular orbital (HOMO) energy level, the lowest unoccupied molecular orbital(LUMO) energy level, energy gap, dipole moment, atomic charge distribution, infrared intensities of CdSe ground state molecule etc. in different intense electric fields. The excitation energy, wavelengths and oscillator strengths in ground state and the first nine different excited states are investigated by the time-dependent density functional (B3PW91) method in external electric fields. Results show that the excitation wavelength is in agreement with the experimental result and the excitation energy is close to the experimental data. With the increase of the external field, the bond length, electric dipole moment, infrared intensities are observed to decrease first, and increase afterwards. But the HOMO energy, LUMO energy, energy gap are seen to decrease. And the total energy and harmonic frequency are found to increase first, and then decrease. In addition, the external electric fields have significant effects on the excitation energy, wavelength and oscillator strengths of CdSe molecule.
2015, 64 (15): 153103. doi: 10.7498/aps.64.153103
Density functional (B3LPY) method has been utilized to optimize the possible structures of PuN, PuO, NO and PuNO molecules using the contracted valence basis set (LANL2 DZ) for Pu atom, and the AUG-cc-pVTZ basis set for N and O atoms. It is shown that the ground state of the PuNO molecules has Cv (Pu-N-O) symmetry and the ground electronic state is 6-. The equilibrium nuclear distances for Pu-N and N-O bonds in the PuNO molecules are RPuN=0.22951 nm and RNO=0.12257 nm, and the dissociation energy is De=8.10537 eV. Furthermore, the other two metastable states of the PuNO molecules are also obtained, and the electronic states of the two configurations are 6- and A with Cv (Pu-O-N) and Cs (O-Pu-N) symmetry, respectively. Then the Murrell-Sorbie potential energy functions of the PuN, PuO and NO molecules have been simulated and the analytical potential energy function of the PuNO molecules has been derived using the many-body expansion theory. The contours of the potential energy functions reproduce exactly the most stable equilibrium structures, the two metastable state structures as well as the dissociation energy of the PuNO molecules. The molecular static reaction pathway, based on the potential energy function, is also discussed.
2015, 64 (15): 153301. doi: 10.7498/aps.64.153301
By using a mid-infrared tunable diode laser and a home-made cooling cell, the N2- and air-broadening coefficients of 13CH4 have been measured at room and low temperatures around 3.38 μm. Four transitions are studied for the 13CH4 diluted with nitrogen and air at temperatures 296, 252, 213, and 173 K. Measurements at low temperatures allow the determination of the temperature dependent parameter of the collisional broadening coefficients. The line parameters are obtained by fitting the experimental profile to the Voigt line shape. The N2- and air-broadening coefficients increase with the drop of the temperature. The collisional broadening coefficients of N2 are always larger than those of air at the same temperature. These data support the remote sensing of the Earth and outer planet atmospheres. According to our knowledge, the line parameters are reported experimentally for the first time.
The Compton profiles of nitric oxide and acetylene molecules have been determined at an incident photon energy of 20 keV. Compton profile measurements are carried out with the beamline BL15U1 at the Shanghai Synchrotron Radiation Facility (SSRF). A dedicated gas cell is used, in which diffuse scattering is effectively suppressed. By considering that the statistical accuracy of 0.2% at pz ≈ 0 is achieved, the Compton profiles of NO and C2H2 determined in this paper can serve as the experimental benchmark data. Furthermore, the density functional theory (DFT) and HF calculation for different basis sets are used to calculate the Compton profiles of nitric oxide and acetylene. It is found that the DFT calculations with the diffuse basis sets are closer to the experimental results, indicating that the electronic density distribution of nitric oxide is more diffuse. For acetylene, the HF calculation is of better agreement with the experimental result. To better understand Compton profiles, we have compared them with distributions of electron density by theoretical calculation. There are clear correspondences between them: diffuse distribution is related to the localized profile and complex structure in electron density distribution, which also shows a subtle structure in profile. The present Compton profiles of nitric oxide and acetylene molecules achieved by synchrotron radiation are the most accurate up to now, as far as we know.
High sensitive photoassociation spectroscopy investigation on 0u+ (6P3/2) long-range state of ultracold cesium molecules
2015, 64 (15): 153303. doi: 10.7498/aps.64.153303
Photoassociation spectroscopy with high resolution for 0u+(6P3/2) long-range state of ultracold cesium molecules has been measured experimentally using the modulated trap loss fluorescence spectroscopy technology. The spectral range has been extended over 60 cm-1 below the 6S1/2 + 6P3/2 dissociation limit as compared with other groups. twenty-five new observations of the Cs2 0u+ in long-range state are reported. The vibrational binding energies of these states are analyzed by using the LeRoy-Bernstein formula. The long-range parameter C3 in molecular 0u+(6P3/2) state is derived for 16.1030.010, and the corresponding molecular potential curve is depicted.
Theoretical study on radiative electron capture and subsequent radiative decay in collision of Xe54+ with Xe
2015, 64 (15): 153401. doi: 10.7498/aps.64.153401
Based on the multiconfiguration Dirac-Fock method and impulse approximation, the electron capture and following radiation decay of the projectile ion are studied theoretically for Xe atom which is bombarded by Xe54+ ion at 197 MeV/u. The radiative electron capture (REC) cross-sections and the corresponding emitted photon energies have been calculated in detail. Meanwhile, the probabilities of the radiative decay and energies of the REC final states are also calculated; combined with the calculated results in this paper, the X-ray spectra structure of radiative decay from projectile ion is further simulated. It is found that the simulated spectra are in good agreement with the newly measured results at Lanzhou Heavy-Ion Accelerator Device.
2015, 64 (15): 153601. doi: 10.7498/aps.64.153601
Alloy nanoparticles exhibit multifunctional properties different from monometallic nanoparticles. Especially, when a third metal is introduced into bimetallic nanoparticles system to form trimetallic nanoparticles, their chemical activities will be further improved. As the catalytic reaction of nanoparticles usually takes place on surfaces, and the activity and stability are closely related to their structures, therefore the research on the stable structure is crucial for understanding their catalytic activities. In addition, the electrochemically synthesized tetrahexahedral nanoparticles bound with highindex facets may exhibit greatly enhanced catalytic activity because of their large density of low coordination sites at the surface. Based on the above reasons, this paper carries out the investigation on the stable structures of tetrahexahedral Au-Cu-Pt trimetallic nanoparticles by using an improved genetic algorithm and the quantum-corrected Sutton-Chen (Q-SC) type many-body potentials. To avoid the genetic algorithm being trapped into premature convergence, two improvement strategies are developed. On the one hand, an atom coordinate ranking operation, which is implemented according to the atomic distance from the core, is proposed for reducing the probability of individual loss. On the other hand, an alternating bit means is introduced into the crossover operation to keep the atomic composition ratio unchanged. Moreover, the performance of genetic algorithm and the influence of original configuration on the stable structures of Au- Cu-Pt trimetallic nanoparticles with different sizes and different compositions also have been investigated. One stochastic distribution structure and three core-shell distribution structures of Au@CuPt, Cu@AuPt and Pt@AuCu are adopted as the initial structures, respectively. Eleven optimization trials on Au-Cu-Pt trimetallic nanoparticles in Au-Cu-Pt system with Au : Cu : Pt of 0:343 : 0:343 : 0:314 with 443 atoms are used to verify that the different original structures should have no effect on the final stable structure. Furthermore, 30 random trails on Au-Cu-Pt trimetallic nanoparticles at Au : Cu : Pt of 0:316 : 0:316 : 0:368 with 443 atoms are conducted to prove that the genetic algorithm can obtain robust results with small standard deviation. Finally, the segregation analysis results show that: In Au-Cu-Pt trimetallic nanoparticles, Au and Cu atoms prefer to aggregate on the surface while Pt atoms are preferential to locate in the core. Furthermore, Cu atoms exhibit stronger surface segregation than Au atoms. For small Au or Cu concentration, Au and Cu atoms would display the maximum segregation. They begin to compete during aggregation, and the Cu atoms have a strong tendency for surface segregation when the number of Au and Cu atoms is bigger than the total number of surface atoms. With increasing number of Au and Cu atoms over those on the surface and sub-surface, Au atoms would display a strong surface segregation than Cu atoms. Additionally, Cu atoms will mix with Pt atoms in the inner layers over the sub-surface after occupying the surface. The distribution of surface atoms has been further examined by the analyses of coordination number: the Cu atoms tend to occupy the vertices, edges and kinks, while the Au atoms preferentially segregate to the flattened surface. This study provides a perspective on structural features and segregation behavior of trimetallic nanoparticles.
ELECTROMAGENTISM, OPTICS, ACOUSTICS, HEAT TRANSFER, CLASSICAL MECHANICS, AND FLUID DYNAMICS
2015, 64 (15): 154101. doi: 10.7498/aps.64.154101
The electron beam produced by an ultra-short, high-intensity laser pulse is of properties of small source size, short duration, and quasi-monoenergetic energy, and will play a unique role in radiographic diagnostics. By analyzing the scattering processes of electrons in materials and performing Monte-Carlo simulations, electron radiography for probing target surface non-uniformities or material interfaces is studied for electron energy ranging from 100 keV to several hundreds of MeV, and the results are compared with those of proton radiography and X-ray radiography, respectively. Features and parameter optimization of electron radiography are obtained, and some applications are suggested. By taking advantage of inelastic scattering or energy loss of charged particles, target surface nonuniformities could be diagnosed by a charged-particle beam whose range is close to the target thickness. Such a diagnosis would produce a higher detection contrast than that by absorption-type X-ray radiography. For a proton beam, a target thickness variation as small as 0.1% could be detected due to a more evident Bragg peak of the stopping power near its range. Nevertheless, the energy of laser-accelerated proton beams being up to 100 MeV would limit the applications. For an electron beam, since a thickness variation of 0.3% could be detected, its energy over 1 GeV has been realized by laser acceleration, the electron radiography could be extended to diagnose thicker targets. When using an electron beam to radiograph a thin or a foil target, for example, of thickness on the order of 100 μm, a spatial resolution of 11 μm or better could be achieved due to the reduced elastic scattering and angular deflection. By taking advantage of elastic scattering of electrons, an electron beam whose range is much greater than the target thickness could be used to diagnose a target interface composed of different materials or even a multilayered capsule, and a higher contrast of the electron fluence modulation at interfaces would be realized than that by absorption-type X-ray radiography, which is caused by stronger scattering of electrons as the electron scattering cross section is several orders of magnitude greater than that of X-ray scattering such as the Thomson scattering. As a laser-produced electron beam is prone to have an ultrafast pulse duration of 100’s of femtoseconds or less, it is anticipated that the electron radiography will produce an ultrasfast temporal resolution. These results and conclusions would be helpful to the applications and parameter optimization of electron radiography.
