Light Scattering Cross Section Research Articles

Light scattering by small hard particles in liquid crystals in anomalous-diffraction approach

The differential and total light scattering cross sections by LCs which contain hard macroscopic spherical and cylindrical particles are theoretically studied using the anomalous-diffraction approach. The influence of form and size of particles, the type and strength of director anchoring on the particle surface as well as light wave polarization on the light scattering is analysed.

Hourly concentrations and light scattering cross sections for fine particle sulfate at Big Bend National Park

Fine particle sulfate was measured continuously for 90 consecutive days, from August through October 1999, at Big Bend National Park. Measurements were made with a prototype integrated collection and vaporization cell, whereby particles are humidified and collected by impaction onto a metal strip and analyzed in place by flash-vaporization and pulsed fluorescence detection of the evolved sulfur dioxide. The time resolution was 12 min. Sulfate values are compared to 24-h integrated filter measurements. Time variation in observed concentrations are compared to hourly measurements of dry particle size distributions and to nephelometry at ambient humidity. For most of the study period the sulfate was the major constituent of the accumulation mode aerosol. The sulfate scattering cross section as a function of relative humidity is inferred by comparisons among these measurements using a two-component fine particle model wherein the nonsulfate accumulation mode volume is attributed to nonhygroscopic components.

Computer modeling of light scattering in filled liquid crystals

The differential and total light scattering cross sections by LCs which contain hard macroscopic spherical and cylindrical particles are theoretically studied using the anomalous-diffraction approach. The influence of the form and size of particles, the type and strength of director anchoring on the particle surface as well as light wave polarization on the light scattering is analyzed.

Second order magneto-optic effects in Brillouin scattering from spin waves in magnetic multilayers

The Brillouin light scattering cross section from a thick magnetic multilayer has been calculated using a theoretical model based on a macroscopic partial waves approach for the calculation of the normal modes. Special attention has been paid to analyzing the effect of inclusion of the second order magneto-optic constant in addition to the first order one (corresponding to Voigt and Faraday effects, respectively). This model has been exploited to analyze the experimental Brillouin spectra measured from a several nm thick Ni/Cu bilayer. Both surface and bulk standing spin waves have been experimentally detected for different incident angles and applied magnetic fields (including field reversal). The measured spectra can be satisfactorily reproduced by our model, allowing the determination of magnetic and magneto-optic parameters of the samples.

LIGHT SCATTERING BY FILLED LIQUID CRYSTALS IN ANOMALOUS-DIFFRACTION APPROACH

The differential and total light scattering cross sections by LCs which contain hard macroscopic spherical and cylindrical particles are theoretically studied using the anomalous-diffraction approach. The influence of form and size of particles, the type and strength of director anchoring on the particle surface as well as light wave polarization on the light scattering is analysed

Laboratory Measurements of Light Scattering by Single Levitated Ice Crystals

Abstract The authors have measured the differential light-scattering cross sections and phase functions of single vapor-grown hexagonal ice particles levitated in an electrodynamic balance. The ice particles, grown at temperatures −5° > T > −10°C, were typically ≈50 μm in diameter and tended to orient with the c axis either nearly vertical (parallel to the scattering plane normal) or horizontal (in the scattering plane). Helium–neon laser light scattered by a levitated crystal was collected in the angular ranges 20°–65° and 115°–160° with a 1024-element linear photodiode array with an angular resolution of about 0.05°. The particle size and orientation were measured a few seconds before and after the scattering measurements with top- and side-view video telemicroscopes. Three basic features are found in the scattering from vertically aligned crystals: (i) a strong “halo” peak between about 21° and 35°, (ii) a secondary peak with ripple structure between about 30° and 70°, and (iii) a weaker peak in the ba...

Light scattering from slightly rough semiconductor surfaces near exciton resonance

The phenomenon of light scattering by a randomly rough surface and fluctuations of the excitonic surface potential is investigated by means of a first-order perturbation theory. We employ the generalized Morse potential to describe both intrinsic (repulsive) potentials and extrinsic near-surface potential wells. Frequency and angle dependencies of the light-scattering cross section are calculated. A considerable increase of the scattering cross section, as the correlation between the surface roughness and the excitonic potential fluctuations diminishes, is observed. Our theory describes very well available experimental results for samples with a repulsive surface potential. Also, the optical manifestation of excitonic bound states, generated within a surface-potential well, is analyzed. We find that the near-surface localized excitons produce a resonance structure in the spectrum of the light-scattering cross section, which is very sensitive to the degree of correlation between surface roughness and potential-well fluctuations.

