Free-carrier absorption in quantum well structures for acoustic phonon scattering

Free-carrier absorption has been studied for quantum well structures fabricated from III-V semiconducting materials where the acoustic phonon scattering is important. The energy band of carriers is assumed to be nonparabolic. We discuss the effect of acoustic phonon scattering on the free-carrier absorption for both deformation-potential coupling and piezoelectric coupling. It is found that the free-carrier absorption coefficient depends upon the polarization of the electromagnetic radiation relative to the layer plane or quantum well, the photon frequency, and the temperature. When the deformation-potential coupling is dominant, the free-carrier absorption coefficient increases with increasing temperature for photons polarized in the layer plane or perpendicular to the layer plane. However, when the piezoelectric coupling is dominant, the free-carrier absorption coefficient increases with increasing temperature for photons polarized in the layer plane, but for photons polarized perpendicularly to the layer plane, the free-carrier absorption coefficient decreases with increasing temperature. Moreover, at high temperatures such as T = 300 K, the free-carrier absorption coefficient oscillates with the film thickness in a small quantum well region and then decreases monotonically with increasing the film thickness. This is different from the result for three-dimensional semiconducting solids.     

Low-frequency electronic resistivity of crystals due to the scattering from dipoles

The screened potential of an electrical dipole impurity in crystalline solids is obtained in the Debye-Hückel approximation. In the one-band effective-mass approximation and Born approximation of the scattering, the low-frequency transport relaxation time of current-carriers, electrical conductivity and the electronic part of permittivity of crystals are calculated when the scattering mechanism is connected with the randomly distributed screened dipole centres. It is shown that in some cases the scattering from dipole impurity centres may be essential and even more effective than the other scattering mechanisms.     

Determination of photoexcited carrier concentration and mobility in GaAs doping superlattices by hall effect measurements

The photo-Hall effect in a new type of periodicp-n doping multilayer structures (superlattices) of GaAs grown by molecular beam epitaxy has been investigated. In these space charge systems electrons and holes are separated in real space. As a consequence, large deviations from thermal equilibrium become quasi-stable. Carrier generation by optical absorption occurs in these doping superlattices even at photon energies far below the gap of the homogeneous semiconductor material. The photoexcitation results in a strong enhancement of the conductivityparallel to the layers and in a substantial photovoltaic response. An increase in carrierconcentration as well as an increase in carriermobility both contribute to the observed enhancement of the conductivity under excitation. The absolute values of changes in free-carrier concentration are very large due to the manyfold active layers of the structure. The measured free-carrier mobilities depend on the population of the multilayer system. A reduction in mobility as compared to bulk material is found to be more pronounced in weakly populated systems. This finding is caused by the larger weight of the boundary regions of the total active layers where the free-carrier density is lower than the density of ionized impurities resulting in an enhanced impurity scattering.     

Kinetic Approach to the Electrical Conductivity in a Partially Ionized Hydrogen Plasma

The electrical conductivity is investigated for a partially ionized hydrogen plasma, starting from a generalized quantum kinetic equation with three particle collision integrals. To take into account plasma effects, screened potentials are used. The transport cross sections of the considered two and three particle scattering processes are calculated by perturbative solution of the Lippmann-Schwinger equations for the T-matrices up to the second Born approximation. In connection with a mass action law the influence of the electron-electron and the elastic electron-atom scattering is discussed. The pressure ionization (Mott-effect) is described by a minimum-behaviour of the electrical conductivity.     

纳米碳纤维导电性对红外消光性能的影响

纳米碳纤维是一种新型吸波材料。为探讨烟幕粒子的红外消光特性,利用矩量法建立了纳米碳纤维感应电流和散射场的计算模型,在此基础上得出了纳米碳纤维吸收、散射和消光截面的计算表达式。通过Matlab编程计算分析了纳米碳纤维红外消光截面与电导率的关系。结果表明,纳米碳纤维红外消光特性与电导率密切相关,随着电导率的增加,消光截面迅速增加,当波长大于纤维本身长度时,消光截面随电导率的变化出现峰值,峰值电导率与波长有关。     

Free-carrier absorption due to electron-(magneto) plasmon interaction?

It is shown in dipole approximation that, contrary to several published calculations, the electron-(magneto) plasmon interaction in a one-component unbounded plasma of carriers, with an energy-independent scalar effective mass in a perfect crystal does not lead to any free-carrier absorption.     