Design and study of the elliptically cylindrical transparent concentrator based on transformation optics
2015, 64 (15): 154102. doi: 10.7498/aps.64.154102
The design of a novel elliptically cylindrical transparent concentrator with functions of both electromagnetic transparency and electromagnetic concentration is put forward based on the form invariance of Maxwell’s equations in different coordinate transformation and transformation optics theory, and through the design of material constitutive parameters (permittivity and permeability) to guide the electromagnetic wave propagation. The electromagnetic wave transparent body does not prevent the transmission of electromagnetic waves that can interact in the cloak. An electromagnetic wave concentrator is an electromagnetic device that controls the electromagnetic waves to focus on an area or at a point to realize the electromagnetic wave energy concentration according to the requirement when the electromagnetic waves are incident on the device. In this paper, the expressions of the relative permittivity and permeability tensors in every layer of the electromagnetic device are derived by compression transformation and extension transformation. Then full-wave simulations for the electromagnetic device are performed by using finite-element software. The distributions of z-component of the magnetic field for the electromagnetic waves incident from different directions are obtained. Simulation results confirm the validity of the design method and the constitutive parameter tensors. Finally, effect of electromagnetic loss on the performance of the device is also discussed. To a certain extent, the function of the device will be weakened when the electromagnetic loss increases gradually. The design method proposed in this paper provides a new approach that can be used to design other novel electromagnetic devices.
2015, 64 (15): 154201. doi: 10.7498/aps.64.154201
In this paper, an interlacing layered infiltration method is proposed, using some liquid material as the common porous fiber with triangular air-hole array in the core region, which can achieve the characteristic of ultrahigh modal birefringence in this circumstance. Förstly, the basic properties of the porous fiber with a porosity of 43.08% are thoroughly analyzed by using a full-vector finite element method, as wellas the dispersion curves of the fiber, modal birefringence, fraction of the fundamental modal power for x and y polarizations, loss characteristics, etc. Secondly, to enhance the asymmetry of the proposed structure, some liquid material with a refractive index of 1.4 is infiltrated into the air holes in the fiber core region, by using interlacing filling method. It is found that the modal birefringence of the fiber dramatically increases. At an operation frequency of 1.1 THz, the peak value of modal birefringence rises from 1.05×10-3 to 1.36×10-2 after the infiltration operation. The fundamental model effective material absorption loss coefficients for x and y polarization modes increase from 0.16 dB/cm to 0.25 dB/cm and 0.28 dB/cm, respectively. And the operation frequency band increases from 1.1 to 1.9 THz. Simulation results indicate that the modal birefringence of the fiber can be remarkably improved by increasing the refractive index of the infiltrated liquid material. With an operation frequency of 2.2 THz and a refractive index of 2, this fiber can realize an ultrahigh modal birefringence of 8.03×10-2. Moreover, to achieve the gradient distribution of the refractive index, an interlacing layered infiltration method to infiltrate the liquid material with different refractive indices in different layers is employed. Results show that the confinement capability to the guided modes has been greatly enhanced. Results also show that the peak value of the modal birefringence for the fundamental modes does not exist in the operation band. It represents a monotonically increasing trend. At an operation frequency of 2.2 THz, the fiber modal birefringence can reach as high as 7.19×10-2. This scheme presents an ultrahigh modal birefringence, and it presents the tunable characteristic as well. This study may be of significance in the practical applications in the field of THz functional devices.
2015, 64 (15): 154202. doi: 10.7498/aps.64.154202
Starting from the nonlocal nonlinear Schrödinger equation in Cartesian coordinates, we also obtained nonlocal nonlinear Schrödinger equation in a rotating coordinate system.Assuming that the response function of media is Gaussian, we obtain the stable solutions of the solitons of nonlocal nonlinear Schrödinger equation in rotating coordinate system by means ot the imaginary-time evolution method. The propagation properties of the (1+2) dimensional spiraling elliptic spatial optical solitons in the media is discussed in different degrees of the nonlocality by using the split-step Fourier algorithm.The elliptic soliton profiles of the major and the minor axes are Gaussian shaped in a strongly nonlocal case, but not in a weakly nonlocal case. It is suggested that (1+2) dimensional elliptic solitons be highly dependent on the degree of nonlocality. The angular velocity for the change of the ellipticity is very sensitive when the nonlocality is strong,but in the weakly nonlocal case, the change of the angular velocity is very small.The angular velocity depends strongly on weakly nonlocal case to different degrees of ellipticity. Oppositely, in strongly nonlocal case, the value of the angular velocity is almost unchanged. In another way, the critical power for the solitons decreases as the nonlocality decreases in different degrees of ellipticity.Similarly,the critical power for the solitons decreases as the ellipticity decreases in different degrees of nonlocality.
2015, 64 (15): 154203. doi: 10.7498/aps.64.154203
With the development of quantum radar technology, the interaction of photons and targets has gradually become a new hotspot. Quantum radar cross section (QRCS) is an important parameter fon describing the visibility of the target illuminated by light quantum. #br#According to the conservation of energy and the finite element method, the expression of QRCS derived by Marco Lanzagorta is extended, which can be applied to QRCS calculations of non-planar convex targets. As the surface elements of the target have different incident and scattering angles, the integral equation can give a higher calculation accuracy and is suitable for bistatic or multistatic situations. #br#The distribution pattern of the target’s atoms is varied. Using the interatomic distance as the only parameter to describe the atomic distribution is inaccurate. In this paper the metal atomic lattice is considered. Simulation of the QRCS that is composed of three kinds of metal atomic lattices (face-centered cubic, body-centered cubic and hexagonal close-packed lattices) with different atomic distributions has been made. The hexagonal close-packed lattice with asymmetrical distribution for different azimuth angles is discussed. Simulation result shows that with different arrangement of atoms, the main lobe of the target QRCS is basically unchanged, while the quantum side-lobes of the target with sparsely arranged atoms are much more significant. This reveals a different characteristic of QRCS, and provides theoretic basis for quantum radar and stealth technique researches.
Research of a novel optoelectronic oscillator based on dual-loop structure with different wavelengths
2015, 64 (15): 154204. doi: 10.7498/aps.64.154204
Microwave generator is the foundation of all the applications in the area in microwave. It is widely used in electronic systems. Note that, the phase noises in microwave sources them are so important that if the phase noises in them are reduced, the performance of many electronic systems will be significantly improved, such as radar and so on. Optoelectronic oscillators, with a characteristic of ultra-low phase noise, have attracted great attention in recent years. In this paper, a novel scheme of optoelectronic oscillator based on dual-loop structure with different wavelengths is demonstrated. In this structure, two beams of continuous-wave light at different wavelengths, emitted by two lasers separately, are combined together by a wavelength division multiplexer and then are injected into an electro-optical modulator. After injection, the optical carriers at different wavelengths are divided into two paths again by using a second wavelength division multiplexer. The two optical beams at different wavelengths go through two optical fibers of different lengths, and then the two paths are combined together by a third wavelength division multiplexer. This constitutes the dual-loop structure. According to Vernier effects, this dual-loop structure can achieve effective side-mode suppression, since only the modes that satisfy the oscillation conditions of the two loops will be selected. Theoretical work has demonstrated that there are few beating noises when the two optical carriers at different wavelengths are combined. Compared with the scheme of dual-loop optoelectronic oscillator with orthogonal states of polarizations (SOPs), the interference between the two beams with different wavelengths in a wavelength division multiplexer system is much less than those with orthogonal SOPs in polarization-beam splitter/polarization-beam combiner devices. Therefore, the scheme in our experiment can reduce the beating noise due to random interference. Meanwhile, the stability in the dual-loop structure with different wavelengths could be achieved by using ordinary single-mode fiber, instead of adopting polarization maintaining fiber in the dual-loop structure with orthogonal SOPs. Hence, the cost of the system is reduced. In this experiment, the high-quality and tunable microwave signal within the X-band (8-12 GHz) is achieved. The measurement results indicate that the side-mode suppression ratio of the signal is 60 dB and the phase noise is -132.6 dBc/Hz@10 kHz. The loop drift of the system is compensated effectively by a fiber stretcher using phase-loop locked technology and the stability of the RF has been improved greatly. Then the frequency drift in terms of the loop drift in the system becomes less than ±84.3 mHz within 2 h. In addition, the linewidth is measured as 5.3 mHz and the Q-factor is on the order of 1012. Therefore, the signal is of a high spectral purity.
