An experimental set-up for the study of electromechanical properties of conducting polymer films

An experimental set-up have been designed and fabricated to study the electromechanical properties of a thick film of conducting polymer under load. Extension of the films versus voltage has been measured in terms of change in capacitance of parallel plate capacitor constituted by metal pan and a fixed metal plate. HP 4284A impedance analyzer measures absolute value of the capacitance. Change in capacitance is related to change in distance by pre-calibrating the assembly using traveling microscope. A computer programme is developed to convert the capacitance value with the corresponding distance and simultaneously plotting the graph between changes in length of polymer film versus applied voltage.The assembly has been used to study the electrochemomechanical behavior of solution cast polyaniline films (∼50 μ thick). During first cycle the length is enhanced by about 6% of original value, while repetitive value of extension is ≅2.8%, in subsequent cycles. This value closely matches to that reported in literatures.     

Transmission and reflection spectra of a photonic crystal with a Raman defects

Features of Raman gain of probe radiation in three-level atoms placed in a defect of a one-dimensional photonic crystal in the presence of laser radiation (pump) at an adjacent high-frequency transition have been theoretically investigated. It has been shown that there is a pump intensity range where narrow peaks (resonances) simultaneously appear in the transmission and reflection spectra of the probe field. Beyond this region, the peak in the transmission spectrum is transformed to a narrow dip. The spectral position of these peaks is determined by the Raman resonance and the transmittance and reflectance can be larger than unity at pump intensities from several microwatts per square centimeter to several tens of milliwatts per square centimeter. The nature of narrow peaks is due to a sharp dispersion of a nonlinear refractive index near the Raman resonance; this dispersion is responsible for a strong decrease in the group velocity of probe radiation. The proposed scheme makes it possible to obtain controlled ultranarrow resonances in the transmission and reflection spectra of the photonic crystal.     

Frequency spectrum and confinement of optical radiation of nanorods in multilayered microsphere

The electromagnetic radiation of a quantum semiconductor nanorod (considered in a dipole model), placed into a multilayered microsphere with alternative spherical stack, is studied. We found that the frequency spectrum consists of series of narrow and well separated peaks even in a lossy case. In the band of such peaks the great part of the field energy is located in the structure of the stack and practically does not leave the microsphere. A possibility of resonant coupling of photon states in the area of nanorods is discussed.     

Transport studies of MBE-grown InAs/GaSb superlattices

We report on the results of transport studies of MBE-grown InAs/GaSb superlattices. We demonstrate that the in-plane mobility is limited by interface roughness scattering by showing that, as a function of InAs layer width L, the in-plane mobility behaves as μ ∝ L^5.3, which closely follows the classic sixth power dependence expected from theory for interface-roughness-limited mobility. Fits to the mobility data indicate that, for one monolayer surface roughness, the roughness correlation length is about 35 Å. Next, we show that the in-plane carrier mobility in InAs/GaSb superlattices is inversely proportional to carrier density in n- and p-type samples, the result of screened interface roughness scattering.     

Chou-Yang model and predictions for high-energy pion-deuteron scattering

Using fits to the available data on the pion and deuteron electromagnetic form factors and the Chou-Yang model, the computed values of differential cross-sections for high-energy pion-deuteron elastic scattering agree reasonably well with the available experimental data. Whereas only a shoulder is expected to appear up to energies of several hundreds of GeV, a dip and a secondary maximum are predicted to be conspicuous only forP _lab≫400 GeV/C. The position of the dip starts at a much lower value, |t _d| ≅ 0.5–0.6 GeV², as compared to the corresponding position in thepp scattering. The positions of the first dip are plotted against total cross-section which can be verified by future high-energy experiments. Limitations of the model predictions for ultrahigh energies are pointed out.     