On the Calculation of the Electrical Conductivity of a Nonideal Hydrogen Plasma

The electrical conductivity of a nonideal hydrogen plasma is calculated within the linear response formalism. The approximation given involves the first term with respect to the density expansion in the low degenerate case. The contributions of the free-free scattering of the particles as well as the bound-bound, bound-scattering, and scattering-scattering parts due to multiple scattering are discussed up to this level.     

Nonlinear absorption and Raman gain in helium-ion-implanted silicon waveguides

We study whether helium ion implantation can reduce the carrier lifetime and thus reduce the density of two-photon-absorption-generated free carriers. Our experiments and theoretical model show that helium ion implantation into silicon waveguides can successfully reduce the free-carrier losses and allow net gain to be attained by cw-pumped stimulated Raman scattering without requiring reverse bias to remove the photogenerated free carriers.     

Electrical Conductivity of Non-ideal Plasmas and the Ion Distribution Function

The theory of electrical conductivity of electron-ion-systems is developed for a density region which reaches from the region of non-ideal plasmas up to the region of liquid metals. The conductivity is expressed by quantum mechanical correlation functions. Different forms of the electron-ion pseudopotentials are considered. The ion distribution function is derived using the mean spherical approximation (MSA) theory or the nonlinear Debye-theory. Higher order scattering effects are treated by introducing scattering phase shifts for the statically screened electron-ion potential. The numerical results for the conductivity show a SPITZER-like behaviour in the low-density non-degenerate limit where higher order scattering is important, and a ZIMAN-like behaviour in the strongly degenerate high-density limit where the ion distribution functions and the form of the electron-ion pseudopotential become more important.     

Theory of the residual electrical resistivity of binary actinide alloys

A system of self-consistent equations has been proposed for the coherent potential approximation of the multiband conductivity model for the case of conduction electron scattering from chaotic electric fields of ions of disordered binary alloy components at zero temperature. It has been qualitatively demonstrated that the deviation of the concentration dependence of the residual electrical resistivity of actinide alloys with multiband conductivity from the Nordheim rule is caused by the explicit dependence of the electrical resistivity of the alloy on the magnitude and sign of the real part of the Green’s function at the Fermi level. The derived system of equations for the multiband coherent potential approximation has been used to calculate the concentration dependence of the density of states and the residual electrical resistivity of the alloys of neptunium and plutonium. The results of the calculations have been compared with the available experimental data.     

The magneto-thermoelectric effect of graphene with intra-valley scattering

We present a qualitative and quantitative study of the magneto-thermoelectric effect of graphene. In the limit of impurity scattering length being much longer than the lattice constant, the intra-valley scattering dominates the charge and thermal transport. The self-energy and the Green's functions are calculated in the self-consistent Born approximation. It is found that the longitudinal thermal conductivity splits into double peaks at high Landau levels and exhibits oscillations which are out of phase with the electric conductivity. The chemical potential-dependent electrical resistivity, the thermal conductivities, the Seebeck coefficient, and the Nernst coefficient are obtained. The results are in good agreement with the experimental observations.     

Spontaneous Raman scattering in ultrasmall silicon waveguides

We report spontaneous Raman scattering at 1550 nm in ultrasmall silicon-on-insulator (SOI) strip waveguides of 0.098-microm2 cross-sectional area. The submicrometer-scale dimensions provide tight optical confinement and, hence, highly efficient Raman scattering with milliwatt-level cw pump powers. The prospect of Raman amplification in such a deeply scaled-down waveguide device in the presence of various loss mechanisms, particularly free-carrier loss that arises from two-photon absorption, is discussed, and the feasibility of high-gain SOI-based fully integrated optical amplifiers is shown.     

Normal quasiparticle scattering and kinetic effects in degenerate semiconductors

The influence of normal processes of electron-electron and phonon-phonon scattering on quasiparticle momentum relaxation in nonequilibrium electron-phonon systems of degenerate semiconductors is investigated. A system of kinetic equations is solved for the electron and phonon distribution functions, and the kinetic coefficients of a semiconductor are calculated in the linear approximation in the degeneracy parameter. The influence of normal scattering of quasiparticles on the electrical conductivity, thermopower, and heat conductivity of a degenerate semiconductor is analyzed. Redistribution of the phonon momentum in N processes within each branch of the vibrational spectrum, as well as among different branches, is taken into account.     