2015, 64 (15): 154205. doi: 10.7498/aps.64.154205
Chaos is a fascinating phenomenon of nonlinear dynamical systems, and optical chaos communication has been one of potential frontier techniques to implement secure transmission of information. In this paper a novel high-speed bidirectional dual-channel chaos secure communication system is proposed based on semiconductor ring lasers (SRLs). In this system, the time delay signatures in chaotic output of clockwise (CW) and counterclockwise (CCW) patterns from a driving SRL (D-SRL) are firstly suppressed by using the double optical cross-feedback frame. Then, the chaotic output of D-SRL is injected into two response SRLs (R-SRLs) to drive the corresponding CW and CCW patterns of R-SRLs that are synchronized and bandwidth enhanced simultaneously. Thus, a bidirectional dual-channel chaos communication could be built based on chaotic synchronization of the two R-SRLs. We theoretically investigated the chaotic characteristics of a D-SRL under double optical cross-feedback and the chaotic synchronization features between R-SRL1 and R-SRL2 under different driving conditions. Results show that the time delay signatures of CW and CCW patterns of D-SRL could be effectively hidden under proper feedback conditions. The bandwidths of CW and CCW patterns of the D-SRL could be enhanced significantly. Furthermore, high-quality isochronous synchronization between R-SRL1 and R-SRL2 can be realized by choosing appropriate injection strength and detuning frequency in D-SRL and R-SRLs. Finally, the communication performances of bidirectional dual-channel chaos secure communication based on this proposed system are preliminarily examined and discussed, and the simulated results demonstrate that for 10 Gbit/s message, the Q factor of decoded message could be maintained above 6 after 10 kilometers distance transmission.
2015, 64 (15): 154206. doi: 10.7498/aps.64.154206
Based on the development of high sensitivity, low cost, high integration and miniaturization demand of the resonant micro-optical gyro(R-MOG), and in order to achieve a resonant micro-optical-mechano-electrical integrative gyro having high sensitivity, a microsphere optical resonator key sensitive element for producting a cavity with high quality value (Q value) and large diameter in the field of integrated optical micro resonator is proposed, for making a resonant micro optical gyro. Microsphere optical resonator is made by means of water-hydrogen flame melting, and the SiO2 microspherical cavity is formed under the natural cooling and contraction surface tension. Microsphere optical resonator with its diameter D ranging from 300 μm to 2200 μm is fabricated by melting method with hydrogen flame as a heat source through controlling the hydrogen flame’s area by regulating the flow of hydrogen gas. The resonator serves as the key unit of the resonant optical gyro sensitive parts, its Q value and diameter D have direct effect on the performance of the resonant angular velocity sensor. Affect parameters on the performance of the microsphere optical resonator with different diameters is tested and processed to obtain the result. The corresponding relationship among Q value, DQ product, resonant micro-optical gyro’s sensitivity and microspherical cavity diameter D is analyzed, and the reason for them is given. With the increase of microspherical cavity diameter D, the Q value and DQ product reduce after rising first, while the gyro sensitivity goes to rise and fall. Based on the microsphere optical resonator DQ product optimization research, the resonant micro-optical gyro’s key sensitive unit with best parameters is obtained. When the microspherical cavity diameter D varies from 600 to 200 μm, the gyro sensitivity can meet the condition that δΩ D is 1260 μm, the Q value of microsphere optical resonator is 7.18×107 and the corresponding optimal limited sensitivity of the resonant micro-optical gyro is almost 10°/h, and this result adequately meets the requirement of business level gyro applications. This work can serve as an experimental foundation in the research of new type resonant micro optical gyro at chip level, high accuracy and low cost, and will also provide a technical reference for further study of high integrated and high precision resonant micro optical gyro.
2015, 64 (15): 154207. doi: 10.7498/aps.64.154207
Electric field distribution, in the wavelength range 1053 nm and 0° high reflection coatings, with different truncated conical pits has been estimated by using the finite difference time domain method (FDTD). Results of simulations indicate that the smaller the angle between the pit’s edge and the normal line, the higher the damage threshold of the mitigation pit. In the experimental process, the dimension of this angle mainly depends on two factors, i.e. the influencing area of the focal spot and the depth of mitigation pits. Because the ratio between them is the angle’s tangent, decreasing the influencing area of the focal spot and increasing the depth of the machined area could yield a mitigation pit with a smaller angle. By optimizing the focal spot size, pulse energy, step size and the number of machining passes of femtosecond laser micromachining, a pit with an angle of 25° and a depth of 14 μm is obtained. The typical damage threshold of the mitigation pit is about 21 J/cm2, which is 2.3 times greater than the fluence-limited defect. Moreover, the laser damage testing results of 50 mitigation pits show that the mitigation process has a good repeatability. The correlation between the cone angle and the damage threshold is also examined, the simulations are in agreement with the experimental results. The ratio of the maximum intensification between 45° and 25° cone angles is ～2.5 and that of the damage threshold between the two angles is ～0.5. At the same time, the relationship between the micromachining pulse width and the damage threshold is also estimated: if other process parameters are kept constant, a longer pulse length tends to produce lower laser-resistant mitigation pits. Compared to the result of 260 fs laser pulse, the truncated conical pit created by 6 ps laser pulse has a smaller depth, which implies that more thermal effect occurs during the miromachining process. However, cracks are not found around the pit. Thus, thermal damage is not the major reason for the decrease of damage threshold. Meanwhile, smaller depth also indicates that the pit has a large cone angle. According to the result of former FDTD simulation, the decrease of damage threshold is mainly caused by electric field enhancement in a pit with a large cone angle.
2015, 64 (15): 154208. doi: 10.7498/aps.64.154208
The phenomenon of electromagnetically induced transparency (EIT) in 85Rb atomic vapor is studied in a -type of four-level system with the coupling laser frequency detuned from the atomic resonance frequency. We find that the EIT intensity grows weaker as the coupling laser frequency is detuned from resonance, but the relative depth of the transparency window increases. The maximum depth appears when the coupling frequency is detuned at about +180 MHz, not at the resonance frequency. We demonstrate that this is a result of the combined effect of the two excited states, and present a theoretical analysis based on the four-level system, which agrees quite well with the experimental results.
2015, 64 (15): 154209. doi: 10.7498/aps.64.154209
Using light to dynamically and stably redirect the flow of another beam of light is a long-term goal for photonic-integrated circuits. However, it is challenging to realize a practically all-optical switching device in silicon owing to its weak optical nonlinearity. Major published work on all-optical switches were using single-photon absorption and two-photon absorption, which requires ultrahigh switching energy. This paper presents a nano-silicon-photonic all-optical switch driven by an optical gradient force, in which a fast switching speed with low power consumption is obtained. Each switching element is composed of a waveguide crossing connection and a micro-ring resonator. The ring resonator is side-coupled to a double-etched waveguide crossing, while the micro-ring resonator is partially released from the substrate and becomes free-standing. When the “drop” port is in “OFF” state, the wavelength of the signal light from the “input” port does not satisfy the resonant condition in the micro-ring. Therefore, light is mainly transmitted to the "thru" port without control light. When a control light is loaded to the “add” port, of which the wavelength satisfies the resonance condition in the micro-ring, a strong optical gradient force is generated by the induced evanescent optical field. The freestanding arc of the ring is then bent down to the substrate, leading to a cavity resonance wavelength shift. As a result, the signal light is diverted to the “drop” port and the corresponding transmission state is switched to the “ON” state. The optical switch is fabricated by nano-photonic fabrication processes using standard silicon-on-insulator (SOI) wafer. The waveguide structures have a width of 450 nm and a height of 220 nm for a single mode transmission; the outer radius of the ring in the switching element is 15 μm; the coupling gap between the ring and the nano-waveguide is 200 nm; the system is fabricated through two-step lithography and plasma dry etching processes while the free-standing arc is released by undercutting the buried oxide layer. #br#A switching time of 180 ns(rise) and 170 ns (fall) is experimentally demonstrated, which is much faster than that of conventional optical switches. The present optical switch can reach a high extinction ratio (10.67 dB) and a low crosstalk (-11.01 dB). In addition, the proposed switch has the advantages of compact size and low power consumption. Potential applications of this optical switch include photonic integrated circuits, signal processing, and high speed optical communication networks.
Effective medium theory of two-dimensional photonic crystal for transverse electric mode beyond the long-wavelength limit
2015, 64 (15): 154210. doi: 10.7498/aps.64.154210
An effective medium theory of two-dimensional photonic crystal for TE mode beyond the long-wavelength limit has been established based on the Mie scattering theory. Först, the proposed theory has been used to study the negative-refractive-index photonic crystals for TE mode. This theory can be used to calculate the effective indices and the effective impedance, and to predict the position of the band gap. Results agree well with the band structures, especially when the equifrequency surface contours are almost circular. Then the proposed theory is used to study the zero-refractive-index photonic crystals for TE mode. It can be seen a triply-degenerate point at Γ point, forming a Dirac cone in the band structures. It has been called an “accidental-degeneracy-induced Dirac point”, where the effective index is zero and the effective impedance is 1. Results calculated using the proposed theory agree well with the band structures. This means that the theory can be used well beyond the long-wavelength limit. Furthermore, the additional impedance information, which cannot be obtained by band structures, can be derived from the proposed theory.
2015, 64 (15): 154211. doi: 10.7498/aps.64.154211
Based on the fact that Bragg reflection grating(BRG) is a special one-dimensional photonic crystal, we propose to apply the one-dimensional photonic crystal band gap theory to the design of Bragg reflection grating and then to make an analysis of its optical performance. According to the above band gap theory, FDTD solution is used to build the elliptical Bragg etched diffraction grating (EDG) based on the Rowland circle. We have studied the spectral characteristics for both TE and TM modes and the angle dispersion due to the variation of the incident angle, at the same time, the optical performances in the air dielectric type Bragg grating and the Al metallic line type Bragg grating are also made to compare with each other. It turns out that by choosing appropriate parameters the diffraction efficiency can be got more than 95% within the scope of 1.465-1.615 μm in both TE and TM modes, and the air dielectric type Bragg grating structure behaves better in the uniformity throughout the whole channel than in the Al metallic line type Bragg grating structure. When the incident angle varies from 30° to 60°, the angle dispersion in TM mode is larger than that in TE mode. This is the foundation of a new type of EDG wavelength division multiplexer with advantages of small size and high diffraction efficiency. It may have the potential to promote the development of high diffraction efficiency dense wavelength division multiplexer in the future.
