Vol. 62, No. 8 (2013)
2013, 62 (8): 080301. doi: 10.7498/aps.62.080301
Using the technique of integration within an ordered product of operators, we derive Weyl ordered expansion for optical field phase operator and inverse of creation and annhilation operators. The results are used to obtain the classical correspondence, new generation function formula of some special functions and new integration formulas. In particular, for the first time we derive complex Gaussian integral formula when the integrand has negative power function.
2013, 62 (8): 080501. doi: 10.7498/aps.62.080501
It is improper to use frequency domain signal-to-noise ratio as a measure to judge whether stochastic resonance happens with chirp signal which is common in engineering. As a result, there is little literature on this subject. Using the energy aggregation in an optimal fractional Fourier transform domain of chirp signal, in this paper we propose a new signal-to-noise ratio defined in an optimal fractional domain to study the stochastic resonance of over-damped bistable system driven by chirp signal and Gaussian white noise. A new phenomenon is found, that is, the stochastic resonance phenomenon weakens gradually as the frequency of chirp signal increases. And it is reasonably explained in this paper. The consistency of simulation results with theoretical analysis verifies the effectiveness of the method proposed in this paper.
2013, 62 (8): 080502. doi: 10.7498/aps.62.080502
How to improve the network traffic capacity (load), in complex networks with the connection edge by which the transmission capacity (bandwidth) is limited, is the main subject of the research in this paper. We propose a heterogeneous bandwidth allocation scheme under the condition of the fixed total bandwidth resource of the network. With an appropriate proportion of 'controlled edge', the bandwidth resource is reallocated in the network. According to the data flow model, with congestion awareness routing strategy, bandwidth allocation we proposed can adjust the data flow and improve the efficiency of bandwidth utilization. Finally, the network traffic capacity is significantly improved compared with the one in the homogeneous bandwidth allocation. Meanwhile, it does not destroy the premise of the network topology by using our bandwidth allocation. In this paper, we make a series of simulation using the heterogeneous bandwidth allocation in the Barabsi-Albert scale-free network and the Watts-Strogats (WS) small world network, and find that there is a strong correlation between the network traffic capacity and the bandwidth of vertex in the WS network.
2013, 62 (8): 080503. doi: 10.7498/aps.62.080503
In this paper, second-moments of the responses are analytically solved by the Laplace transform in a coupling two-degree-of-freedom system with a biexponentical dissipative memory kernel function driven by a thermal broadband noise. The mean square displacement x2(t)> is different from anomalous diffusion (i.e. x2(t)> ∝ tα (0αα≠1)), which is produced by the single-degree-of-freedom generalized Langevin equation. The oscillation-diffusion of x2(t)> with the change of time and noise parameters is observed generally. According to our analysis, a particle confined by the harmonic potential can escape with the help of the coupling-damping factor B. The diffusion of x2(t)> aggravates with B increasing. However, x2(t)> tends to the stationary state with the increase of the friction coefficient Further, if the two thermal noises are in cross-correlation, smaller cross-correlation time has a deeper influence on second-moments. Meanwhile, the diffusion aggravates and the cross-correlation between two displacements strengthens markedly with cross-correlation strength increasing. It is consistent with physical intuition.
2013, 62 (8): 080504. doi: 10.7498/aps.62.080504
Recently, heat conduction in low-dimensional materials has attracted much attention. The energy carriers responsible for the heat conduction in the Fermi-Pasta-Ulam (FPU) β lattice is still in debate. To the best of our knowledge, the sound velocity of energy transfer has been measured to examine the properties of the energy carriers, by using both nonequilibrium and equilibrium approaches. Nevertheless, the uncertainty of the computational data is too large to distinguish between the two predictions based on soliton theory and effective-phonon theory. In this paper, the spatiotemporal propagation of a momentum excitation traveling along the FPU-β lattice is investigated. The harmonic and anharmonic couplings induce the dispersion and localization of a energy packet on the lattice, respectively. The bifurcation of a solitary wave takes place as the energy of the packet exceeds a threshold. Based on the "acoustic vacuum" phenomenon in the pure quartic lattice, a two-segment lattice composed of the FPU-β chain and the pure quartic chain is constructed. The heat flux in the two-segment lattice is studied to compare with that in the FPU-β lattice. The ratio of the heat flux of the two-segment lattice to the FPU-β lattice increases monotonically as temperature rises. We conclude that phonons are the majority energy carriers in the low temperature regime, while solitons become dominant carriers as the temperature increases. The spatiotemporal propagation of a momentum excitation traveling along the two-segment lattice is also investigated. Phonon packets excited in the FPU-β part are reflected at the interface while solitary waves can pass through the interface. This supports microscopically our conclusion on the energy carriers in the FPU-β lattice.
Chaotic behaviors and fractal self-similar analysis of particles transport properties in RIKEN mesoscopic devices
2013, 62 (8): 080505. doi: 10.7498/aps.62.080505
The theoretical model of RIKEN mesoscopic device in our study is one kind of the two-dimensional Sinai billiards, which is an ideal model to investigate the chaotic and fractal behaviors in particle escape curves. In the analysis of the escape curves, we use two methods:qualitative comparison and quantitative calculation of the fractal dimensions. Then we obtain the influence of the distribution of chaotic areas caused by the opening width, cavity length, corner positions, arc radius, etc. In the results, we find the fractal self-similar structure of the escape curves in which the similarity is very good, and they display the chaotic property of the transmission in the RIKEN device. Moreover, we use the eye-style structure analysis and the comparation between similar ratios to investagate the fractal self-similar structure.
2013, 62 (8): 080506. doi: 10.7498/aps.62.080506
In this paper a method is proposed to estimate the unknown parameters of nonlinear map based on the second-order discrete variational method. On the basis of adjoint equations and gradient expressions of cost functional derived from variational method, the second-order adjoint equations and the exact formulas for the Hessian-vector product are educed using the second-order discrete variational method. A new algorithm is given for estimating the unknown parameters of nonlinear maps. The numerical simulations show that all unknown physical parameters of Hyperhenón map and two-dimensional parabolic map are estimated successfully and precisely. Simulation results also demonstrate the validity and advantages of second-order discrete variational method of estimating the parameters of the discrete chaotic systems.
The synchronization of fractional order chaotic systems with different orders based on adaptive sliding mode control
2013, 62 (8): 080507. doi: 10.7498/aps.62.080507
In this paper, based on sliding mode control and adaptive control theory, the synchronization of two different fractional order chaotic systems is investigated. First, a fractional sliding surface with strong robustness is designed and a suitable adaptive sliding controller is constructed, then the error states of the systems are controlled to the sliding surface via the method to guarantee the synchronized behaviors between two fractional chaotic systems. Numerical simulations on the hyper Chen chaotic systems and Chen chaotic system are also carried out respectively. Simulation results show that the generalized errors tend to zero after a short time, and the effectiveness and feasibility of this method are well verified.
ATOMIC AND MOLECULAR PHYSICS
Theoretical study on the electronic structures and photophysical properties of carbon nanorings and their analogues
2013, 62 (8): 083101. doi: 10.7498/aps.62.083101
Density functional theory (DFT) is used in a series of hexacene nanoring (CA), its boron nitride analogue (CA-BN) and lithium ion doping derivatives to obtain an insight into electronic structure, aromaticity property, energy gap, ionization potential, electron affinity and reorganization energy. DFT calculations of these nanorings indicate that the energy gaps of the carbon nanorings are smaller than those of the boron nitride nanorings. The lithium ion doping will remarkably reduce the HOMO and LUMO energy. The aromaticities of the rings are investigated though nucleus-independent chemical shift (NICS) values. The NICS scan suggests that the aromaticities of carbon nanoring systems are more than those of boron nitride analogues, the aromaticities of boron nitride compounds are very weak due to orbital localization. We also calculate the reorganization energy to investigate the charge transport properties. The results show that the carbon nanoring and their analogues could serve as bipolar carrier transport materials in photoelectric functional materials, and the lithium ion doping significantly improves the charge transport properties. The CA-BN nanorings serve as better electron-transport materials. Furthermore, the lithium ion doping significantly affects the charge transfer property of CA-BN nanoring, making it used as bipolar carrier transport materials. The time dependant DFT investigations show that the boron nitride substitution leads to an important change in absorption spectrum with blue-shift. And lithium ion doping has no obvious influence on absorption spectrum.