Light scattering from self-assembled quantum disks

We present results of calculations and experiments probing the electronic structure of self-assembled Ga 0.8In 0.2As semiconductor quantum dots by inelastic light scattering. The inelastic light scattering signal is strongly enhanced when the energy of the incident light is resonant with the first excited subband. Both the optical phonon and the low frequency scattering spectra exhibit a strong effect of optical alignment, i.e., when the exciting and the scattered light have the same linear polarization. To understand the results of measurements we calculate both the single particle energy levels and excitonic states in the disk. The calculations of the resonant light scattering cross section are interepreted in terms of the one-phonon resonant Raman process involving bulk acoustical phonons interacting with excited subband excitons of the quantum dot.

Light scattering from a periodically modulated two-dimensional electron gaswith partially filled Landau levels

We study light scattering from a periodically modulated two dimensional electron gas in a perpendicular magnetic field. If a subband is partially filled, the imaginary part of the dielectric function as a function of frequency contains additional discontinuities to the case of completely filled subbands. The positions of the discontinuities may be determined from the partial filling factor and the height of the discontinuity can be directly related to the modulation potential. The light scattering cross section contains a new peak which is absent for integer filling.

Multiband electronic Raman scattering in bilayer superconductors.

A theory of electronic Raman scattering in the presence of several energy bands crossing the Fermi surface is developed. The contributions to the light scattering cross section are calculated for each band and it is shown that the cross section can be written in terms of the sum of the single band contributions and a mixing term which only contributes to the fully symmetric channels $(A_{1g})$. Particular emphasis is placed on screening in bilayer superconductors. Since any charge fluctuation with long range character in real space is screened by the Coulomb interaction, the relevant fluctuations in a single layer case are induced between different parts of the Fermi surface. In a single band $d$-wave superconductor the scattering at energy transfer twice the maximum gap $\Delta_{max}$ is dominated by those parts of the Fermi surface where $\Delta^2({\bf k})$ is largest. As a consequence, the fully symmetric ($A_{1g}$) scattering is screened. In the case of a bilayer superconductor however, the charge transfer is possible between layers inside the unit cell. Therefore a formalism is considered which is valid for general band structure, superconducting energy gaps, and inter-layer hopping matrix elements. The spectra is calculated for La 2:1:4 and Y 1:2:3 as representative single and bilayer superconductors. The mixing term is found to be negligible and thus the response is well approximated by the sum of the contributions from the individual bands. The theory imposes strong constraints on both the magnitude and symmetry of the energy gap for the bilayer cuprates, and indicates that a nearly identical energy gap of $d_{x^{2}-y^{2}}$ symmetry provides a best fit to the data. However, the $A_{1g}$ part of the spectrum depends sensitively on many parameters.

Computed light scattering cross sections of oxide particles in silicon

We have used the discrete-dipole approximation (DDA) method, implemented as a modified version of the DDSCAT program of Draine and Flatau, to calculate 3D light scattering intensity diagrams for individual oxide particles present in Czochralski silicon wafers. The particles were either octahedra or plates with normalized sizes x=0.01–5. The signals that would be measured for light scattered through 90° or 180° were determined from the diagrams, these angles corresponding to the cases of light scattering tomography (LST) and reflection confocal (RC) scanning infrared microscopy (SIRM), respectively. The results show that as x increases, the signal (particle image contrast) increases ∝x6 for x<∼1, but increased more slowly and in an irregular manner for x≳∼1. The signal also depends markedly on the particle shape and orientation. These findings demonstrate the difficulty of deducing quantitative data, e.g., individual particle sizes, from SIRM image contrasts. However, they also indicate the type of measurements that need to be made and provide a basis for the quantitative interpretation of the experimental results.

Discriminative temperature dependencies of differential light-scattering cross sections from an electron gas in semiconductors with a nonparabolic dispersion of energy bands.

We present the results of a direct comparison between experiment and theory of the temperature dependencies of the integrated intensities and spectral linewidths in the quasielastic light-scattering spectra from the electron gas in III-V semiconductors. An explicit expression for the inelastic-electronic light-scattering cross sections in semiconductors with nonparabolic dispersion of energy bands is given for a realistic hydrodynamic free-electron-gas model. We find rather different effects of single-particle electronic excitations on polarized and depolarized scattering in n-InP in the temperature range 27--300 K with near infrared excitation of the spectra. The results provide clear evidence for the existence of strong temperature-dependent free-electron-gas fluctuations and prove unambiguously the observation of light scattering from energy- and momentum-density fluctuations.

Classical centrifugal-sudden calculations of transport and relaxation cross-sections for Ar−N2

Classical limits of the quantal centrifugal-sudden approximation to generalized cross-sections have been derived and compared with the exact classical results. Detailed comparisons between the classical centrifugal-sudden approximation (cCSA) values, the exact classical trajectory results, and the corresponding quantal calculations are then carried out for the Ar−N2 system, and contrasted with similar comparisons for He−N2. Above 150 K the cCSA is found to be accurate to within 1% for the temperature dependent viscosity and diffusion cross-sections and 5% for the viscomagnetic and depolarized Rayleigh light-scattering cross-sections.