Modulation of Fano resonances in symmetry-broken gold-SiO2-gold nanotube dimers

Fano resonances in the symmetry-broken gold-SiO2-gold(BGSG)nanotubes and the associated dimers have been investigated based on the finite element method.In the BGSG nanotube,the symmetry breaking induced the interactions of the inner gold core and outer gold nanoshell plasmons of all multipolar orders and hence the red-shifts of the plasmon resonance modes and the enhanced quadrupole mode peaks were observed.The interference of the quadrupole mode peak with the subradiant dipole mode caused a Fano-dip in the scattering spectrum.By increasing the core offset-value in the BGSG nanotube,the Fano dip with low energy showed a red-shift and became deeper.Unexpectedly the plasmon coupling between a GSG nanotube and a BGSG nanotube can lead to two strong Fano dips in the scattering spectra of the dimer.It was further noted that the thin side of the BGSG nanotube located at two sides of the dimer gap can lead to the strong near-field coupling between two BGSG nanotubes and hence a deeper and broader Fano dip.     

量子线/铁基超导隧道结中隧道谱的研究

考虑铁基超导中能带间的相互作用和界面对每一个能带的散射作用, 利用推广的Blonder-Tinkham-Klapwijk模型, 并通过求解Bogoliubov-de Gennes 方程研究了具有不同类型双能隙系统的量子线/铁基超导隧道结中准粒子的输运系数和隧道谱. 研究表明: 1)在弹道极限时, 随着带间相互作用的增大, s_± 波隧道谱中零偏压附近的平台演变成电导峰; s₊₊ 波的平台演变成凹陷; p波的零偏压电导峰被压低. 2)界面对两个能带的散射作用不为零时, 随着带间相互作用的增大, s_± 波和s₊₊ 波两能隙处的峰值将降低, 而两峰间的凹陷值将变大; p波的零偏压电导峰被压低, 非零偏压电导增大. 3)界面对每个能带的散射, 可使其产生的电导峰变得更加尖锐, 但可压低和抹平另一个带产生的电导峰值. 这些结果对于澄清铁基超导体的能隙结构和区别不同类型铁基超导体有所帮助.     

Investigation of multibaryonic resonances with strangeness equal to and different from zero

Results on a search for Λp, ΛΛ, Λpp, K^op, pp, 3p and 4p resonances are presented. The Λp effective-mass spectrum reveals three significant peaks (more than 4 s.d.) at 2058, 2127 and 2252 MeV. The first two peaks are shown to be due to effects of the negative Λp and Σp scattering lengths respectively, whereas the third peak is interpreted as a Λp resonance of (2251.4 ± 3.9) MeV mass and Λ = (21.1 ± 5.4) MeV full width. The two enhancements observed in the ΛΛ effective-mass spectrum may be due to the effect of the negative ΛΛ scattering length and to a ΛΛ resonance of ～2370 MeV mass. No enhancements in other effective-mass spectra are observed. The results of this work suggest a condition for the possibility of resonance formation depending on the total hypercharge of the system considered.     

高光谱分辨率激光雷达鉴频器的设计与分析

提出了利用Fabry-Perot干涉仪的反射场实现高光谱分辨率激光雷达精细探测大气光学参量的新方法和思路.设计了高光谱分辨率的分光系统,并分析了干涉仪反射场的光谱透过特征曲线.结合高光谱激光雷达探测信号特征,讨论分析了谱分离比和瑞利信号透过率随反射率和腔长的变化曲线,同时结合误差传递公式,建立了仿真分析模型,讨论了回波光束发散角和入射角变化对激光雷达探测结果的影响.结果表明,所提出的Fabry-Perot干涉仪反射场可以实现高光谱分辨率激光雷达探测系统的精细分光,同时探测结果误差随回波光束发散角变化不敏感,控制发散角在10 mrad以内,入射角在1.5 mrad以内时,可以实现气溶胶光学参数廓线的高精度探测.     

Multiphoton intraband absorption of electromagnetic waves and stimulated scattering by optical phonons in the superlattice

Multiphoton intraminiband absorption of the electromagnetic wave and stimulated scattering by optical phonons in the semiconductor superlattice are considered. The absorption coefficient is found without using the quantum kinetic equation. The absorbed energy is plotted versus the electric field of the wave. It is found that, as the electric field grows, the absorbed energy reaches a maximum and then starts decreasing in an oscillatory manner. An explanation for such a run of the absorbed energy with electric field is given. It is shown that multiphoton absorption must be taken into account if the condition eEd ≥ ?ω is satisfied. Stimulated multiphoton scattering by optical phonons in superlattices is shown to produce harmonics of the scattered light at frequencies both higher and lower than those of the incident light. Scattering partial cross sections are found and plotted versus the incident wave intensity.     