2013, 62 (8): 083102. doi: 10.7498/aps.62.083102
Using first-principles calculations based on density functional theory and projector augmented wave method, we investigate the geometric structures, magnetic properties and electronic structures of h-BN monolayer with nitrogen vacancy (VN), nitrogen substituted by oxygen (ON) or sulfur (SN). The results show that the h-BN monolayer with SN presents a larger geometric distortion than with ON and VN. Although the pure h-BN is non-magnetic, all the systems with defects are magnetic with the magnetization value of 1 μB per cell. Spin polarized density of states and energy band structures are presented. By using symmetrical analyses and molecular orbit theory, the formations of impurity energy levels and local magnetic moment are well explained.
2013, 62 (8): 083201. doi: 10.7498/aps.62.083201
The L-shell X-rays of Si, induced by 50-250 keV proton and 1.0-3.0 MeV Ar11+ ions impacting are measured. It is found that the X-ray induced by Ar11+ is about 36 eV higher than that induced by proton. That indicates that 3, 4 L-shell electrons of Si atom are multiply-ionized by Ar11+ ion impact. The X-ray production cross section is extracted from the yield data and compared with the results from the BEA, PWBA and ECPSSR models. With the same unit incident energy, the cross section induced by Ar11+ is about 3 orders of magnitude larger than that produced by proton. For proton impact, the ECPSSR model gives an accurate prediction to the cross section data. However, the BEA model, considering the change of fluorescence yield due to the multiple-ionization, presents the results that are in better agreement with the experimental results for Ar11+ ions.
2013, 62 (8): 083202. doi: 10.7498/aps.62.083202
By numerically solving the time-dependent Schrödinger equation with the interaction between one-dimensional multi-well potential and the mid-infrared few-cycle femtosecond pulses, we theoretically investigate the high-order harmonic generation spectra in a crystal, further find the cutoff frequency formula under the new condition. Our results clearly show that the high order harmonic generation in the crystal is fundamentally different from that in the atomic case, owing to the high density and periodic structure. The harmonics spectrum shows a cutoff position that scales linearly with the peak amplitude of electric field of the drive laser and the lattice parameters. Based on the important role of the three-step model obtained by quasi-calssical mechanics method in gas harmonic generation, in this paper, this method is also well used to verify the cutoff position law in crystal harmonic generation.
2013, 62 (8): 083301. doi: 10.7498/aps.62.083301
The potential energy curves (PECs) for ground electronic state (X1∑+) and seven excited electronic states (a3∏, A1∏, b3∑+, c3∑+, B1∏, C1∑+, d3∏) of LiAl are obtained using the multi-configuration reference single and double excited configuration interaction method. Equilibrium bond length Re, adiabatic excited energy Te and vertical excited energy Tv are obtained. It is shown that c3∑+ is an unstable repulsive state, A1∏ is a weak bound state and the others are all bound states. Predissociation can be found between b3∑+ and c3∑+ states. Eight electronic states are dissociated along two channels, Li(2S)+Al(2P0) and Li(2P0)+Al(2P0). And then PECs are fitted to analytical Murrell-Sorbie (MS) potential function to deduce the spectroscopic parameters:the Re is 0.2863 nm, ωe is 316 cm-1 and De is 1.03 eV for the ground state; the values of Tv of excited states are 0.27, 0.83, 1.18, 1.14, 1.62, 1.81 and 2.00 eV; the values of De are 1.03, 0.82 and 0.26, repulsive state, 1.54, 1.10, 0.93 eV, and the values of corresponding frequency ωe are 339, 237, 394, repulsive state, 429, 192, 178 cm-1. By solving the radial Schrödinger equation of nuclear motion, the vibration levels, inertial rotation constants (J=0) are reported for the first time.
2013, 62 (8): 083601. doi: 10.7498/aps.62.083601
A kind of spintronics is designed by doping the transition metal into Si clusters. Their spin-polarized electron transport properties are investigated by using the first principle analysis. Calculation shows that Fe, Cr and Mn atom doped clusters give the largest spin-polarized transmission coefficients in all the clusters. From Sc to Ni doped clusters, spin filter efficiencies of the systems increase gradually, and the maximal spin filter coefficiency appears in the Fe doped system. The ability to induce the spin-polarized electron transport of the cluster in junction is not cosistent with the magnetic moment of cluster under isolated states.
2013, 62 (8): 083602. doi: 10.7498/aps.62.083602
The melting and freezing behavior of small-sized aluminum nanoclusters with radii ranging from 0.3 nm to 1.3 nm are investigated by molecular dynamics simulation. Based on the potential-temperature curves and heat capacity-temperature curves, the size dependences of melting point and freezing point are obtained and the results are analysed by the surface energy theory and small size effect. The results show a non-monotonic size-dependence of the melting temperature when the atom number of nanoclusters is less than 80. For those clusters with atom number more than 80, the melting and freezing point drop down monotonically with size decreasing. For some special cases, such as aluminum nanoclusters with atom number 27, the melting point is nearly 40 K higher than the bulk melting point. Besides, we observe a rather strong hysterisis of the liquid-solid transition, which states that it is much easier for a cluster to go from ordered to disordered than for the opposite process.
ELECTROMAGENTISM, OPTICS, ACOUSTICS, HEAT TRANSFER, CLASSICAL MECHANICS, AND FLUID DYNAMICS
2013, 62 (8): 084101. doi: 10.7498/aps.62.084101
Based on Maxwell's curl equations, the electric component and the magnetic component in magnetoelectric metamaterial plate are equivalent to the surface electric current and the surface magnetic current respectively. By calculating the total electric field and the total magnetic field in a magnetoelectric metamaterial plate generated by these periodic surface electric currents and magnetic currents, we obtain two equations about the surface current density and the surface magnetic current density, and thus deduce the analytical formulas for relationship between the refractive index of periodic magnetoelectric metamaterial and permeability of the magnetic component, and that between permittivity of the electric component and spatial dispersion. Unlike traditional index formula, the analytical formulas fully consider the spatial dispersion and the interaction of the electrical component and the magnetic component. The theoretical curves for refractive index are found to be in good agreement with the retrieval curves from simulation data, which shows that the analytical formulas for the refractive index can correctly describe the negative refraction characteristics of the magnetoelectric metamaterials. Our work will provide important theoretical reference for researchers to analyze interaction between electromagnetic components and to design magnetoelectric metamaterials with negative refractive indexes which meet certain requirements.
2013, 62 (8): 084102. doi: 10.7498/aps.62.084102
Honeycomb sandwiches are widely used as electromagnetic transparent materials for radomes. However, the electric anisotropy has a significant influence on the transmission performance. This work aims to investigate the electromagnetic transmission characteristics of the anisotropic sandwich panel. First, we deduce the effective permittivity of multilayered anisotropic sandwich material in the respect of the horizontal polarization and the perpendicular polarization components of the incident wave. Second, the transmission line network method related to the multilayered homogeneous medium is improved to simulate the electromagnetic transmission through honeycomb sandwiches and to calculate the transmission ratio. As the proposed method takes into account the three-dimensional anisotropy of each slab, it can simulate the transmission of plane wave with arbitrary incident direction in multilayered anisotropy sandwich panels, moreover, it can reveal the influence of material orientation on the transmission characteristics. Since the multilayer configuration is simulated by transmission line network, the proposed method is far more efficient than the finite element method. Numerical experiments indicate that the influence of the electric anisotropy on the transmission performance of honeycomb sandwich materials can be well revealed. In an incident angle range between 0 and 80 degrees, the simulation results fit well to the results obtained by the finite element method.
2013, 62 (8): 084103. doi: 10.7498/aps.62.084103
More and more overmoded structures have been adopted in electronic vacuum devices because they can increase the power capacity and the frequency of the output signals. Overmoded structure device can more easily work in a low magnetic field. Overmoded structure may cause mode competition, which will reduce the output power. Mode selection in overmoded structure is studied in this article. Dispersion relation is obtained and solved numerically. The results show that the appropriate selection of working point that ensures the device working in a traveling wave state can realize transverse mode selection and appropriation selection of parameter of electron beam and the structure can realize the axial mode selection. The calculation results are verified by particle simulation.
2013, 62 (8): 084104. doi: 10.7498/aps.62.084104
In the Raman free-electron laser as a high-power radiation source with megawatt in millimeter and terahertz wave ranges, the stability of relativistic electron motion is of importance for the performance of the device. By making use of the reported MIT experimental data and Kolmogorov entropy, comparative study is carried out on the stability of the relativistic electron motion in a Raman free-electron laser with positive/reversed guide magnetic field. Results show that the wiggler adiabatic field has trivial influence on the stability of electron motion but substantially affects the electron motion itself in both positive and reversed guide magnetic field cases; the self-field of the electron beam deteriorates the motion stability in the case of a positive guide magnetic field, but favors the motion stability in the case of a reversed guide magnetic field.