Light scattering cross section from surface acoustic phonons in multilayer films

We calculate the Brillouin cross section from surface and guided acoustic modes in opaque multilayer supported films by including the ripple scattering at all the interfaces. The results are compared with the usual approximate calculations. The superlattices Ag/Pd and Mo/Si are studied. We investigate in detail the dependence of the cross section on the top layer and the limit of validity of the effective-medium approximation. It is found that this approximation is no longer valid in case of a long-period metal-semiconductor superlattice such as Mo/Si.

Theory of surface acoustic phonon normal modes and light scattering cross section in a periodically corrugated surface.

We have developed a theory of surface acoustic phonon normal modes and of Brillouin-scattering ripple cross section for a periodically corrugated surface of an opaque material, in the small corrugation limit. Both the discrete and continuous spectra of acoustic modes on a grating have been studied within the elasticity theory. In the continuum, the Rayleigh wave becomes a resonance and hybridizes with the longitudinal pseudomode of the flat surface, giving rise to a gap. The theory explains quantitatively recent experimental results obtained for a shallow grating on a Si(001) surface.

Theory for Brillouin light scattering from dipole-exchange surface spin waves in ferromagnetic films

A theory is presented for Brillouin light scattering from spin-wave excitations in ferromagnetic films. The approach is based on a tensorial Green-function theory of the dipole-exchange spin wave spectrum in magnetic films. Mixed exchange boundary conditions are used, and the particular case of antisymmetric pinning in perpendicularly magnetized ferromagnetic films is discussed in detail. Explicit analytical expressions for the light-scattering cross sections are obtained in this case of scattering from both surface and bulk spin-wave modes. Applications of the theory to films of Fe and EuO are discussed. >

Raman intensities near second-order transitions: RP5O14 ferroelastics (where R is a lanthanide).

This paper provides a theory of Raman intensities for soft optic modes at second-order structural phase transitions. It combines the general theory of Fleury [Comments Solid State Phys. 4, 167 (1972)], which gave expressions for the total light scattering cross section (including Raman, Brillouin, and central-mode components) with the theory of Errandonea [Phys. Rev. B 21, 5221 (1980)], which described a specific form of coupling between the optic-mode order parameter Q and certain elastic strain components ${e}_{j}$ for the R${\mathrm{P}}_{5}$${\mathrm{O}}_{14}$ family of ferroelastics. The result of combining these two theories is a set of explicit expressions for the Raman intensities of the soft optic mode alone as a function of temperature, in both phases, and for all Raman tensor components. The predictions are compared with experimental data for both ZZ, YY, XX, and ZX polarizability components above and below ${T}_{c}$ for both ${\mathrm{LaP}}_{5}$${\mathrm{O}}_{14}$ and ${\mathrm{TbP}}_{5}$${\mathrm{O}}_{14}$; good agreement between theory and experiment is obtained.

Influence of entropy-fluctuation on the Rayleigh spectrum

We discuss the light scattering cross section due to entropy-fluctuation on the region of the central peak of the spectrum. A back-scattering oblique incidence geometry is used in our theoretical calculations which are based on the linear response function approach. Our results show that the power spectrum varies significantly with the incident angle and frequency and so does the light scattering cross section. The results are illustrated with numerical calculations for silicon.

Measurement of the resonantly enhanced quasielastic light scattering cross sections of heme proteins

We have measured the absolute cross sections for resonantly enhanced quasielastic light scattering in the heme proteins: cytochrome-c and metmyoglobin. Measurements of the scattered intensity were made as a function of protein concentration using the Brillouin scattering of the solvent as an internal standard. Our choice of scattering standard permits the measurement of absolute cross sections without employing correction factors for the reabsorption of scattered light. Incident laser excitation was on the red edge of the ‘‘Soret’’ absorption band of these proteins (21 839–23 529 cm−1). Scattering cross sections are nearly a factor of 3 larger than predicted with the Kramers–Kronig transform for both cytochrome-c and metmyoglobin.

Theory of Raman scattering by plasmon-polaritons in semiconductor superlattices

A theory of light scattering by superlattice plasmon-polaritons is developed using linear response techniques. The general polariton Green functions are adapted to the case of plasmon-polaritons in GaAs/AlxGa1-xAs superlattices, and these are used to calculate the light scattering cross section. The fully retarded theory yields mode frequencies, spectral lineshapes, polarisation selection rules and integrated intensities of the high-frequency transverse magnetic and transverse electric modes, as well as the low-frequency transverse magnetic modes. The nonretarded limit is appropriate for the description of the low-frequency modes, which have been investigated in recent Raman experiments, and the theory simplifies in this case. Numerical examples of GaAs/AlxGa1-xAs superlattices are given. Comparison is made with the available experimental data and further experimental work is suggested.