Analysis of the vibrational spectrum of crystalline polypropylene

The effect of intermolecular interactions on the skeltal vibration frequencies of the isotactic polypropylene crystal active in the IR spectrum is theoretically estimated. A simplified model of the molecule is employed. The Kitaigorodskii potential function is used for the intermolecular atom—atom interactions. The results are given in the form of a frequency table.Ioffe Physicotechnical Institute, Academy of Sciences of the USSR, Leningrad. Translated from Mekhanika Polimerov, No. 2, pp. 205–211, March–April, 1971.     

Bragg diffraction of light in synthetic opals

Three-dimensional light diffraction from the crystal structure, formed by closely packed a-SiO₂ spheres of submicron size, of samples of synthetic opals was visualized. The diffraction pattern of a monochromatic light beam was established to consist of a series of strong maxima whose number and angular position depend on the wavelength and mutual orientation of the incident beam and the crystallographic planes of the sample. The diffraction patterns were studied under oblique incidence on the (111) growth surface of the sample and with light propagated in the (111) plane in various directions perpendicular to the sample growth axis. The spectral and angular relations of diffracted intensity were studied in considerable detail in both scattering geometries. The experimental data are interpreted in terms of a model according to which the major contribution to the observed patterns is due to Bragg diffraction of light from (111)-type closely packed layers of the face-centered cubic opal lattice. The model takes into account the disorder in the alternation of the (111) layers along the sample growth axis; this disorder gives rise, in particular, to twinning of the fcc opal lattice.     

Acoustic,optical, and interface phonons in BeTe/ZnSe superlattices

Raman scattering in a number of BeTe/ZnSe type-II superlattices which share no common cations or anions in the interfaces was studied. Folded acoustic phonons; LO phonons of the first, second, and third order in the ZnSe layers; and Kliewer-Fuchs-type electrostatic interface phonons were observed when excited in resonance with the direct exciton transition in the ZnSe layers. Nonresonant excitation produced LO phonons in the ZnSe and BeTe layers and a high-frequency mechanical interface mode, assigned tentatively to a local vibration of the interface Be-Se bond.     

Local-field effects in reflectance anisotropy spectra of the (001) surface of gallium arsenide

Characteristic reflectance anisotropy spectra of the naturally oxidized (001) surfaces of GaAs undoped crystals and Ga_0.7Al_0.3As epitaxial films are measured in the energy range 1.5–5.7 eV. The spectra are interpreted in the framework of the microscopic model proposed for a GaAs(001)/oxide interface and the reflectance anisotropy (difference) theory developed for a multilayer medium with a monolayer of atomic dipoles located near one of the interfaces. The anisotropy of dipole polarizability and the anisotropy of the plane lattice formed by dipoles are taken into account within the unified Green function approach of classical electrodynamics. A good agreement between the measured and calculated reflectance anisotropy spectra of the oxidized GaAs(001) surfaces shows that the local field effects at the semiconductor-oxide interface make the main contribution to these spectra.     

Estimation of the length distribution of through polymer chains

The contributions of entropy and elastic forces to the tensile modulus of a polymer chain are estimated. The chain length-distribution function in the amorphous region of highly oriented Lavsan is approximately determined from macroscopic stress-strain dependence. It is found that the length distribution has a width of the order of several percent of the length of the amorphous region of the fibril.A. F. Ioffe Physicotechnical Institute, Leningrad. Translated from Mekhanika Polimerov, No. 5, pp. 922–925, September–October, 1974.     

Propagation of quasi-two dimensional exciton polaritons in a quantum well waveguide

The propagation of exciton polaritons in an optical waveguide with a quantum well is studied. Spatial dispersion of the excitons causes the wave vector of the exciton polaritons to split between waveguide and exciton modes at resonance. The magnitude of this splitting is determined by the radiative decay parameter of excitons with corresponding polarization in the quantum well. The group velocity of the waveguide exciton polaritons in the resonance region can be three or four orders of magnitude lower than the speed of light in vacuum. Fiz. Tverd. Tela (St. Petersburg) 40, 362–365 (February 1998)     

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.

     

Propagation of circularly polarized waves in one-dimensional bragg structures (magnetophotonic crystals)

A theory of electromagnetic wave propagation in structures formed by alternating magnetic and dielectric layers is proposed. Models of macroscopically thick and atomically thin layers magnetized perpendicular to their plane are considered. Transfer matrices of circularly polarized waves and characteristics of light propagation in periodic magnetic structures under normal incidence are obtained by the self-consistent electrodynamic Green’s function method in analytic form. The results obtained are employed to analyze linear-in-magnetization magneto-optical effects in the transmittance and reflectance spectra of one-dimensional magnetic Bragg structures called magnetophotonic crystals. For structures of finite thickness, Faraday rotation and other observable magneto-optical quantities are shown to vary appreciably in the spectral region of the stop bands of a magnetophotonic crystal. This is paralleled by a substantial enhancement of the magnetic-fieldinduced modulation of the reflectance of light polarized in the analyzer plane.