2013, 62 (8): 084105. doi: 10.7498/aps.62.084105
From molecular current viewpoint, demagnetizing field and process of permanent magnet, taking rectangular permanent magnets for example, the various factors that affect inhomogeneity of external magnetic field for permanent magnet are analyzed. The results indicate that the macroscopic inhomogeneity of external magnetic field for permanent magnet is closely related to space distance and shape of permanent magnet design. Demagnetizing field of permanent magnet has a complex influence on micro-inhomogeneity of external magnetic field. Technology of permanent magnet such as powder particles, orientation degree, sintering solidification, mechanical processing will affect inhomogeneity of external magnetic field of permanent magnet such as magnetic degree, symmetry, smoothness, etc.
Z-transform algorithm in the finite-difference time domain analysis of ferrite subject to an arbitrary direction of external magnetic field
2013, 62 (8): 084106. doi: 10.7498/aps.62.084106
The susceptibility tensor of saturated magnetized ferrite in frequency domain in laboratory coordinate system is gained by using the transfer matrix between the principal and the laboratory system, and then its form in Z-domain is derived by Z-transform (ZT). The ZT-finite-difference time domain method applied to the ferrite subject to an arbitrary direction of external dc magnetic field is obtained. The co-polarized and cross-polarized backward radar scattering cross-section for a saturated magnetized ferrite sphere are obtained by the presented algorithm. The computed results indicate the correctness and feasibility of the method.
2013, 62 (8): 084201. doi: 10.7498/aps.62.084201
Airy accelerating beams have aroused a great deal of interest because of their non-diffracting and self-accelerating properties, which have many potential applications such as in optical micromanipulations, nonlinear optics and vacuum electron acceleration. A key issue in research of the acceleration beam and its applications is how to generate this specific type of beams with high energy efficiency. In this paper, the spatial oscillation properties of the complex amplitude of an accelerating beam are analyzed and a theory describing the accelerating properties of the beam based on its local spatial frequency is proposed. The local spatial frequency of a general Airy beam is calculated through the zero-point coordinates of the Airy function, and an analytical formula accurately describing the local spatial frequency distribution is given. The relationship between the local spatial frequency and the accelerating trajectory of the beam is also given, based on which a simple algorithm for finding the pure-phase expression of an acceleration beam from its given accelerating trajectory is presented. Finally an analytical expression of the pure-phase function of an acceleration beam for generating a circle-arced trajectory is found out, based on which a pure-phase diffractive optical element is designed and demonstrated successfully.
Research on the algorithm of multiple-image encryption based on the improved virtual optical imaging
2013, 62 (8): 084202. doi: 10.7498/aps.62.084202
In this paper we propose an algorithm for multiple-image encryption based on the improved virtual optical imaging (VOI). The improved algorithm enhances the security by integrating several images into one as an encrypting object and increasing key space. It focuses on three issues. First, it encrypts the phase information, which can improve the performance of decryption effect of the system. Second, it combines random phase plate with a random amplitude board at the position of the random phase plate to increase the dimension of the key space and the ability to resist the attacks. Finally, it adds a random phase plate before the image place, which enhances the security of the system. It uses the peak signal-to-noise ratio to evaluate the performance of the proposed algorithm. Compared with the traditional VOI-based encryption method, the simulations show that the proposed method demonstrates a good performance in the sense of feasibility, robustness and security.
Colligated multiplexing approach to expanding the field of view in the recording process of digital holographic microscopy
2013, 62 (8): 084203. doi: 10.7498/aps.62.084203
We present a method to expand the record field of view in the recording process of digital holographic microscopy systems without loss of resolution. A series of incoherent sub-holograms covering different regions of sample can be recorded in a single frame of the CCD synchronously based on wavelength and polarization multiplexing. The reconstructed images can be obtained with information about different parts of the sample. Thus, the synthetic reconstructed image with a wider field can be achieved by image stitching and fusion technology without any form of scanning. Based on single-exposure working principle, this approach can be used for real-time recording of the dynamic samples without moving CCD, point source and tested samples. Experimental results show that the final synthetic image produced by the system in this paper can be achieved to be close to four times as large as the original record field of view with original resolution.
2013, 62 (8): 084204. doi: 10.7498/aps.62.084204
The balanced homodyne detection technique is an important method to detect the quantum light field. The signal light injected into the phase sensitive optical parameter amplifier (OPA) is phase modulated. The output squeezed light of OPA is detected by the balanced homodyne detection system. When the phase sensitive OPA is operated at parametric amplification and deamplification respectively, we can judge which quadrature component of the quantum light field is measured by observing the modulation signal. We can demodulate the phase modulation signal to achieve the error signal, which is used to lock the relative phase of zero between the local and the quantum light fields (corresponding to the quadrature-amplitude of the unknown light field).
2013, 62 (8): 084205. doi: 10.7498/aps.62.084205
Curviform surface breaks the symmetrical shape of silicon quantum dots on which some bonds can produce localized electronic states in band gap. The calculation results show that the bonding energy and electronic states of silicon quantum dots are different on various curved surfaces, for example, an Si–O–Si bridge bond on curved surface provides the localized levels in band gap and its bonding energy is shallower than that on facet. The red-shifting of PL spectrum on smaller silicon quantum dots can be explained by curved surface effect. Experiments demonstrate that silicon quantum dots are activated for emission due to the localized levels formed in the band gap.
2013, 62 (8): 084206. doi: 10.7498/aps.62.084206
Large mode area photonic crystal fibers (PCFs) have attracted much attention, owing to their applications in such as high power delivery, high power fibre amplifiers and fiber lasers. As an ideal candidate for fabricating mid-infrared fibers, chalcogenide glass possesses some unique advantages, such as high refractive indices (2.0-3.5), low photon energies (lower than 350 cm-1), tailorable compositions, and large infrared transmission window(from 1.0 to 20 μm). In this paper, we present a novel type of effectively single-mode chalcogenide glass Ge20Sb15Se65PCF with utra-large mode area as well as small confinement loss (lower than 0.1 dB/m for the fundamental mode, high confinement loss exceeding 2 dB/m for the higher-order mode at 10.6 μm).
Low-threshold electrically pumped ultraviolet random lasing from ZnO film prepared by pulsed laser deposition
2013, 62 (8): 084207. doi: 10.7498/aps.62.084207
ZnO films on silicon substrates are prepared by reactive sputtering and pulsed laser deposition, respectively. Their crystallinities, surface morphologies and photoluminescence actions are characterized using X-ray diffraction, scanning electron microscopy and photoluminescence spectroscopy correspondingly. Furthermore, the electrically pumped random laser actions of the two metal-insulator-semiconductor structured devices based on the sputtered and pulse laser deposition ZnO films respectively are comparatively investigated. It is found that the device fabricated using the pulse laser deposition ZnO film possesses a much lower threshold current for random lasing and higher output optical power. This is due to the fact that the pulse laser deposition ZnO film has much fewer defects, leading to remarkably lower optical loss during the multiple scattering within such a ZnO film.
2013, 62 (8): 084208. doi: 10.7498/aps.62.084208
In this paper, multilayer films Si/[TiO2/Al2O3]2TiO2 and Si/[TiO2/MgO]2/TiO2 with thickness values from microns to tens of microns are fabricated by spin-coating method. The transmission spectra of these films are obtained by terahertz time-domain transmission spectrum system (THz-TDS). The phase shifts of reflection and phase penetration depths of Si/[TiO2/Al2O3]2TiO2 and Si/[TiO2/MgO]2/TiO2 are simulated by the transfer matrix method. On this basis, two kinds of symmetrical THz microcavities each with a structure of DBR/LT-GaAs/DBR are designed and the radiation spectra are also simulated. The results show that the intensities of two microcavities are enhanced by 19 and 14 times at resonance wavelength, respectively. There are two resonance peaks in the emission spectrum of the structure Si/[TiO2/Al2O3]2TiO2/LT-GaAs (12 μm)/[TiO2/Al2O3]2TiO2, which are located at 208 μm and 248 μm, respectively. The reason is discussed based on the effective cavity length. The feasibility to regulate the emission properties of the THz source by introducing dielectric microcavities is discussed.