2015, 64 (15): 154212. doi: 10.7498/aps.64.154212
A novel method is presented in this paper to realize terahertz even beam splitting by using a subwavelength binary simple periodic rectangular structure, for making comprehensive application of both the RCWA (Rigorous Couple-Wave Analysis) and the GA (Genetic Algorithm). By applying RCWA, the efficiency of each diffraction order can be numerically solved by using the structure parameters. To design an even beam splitter with a subwavelength structure is to find the optimal duty cycle f, period d, the grating depth h1 and the substrate thickness h2 to approach the minimum zero-order diffraction efficiency, the maximum sum of each non-zero-order diffraction efficiency, and the uniform distribution. Considering the three goals above, an evaluation function is established. GA is applied to optimize the evaluation function F, and then the optimal parameters of the splitter are obtained. When its period, groove depth, substrate thickness and duty ratio respectively equal to 269.7 μm, 175.2 μm, 18.1 μm and 0.409, the normal-incident TE-polarized terahertz plane wave with its frequency equal to 2.52 THz is divided evenly into the diffraction orders±1 and±2. It has a total efficiency up to 92.23% with a preferable result of reducing zero-order diffraction efficiency to 0.192% and an error of uniformity down to 6.51×10- 6, indicating an excellent performance of diffraction efficiency, uniformity and zero-order suppression as a terahertz even splitter. These results break the limitation of realizing even beam splitting in which the traditional scalar theory applies. In addition, this paper exposes the law of influence of the structure parameters, including ridge width, groove width, groove depth, duty ratio and substrate thickness, on the diffraction efficiency and its uniformity. It is found that only a small deviation of the structure parameters, corresponding to a deviation of ridge width a, groove width b, groove depth h1, and substrate thickness h2, less than 1 μm from the optimum design values, the element maintains good beam splitting performance. P0 is permitted to suppress to less than 2%, the error of uniformity U is better than 19.60%, and the diffraction efficiency maintains higher than 79.10%. With a substantial deviation from the design values of the structure parameters, the performance of the splitter will severely degrade and need to be redesigned.
Experimental study on a low switching energy and high-linearity all-optical sampler based on terahertz optical asymmetric demultiplexer
2015, 64 (15): 154213. doi: 10.7498/aps.64.154213
We demonstrate experimentally a low switching energy and high-linearity all-optical sampler based on terahertz optical asymmetric demultiplexer (TOAD) composed of a nonlinear semiconductor optical amplifier (SOA) with a multiple quantum well structure. Effects of the sampling pulse power and asymmetric offset of SOA on the shape, width and amplitude of sampling windows are analyzed in detail respectively. It is found that the sampling pulse power has no effect on both the shape and the width of sampling windows, but has a significant effect on the window amplitude. Meanwhile there exists an optimal power which maximizes the sampled output and determines the switching energy of TOAD. The asymmetric offset of SOA from the center position in the loop determines the width of sampling windows and has great influences on both the shape and the amplitude of the sampling window. The sampling windows with different widths have approximately the same rise edge due to the fast response of SOA for the sampling pulse. However, the normalized amplitude of sampling windows firstly increases sharply with the increase of the asymmetry, then gradually flattens out, and tends to be stable in the end. In addition, the switching energy and linearity of TOAD are studied. The switching energy is as low as 25 fJ, and the linearity is as high as 0.99. Moreover, at different window widths, the switching energy of TOAD remains the same and the sampling windows have a very good linearity. However, the sensitivity of a TOAD sampler with different width is different: the wider the sampling window, the higher the sensitivity and the larger the corresponding dynamic range.
Effect of coating-induced polarization aberrations on the focusing properties in high numerical aperture optical system
2015, 64 (15): 154214. doi: 10.7498/aps.64.154214
The incident angles of the optical systems with high numerical apertures, such as lithography or microscope, are larger than those of normal ones. For these systems, polarized illumination is widely adopted. The coatings on their surfaces will make s and p polarization components of oblique incident light experience diverse amplitudes and phase modulations, and induce extra polarization aberrations. We apply the vectorial diffraction theory to assess the effects of coating-induced polarization aberrations on the focusing properties of these systems. By applying the generalized Debye integral, the relationship between the parameters of coating and electric field vector near the focal spot is established. Considering x linearly polarized light as the incident light field, we evaluate the potential influence of the coatings on the intensity and the full width at half maxium of focal spots. In the further discussion, we compare the results of different coatings when the various optimization goals are set, and certify that the phase difference caused by coating has more effect on focusing property. Based on this, the additional constraint conditions of coating design are proposed to suppress such disturbance, i.e. to properly increase relative weight of phase constraint conditions. With this proposed constraint conditions, we design and optimize an anti-reflection coating with low polarization aberrations. By applying this designing, the central intensity of focal spot can be enhanced by 12.5%, and the light energy utilization will be improved effectively.
Research on the frequency stabilization of pulsed differential absorbing lidar for CO2 detection based on matching algorithm
2015, 64 (15): 154215. doi: 10.7498/aps.64.154215
The differential absorption lidar (DIAL) can help us to obtain the vertical distribution of the atmospheric CO2 concentration, which is important to the study of carbon sources and carbon sinks. We design a seeder injected pulsed laser system, working as the laser source of the CO2 DIAL. Unlike the other CO2 DIALs, our laser source is the result of difference frequency of two lasers at the wavelengths of 1064 nm and 634 nm, respectively. It should be pointed out that the high frequency (wavelength) accuracy and stability of the emission laser, especially the on-line one, are greatly required in the CO2 DIAL system. However, the mechanical properties of the dye laser (634 nm) and the application of laser difference frequency technique make the wavelength drift constantly; besides, the extremely unstable energy of the pulsed laser increases the difficulty in identifying and stabilizing the on-line wavelength. Hence, a fast and efficient frequency (wavelength) stabilization method is needed to achieve a stable emission wavelength. Aiming at the research gap of the high precision requirements of on-line laser for this kind of pulsed DIAL, we propose a frequency stabilization method based on matching algorithm. The basic idea is to utilize the saturable absorption of CO2 molecule, by measuring the differential residual-intensity after the laser passing through dual absorption cells to calculate the optical depth (OD) and obtain the so-called pseudo CO2 absorption spectrum, which can be used to identify the on-line laser accurately. Finally, based on the matching algorithm of one-dimensional image, treating the OD as the gray value in the image, we implement the OD matching as a most important part in the process of frequency stabilization, and determine the exact position of the real-time output laser in the measured pseudo absorption spectrum. Thus, when some errors happen to the monitored ODs, by continuously adjusting the wavelength of the laser, the proposed method can fulfill the wavelength adjustment and accomplish the continuous frequency stabilization for on-line laser. Experimental results show that the frequency stabilization algorithm based on OD matching can satisfy the requirements for pulsed on-line laser frequency stabilization, and the sum of squares of deviation method is the optimal one in this application, with a stabilization accuracy of 0.3 pm. Besides, the proposed method can also be introduced in other laser frequency stabilization.
Third-order optical nonlinearity at communication wavelength and spectral characteristics of Ge-Se based chalcogenide glasses
2015, 64 (15): 154216. doi: 10.7498/aps.64.154216
A series of Ge-Se chalcogenide glasses incorporated with same molar percentage of Ga, Sn, Sb and Te are synthesized by melt-quenching method. The variations of optical band gaps doped with different elements are investigated by absorption spectra, and the relationship of optical band gap with glass network structure is studied by Raman spectra The results show that the doping of heavy metallic elements (except Ga) could reduce the optical band gap of the Ge-Se glass due to the decrease of the number of Se-Se chains or ring bonds. Third-order optical nonlinearities of the glasses are studied by femtosecond Z-scan method at a telecom wavelength of 1550nm. The results show that the performance of third-order optical nonlinearity of the Ge-Se glass could be improved by doping the above-mentioned elements. By comparison, the Sn-doped Ge-Se glass has a maximum nonlinear refraction index (n2) of 6.36× 10-17 m2/W and a figure of merit of over 23. By combining the experimental results from Raman spectra, the enhancement of third-order optical nonlinearity after the introduction of Sn can be ascribed to the formation of Sn(Se1/2)4 tetrahedra that enters into the main frame of Ge-Se glass and results in a stable Ge-Sn-Se network. Te doping could also remarkably enhance the n2 value of the Ge-Se glass, however, it could cause large two-photon absorption, leading to a poor value of figure of merit. The research result shows that chalcogenide glass in Ge-Sn-Se ternary system is an ideal candidate material for designing and fabricating infrared devices with high performance and environmental friendness.