Photoacoustic effect upon material melting and evaporation by laser pulses

Photoacoustic signals in laser-irradiated samples were mathematically modeled for the cases when melting and evaporation occur. In particular, it was shown that rapid melting processes induced by nanosecond laser pulses result in a rather narrow pressure peak or dip in photoacoustic pressure signals due to density changes in the moving melting front. Amplitudes of these peaks or dips depend on the melting front velocity, as well as on the magnitude of density change. Experimental detection of this effect using a piezoelectric transducer requires a uniform laser intensity distribution over the irradiation spot.     

Spectroscopy of Plasmon-Excitons in Semiconductor-Metal Nanostructures

The results of the theory considering mixed plasmon-excitonic modes and their spectroscopy are presented. The plasmon-excitons are formed owing to strong Coulomb coupling between quasi-two-dimensional excitons of a quantum well and dipole plasmons of nanoparticles. The effective polarizability associated with a nanoparticle is calculated in a self-consistent approximation taking into account the local field determined by in-layer dipole plasmons and their image charges due to the excitonic polarization of a near quantum well. The spectra of elastic scattering and specular reflection of light are investigated in cases of a single silver nanoparticle and a monolayer of such particles situated in close proximity to a quantum well GaAs/AlGaAs. The optical spectra show a two-peak structure with a deep and narrow dip in the resonant range of plasmon-excitons. Propagation of plasmon-excitonic polaritons is discussed for periodic superlattices whose unit cell consists of a quantum well and a layer of metal nanoparticles. The superradiance regime originating in the Bragg diffraction of plasmon-excitonic polaritons by the superlattice is investigated. It is shown that the broad spectrum of plasmonic reflection depending on the number of unit cells in a superlattice also has a narrow dip at the exciton frequency.

     

三温与多温电子等离子体的自由膨胀与快离子的产生

本文介绍三温与多温电子等离子体自由膨胀的理论模型。自相似解成功地再现了在激光等离子体自由膨胀中离子速度分布呈现的三峰和多峰结构,这些结构已在激光打靶的实验中频繁地观察到。计算结果表明:快离子谱峰值的数目与构成等离子体的电子分量的数目密切相关;相邻两峰之间坑的深度对相应电子温度的比值非常敏感;坑的位置依赖于相应电子数密度的比值,它们也受其它电子分量耦合的影响。最后还讨论了自相似解的适用范围。 关键词：     

Dynamic light scattering at the ice water interface during freezing

The 488 nm radiation of an argon laser is scattered quasi-elastically at the [0001]-face (basal plane) of a growing ice crystal. The scattering plane is the basal plane. The Rayleigh-linewidth is proportional to the square of the scattering vector. One measures about 2 krad/s at a scattering angle of 90°. The linewidth does not depend on the growth rate. Scattering is only observed once a critical growth rate ν_crit = 1,5 μ m?s has been exceeded. Then the scattering intensity depends linearly on the growth rate in the range between 0,03 μ m?s and 2,5 μ m?s. Once the surface is molten scattering vanishes and does not reappear until the growth rate has again exceeded the threshold ν_crit. The coherence properties of the scattered light indicate that the thickness of the scattering layer is less than 6 μ m. The observations are interpreted in terms of a fluctuating interface. The decay time of the fluctuations has been calculated. Satisfactory agreement with the observed Rayleigh linewidth is obtained.     

Transmittance of partially coherent cosh-Gaussian, cos-Gaussian and annular beams in turbulence

Average relative power transmittance is evaluated, by incorporating atmospheric turbulence, for partially coherent cosh-Gaussian, cos-Gaussian, Gaussian and annular beams. For all the collimated versions of these beams, against the increasing propagation length, there is a typical trend of the decrease in the relative average power transmittance with incremental drop being much less for partially coherent cos-Gaussian beams. The change in the transmittance versus the propagation length will be similar to the corresponding collimated cases, when these beams are focused at a certain focal length. Also partially coherent beams are less sensitive to propagation length changes, except for cos-Gaussian case. Partially coherent cosh-Gaussian beams exhibit a drop in the transmittance as the displacement parameter of the beam is made larger, whereas this trend is just the opposite for partially coherent cos-Gaussian beams. When examined versus the source size, for all the four types of beams, the transmittance has a similar behavior, i.e., it becomes high at small source sizes, falling with increasing source size, and following a dip, it starts to rise, eventually approaching the plane wave limit of unity. The occurrence of the dip coincides with the smallest source size for cosh-Gaussian, with the largest for cos-Gaussian, and about the same source size for Gaussian and annular beams. In general, the average relative power transmittance of coherent beam is affected much more than the partially coherent beams against the variations in source properties.     