All-optical format conversion from non-return-to-zero to return-to-zero based on dual-pump four-wave mixing in photonic crystal fiber
2013, 62 (8): 084209. doi: 10.7498/aps.62.084209
A novel all-optical format conversion scheme based on dual-pump four-wave-mixing (DP-FWM) in dispersion-flattened highly nonlinear photonic crystal fiber (DF-HNL-PCF) is proposed and experimentally demonstrated. The original non-return-to-zero (NRZ) format is converted into double return-to-zero (RZ) format by injecting NRZ signal and double synchronized clock signals into the DF-HNL-PCF. The DP-FWM effect generates two sideband components, which carry the same data information as the original NRZ signal with RZ format. The wavelength tunability and dynamic range of format converter are investigated. The optimum extinct ratio and Q factor of converted signals are 15 dB and 5.4, respectively. The advantages of this scheme are that the each wavelength of double channel multicasting signals is dependent and flexible. Moreover, the system is transparent to bit rate as well as modulation format, and achieves all-optical wavelength conversion and wavelength multicasting.
2013, 62 (8): 084210. doi: 10.7498/aps.62.084210
Based on the division Fourier algorithm and the rapid virtual time evolution (AITEM) iterative method, the transmission characteristics of bullet in linear and nonlinear scattering out of phase modulation Kummer-Gauss optical lattice are studied. The results show that the linear and nonlinear phase modulation significantly change the bullet shape and its range of stability, and the nonlinear modulation depth through the propagation constant controls the stability region width. It is shown that stable space-time soliton energy will grow with nonlinear modulation depth strengthening.
Two-dimensional asynchronous fractional Fourier transform and propagation properties of beams in strongly nonlocal nonlinear medium with an elliptically symmetric response
2013, 62 (8): 084211. doi: 10.7498/aps.62.084211
Propagation of beams in strongly nonlocal nonlinear medium with an elliptically symmetric response is investigated. The result shows that the propagation of beams in this type of medium can be regarded as two-dimensional asynchronous fractional Fourier transform. With this powerful mathematical tool, the analytical solution and the propagation properties of beams can be conveniently obtained. Based on the properties of the two-dimensional asynchronous fractional Fourier transform, the general propagation properties of beams are discussed, the formation conditions of solitons and two-dimensional asynchronous breathers are analyzed, and the law for interaction among solitons and/or breathers is obtained.
2013, 62 (8): 084212. doi: 10.7498/aps.62.084212
A novel mechanism of cascaded parametric oscillation is proposed in this paper to solve the problem of low-efficiency in terahertz (THz) parametric oscillation. The cascaded parametric oscillation is theoretically analyzed based on PPLN crystal as an example. The tuning characteristics of the THz wave, which are affected by the parameters of pump wavelength, poling period of PPLN and operating temperature, are investigated. The characteristics of THz gain and absorption are deduced in the case of collinear interaction of the three mixing waves. The results indicate that the THz conversion efficiency can be greatly enhanced and the widely tunable THz wave can be realized in cascaded parametric oscillation. The experiment on cascaded parametric oscillation generating high-efficiency, wide-tuning, narrow-linewidth, continuous THz wave is designed based on the analysis above.
Photonic band gaps of two-dimensional hexagon-lattice photonic crystals based on Taiji-shaped dielectric rods
2013, 62 (8): 084213. doi: 10.7498/aps.62.084213
We present a novel structure of two-dimensional (2D) hexagon-lattice photonic crystal with asymmetrical scatterers-Taiji-shaped scatterers. The properties of photonic band gap (PBG) and the influence of parameter on absolute photonic band gap are analyzed by plane wave expansion method. The calculation results demonstrate that the reduction of scatterer symmetry can produce an increase in the number of PBG and a broadening of PBG width for both TE and TM model, which is conducive to obtaining wider and more absolute PBG. By optimizing the parameters of structure, we obtain the widest absolute PBG 0.0541(ωa / 2πc) at ε = 17, R= 0.38 μm, r=0.36R, and θ = 0° and the maximum of 8 absolute PBGs at ε = 16, R=0.44, r=0.2R, and θ = 0°.
2013, 62 (8): 084214. doi: 10.7498/aps.62.084214
Polarization control is of vital importance in optical communications, which can affect the stability and bit error rate of a system. We demonstrate an all-fiber high-speed polarization control scheme based on a bidirectional Sagnac ring. The polarization direction can be accurately controlled by adjusting the phase difference of an electro-optic fiber phase modulator in the ring, meanwhile it is realized that only a single beam port outputs different polarization states. The phase can be adjusted accuratly to 10-3 rad, giving an extinction ratio as high as 30 dB, and the modulation speed can reach 2 GHz. The system has excellent stability, and as the optical circuit consists of simple, low cost components which can be easily integrated, there are good prospects for its application in quantum cryptography and other optical communcations fields.
2013, 62 (8): 084215. doi: 10.7498/aps.62.084215
A kind of highly birefringent and highly nonlinear photonic crystal fiber (PCF) is proposed. The fundamental mode field, birefrin-gence, nonlinear coefficient, effective mode area and dispersion characteristic of the fiber are studied by the full-vector finite elementmethod. Simulation results show that the birefringence becomes larger by reducing the air hole pitch, and the birefringence reachesup to 10-2 at a wavelength of 1550 nm, which is about two orders of magnitude higher than that of the regular elliptic polarizationmaintaining fiber. At the same time, a nonlinear coefficient of 42 W-1·km-1 is obtained. There are two zero dispersion wavelengthsin the visible and the near-infrared region respectively, and flattened chromatic dispersion from 800 nm to 2000 nm is obtained. Sucha design provides a new approach to obtaining highly birefringent and highly nonlinear PCF, and the fiber has a broad prospect ofapplication in the polarization control, nonlinear optics and dispersion control.
Design and fabrication of a novel side-leakage photonic crystal fiber and its propagation properties
2013, 62 (8): 084216. doi: 10.7498/aps.62.084216
A novel side-leakage photonic crystal fiber (SLPCF) is proposed and fabricated by introducing a central elliptical Ge-doped core and side-leakage channel. The propagation properties of the ideal and actual structure are modeled by using full-vectorial finite element method for the rebuilt structure. This SLPCF exhibits good compatibility with the standard single mode fiber (SMF) due to its modal diameter of 9.275 μm which is very close to that of SMF at a wavelength of 1550 nm. Modal birefringence of 0.837× 10-4 and the group birefringence of 1.508× 10-4 are obtained at a wavelength of 1550 nm. Based on the side-leakage PCF, a Sagnac interferometer is constituted for evaluating the properties of the actual SLPCF. Experimental results demonstrate that the second order mode can be efficiently confined and thus single mode operation can be realized in a wavelength range from 1450 nm to 1750 nm when this fiber reaches a certain length. In addition, the average measuring value of group birefringence accords with the numerical result. The proposed SLPCF has a number of potential applications in fiber sensor and fiber components with high performance since the introduction of the side-leakage channel enhances its sensitivity, the environmental parameters such as torsion, curve and strain and so on.
2013, 62 (8): 084217. doi: 10.7498/aps.62.084217
An all-fiber polarizing controller scheme with high speed and high accuracy is proposed. The stability is tested in detail and the system can compensate for the birefringence effects induced by the deformation of the fiber automatically. The system is simple and has high accuracy and efficiency. A preliminary experiment demonstrates that it reaches an high extinction ratio of 31 dB. The experimental results show that this system has prospects for practical applications.
2013, 62 (8): 084218. doi: 10.7498/aps.62.084218
We propose a high sensitivity fiber optical temperature sensor with simple structure. The sensor is composed of a multimode fiber (MMF) and a short single mode fiber (SMF) whose end is coated with silver film as a reflective mirror. Due to the mismatch between the MMF core and the SMF core, part of guided mode is coupled with the cladding modes of downstream SMF cladding. The mode index difference between the fiber core and cladding is attributed to the phase difference, resulting in interference. When ambient temperature increases, the interference spectrum presents red-drift because of the difference in thermo-dependence. A high temperature sensitivity of 120 pm/℃ is achieved, and the linearity is 99.5%. The configuration is simple and has a compact size, making it a good candidate for distant temperature sensing and oil prospecting.