Photoluminescence spectra of 1 MeV electron beam irradiated In0.53Ga0.47As/InP quantum well and bulk materials
2015, 64 (15): 154217. doi: 10.7498/aps.64.154217
Minimizing the impact of radiation-induced degradation on optoelectronic devices is important in several applications. Satellites and other spacecraft that fly in near-earth orbits (below 3.8 earth radius) are extremely susceptible to radiation damage caused by the high flux of electrons trapped in the earth’s magnetosphere. Optoelectronic devices are particularly vulnerable to displacement damage caused by electrons and protons. Effects of 1 MeV electron beam irradiation on the photoluminescence properties of In0.53Ga0.47As/InP quantum well (QW) and bulk structures, which are grown by metal-organic vapor phase epitaxy, are investigated. Samples are irradiated at room temperature using an ELV-8II accelerator with 1 MeV electron at doses ranging from 5×1012 to 9×1014 cm-2, and a dose rate of 1.075×1010 cm-2·s-1. Photoluminescence measurements are made using a 532 nm laser for excitation and a cooled Ge detector with lock-in techniques for signal detection. Photoluminescence intensity of all the structures is degraded after irradiation, and its reduction increases with increasing total dose of irradiation. Electron beam irradiation causes a larger reduction in the photoluminescence intensity and carrier lifetime of the bulk than that of quantum well. Photoluminescence intensity of five-layer quantum wells degenerates to 9% that before irradiation as the fluence reaches 6×1014 cm-2. As the electron beams bombard into the sample, the destruction of the lattice integrity will cause the decrease in the number of excitons and intensity of photoluminescence. Electron beam irradiation introduces defects in the samples, increases the density of the nonradiative recombination centers, and results in the decrease of carrier mobility. In a quantum well structure, due to the two-dimensional confinement, the probability of carrier nonradiative recombination at radiation-induced defect centers will be reduced. The reduction of photoluminescence intensity in the bulk is severer than in the quantum well while the cross-sectional area which is sensitive to radiation is kept the same. The number of interface defects which are produced by electron irradiation will increase with the number of layers in quantum well and the heterojunction interface of quantum wells, so is the degration of photoluminescence intensity. The degration is mainly due to the increase of non-radiative centers in the samples. By comparing the different structures, the quantum well structure shows a better radiation resistance.
2015, 64 (15): 154301. doi: 10.7498/aps.64.154301
Introducing the decoupling coating is an effective way to reduce sound radiation from underwater structures. In order to investigate the decoupling mechanism of a viscoelastic coating layer with horizontal cylindrical cavities, such a coating layer is approximated to a homogeneous layer with equivalent material properties, and a theoretical model is also developed to predict the sound radiation from a finite plate with such a decoupling coating layer. #br#The validity of the theoretical model is confirmed by comparison with the finite element method; and the decoupling mechanism of the coating layer is discussed. Numerical analysis shows that: (1) The energy flow across the interface between the plate and coating layer is mainly conveyed by longitudinal waves. (2) At a low frequency, the coating layer has nearly no decoupling effect. (3) In contrast with a homogeneous coating layer, the coating layer with horizontal cavities can greatly enhance the mechanical impedance in the mid- and high-frequency areas; hence the mean square velocity is effectively suppressed in the same area. (4) Compared with the homogeneous coating layer, the coating layer with horizontal cavities has a larger impedance mismatch with water, thus it exhibits great vibration transmission loss. Therefore, in general, the coating layer with horizontal cylindrical cavities has a better decoupling performance than the homogeneous coating layer in the mid- and high-frequency areas.
2015, 64 (15): 154302. doi: 10.7498/aps.64.154302
Taking the flexural wave propagating in elastic thin plate as an example, we investigate the mechanism for gap opening in the resonator-based acoustic metamaterials. Results show that the band gap in such a kind of structure depends not only on the abrupt phase change of the wave when it is scattered by the resonators, but also on the retarded phase of wave when it is propagating in host. This means that the dispersion of wave in the structure can be adjusted either by the scattering or by the propagating phase. Based on this understanding, we show that the defect state at subwavelength scale (obtained either by changing locally the resonating property of the resonator or by changing locally the distance between the resonators) can be understood simply by the band gap condition. We show further in this paper that, because the dispersion of the metamaterial can be adjusted by the propagating phase, the structures with negative band at a subwavelength scale can also be achieved by arranging the resonators into a compound lattice.
Source ranging based on frequency band decomposition and distance weighting using a single acoustic vector sensor in shallow water
2015, 64 (15): 154303. doi: 10.7498/aps.64.154303
A novel method is proposed for the passive source range estimation based on union processing of pressure and particle horizontal velocity. Autocorrelation functions’ warping spectra of pressure and particle velocities have frequency invariability. The spectra of the warped autocorrelation functions of pressure and particle horizontal velocity have the same line spectrum feature, while the spectrum of the warped autocorrelation function of particle vertical velocity possesses both line and broadband spectrum features. Moreover, the warped autocorrelation function’s spectrum of particle vertical velocity has more peaks, and the peak width is broader than those of pressure and particle horizontal velocity. In this paper, source ranges are estimated based on frequency band decomposition and distance weighting, and a guided source with a known range is employed to provide the crucial frequency invariant features. Experimental data in shallow water with an iso-speed profile are used to verify the approach which can reasonably estimate source ranges with the relative errors of the source ranging basically less than 7%. This method can effectively reduce the mainlobe width and background level of the cost function, and can significantly improve the resolution of source range estimation, compared with the results of conventional source ranging approach that uses a single pressure hydrophone.
Sparse channel estimation and pilot optimization for underwater acoustic orthogonal frequency division multiple access uplink communications
2015, 64 (15): 154304. doi: 10.7498/aps.64.154304
Considering that the conventional channel interpolation method with sparse and irregular spaced pilots will lead to an error floor in underwater acoustic (UWA) orthogonal frequency division multiple access (OFDMA) uplink communications, a method for sparse channel estimation and pilot optimization is proposed in this paper. A compressed sensing (CS) algorithm is utilized for sparse channel impulse response estimation, which performs well in sparse and irregular spaced pilots and significantly decreases the channel estimation error. Besides, the pilots’ pattern and power joint optimization algorithm based on the random search technique is proposed for the minimum mutual coherence criterion in CS theory, which further improves the performance of CS estimation algorithm. During each iteration step, we randomly pick a pilots’ pattern from the subcarrier index set and a pilots’ power subset from the available power set. Then we perform this step iteratively within a certain searching time. Finally, the local optimal solution of the objective function for minimizing mutual coherence is considered as the feasible pilots’ pattern and power. Simulation results show that the convergence performance of the pilots’ pattern and power joint optimization algorithm is much better than that of the pilots’ pattern optimization algorithm. Furthermore, the channel estimation error of the proposed method is much lower than that of conventional least-squares channel estimator based on linear interpolation, CS channel estimator without pilot optimization, and CS channel estimator merely with pilots’ pattern optimization in channels of different multipath delay spreads. Finally, performance of the proposed method is demonstrated in the UWA uplink OFDMA systems with interleaved and generalized carrier assignment schemes respectively in the two-user case in a pool experiment. Experimental results show that the proposed method decreases dramatically the bit error rate in both carrier assignment schemes, and simultaneous reception for two users is achieved when signal noise ratio is larger than 10 dB.
2015, 64 (15): 154305. doi: 10.7498/aps.64.154305
Study on underwater acoustic scattering is very important for detection, location, and recognition of underwater targets. In the past decades, most investigations in this respect were focused on the case of plane wave incidence. But the Bessel beam is a kind of approximate non-diffracting beam with an excellent directing property, so more attention should be paid on it. So far, according to the literature, the studies about underwater acoustic scattering of a Bessel beam mainly focused on spheres and spherical shells using the partial wave series form. When the scatterers become complex objects, the partial wave series form fails to deal with these kinds of problems. To overcome this shortage, the T-matrix method has been introduced to calculate the underwater scattering of a Bessel beam by complex rigid objects. #br#In this paper, the underwater acoustic scattering of a Bessel beam by rigid objects with arbitrary shapes calculated by T-matrix method is studied. By means of the harmonic expansion of Bessel beam, the expression of the incident coefficient can be derived. Through the transmission matrix that relates the known coefficients of expansion of an incident wave to the unknown expansion coefficients of the scattered field, the acoustic scattering formula of a Bessel beam by a rigid scatterer with arbitrary shape is established. In this paper, the backscattering fields of rigid spheroids and finite cylinders with two spheroidal endcaps are discussed, and the backscattering form function modulus |F| is curved as a function of dimensionless frequency ka. Subsequently, the peak to peak intervals in backscattering form function caused by the interference of the specular wave and the Franz wave are also analyzed in geometry. The calculated results show that the frequency interval obtained from the curves agrees well with those obtained by geometric analysis for the rigid objects. Meanwhile, for both the rigid spheroid and finite cylinder, the highlight model is successfully applied to explain the phenomenon in which the amplitude of backscattering form function changes with the cone angle of the Bessel beam. From the above numerical results and analysis, the T-matrix method has been successfully introduced to calculate the acoustic scattering of the Bessel beam by complex objects, which extends the application of the T-matrix method and provides a useful tool to explore the characteristics of the Bessel beam.
2015, 64 (15): 154306. doi: 10.7498/aps.64.154306
The purpose of this study is to develop an integrated analytical method of flow-induced noise for vector hydrophones in towed arrays and discuss the parameters that influence flow-induced noise character. Based on Carpenter turbulent boundary layer pressure fluctuation model for slender cylinder, the power spectra and cross power spectra of acoustic pressure and particle velocity of flow-induced noise, are deduced by utilizing the wavenumber-frequency spectral analysis. It is shown that the flow-induced noise is determined by the towed speed, the size of both vector hydrophones and elastomer tube, the material parameters of elastomer tube and so on. In addition, the condition that cylindrical vector hydrophones are distributed non-axially in elastomer tube is also taken into account. Considering the influence of the axis-off distance on acoustic pressure, axial particle velocity and radial particle velocity, a set of numerical results show that the influence of the axis-off distance on the high-frequency component of the acoustic pressure and axial particle velocity is greater than that on the low-frequency component. The radial particle velocity is influenced by the axis-off distance within the full frequency range. The impact of the axis-off distance on the particle velocity is far greater than that on acoustic pressure.