Zeeman splitting of zero-bias anomaly in Luttinger liquids

Tunneling density of states (DOS) in Luttinger liquid has a dip at zero energy, commonly known as the zero-bias anomaly. In the presence of a magnetic field, in addition to the zero-bias anomaly, the DOS develops two peaks separated from the origin by the Zeeman energy. We show that these finite-bias anomalies are characterized by a power-law behavior of the DOS and the differential conductance, and find the corresponding exponents at arbitrary strength of the electron-electron interaction. The developed theory is applicable to various kinds of quantum wires, including carbon nanotubes.     

On the interpretation of rainbow scattering in gas atom/surface collisions

A model is proposed to explain the scattering of thermal energy atoms from surfaces. The model allows the behaviour of the scattering trajectories to be assessed. It is shown that when a collimated beam of Neon atoms is scattered from the LiF(100) surface, the peaks observed in the inplane distribution of scattered atoms versus angle of reflection arise from trajectories with one and two repulsive collisions with the same surface atom.     

The scattering matrix of transparent particles of random shape in the geometrical optics approximation

A numerical model that allows one to calculate elements of the scattering matrix for transparent particles of random shape in the geometrical optics approximation is presented. It is shown that a deviation from sphericity, which, in particular, is modeled by a reduction of the number of triangular facets approximating a sphere, essentially affects the magnitude, position, and width of peaks of the photometric and polarimetric indicatrices. Thus, when 1500 facets were used for the approximation, the amplitude of the polarimetric peak associated with the first rainbow, which is located close to the scattering angle 160°, decreases by a factor of two. Calculations showed that, in the region of backscattering, for particles of an arbitrary shape, the linear polarization ?F ₁₂/F ₁₁ has no negative branch, which is well observed for spherical particles. In going from spherical to nonspherical particles, the backscattering peak also disappears. The indicatrices for particles of irregular shape that were calculated for small distances from the center of a particle noticeably differ from the indicatrices at infinity. Thus, when simulating multiple scattering in dense powderlike media, the use of particle scattering indicatrices that were calculated for infinite distances is incorrect even in the geometrical optics approximation.     

Spin-dependent conductance in FM/quantum-dot/FM tunneling system

We present a theoretical study of the spin-dependent conductance spectra in a FM/semiconductor quantum-dot (QD)/FM system. Both the Rashba spin-orbit (SO) coupling in the QD and spin-flip scattering caused by magnetic barrier impurities are taken into account. It is found that in the single-level QD system with parallel magnetic moments in the two FM leads, due to the interference between different tunneling paths through the spin-degenerate level, a dip or a narrow resonant peak can appear in the conductance spectra, which depends on the property of the spin-flip scattering. When the magnetizations of the two FM leads are noncollinear, the resonant peak can be transformed into a dip. The Rashba SO coupling manifests itself by a Rashba phase factor, which changes the phase information of every tunneling path and can greatly modulate the conductance. When the QD has multiple levels, the Rashba interlevel spin-flip effect appears, which changes the topological property of the structure. Its interplay with the Rashba phase can directly tune the coupling strengths between dot and leads, and can result in switching from resonance into antiresonance in the conductance spectra.     

Bragg scattering induces Fano resonance in photonic crystals

We present a study of a Fano resonance between a narrow Bragg band and disorder-induced continuum in photonic crystals where the continuum is either of the broad band Fabry-Pérot scattering in an imperfect one-dimensional photonic crystal or Mie scattering in an imperfect three-dimensional photonic crystal. Our experimental studies of synthetic opals have demonstrated how the Fano resonance may lead to a transmission spectrum exhibiting a Bragg dip with an asymmetric profile or a Bragg rise.