Experimental investigation on aero-optical aberration of the supersonic flow passing through an optical dome with gas injection
2013, 62 (8): 084219. doi: 10.7498/aps.62.084219
During the flight in the atmosphere, the optical window of an optical dome needs to be cooled, and supersonic film cooling is one of the economic ways. After traversing through the complex flow field above the window, the optical wave would be distorted by fluctuations in the density field due to the expansion wave, shockwave, mixing layer, turbulent boundary layer, etc. The aero-optical aberrations induced by the flow field of an optical dome in the presence and in the absence of the gas injection at Mach 3.8 are investigated experimentally. Based on the nano-tracer planar laser scattering (NPLS) technique, the density field with high spatial-temporal resolution is first obtained by the flow image calibration, and then the optical path difference (OPD)fluctuations of the original 532 nm planar wavefront perpendicular to the window are calculated using Ray-tracing theory. Also the OPD fluctuations caused by the near-wall region flow structures are presented. In the absence of the gas injection, the flow structure is relatively simple with a long recirculation and laminar region, while in the presence of the gas injection, there appear more complex structures such as shear layer, mixing layer and turbulent boundary layer and the flow is converted into turbulence quickly. Clearly, the optical aberration in the presence of the gas injection is degraded more. For example, the values of root-mean-square OPD (OPDrms) in the absence of the gas injection are 0.038 μm and 0.0356 μm, and they are 0.0462 μm, and 0.0485 μm in the presence of the gas injection during the interval 5 μs.
Sound field separation technique based on equivalent source method and double-layer particle velocity measurements
2013, 62 (8): 084301. doi: 10.7498/aps.62.084301
Two sound field separation techniques based on equivalent source method are proposed:one is based on the double-layer pressure measurements and the other is based on the single-layer pressure-velocity measurements. In this paper, a sound field separation technique based on equivalent source method but using the double-layer velocity measurements is proposed. First, the theoretical description of this technique is given, and then numerical simulations and experiments are conducted to demonstrate the validity of this technique. By comparison with the technique based on double-layer pressure measurements, the superiority of the proposed technique in separating particle velocity field is shown. And also, in the numerical simulations, the influences of the strength of disturbing source and the distance between two measurement planes on separation accuracy are investigated.
2013, 62 (8): 084302. doi: 10.7498/aps.62.084302
Clutter caused by reel and ocean animals is the primarily interference for active sonar. How to classify and identify underwater vehicle echo between reel and ocean animal echo is the most confusing problem that affects the performance of active sonar. Characterization and application mode of underwater target omnibearing echo characteristic are studied for the application of active sonar target identification. A method of estimating the acoustic scattering function based on highlight model is presented, through which underwater target acoustic echo characteristics could be well characterized by statistical feature, and the method is more suitable for the application in active sonar classification. Based on the method studied in this paper, statistical feature of echo highlight is extracted from lake acoustic scattering test of scalled benchmark submarine model.
2013, 62 (8): 084303. doi: 10.7498/aps.62.084303
There are a few references on thunder at present, moreover they are all about thunder frequency spectrum at observation location. However, thunder frequency spectrum in observation location is a distorted signal because of noises and sound attenuation effects. In this paper, thunder signal from lightning discharge channels is obtained in Lanzhou of China. By using digital filter and Fourier transform, thunder frequency spectrum at the observation location is gained. Then, thunder frequency spectrum near lightning discharge channel is computed with the theory of sound propagation effects.
The implement and influence of different boundary conditions in direct simulation on particle sedimentation
2013, 62 (8): 084401. doi: 10.7498/aps.62.084401
In this paper, we present a direct numerical simulation of particle sedimentation in two-phase flow with four different boundary conditions. We demonstrate that different boundary conditions can result in quite different flow behaviors. Some interesting results are obtained. By redefining the computational domain at each time step according to the position of the particle, we construct an infinite channel, which can simulate the particle sedimentation accurately; the flow pattern of periodic boundary is quite different from the infinite channel because of the disturbed flow field; if the settlement is reached steadily before the closed bottom, the closed channel can also simulate the particle settled in the infinite channel; the fluidized condition slows down the particle sedimentation, which is very helpful for better using the boundary conditions.
2013, 62 (8): 084501. doi: 10.7498/aps.62.084501
Forcing currents are generated by using two types of constant currents to stimulate different areas of a medium. The external current is mapped into transmembrane current and stable gradient force is induced to drive the stable rotating spiral wave. The medium becomes homogeneous when the spiral wave is removed and the time series of sampled membrane potentials becomes stable value without considering noise. The scheme is confirmed to be effective in the presence of Gaussian white noise.
2013, 62 (8): 084701. doi: 10.7498/aps.62.084701
We use the close return method combined with adaptive threshold selection to detect the unstable periodic orbits from the signals measured from experimental oil-gas-water three-phase slug flows, and find that the period of the emulsion type slug flow is longer than that of oil in water type slug flow. Especially, the orbit of oil in water type slug flow is basically composed of one big loop and one small loop, and the emulsion type slug flow orbit is composed of two big smooth loops. In addition, we employ the method of adaptive optimal kernel time-frequency representation to investigate the flow behaviors of two typical oil-gas-water three-phase slug flows and indicate that the energy of the oil in water slug flow exhibits a dispersed distribution and its frequency spectrum consists of various components distributes in a wide range. In contrast, the energy of emulsion type slug flow is distributed in a rather concentrated region and the high frequency component in its frequency spectrum is much less than that of the oil in water slug flow. These results are well consistent with the detected structure of unstable periodic orbit, further suggesting that the fluid mechanism underlying oil in water type slug flow is more complicated than that of the emulsion type slug flow.
2013, 62 (8): 084702. doi: 10.7498/aps.62.084702
Based on the lattice Boltzmann method, the motion of single suspended particle in tapered tube is simulated numerically. The distributions of velocity and pressure in the flow field are obtained. The hydrodynamic force on the particle boundary is evaluated by conventional momentum exchange method (CME), lattice-type-dependent momentum-exchange method (LME) and stress tensor integral method (STI) separately. The variations of velocity and the trajectory of the particle which starts at different places are analyzed, the results evaluated by LME are in excellent agreement with those by STI and the results evaluated by CME are slightly different from those by the former two methods.
2013, 62 (8): 084703. doi: 10.7498/aps.62.084703
We investigate numerically the sedimentations of solid particles in a fluid with different temperatures using a lattice Boltzmann method. The sedimentation processes of a single particle in an isothermal, hot, or cool fluid are first simulated, and then the sedimentations of two particles with different temperatures are carefully studied. It is found that the dynamics of the two particles with the same temperature is closely related to the Reynolds number and the Grashof number, while the process of two particles with different temperatures are different from that of two particles with the same temperature. The cold particle will eventually descend under the hot particle, and the drafting and kissing phenomenon occurs as Re is large, while the cold particle falls far from the hot one as Re is small.
2013, 62 (8): 084704. doi: 10.7498/aps.62.084704
The deflecting oscillation of planar opposed jets is experimentally studied and numerically simulated by large-eddy simulation (LES) at 25 ≤ Re ≤ 10000 (Re= U0hρ/μ, where U0 is the bulk velocity of the jet at the nozzle exit, h is the height of the slit of the planar nozzle, and ρ and μ are the density and dynamic viscosity of fluid, respectively) and 4h≤ L ≤ 40h, where L is the nozzle separation. The numerical results are validated by comparing with the experimental results of planar opposed jets. Maps of parameter space describing the deflecting oscillation of planar opposed jets at various nozzle separations and exit Reynolds numbers are presented. And the variation features of deflecting oscillation periods and velocity-pressure of turbulent planar opposed jets are primarily investigated. The results of the study show that the LES can effectively predict the deflecting oscillation of planar opposed jets. The velocity and pressure at specific points vary periodically while the deflecting oscillation of planar opposed jets happens. Furthermore, the variation periods of velocity and pressure are in accordance with the periods of the deflecting oscillation. In essence, the deflecting oscillation of planar opposed jets is caused by periodical variation and transformation of the velocity and pressure.