Coupling double-distribution-function thermal lattice Boltzmann method based on the total energy type
2015, 64 (15): 154401. doi: 10.7498/aps.64.154401
Micro-scale flow is a very important and prominent problem in the design and application of micro-electromechanical systems. With the decrease of the scale, effects, such as viscous dissipation, compression work and boundary slip etc., which are ignored in a large-scale flow, play important roles in a microfluidic system. #br#With its certain advantages such as high numerical efficiency, easy implement, parallel algorithms etc., the lattice Boltzmann method is a powerful numerical technique for simulating fluid flows and modeling the physics in fluids. The double-distribution-function lattice Boltzmann method has been widely used in a micro-scale thermal flow system, since it utilizes two different distribution functions to take account of the viscous dissipation and compression work. However, most of the existing double-distribution-function lattice Boltzmann methods are “decoupling” models, and decoupling will cause the models to be limited to Boussinesq flows in which temperature variation is small. In order to overcome the above problem, based on the low-order Hermite expansion of the continuous equilibrium distribution function, we propose a coupling double-distribution-function thermal lattice Boltzmann method. This method introduces temperature changes into the lattice Boltzmann momentum equation in the form of the momentum source, which can affect the distribution of flow velocity and density, so as to realize the coupling between the momentum field and the energy field. In the process of fluid flow, the temperature change of the energy field includes two parts: one is for different times at the same lattice which can cause the change of the fluid characteristic parameters, such as the viscosity coefficient and the thermal diffusivity; the other is for the same time at different lattices which mainly affects the distribution of the velocity. In the collision and the migration processes, temperature change is introduced into the fluid flow to achieve the effect of temperature changes on the flow field and the coupling between the energy field and the momentum field. This method can break up the limitation of the Boussinesq flows and expand the application scope of the lattice Boltzmann method. #br#Two natural convection models (one takes into consideration the viscous dissipation and compression work, and the other does not) are studied in this paper to verify the effectiveness and accuracy of the coupling double-distribution-function thermal lattice Boltzmann method. Flow field and the changing trend in temperature, velocity and the averaged Nusselt number are analyzed emphatically at different Rayleigh number and Prandtl number. Results of this paper are excellently consistent with those in papers published, confirming the validity and accuracy of this method. Results also show that the convective heat transfer gradually enhances with increasing Rayleigh number and Prandtl number in the cavity, and the boundary layer is obviously formed in the regions very close to the walls; the heat transfer is greatly enhanced if viscous dissipation and compression work are considered; and these effects should not be neglected in the micro-scale flow system.
2015, 64 (15): 154501. doi: 10.7498/aps.64.154501
Based on the combination of linear contact model, Coulomb slip contact model and parallel bond contact model, a discret element model (DEM) of wet granule agglomerates with coating structure is constructed. Disaggregation processes of wet agglomerates in impacting to a horizontal plate are performed by applying particle flow code (PFC). Three failure patterns are obtained corresponding to those in experiment. The variation of velocities and rupture characteristics of liquid bridge in disaggregation process are investigated. Effects of impact velocity, gravity of adhered granules, and rotation of core granule are analyzed. DEM simulations show that there are three disaggregation patterns in the coating structure of agglomerates: impact disaggregation, gravity-impact disaggregation and shear-impact disaggregation, depending on the size of primary particles and the rotation of the core granules. With the enlargement of size, gravity plays an increasingly important role and the impact disaggregation pattern shifts to gravity-impact disaggregation. The rotation of core can generate a shear force to separate the fine and disaggregation pattern to turn to shear-impact disaggregation. Impacting results in a heterogeneous distribution of granule velocities and a tendency of relative movement in agglomerates. Relative movement will bring about the stretch of liquid bridge between granules. If the maximum separation distance of wet granules exceeds the rupture distance of liquid bridge, disaggregation happens. The ruptures of liquid bridge start from impact point and expand to outward, from bottom to up, from inside to outside in coating agglomeration. It is found that the rupture of liquid bridge needs time for accumulation and goes through three stages termed as slow rupture stage, quick rupture stage and entire rupture stage. With the increase of impact velocity, particle gravity, and rotating speed of core granules, disaggregation processes of wet granule agglomerates become fast and thorough. Impact velocity plays a primary role in disaggregation. DEM simulations are consistent with the experimental results.
2015, 64 (15): 154502. doi: 10.7498/aps.64.154502
Mechanical response of mixtures composed of glass and rubber particles are investigated in direct shear experiments in laboratory and by means of discrete element method simulations. The mixtures are prepared with different contents of rubber fractions. It is found that, with increasing rubber particles, volume phase transition occurs from dilatancy to reduction, and the elastic properties of the mixtures are improved. Experimental results show that, as the rubber particles (up to 30% in volume) are added, the value of the shear stress falls, and the volume phase transition occurs, but the critical states are the same. The shear stress is independent of shear rates, however, it grows with the normal force. We have obtained the consistent results in the simulation. Furthermore, statistical analysis of the simulation results shows that the average coordination number is raised with the increase of rubber particles. Volume phase transition occurs at low rubber fraction when the normal force is large. It is very important to keep in mind that the average coordination number is always between 5.6 and 5.9 at the phase transition points even under different normal forces. When the rubber fraction is less than 30%, the residual shear strength is nearly the same as in the system of glass beads. However, the residual shear strength decreases when the rubber particles increase to the fraction larger than 30%. Meanwhile, the residual shear strength increases with the normal pressure.
Analysis of evolution mechanism of traffic energy dissipation by considering driver’s forecast effect
2015, 64 (15): 154503. doi: 10.7498/aps.64.154503
Starting from the full velocity difference model, an extended car-following model is proposed by considering the influence that in real traffic the driver’s forecast has an effect on car-following behavior of traffic flow. The mechanism how the stability and energy dissipation of traffic flow are in fluenced by the driver’s forecast effect is revealed by the application of the proposed new model. The linear stability condition of the new model is derived theoretically through linear stability theory. The phase diagram of linear stability condition is divided into two regions by each stability curve: the stable and unstable regions. And the corresponding stable region will be enlarged with the increase of driver’s forecast time, hence the traffic condition will be improved through considering driver’s forecast effect. By numerical simulation method, the space-time evolution relation between the velocity and headway of vehicles in car-following queue is investigated systematically under the influence of driver’s forecast. In the same time, the evolution mechanisms of the overall average energy dissipation of traffic flow and individual vehicle energy consumption with the addition of small disturbance are discussed explicitly under a periodic condition, and it is discovered that the overall average energy consumption in traffic flow and the energy dissipation of individual vehicle is accompanied by a complex critical phase transition process. Good agreement between the numerical simulation and the theoretical analysis show that by considering of driver’s forecast effect, not only the stability of traffic flow is enhanced obviously, but the energy consumption is reduced remarkably as we expect. Furthermore, it is verified that both the overall average energy consumption of the considered traffic flow and the energy consumption of an individual vehicle are reduced gradually along with the increase of driver’s forecast time. On the other hand, numerical simulation results verify that the shortcoming of negative speed appearing in the full velocity difference model with low reaction coefficient can be effectively avoided by increasing the driver’s forecast time in the improved model, which means that the dynamic characteristics of traffic flow can be described more precisely by the proposed model.
2015, 64 (15): 154701. doi: 10.7498/aps.64.154701
In this paper, we simulate numerically the dissolution and precipitation in porous media by using the lattice Boltzmann method (LBM). The fluid flow in porous media is simulated by using a multiple-relaxation-time (MRT) LBM, while a D2Q9 lattice BGK model is used for reactive solute transport. Frst, the code of LBM is tested by simulating the diffusion and reaction at a boundary in an open rectangular domain, and comparing the results with the analytic solution. Then, the effects of the Reynolds number (Re), the Schmidt number (Sc) and the Damkohler number (Da) on the variations of the geometry of the porous media and the concentration field are carefully studied. It can be found that for the dissolution (precipitation), as Re is increased, the porosity of the porous media will be increased (decreased), and the average concentration will be decreased (increased). Besides, at low Damkohler numbers or Schmidt numbers, the dissolution and precipitation will be reaction-controlled and are highly uniform. However, as Da or Sc is high, the dissolution and precipitation will be diffution-controlled, and mainly occur in the upstream and large pore space.
Direct solution method of efficient large-scale parallel computation for 3D turbulent Rayleigh-Bénard convection
2015, 64 (15): 154702. doi: 10.7498/aps.64.154702
Research of turbulence Rayleigh-Bénard convection with high Ra number is a hot topic in physics research in the world. DNS simulation is one of the important means to study the subject. The computing work is hard to achieve when the calculation size is increased and the grid number is bigger. Numerical simulation for high Ra turbulent convection faces some major challenges. So the direct (non iterative) solution method of efficient large-scale parallel computation for the 3D turbulent convection is created in this paper. Main difficulties are the parallel computing technology for the pressure Poisson equation. The mass efficient parallel approximate solution with the block tridiagonal equations of OpenMP and MPI used simultaneously after decoupling pressure Poisson equation using FFT is presented. Through the validation of the efficiency of this method in parallel computing, the new method for direct solution of parallel computing have good parallel efficiency and computational time. Results of thermal convection in 3D narrow cavity show that the convection characteristics calculated by using the new method is reasonable. The direct solution method for efficient large-scale parallel computation of 3D turbulent convection created in this paper also is likely to be a breakthrough in computing technology about efficient large-scale parallel computing incompressible NS equations in some special cases.
