Diffusing-wave spectroscopy in randomly inhomogeneous media with spatially localized scatterer flows

Multiple scattering of laser radiation in a randomly inhomogeneous turbid medium with a spatially localized flow of particles is studied. The time autocorrelation function of backscattered light is calculated for the case of a laminar flow of scatterers in a cylindrical capillary embedded in the medium. A new method is proposed and tested experimentally for determining the position of the dynamic region and the dominant form and characteristic velocity of the particle motion there. Zh. éksp. Teor. Fiz. 113, 1213–1222 (April 1998)     

Correlation effects in electron-ion collisions in a strong laser field

We examine elastic and inelastic scattering of electrons by ions in intense laser light. A method of numerical investigation of the scattering characteristics based on regularizing the Coulomb singularity is proposed. We show that over a broad range of parameter values the transport scattering cross section is weakly dependent on the intensity of the high-frequency field. We detect a significant modification of the dependence of the effective inelastic scattering cross section. We also show that the energy exchange with the field is determined by a fairly small group of electrons, called the representative electrons. Finally, we propose a qualitative model that explains our results by the fact that the leading contribution is provided by inelastic collisions of electrons with relatively small impact parameters traversing the region important for the interaction at large angles. Zh. éksp. Teor. Fiz. 115, 463–478 (February 1999)     

Quasielastic light scattering in the near IR from photoexcited electron-hole plasma created in a GaAs layer with embedded InAs quantum dots

The paper reports the development of a high-sensitivity technique for measurement of inelastic electronic light-scattering spectra in the near-IR region, which are excited by a stable single-mode cw YAG:Nd laser operating at 1064.4-nm. This technique has permitted detection for the first time of quasielastic scattering of light by a photoexcited electron-hole plasma generated in a GaAs layer with an embedded self-organized ensemble of InAs quantum dots. A considerable resonant enhancement of the quasielastic electronic scattering intensity exceeding the level characteristic of the bulk material by two orders of magnitude has been revealed. The main scattering mechanism, involving joint diffusion of electrons and holes, has been elucidated. Fiz. Tverd. Tela (St. Petersburg) 41, 844–847 (May 1999)     

Modeling of light propagation in turbid medium using Monte Carlo simulation technique

The aim of the current study is to simulate the laser photon through biological tissue during PDT therapy using Monte Carlo simulation technique. The model is coded using MATLAB. Interaction of laser light with turbid medium e.g. human tissue depends on the optical properties of the medium i.e. refractive index n, absorption coefficient μ _a , scattering coefficient μ _s and anisotropy factor g. Laser light transport through tissue is governed by the radiative transport equations based on absorption and scattering. Direct sampling is used for step-size generation before interaction via absorption or scattering with the transmitting medium, for deflection and azimuthal angle (θ and ϕ) when the scattering even occurs. The tissue medium considered is divided into radial, axial and angular grid elements and an infinite narrow beam with normal incidence on the tissue is considered. The laser light absorbance inside the tissue, reflectance at the top boundary of the tissue and transmittance at the bottom are estimated and these quantities are shown varying radially and angularly. Results of reflectance, transmittance and fluence are compared with the already published results to confirm the authenticity of our coding and these results are found to lie at only 3–4% error.     

Laser light scattering in micellar solutions

Summary The laser light scattering apparatus used for both total intensity and quasi-elastic light scattering experiments on micellar solutions is described in detail. The absolute calibration of the scattered intensity is obtained by exploiting the existence of a lower critical consolution point in a low-concentration aqueous solution of a nonionic amphiphile (C₁₂E₈). The obtained results suggest that micellar solutions may represent a very convenient light scattering standard for both static and dynamic experiments. Paper presented at the “Meeting on Lyotropics and Related Fields”, held in Rende, Cosenza, September 13—18, 1982.     

Raman scattering by hot and thermal polaritons in crystal quartz

Summary Nonlinear mixing of IR and visible radiation,i.e. coherent Raman scattering by polaritons driven by a CO₂ laser, has been used to obtain the dispersion curve and its width inq-space of the polariton associated to theE-phonon at 1065 cm⁻¹ in crystal quartz. It is shown in this paper that a direct method to determine indipendently, with high precision, the refractive index and absorbance of a crystal can be obtained in this way. The results are compared with accurate data obtained from Raman scattering by polaritions in thermal equilibrium and very good agreement is found between the two measurements. It is finally shown that nonlinear-mixing techniques turn out to be completely consistent with the simple picture of scattering of light by hot polaritons. This work was supported by the Italian Consiglio Nazionale delle Ricerche del Ministero della Pubblica Istruzione.     