Wave-making experiments and theoretical models for internal solitary waves in a two-layer fluid of finite depth
2013, 62 (8): 084705. doi: 10.7498/aps.62.084705
A laboratory wave-making method is developed for the internal solitary wave under the condition of giving its amplitude produced by oppositely and horizontally pushing two vertical plates placed separately in the upper- and lower-layer fluids of a large-scale density stratified tank where based on the Miyata-Choi-Camassa (MCC) theoretical model, the layer-mean velocities of the upper- and lower-layer fluids induced by the internal solitary wave are used as the velocities of the two plates. On this basis, a series of experiments is conducted to explore the applicability conditions for internal solitary wave theories with stationary solutions which are Korteweg-de Vries (KdV), extended KdV (eKdV), MCC and modified KdV (mKdV) models in a two-layer fluid of finite depth respectively. It is shown that for the nonlinear parameter ε and the dispersion parameter μ defined by the total water depth, there exists a critical dispersion parameter μ0, in the case of μ μ0, the KdV model is applicable for ε ≤μ, the eKdV model is applicable for μ ε ≤√μ, as well as the MCC model is applicable for ε > √μ. However, in the case of μ ≥ μ0, the MCC model is still applicable for a wide range of ε. Furthermore, for the case where the ratio of depth between the upper- and lower-layer fluids is not close to its critical value, the mKdV model is mainly applicable for the case where the amplitude of the internal solitary wave is close to its theoretical limiting amplitude, however, the MCC model is also applicable for such a case. The investigation quantitatively characterizes the applicability conditions for four classes of internal solitary wave theories, and provides an important theoretical foundation for what kinds of theories can be chosen to model internal solitary waves in the ocean.
2013, 62 (8): 084706. doi: 10.7498/aps.62.084706
Rectangular under-expanded supersonic jet collision experiment is carried out under different nozzle distances and jet pressures and compared with that in the case of free jet. Experiments indicate that there are four screech tone modes of supersonic jet collision, switched from one mode to another depending on the nozzle distance and jet pressure. Two normal shock waves are present between nozzles as jet pressure is more than 0.5 MPa and nozzle distance is less than 50 mm, radiating a stable screech tone with a frequency of about 3 kHz. With nozzle distance increasing or jet pressure decreasing, a bow shock is present at one nozzle exit and a normal shock wave appears at the other exit with the collision surface oscillating between them. Collision surface might be kept balanced in the centre of two nozzles with a 9 kHz frequency screech tone, however, it is vulnerable to disturbance and would return to the equilibrium position near nozzle exit or oscillate between nozzles with large amplitude. When jet pressure is less than 0.36 MPa and nozzle distance greater than 70 mm, the collision surface substantially oscillates between the nozzles, radiating a screech tone with a frequency of about 1 kHz which decreases with jet pressure decreasing and nozzle distance increasing.
2013, 62 (8): 084707. doi: 10.7498/aps.62.084707
The phenomena of droplet impact on an inclined solid surface covered with a pre-existing liquid film are observed using high speed camera at 10000 frames per second. The processes of droplet spreading, liquid sheet formation, splashing and droplet oscillation are observed and analyzed. From the results the relationship between spreading velocity and time is discussed quantitatively. Besides, the effects of impact velocity and impact angle on front and back spreading factors and initiatory spreading velocity are also revealed. At the impact angles ranging from 28.0° to 74.7°, it is found that with the decrease of impact angle, the deformation degree of the droplet oscillation on the film surface increases. The results also indicate that the front spreading factor can increases by increasing impact velocity or by reducing the impact angle, whereas the back spreading factor increases with impact angle increasing. The impact velocity almost has no influence on it. The initial spreading velocity can increase by increasing the impact velocity and the impact angle.
2013, 62 (8): 084708. doi: 10.7498/aps.62.084708
Using a long-distance microscope imaging system and a lock-in phase integrate technique, optical measurements demonstrate that the evolutional behaviors of a phospholipid-shelled micro-bubble, a kind of ultrasound contrast agent, under two ultrasound frequencies and different pressures. Dozens of frames of an oscillating microbubble for one acoustic cycle are captured. The experimental data of micro-bubble diameter are fit by the numerical calculations of the Hoff model and the Rayleigh-Plesset model, respectively. The results show the good agreement between experimental data and the theoretical calculation in the Hoff model. In addition, the spectral analysis based on the experimental data indicates that the relative intensity of the second harmonic increases with the driving pressure amplitude.
CONDENSED MATTER: STRUCTURAL, MECHANICAL, AND THERMAL PROPERTIES
High-quality InGaN epilayers grown by PA-MBE and abnormal incorporation behavior of Indium into InGaN
2013, 62 (8): 086101. doi: 10.7498/aps.62.086101
Growth behaviors of InxGa1-xN (x ≤ 0.2) materials by plasma-assisted molecular beam epitaxy (PA-MBE) are investigated in detail. A precise control of the incorporation of indium into InxGa1-xN at a growth temperature of 580 ℃ is realized. The In0.19Ga0.81N shows a very narrow width of 587 arcsec for the (10.2) asymmetrical reflection from high-resolution X-ray diffraction and the background electronic concentration is 3.96× 1018 cm3. In the region of metal-rich growth, no negligible indium incorporation is observed even if the Ga beam flux is much larger than the equivalent N flux. This growth behavior might be ascribed to an incomplete Ga incorporation during InGaN growth. In addition, a slight increase of In flux results in crystalline quality degradation of InGaN epilayers.
Influence of growth conditions on the lateral grain size of AlN film grown by metal-organic chemical vapor deposition
2013, 62 (8): 086102. doi: 10.7498/aps.62.086102
In this paper, we investigate the effect of growth conditions on the quality of AlN film grown by metal-organic chemical vapor deposition. We test and analyze the influence of different growth conditions, such as nitridation time, growth time of AlN buffer layer and the flow rate of carrier gas, on the lateral grain size of AlN film. It is found that the redution of nitridation time, the increase of growth time of AlN buffer layer, and the reduction of the flow rate of carrier gas can enhance the lateral growth of AlN film and coalescence of islands and increase the lateral grain size of AlN film. So the quality of AlN film is improved.
2013, 62 (8): 086201. doi: 10.7498/aps.62.086201
In order to investigate the mechanism and regular pattern of metal magnetic memory (MMM) signal, from the angle of electron spin, the magnetomechanical model of MMM is set up, and the relationship between stress concentration state and self magnetic flux leakage (SMFL) signal is calculated by the plane wave and pseudo-potential method based on the density functional theory. The research results show that the fundamental reason for SMFL is lattice distortion induced by loads, and the theoretical calculations are in very good agreement with the experimental observations. The present work is helpful for testing the mechanism of MMM.
2013, 62 (8): 086601. doi: 10.7498/aps.62.086601
The viscosity and surface tension of metal melt are all sensitive physical properties that relate to the liquid structure and also have a certain correlation between them. For electronic packaging materials, both viscosities and surface tensions are very important parameters affecting the processing properties. In this study, the viscosities of Sn-xCu (x = 0.7, 1.5, 2) solder melts are measured by using a torsional oscillation high-temperature viscometer. Abrupt change in viscosity occurrs in a certain range of temperature. The temperature range can accordingly be divided into a low temperature zone and a high temperature zone. The relationship between viscosity and temperature can fit to the Arrhenius equation very well in each temperature zone. The structure characteristics and evolutions of the liquid solders are then discussed. Meanwhile, the surface tensions of the Sn-xCu solders are calculated according to the viscosity values at the corresponding temperatures. The test results of the wetting angle and the spreading rate are in good agreement with the calculations, indicating that the method of using the viscosity values to calculate the surface tensions of binary lead-free solder alloys and evaluate their wettabilities is feasible.
CONDENSED MATTER: ELECTRONIC STRUCTURE, ELECTRICAL, MAGNETIC, AND OPTICAL PROPERTIES
2013, 62 (8): 087101. doi: 10.7498/aps.62.087101
Occupancy sites of uranium atom in goethite are studied by first principles calculations. It is found that the formation energies of substitution (S), octahedral (O), and tetrahedral (T) sites of uranium in goethite are -13.49, -3.86, and -1.60 eV, respectively. The formation energies of dual uranium in S and O sites in goethite are -27.392 and -16.214 eV, respectively, and corresponding binding energies are -0.417 and 1.131 eV, respectively. It is concluded that dual uranium atoms can aggregate together in S site formation in goethite, but not in O site formation.
2013, 62 (8): 087102. doi: 10.7498/aps.62.087102
The elastic and optical properties of face-centered cubic IVB group transition metal nitrides such as TiN, ZrN, and HfN are calculated using the plane wave pseudopotential method based on first-principle density function theory. Electronic property is a combination of covalent, ionic, and metallic property. And band structures of these compounds show metallicities in the ground state. The obvious numerical differences between E and E indicate elastic anisotropy. The phenomena result in lattice distortions and microcracks in these metal nitride films. With red shift of pseudogap, the metallic properties of TiN, ZrN, and HfN are improved in turn. And the elastic and optical properties change with metallic properties improving:brittleness reduces, the degree of anisotropy increases, the critical energy between intraband and interband transitions increases, and the solar-optical selectivity decreases. So reducing the number of conduction electrons to strengthen covalency is an effective method to improve elastic isotropy and optical selectivity.