2015, 64 (15): 154703. doi: 10.7498/aps.64.154703
Particle focusing induced by viscoelasticity of fluids has attracted increasing interest in recent years. However, the regulation mechanisms of critical parameters affecting the particle focusing behaviors are still unclear. This paper systematically characterized the dynamics of particle migration in non-Newtonian fluid flows, and analyzed the effects of flow rate and channel length on particle focusing behaviors. Först, the lateral migration behaviors of particles suspended in Newtonian fluids (e.g., pure water and 22 wt% glycerol aqueous solution) are compared with those in non-Newtonian fluids (8 wt% polyvinylpyrrolidone aqueous solution). It is found that the particles suspended in non-Newtonian fluids would migrate towards the channel centerline and form a single-line particle array under the action of elastic force while the particles suspended in Newtonian fluids would migrate to form a famous Segré-Silberberg particle annular ring due to the effects of inertial lift forces. Second, the effects of particle size and driving flow rate on particle viscoelastic focusing are quantitatively analyzed. Results show that with increasing flow rate the focusing degree increases and finally stabilize at a certain value, and the large particles have better focusing quality than the small ones. Finally, the dynamic focusing process of particles along the channel length is investigated. A mathematical model of safe channel length for achieving particle focusing is derived and validated by experiments. It is found that the safe channel length for large particles is significantly shorter than that for small ones. The obtained results would improve the understanding of particle focusing processes and mechanisms, and help realize the flexible control of particle migration behaviors in non-Newtonian fluids.
Evaporation enhancement effect of TiO2 nanoparticles on silicone oil in electrorheological fluid suspension
2015, 64 (15): 154704. doi: 10.7498/aps.64.154704
Electrorheological (ER) fluids are suspensions which consist of dielectric particles and insulation fluid. The ER fluids can change from liquid-like to solid-like state under the applied electric field. For traditional ER fluids, the maximum yield/shear stress is only several kPa and the size of dielectric particles is generally of micron. Since 2003, a series of new type ER fluids have been discovered, of which the yield/shear stress is as high as several hundred kPa. Such a type of ER fluid is called giant ER fluid or polar molecule-dominated ER fluid (PM-ER fluid), in which the size of dispersed particles is of nanoscale. Dimethyl silicone oil is the most commonly used dispersing agent in ER fluids, because of its stable physical and chemical behaviors. There is no obvious evaporation in traditional ER fluids when it is mixed with micron grade particles. However, when it is mixed with nanoparticles to prepare giant ER fluids, the silicone oil volatilizes easily in atmosphere. If time is long enough, the silicone oil in ER suspension can even be evaporated completely. In this paper, the existence of TiO2 nanoparticles in ER suspensions enhances the volatilization phenomenon has been studied through experiment. Analysis shows that the nanoparticles caused convex nanoscale curved surfaces on the gas-solid interface makes the vapor pressure increase greatly at the silicone oil surface, and leads to the enhancement of its volatilization. Influence of particle concentration, environmental temperature and viscosity of silicone oil on the evaporation enhancement effect is also studied and analysed systematically. Results show that the increase of the fraction of nanoparticles, viscosity of silicone oil as well as the temperature would promote the effect of evaporation enhancement of silicone oil in the suspensions.
2015, 64 (15): 154705. doi: 10.7498/aps.64.154705
Designed microtextured surfaces have shown promising applications in tuning the wettability of a liquid droplet on the surfaces and attracted great attention over the past decade; unfortunately, the effect of surface geometry on wetting properties is still poorly understood. In this work, two- and multi-stage pillar microtextures are designed to construct gradient surfaces by altering pillar width and spacing. Then, the multi-phase lattice-Boltzmann method (LBM) is used to investigate the wetting dynamics of a liquid droplet on the gradient surface. Results show that for the two-stage gradient surface with variable pillar spacing, the contact angle hysteresis is found to be proportional to the roughness gradient when droplet/surface system is in the Cassie-Baxter state. However, this proportional relation is no longer correct when the system is in the transition state between the Wenzel and Cassie-Baxter states. For the two-stage gradient surface with variable pillar spacing, the contact angle hysteresis always increases linearly with increasing roughness gradient. Results also show that when a larger droplet is placed on the multi-stage gradient surface, stronger droplet motion is observed due to the smaller contact angle hysteresis. The present LBM simulations provide a guideline for the design and manufacture of the microtextured surfaces to tune the droplet wettability and motion.
CONDENSED MATTER: STRUCTURAL, MECHANICAL, AND THERMAL PROPERTIES
2015, 64 (15): 156101. doi: 10.7498/aps.64.156101
In the paper the coupling impurities theory is used, and both s-d interaction effect and phonon effect in dilute magnetic alloys are discussed . The Green’s function is used to analyse the Hamiltonian of the system. In copper-iron dilute magnetic alloy the magnetic impurities interaction has a huge impact on thermoelectric power in the condition of high concentration of iron. Theoretic value of thermoelectric power of dilute magnetic copper-iron alloy with high concentration of iron changing with temperature is given. We have chosen three typical copper-iron dilute magnetic alloys and calculated the thermoelectric power under the effect of impurities and the effect of impurities interaction. Their atomic percentage concentrations are 0.1%, 0.13% and 0.15% respectively. Theoretical value of the thermoelectric power under the effect of impurities interaction in copper-iron alloy complies with experimental value. This paper provides the basic theoretical analysis for promoting the application of low-temperature copper-iron dilute magnetic thermocouple.
Effect of Cu on the structure and phase-change characteristics of Sb2Te film for high-speed phase change random access memory
2015, 64 (15): 156102. doi: 10.7498/aps.64.156102
In this paper, in-situ X-ray diffratometer, Raman spectrometer, and X-ray reflectometer are employed to study the crystal structure, bonding states, and density change upon crystallization of Cu-Sb2Te films. It is shown that the crystallization temperature increases with increasing Cu content due to much more energy being required to overcome the rigid atomic network for the atoms rearrangement as a result of the complex branching and cross links. In X-ray diffraction pattern, both hexagonal Cu7Te4 and Sb2Te peaks have nearly the same peak positions, while the rhombohedral Sb peaks shift obviously their positions toward a small angle upon heating, suggesting a significant increase in lattice parameters of Sb phase. A Cu-Te bond is formed in Sb2Te films containing 10 at% and 14 at% Cu which are crystalized into hexagonal Cu7Te4, rhombohedral Sb and hexagonal Sb2Te three phases. When Cu concentration increases to 19 at%, Cu-Te bond becomes full, and the excess of Cu will bond with Sb. Compared with Ge2Sb2Te5 (GST), Sb2Te films with 10 at% and 14 at% Cu have lower density changes upon crystallization which are about 3.2% and 4.0%, respectively. Phase change random access memory (PCRAM) based on Cu-Sb2Te is successfully fabricated and characterized. Operations of set-reset can be realized in a 10 ns pulse for Cu-Sb2Te based PCRAM. The value of set and reset operation voltage decreases with increasing Cu content. The endurance test shows that the operation cycle numbers can reach 1.3×104 and 1.5×105 for the 10 at% and 14 at% Cu-based PCRAMs, respectively. The resistance ratio of reset and set states maintains a balance of about 100. Cu-Sb2Te film may be considered as one of the promising candidates for high-speed PCRAM.
2015, 64 (15): 156301. doi: 10.7498/aps.64.156301
By using projection augmented plane wave method (PAW) and based on the density functional theory, the stability of lattice dynamics and the magnetism of ordered crystalline alloy -Fe4N are studied at high external pressures. In comparison with the phonon spectrum of -Fe4N without considering the spin-polarization, it is found that the ground-state lattice dynamics stability of the ferromagnetic phase -Fe4N is induced by the spontaneous magnetization at pressures below 1 GPa. The phonon spectra at point (0.37, 0.37, 0) in line , points X and M become softening at pressures between 1.03 and 31.5 GPa. The pressure-induced effect and the spontaneous magnetization effect on the atoms reach a stable equilibrium state at the pressures between 31.5 and 60.8 GPa, which result in the phonon spectrum stability. As the pressure exceeds 61.3 GPa, the system becomes more instable dynamically with the increase of the external pressure. The softening at point M of the acoustic phonon is treated by the soft-mode phase theory at 10 GPa, and a new dynamic stability high-pressure phase with a space group of P2/m is found. This new phase is thermodynamically stable and possesses the same magnetic moments as that of -Fe4N at pressures below 1 GPa. The enthalpy value of the phase P2/m is less than that of phase at the pressures between 2.9 and 19 GPa, therefore its ground-state structure is more stable. As the pressure exceeds 20 GPa, both phases possess almost the same magnetic moments.
2015, 64 (15): 156401. doi: 10.7498/aps.64.156401
A wide-range semi-empirical equation of state is constructed for numerical simulation of high-energy density experiments, such as, wire-array Z-pinch etc. The equation of state consists of zero-temperature free energy term, and thermal contributions of electron and ion. Thomas-Fermi model, which was firstly put forward by Thomas and Fermi, is initially developed to study the electron distribution of multi-electron atoms. Since its advent, this model has been widely used in solid-state physics, atomic physics, astrophysics and equation of state computations. It is a particularly important model to describe the behavior of matter under extreme conditions of high temperature and high density. This model provides reasonably accurate results that are validated experimentally for some thermodynamic quantities, such as the pressure. However, the Thomas-Fermi model yields a pressure of a few GPa under normal density even at very low temperature, and the pressure is always positive, indicating an obvious limitation of this model. Kirzhnits has evaluated the influence of quantum effect and exchange effect on temperature-dependent Thomas-Fermi model and their contributions to the Thomas-Fermi equation of state. Basically, the Thomas-Fermi model with its quantum and exchange corrections which is called Thomas-Fermi-Kirzhnits model, can be applied to calculate the thermal contribution of electrons to the thermodynamic functions, which can lower the pressure given from the Thomas-Fermi model. The zero-temperature free energy term in the semi-empirical equation of state is described by a polynomial expression. The coefficients of the polynomial expression is calculated by using zero-temperature Thomas-Fermi-Kirzhnits model and the relation between thermodynamic quantities. A quasi-harmonic model is adopted to describe the behavior of ions. It is originally applied to calculate the contribution of ions in the condensed state. However, the quasi-harmonic model is close to an ideal equation of state in the high-temperature and low-density region. This model makes the description of the behavior of ions in the phase transition from the solid state to plasma state be approximated. Thomas-Fermi-Kirzhnits model is adopted to calculate the thermal contribution of electrons. The semi-empirical equation of state has the advantages of less calculation and clear physical concepts. Experimental data of isothermal compression at 300 K is fruitful and accurate. They can be used to verify the results of the semi-empirical equation of state. An isothermal compression curve is calculated by the present work and compared with experimental data. The pressures over a wide-range of temperature and density are derived and compared with corresponding data of SESAME database. The trajectory of the electrical explosion of aluminum is demonstrated from solid state to ideal plasma state.