On the Theory of Brillouin Scattering in Piezoelectric Semiconductors with Acoustic Phonon-Conduction Electron Interaction

The theory of nonresonant Brillouin scattering in anisotropic piezoelectric semiconductors with deformation potential coupling and piezoelectric coupling between excited systems of acoustic phonons and conduction electrons is reviewed. The scattering efficiency is calculated using the appropriate dyadic electromagnetic GreeN′s function. The depletion of the scattered intensity arising from a non phase-matched scattering kinematics and from a spatial exponential decay of the sound amplitude is taken into account. The contributions to the Brillouin scattering from the free-carrier-screened indirect photoelastic effect and from the free-carrier density modulation are expressed in terms of the self-consistent electric field. This field is obtained from a Boltzmann-equation calculation of the effective ac conductivity tensor. The acoustic dispersion of the Brillouin-scattering efficiency is considered, and some possibilities of determining electronic transport properties by means of Brillouin scattering are outlined.     

Electron density of states and electrical conductivity of ordered alloys: Extension beyond the single-band approximation, taking into account scattering by clusters

A method is developed for going beyond the single-band approximation and taking into account scattering by clusters. The method is based on a cluster expansion of the averaged Green’s function of the alloy. It is shown that the contributions of scattering processes diminish with respect to a certain small parameter as the number of particles in the cluster increases. The prominent characteristics of the electronic structure and electrical conductivity of ordered alloys are analyzed numerically in the diagonal disorder approximation in the multiband s-d model. Fiz. Tverd. Tela (St. Petersburg) 39, 401–411 (March 1997)     

Simple relations between correlation functions in a closed system

From conservation of total momentum of a general closed system relations between certain correlation functions are derived. The procedure is then applied to express the electrical conductivity of an arbitrary solid or liquid by the dynamics of its nuclei. The possibility to get the wanted information about nuclear motion from inelastic thermal neutron scattering is discussed. For a solid the dynamics of the nuclei are calculated in the harmonic approximation.     

THz Generation by Optical Rectification and Competition with Other Nonlinear Processes

We present a study of the competition between tera-hertz （THz） generation by optical rectification in （110） ZnTe crystals, two-photon absorption, second harmonic generation and free-carrier absorption. The two-photon nonlinear absorption coefficient, second harmonic generation efficiency and free-carrier absorption coefficient in the THz range are measured independently. The incident pump field is shown to be depleted by two-photon absorption and the THz radiation is shown to be reduced, upon focusing, by free-carrier absorption. The reduction of the generated THz radiation upon tight focusing is explained, provided that one also takes into account diffraction effects from the sub-wavelength THz source.     

Revisiting the normalized Born approximation: effects of scattering

The normalized Born approximation has been suggested as a ratiometric method in fluorescence molecular tomography (FMT) applications, to account for heterogeneity variations. The method enabled practical inversions, as it offered fluorescence reconstruction accuracy over a wide range of absorption heterogeneity, while also accounting for unknown experimental factors, such as the various system gains and losses. Yet it was noted that scattering variations affect the robustness and accuracy. Herein we decompose the effects of absorption and scattering and capitalize on the recent development of hybrid FMT/x-ray computed tomography imaging methods to proposed amendments to the method, which improve the overall accuracy of the approach.     

Influence of topological transitions in a quantizing magnetic field and anisotropy of current carrier scattering by acoustic phonons on the longitudinal electrical conductivity of layered crystals with open fermi surfaces

It is demonstrated that the dependence of Fermi’s energy on the magnetic field causes a set of the Shubnikov – de Haas (SDH) oscillation frequencies to change, and their relative contribution to the total longitudinal conductivity of layered crystals depends on whether the scattering of current carriers is isotropic or anisotropic. Owing to the topological transition in a strong magnetic field, Fermi’s surface (FS) is transformed from open into closed one and is compressed in the magnetic field direction. Therefore, in an ultraquantum limit, disregarding the Dingle factor, the longitudinal electrical conductivity of the layered crystal tends to zero as a reciprocal square of the magnetic field for the isotropic scattering and as a reciprocal cube of the magnetic field for the anisotropic scattering. All calculations are performed in the approximation of relaxation time considered to be constant versus the quantum numbers for the isotropic scattering and proportional to the longitudinal velocity of current carriers for the anisotropic scattering.