2013, 62 (8): 087103. doi: 10.7498/aps.62.087103
Based on density functional theory of first-principle theory, we systematically investigate the electronic structure and optical property of V-, N- monodoping and V-N codopiong and compare with pure ZnO. The results show that the absorption in the visible light region is enhanced for the doped system, especially for V-N codoped ZnO. The calculated biding energy indicates that the V-N codoping is the most stable system, thus, V-N codoped ZnO is considered as an ideal photocatalyst. Moreover, the method of anion-cation codoping could be better used in photoelectrochemistry and also in the preparation of stable and high performance short wavelength photoelectron devices.
First-principles study of pressure induced phase transition, electronic structure and elastic properties of CdS
2013, 62 (8): 087104. doi: 10.7498/aps.62.087104
In this work, phase stabilities, phase transitions, electronic structures and elastic properties of wurtzite structure (WZ), zinc-blende structure (ZB) and rocksalt struture (RS) phase of CdS are studied by first principles method. Results indicate that WZ and RS phases could be stable in corresponding pressure areas. However, ZB phase could not be stable. Pressure-induced metallic phase transition from WZ to RS will occur at 2.18 GPa. Electronegativity of S atom in WZ phase is much more than that of Cd atom, and the difference in electronegativity between S and Cd is less than 1.7, which induces covalent crystal of CdS. Under the condition of high pressure, radius of S is reduced sharply, which causes the increase of effective nuclear charge. Large nuclear charge will enhance the ability to attract electrons of outer shell, which will cause larger electronegativity. When pressure is higher than 2.18 GPa, the difference in electronegativity is more than 1.7. Then, CdS will be ionic crystal. C44 of WZ phase decreases with pressure, resulting in mechanical instability. And then,the WZ-to-RS phase transition occurs at 2.18 GPa. Moreover, C11 and C12 of RS phase increase with pressure. At the same time, C44 of RS is stable with pressure increasing entirely, all of which shows that RS phase has excellent stability and mechanical property under high pressure.
2013, 62 (8): 087201. doi: 10.7498/aps.62.087201
The phase matching of the quantum paths in high-order harmonic generation is investigated in two-color laser fields. The results show that the spatial distribution of the laser field is optimized by adjusting the laser beam waist, the phase-matching properties of the long trajectories can be modulated effectively, and then good phase matching for both the two paths can be achieved from on-axis region to off-axis region. A radial clear interference fringe induced by the well phase-matching of two paths can be observed by placing a near-field filter, which has great potential for distinguishing clearly different interference effects and observing higher order trajectories with improved detection sensitivity.
The limited graphene means that two directions of graphene are limited, one is zigzag type boundary and the other is armchair type boundary. Based on the tight-binding model, the electronic state and band of the limited graphene are given analytically. The results show that there are only two kinds of electronic states, i.e., the standing wave state and edge state. For the standing wave state, the wave function is in the form of sine function in two directions; for the edge state, the wave function is in the form of hyperbolic sine function in the direction of armchair boundary and in the form of sine function in the direction of zigzag boundary. The band is composited of total carbon atom number N discrete eigenvalues. The expression of quantitativly calculating the number of eigenvalues of edge state is deduced. Through the density of states of the limited graphene we analyze the existence of the edge state and the consistency in the infinity case. The results from the analitical method are the same as the numerical resullts. When the width of two restricted boundary goes into infinity, the result of the limited graphene tends to that in the infinity case.
2013, 62 (8): 087302. doi: 10.7498/aps.62.087302
Highly efficient organic light-emitting diode is fabricated with a novel double hole injection layer consisting of MeO-TPD/CuPc. We observe that the insertion of such a double hole injection layer leads to a striking enhancement in the electrical property:higher luminance, power efficiency and lower driving voltage. It has the configuration of ITO/MeO-TPD (15 nm)/CuPc(15 nm)/NPB(15 nm)/Alq3 (50 nm)/LiF(1 nm)/Al(120 nm). Its turn-on voltage is 3.2 V, which is 2, 0.3 and 0.1 V lower than those of the device without hole injection layer (device A) and the devices using MeO-TPD (device B), CuPc (device C) as hole injection layer, respctively. The highest luminance of the novel device reaches 23893 cd/m2 at a drving voltage of 10 V. The maximum power efficiency of the novel decive is 1.91 lm/W, which is 43% (1.34 lm/W), 22% (1.57 lm/W) and 7% (1.79 lm/W) higher than those of devices A, B and C, respectively. The improvement is ascribed to its high hole injection and transport ability. The results are verified by using the J-V curves of "hole-only" devices.
2013, 62 (8): 087303. doi: 10.7498/aps.62.087303
Tunneling time is an important factor to describe quantum electronic device. In this paper, the dynamic problem of spin-dependent tunneling is investigated by solving the time-dependent Schrödinger equation. The transmission coefficient and tunneling lifetime are discussed by use of mixing transfer-matrix and Runge-Kutta method. The k3 Dresselhaus term is considered to correct the effective Hamiltonian of the system in our calculation. The results show that the transmission peak of the electrons with different spin orientations split obviously. The building time and the tunneling lifetime through the double-barrier structure of AlxGa1-xSb material are different for the spin-down electron and spin-up electron. These time-dependent properties depend on the electronic spin orientation. It can be considered as one of reasons for spin polarization to appear. Additionally, the steady spin-polarization emerges in the well due to the k3 Dresselhaus spin-orbit coupling.
2013, 62 (8): 087501. doi: 10.7498/aps.62.087501
In this paper, band structures and defect modes of two-dimensional magnonic crystals with line defect are calculated by using the plane-wave expansion method under supercell approximation. The obtained results show that line defect modes can be produced in the band gap by introducing the line defect bodies into the two-dimensional magnonic crystal. The presence of the line defect modes leads to the spin waves propagating along the direction of the line defects in these structures. Utilizing the wave-guide characteristic of line defect modes, two-dimensional magnonic crystals can be used as the materials for fabricating spin-wave waveguides.
2013, 62 (8): 087701. doi: 10.7498/aps.62.087701
The DC conductivity of the CaCu3Ti4O12 ceramic is considerable at low frequency. The dielectric properties of the CaCu3Ti4O12 ceramics are analyzed by dielectric spectrum, and the two relaxation processes are characterized by the dielectric modulus. The two relaxation processes are considered which are dominated by the electronic relaxation of deep bulk traps at the depletion layer edge. The low-frequency and high-frequency relaxation processes are attributed to oxygen-vacancy-related defect and native defect, respectively. It is proved that the modulus response of the CaCu3Ti4O12 ceramic is equivalent to conductivity response at high temperature (low frequency), and the peak value of the M" is inversely proportional to capacitance. The activation energies calculated by conductivity and modulus are equivalent to each other. The modulus spectrum is more effective to the material which has high DC conductivity at low frequency such as CCTO ceramic.
2013, 62 (8): 087702. doi: 10.7498/aps.62.087702
The dielectric loss of the CaCu3Ti4O12 ceramic is high, and the mechanism of the loss is not clear, which restricts its application. The CaCu3Ti4O12 ceramic samples are synthesised by solid state reaction method and coprecipitation method. The electronic relaxation of deep bulk traps at the depletion layer edge, carrier relaxation and the dielectric loss of CaCu3Ti4O12 ceramic are investigated. Both perfect double Schottky barrier and low impurity density can reduce the DC conductivity, thus reducing the low-frequency dielectric loss. High-frequency dielectric loss is controlled by deep bulk trap density, especially in the one whose activation energy is 0.12 eV. At room temperature, when the frequency is 1 kHz, the dielectric constant and loss of CaCu3Ti4O12 ceramic prepared by coprecipitation method are 1.4× 104 and 0.037, indicating a good improvement.
Research of metamaterial absorbers and their rectangular waveguide matching terminal applications based on the electric resonators
2013, 62 (8): 087801. doi: 10.7498/aps.62.087801
To cope with the high complex properties and high costs of measuring the metamaterial absorber in the free space, in this paper we experimentally discuss the absorption characteristics of four kinds of metamaterial absorbers based on different electric resonators in a closed X-band (8-12 GHz) rectangular waveguide. The measured results indicate that the four metamaterial absorbers exhibite similar absorption characteristics and mechanisms in comparison with the results obtained from free space. Further, in this paper we also discuss the rectangular wave matching terminal applications based on the proposed four metamaterial absorbers. And the results show that these new rectangular waveguide matching terminals can possess the advantages including the compact dimensional size, flexible controllability of the operating frequencies, and low costs. Moreover, the matching frequency bands of the novel terminals can be realized by designing the broadband metamaterial absorbers.