2015, 64 (15): 156402. doi: 10.7498/aps.64.156402
The one-dimensional system interacting via a delta-function interparticle interaction is a very important one in cold atomic systems and has fundamental importance in many-body physics. In one dimension, due to the geometric confinement induced quantum correlations and quantum fluctuations, there may exist a number of unusual phenomena, such as spin-charge separation, effective fermionization and quantum criticality. This paper studies the basic properties of a uniform one-dimensional Gaudin-Yang model for fermions by solving the thermodynamic Bethe-ansatz equations by a numerical method. Numerically, we use the many-variable Newton’s method to solve the coupled equations. We analyze the physical properties, including density, interaction, temperature and entropy at a given temperature and a given interaction, separately. We know that a lot of researches are limited to zero temperature. However, we cannot reach the absolute zero temperature in the real cold atomic experiment. So it is important to deal with the finite temperature problems. We study the density and entropy as a function of the chemical potential, temperature and interaction and, then give the phase diagrams, respectively. We found that there is a quantum critical zone in the phase diagram of entropy, including the high temperature zone with thermal fluctuations and the Luttinger liquid zone with quantum fluctuations. For a given temperature and low chemical potential, the thermal fluctuations are the main factor in the entropy. With the increase of chemical potential, the system enters the quantum critical zone where the competitive effect between the thermal fluctuations and the quantum fluctuations exists. When the chemical potential is large enough, the quantum fluctuations become the main factor in the system’s entropy, and we get the Luttinger liquid phase. Our results can be further used in the finite temperature density-functional theory and to analyze the collective phenomena at a finite temperature.
CONDENSED MATTER: ELECTRONIC STRUCTURE, ELECTRICAL, MAGNETIC, AND OPTICAL PROPERTIES
2015, 64 (15): 157101. doi: 10.7498/aps.64.157101
Nowadays, the studies on absorption spectra of Ag-doped ZnO have given two distinctly different experimental results, that is, the red shift or blue shift when the mole fraction of the impurity increases in a range from 0.0278 to 0.0417. To solve this contradiction, according to the first-principles plane-wave pseudopotential of the spin-polarized density functional theory (DFT), we set up three models for Zn1-xAgxO (x=0, 0.0278, 0.0417) to calculate the geometric structure and energy via the method of generalized gradient approximation (GGA+U). Calculated results indicate that compared with the Zn-O bond in pure ZnO system, the value of population decreases, and the bond length of Ag-O in Ag-doped ZnO system increases, this means covalent bond weakens and ionic bond strengthens. With the mole fraction of impurity increases in a range from 0.0278 to 0.0417, the orbital charges of O-2p, Zn-4s and Zn-3d keep unchanged, while the orbital charge of Ag-5s increases, and that of Ag-4d is reduced; the volume and total energy of the doped system increases, causing the system more unstable. Moreover, the formation energy of the doped system becomes lower, thereby making the doping difficult. Meanwhile, the band gap in the system narrows, and its absorption spectra exhibits a redshift. The calculated results are consistent with the experimental data, and can explain the phenomena reasonably. These results may be used in future design and preparation of new type photocatalyst from Ag-doped ZnO as a theoretical basis.
Effects of spin polarization on absorption saturation and recombination dynamics of carriers in (001) GaAs quantum wells
2015, 64 (15): 157201. doi: 10.7498/aps.64.157201
In this paper, the ultrafast dynamics of spin relaxation and recombination of photoexcited carriers has been studied in (001) GaAs quantum wells using a time-resolved pump-probe absorption spectroscopy under a nearly resonant excitation of heavy-hole excitons. It is found that the spin polarization of carriers influences both absorption saturation of linear polarized light and recombination dynamics of carriers. Pump fluence dependence of the ultrafast dynamics of spin relaxation and recombination of carriers is further studied, which shows that the effect of spin polarization on linearly polarized absorption saturation is reduced with lowering pump fluence. Spin-polarization-dependent absorption saturation effect can be ignored only as the pump fluence is weak. However, spin-polarization dependence of recombination dynamics is presented in turn at low pump fluence. Our analysis shows that such dependence originates from the spin-polarization dependence of the density of excitons formed in the excited carriers because recombination time constants of excitons and free carriers are very different so that the ratio of exciton density to free carrier density can influence the recombination dynamics. The spin-polarization dependence of ultrafast recombination dynamics of photoexcited carriers implies that the recombination time constant in the calculation of spin relaxation time from spin relaxation dynamics should be the recombination time of spin-polarized carriers, rather than the recombination lifetime of non-spin-polarized carriers as done currently. Exciton density is estimated based on 2D mass action law, which agrees very well with our experimental results. The good agreement between theoretical calculation and experimental results reveals that the effect of Coulomb screening on the formation of excitons may be ignored for a lower excited carrier density.
The AdS/CFT correspondence has provided us a useful approach to describe strongly interacting systems holographically through weakly coupled gravitational duals. One of the mostly studied gravity duals is the holographic superconductor, which is constructed by a scalar field coupled to a Maxwell field in an AdS black hole background. It is shown that when the Hawking temperature of a black hole drops below a critical value, the black hole becomes unstable and this instability in the (d+1) dimensional AdS black hole corresponds to a d-dimensional phase transition at the boundary, called holographic superconductor model. Generally speaking, the instability of the gravity systems belongs to the second-order phase transition. Lately, it was stated that the holographic superconductor with the spontaneous breaking of a global U(1) symmetry via the Stckelberg mechanism allows the first-order phase transition to occur. Some further studies are carried out by considering new forms of the Stckelberg mechanism. So it is very interesting to extend the discussion to other new forms of Stckelberg mechanism to explore the rich properties of holographic superconductors. By considering new higher correction terms of the scalar fields, we investigate a general class of holographic superconductors via Stckelberg mechanism in the background of four-dimensional AdS black hole. We obtain richer structures in the metal/superconductor phase transitions. We study the condensation of the scalar operator and find that when the model parameter is above a threshold value, this new model allows first-order phase transition to occur. We also examine the effects of the backreaction on the threshold model parameter and find that backreaction makes the first-order phase transitions easier to happen (or smaller threshold parameters above which the phase transition changes from second to first order). We may conclude that the model parameter coupled with the backreaction can determine the order of phase transitions.
2015, 64 (15): 157901. doi: 10.7498/aps.64.157901
Relationship between secondary electron yield (SEY) and electron incident angle has been measured for a polyimide sample. SEY as a function of incident angle at different incident electron energy is measured by use of a system with a single pulsed electron beam and a special surface charge neutralization technology based on the negatively biased collector. Measured results show that the SEY may deviate from the traditional law of monotonic increase with the incident angle when the angle is higher than a certain critical value. This deviation is even more obvious at lower incident electron energy. The critical incident angle decreases with decreasing incident energy. A theoretical analysis on the deviation is given in a simplified electron elastic scattering process. The distribution of the scattering region has an important effect on the relation of SEY versus incident angles. A sector region is introduced to describe the electron scattering region. Due to the limit of sample surface, the electron scattering region will decrease if the angle between the incident direction and the sample surface is smaller than half of the central angle of the sector. Corresponding SEY might no longer increase. Based on the Rutherford’s elastic scattering formula, a formula for the critical incident angle is derived as a function of incident electron energy, which is also confirmed by our measurement results. Finally, a revised SEY computation formula is developed which can give more accurate results at high incident electron angle.
INTERDISCIPLINARY PHYSICS AND RELATED AREAS OF SCIENCE AND TECHNOLOGY
2015, 64 (15): 158101. doi: 10.7498/aps.64.158101
In this paper, we propose a flexible, non-directional lowering scattering 1 bit coding metasurface which can significantly reduce the radar cross section (RCS) within an ultra wide terahertz (THz) frequency band. The total thickness of the coding metasurface is only 40.4 μm. The 1 bit coding metasurface is composed of “0” and “1” elements. And the “0” and “1” elements of metasurface are realized separately by a substrate without any metallic covering and that with a square metallic ring covering, the reflection phase difference of the two elements is about 180 degree in a wide THz frequency range. The theoretical, analytical, and simulation results show that the coding metasurfaces simply manipulate electromagnetic waves by coding the “0” and “1” elements in different sequences. Specific coding sequences result in the far-field scattering patterns varying from single beam to two, three, and numerous beams in THz frequencies. The metasurface with the numerous scattering waves can disperse the reflection into a variety of directions for non-periodic coding sequence way, and in each direction the energy is small based on the energy conservation principle. Full-wave simulation results show that the reflectivity less than -10 dB for coding metasurface can be achieved in a wide frequency range from 1-1.4 THz at normal incidence, and the RCS reduction as compared with a bare metallic plate with the same size is essentially more than 10 dB, in agreement with the bandwidth of reflectivity being less than -10 dB; the maximum reduction can be up to 19 dB. The wideband RCS reduction results are consistent with the bandwidth of 180 degrees phase difference between the two elements “0” and “1”. This wideband characteristic of RCS reduction can be kept up as the coding metasurface is wrapped around a metallic cylinder with a diameter of 4 mm. The presented method opens a new way to control THz waves by coding metasurface, so it is of great application values in stealth, imaging, and broadband communications of THz frequencies.