Using scattered light to amplify the photoacoustic spectroscopic signatures of the main absorbing material in a weakly light-absorbing solid mixture
2013, 62 (8): 087802. doi: 10.7498/aps.62.087802
Scattered light by the sample is usually one of the main error source in conventional transmission spectroscopy and diffuse reflection spectroscopy. However, scattered light may be useful for photoacoustic spectroscopy measurement. In the present experiment, the photoacoustic spectra of an olanzapine drug tablet and its powder are studied. The olanzapine tablet is a weakly light-absorbing solid mixture and its main constituent is olanzapine. In order to eliminate the background signal produced by the photoacoustic cell wall due to absorbing the scattered light by the sample, the difference between the photoacoustic spectrum of the sample and that of its base is taken and normalized with the photoacoustic spectrum of carbon black. It is found that the photoacoustic spectroscopic signatures of olanzapine in the olanzapine table become obvious when the olanzapine table is powdered. This is the result of the internal light scattering effect in the sample. The scattered light which serves as an obstacle in conventional optical spectroscopy techniques, however, facilitates the photoacoustic spectroscopy measurement. This indicates the particular advantage of photoacoustic spectroscopy. A new method to preliminarily fast identify the main absorbing component in a weakly light-absorbing solid mixture is proposed based on the above experiment. This method can be applied to the field of solid drug and mineral and soil and so on.
Photoacoustic spectroscopy based on the photoacoustic effect is the combination of optical imaging and acoustical imaging, which has become a powerful medical diagnosis tool to distinguish different tissues and components with several different wavelength photoacoustic images. But photoacoustic spectroscopy is limited by the scanning speed, system stability and signal accuracy. To solve these limitation problems, in this paper we propose a new method called photoacoustic double spectrum analysis which can greatly improve the image contrast and identification capability with quantitative analysis of the detected photoacoustic signal frequency. The final experimental results indicate that this method has the feasibility to distinguish different tissues quickly and easily with better contrast, which will be helpful for improving the applications of photoacoustic imaging in various branches of physics, biology, engineering, medicine, etc. We also expect the theoretical and experimental research proposed in this paper to establish the foundation and method for photoacoustic frequency imaging.
2013, 62 (8): 087804. doi: 10.7498/aps.62.087804
In this paper, AlON phosphors doped with Ce3+ are prepared by solid-state reaction method. Phases, microstructures and optical properties of the synthesized phosphors are characterized by X-ray diffraction and scanning electron microscopy and photoluminescence. The results indicate that AlON3+:Ce phosphors emit the blue light centered at 390 nm under the excitation of 305 nm. The emission intensity of AlON:Ce3+ at 150 ℃ is about 86% of that measured at room temperature. It demonstrates that the AlON:Ce3+ has a great potential in applications of high-density white light-emitting diodes.
2013, 62 (8): 087805. doi: 10.7498/aps.62.087805
The single-bubble sonoluminescence spectra of the terbium chloride aqueous solutions dissolved with the noble gas Ar are studied in this paper. Under the condition of fixed driving ultrasonic frequency and different sound pressures, a series of line spectra is identified as emissions from the transitions of OH radical vibrational levels from the first excited state A2∑+ to the ground state X2Π, including (0, 0) transition at 307 nm, (0, 1) transition at 335 nm and (1, 0) transition at 276 nm and so on. The experimental results show that higher sound pressure is conducive to the appearance of the line spectrum at 276 nm while lower sound pressure is favorable for the appearance of the line spectra at 307 nm and 335 nm. The relative intensity of the line spectrum in the total spectrum is expressed quantificationally by defining an intensity ratio of the line spectrum to the continuous spectrum. In addition, the effects of the driving sound pressure on the intensity ratios of different line spectra are given.
2013, 62 (8): 087901. doi: 10.7498/aps.62.087901
The models of the true secondary electron emission and backscattered electron emission for metal are provided, based on the physical process of secondary electron emission from metal at high incident electron energy. The formula for the true secondary electron yield at high incident electron energy is derived with the Bethe equation known to provide accurate and simple analytical expressions for the stopping power and the probability that a secondary electron produced in a material reaches the surface and is emitted into the vacuum. The formula for the backscattered electron yield at high incident electron energy is derived with the absorbing rule for the incident electron in the material. All the above results lead to a new model for secondary electron emission from metal at high incident electron energy. The secondary electron yields of Au, Ag, Cu and Al derived with the new model are in good agreement with the results obtained from the scatters process simulation code-Casino.
INTERDISCIPLINARY PHYSICS AND RELATED AREAS OF SCIENCE AND TECHNOLOGY
2013, 62 (8): 088701. doi: 10.7498/aps.62.088701
The evaluation of bone quality, especially cortical bone, is very important for diagnosing and treating the bone diseases. Because of the rapidly aging population of the global society, noninvasively, precisely and feasibly evaluating the bone quality has become a hot topic in the contemporary medical physics studies. Among the several available methods of evaluation, the bone mineral density (BMD) measured with dual-energy X-ray absorptiometry is currently considered to be the gold standard in clinical applications. However, the BMD is limited by its incapability of assessing the organic matrix, microstructure, porosity and perfusion of bone. In addition, the BMD can neither provide a definite diagnose of osteoporosis nor predict fractures precisely. Cortical bone shows near zero signal with all conventional clinical magnetic resonance imaging (MRI) sequences, because of the rapid decay of the magnetic resonance signal in the bone. Due to the recently developed theories, methods and hardware, ultrashort time echo (UTE) sequences with nominal TE of less than 100 s have aroused the increasing research interest. In this paper an introduction to the basic physics of UTE MRI of cortical bone is presented. The newly proposed qualitative and quantitative UTE MRI methods are reviewed with an introduction to the research work in the authors laboratory. The features, application scopes and limitations of those methods are also summarized. Finally, the authors point out the directions and steps of further studies. The paper will be helpful for understanding theoretical research and the clinical applications of UTE imaging of cortical bone.
Microcalcification clusters processing in mammograms based on relevance vector machine with adaptive kernel learning
2013, 62 (8): 088702. doi: 10.7498/aps.62.088702
Using the method of adaptive kernel learning based relevance vector machine (ARVM) and combining the morphological filtering and the clustering criterion recommended by Kallergi, a new algorithm for microcalcification (MC) clusters processing in mammograms is investigated. Firstly, the detection of MC is formulated as a supervised-learning problem. Then the ARVM is used as a classifier to determine whether an MC object is present at each location in the mammogram and a morphological processing is used to remove the isolated spurious pixels. Finally, the identified MC clusters are obtained by Kallergi criterion. To improve the computational speed, a fast processing method based on ARVM is developed, in which the whole image is decomposed first into sub-image blocks for parallel operation. Experimental results indicate that the ARVM method outperforms the RVM method and, in particular, the fast processing method could greatly reduce the testing time.
2013, 62 (8): 088801. doi: 10.7498/aps.62.088801
On the basis of the traditional bulk heterojunction structure, we introduce a method of gradient doping in the mixing layer to improve photoelectric conversion efficiency (ηp) of organic photovoltaic device (OPV). One of the devices with the structure of ITO/CuPc (10 nm)/CuPc:C60 (2:1) (4 nm)/CuPc:C60 (1.5:1) (4 nm)/CuPc:C60 (1:1) (4 nm)/CuPc:C60 (1:1.5) (4 nm)/CuPc:C60 (1:2) (4 nm)/C60 (30 nm)/Bphen (8 nm)/Al (100 nm) show the improvement on performance:short-circuit current JSC = 9.18 mA/cm2, photoelectric conversion efficiency ηp = 1.35% under AM1.5 solar illumination. Compared with ηp of the traditional bulk heterojunction OPV ITO/CuPc(10 nm)/CuPc:C60 (1:1) (20 nm)/C60(30 nm)/Bphen(8 nm)/Al(100 nm), the ηp of the present structure is improved by 25%, which is attributed to the improvement on percolating paths of donor and acceptor molecules across the entire mixing layer, thus increasing the charge collection efficiency. Consequently, the overall device series resistance is reduced and the photoelectric conversion efficiency is enhanced.