Resonant Mandelstam-Brillouin light scattering and Raman scattering in semiconductors with intermediate exciton states belonging to discrete exciton bands and the continuous spectrum

The results of a theoretical and experimental investigation of resonant Mandelstam-Brillouin light scattering by thermal acoustic phonons with k=0 near the direct absorption edge (in the case of ZnSe crystals) are analyzed. The appearance of a new type of resonant increase in the intensity of Raman scattering by optical phonons with k≠0, which corresponds to resonance with the scattered light in the output channel, near the indirect absorption edge (in the case of semi-insulating GaP:N crystals) is also reported. The resonant gain reaches ∼4×10³ at frequencies corresponding to overtone scattering assisted by LO(X) and LO(L) phonons. Exciton states belonging to both discrete exciton bands and to the continuous spectrum are considered as the intermediate states involved in the scattering processes in calculations of the resonant scattering tensors. In addition, all the intraband transitions, as well as the interband transitions between the conduction band, the valence bands, and the spin-orbit split-off band are taken into account, and good agreement with the experimental results is obtained. Fiz. Tverd. Tela (St. Petersburg) 40, 938–940 (May 1998)     

On the theory of the acceleration of plasma electrons during stimulated scattering of an intense laser wave

The mechanism of electron acceleration in the stimulated Raman forward scattering of a monochromatic laser wave in a cold plasma is investigated theoretically. It is shown that as a result of the stochastic interaction of the electrons with the ponderomotive wave and with plasma waves excited in the scattering process, some of the electrons are accelerated to relativistic energies. Zh. Tekh. Fiz. 69, 3–8 (January 1999)     

Enhancement of the quantum characteristics of light from an inversionless three-level Λ-laser in cascade schemes

We show that when a three-level laser is excited by Poisson light the lasing remains sub-Poisson, as asserted by H. Ritsch, M. A. M. Marte, and P. Zoller [Europhys. Lett. 19, 7 (1992)], only if there is no reciprocal action on the exciting laser (weak coupling). In cascade schemes in which each three-level Λ-laser excites each succeeding Λ-laser with its light, any desired degree of shot-noise suppression can be achieved. Zh. éksp. Teor. Fiz. 114, 1971–1992 (December 1998)     

Nanoparticle size characterization by laser light scattering

Spherical semiconductor nanoparticles (ZnS) were specially fabricated by an inexpensive chemical route. The scattering profile of the nanoparticles was investigated by laser light scattering technique. A beam of polarized light from a diode laser (λ ₀ ≈ 630 nm) was allowed to fall on the nanospheres embedded in flexible host matrix Polyvinyl Alcohol (PVA). The light scattered from the samples were detected by means of analyzer mounted photodiode array from 10° to 170° in steps of 1°. Signals from the detectors were interfaced with a high resolution data acquisition system and the whole experiment was carried out in differential mode. Size of the nanoparticles was obtained by using Mie theory and verified by T-matrix approach. The results obtained agree with the XRD and TEM results.      

Four-wave polariton scattering of light in LiNbO3

Four-wave Stokes k-spectra for light scattering on polaritons in lithium niobate crystals with an Mg impurity are studied experimentally. The mechanisms for direct, cascade, coherent, and incoherent four-wave mixing of light are discussed in the course of interpreting the angular dependences of the scattered light intensity. It is shown that the dispersion of the real part of the polariton wave vector and the refractive index of the crystals at the polariton frequencies can be measured with an order of magnitude greater accuracy than by spontaneous three-wave polariton light scattering. A significant discrepancy is found between determinations of the polariton absorption coefficient from the angular spectra of three-wave scattering and four-wave scattering in terms of the model employed here. Zh. éksp. Teor. Fiz. 112, 441–452 (August 1997)     

Direct measurement of the spatial distribution of light intensity in a scattering medium

A direct nonperturbative measurement of the spatial distribution of the light intensity in a strongly scattering medium is performed using an optoacoustic method. It is shown that near a surface the intensity can be five times greater than the incident intensity, and the absolute maximum of the intensity is observed at a depth ℓ(1–R)(1–4.0R) determined by the photon transport mean free path ℓ and the effective light reflection coefficient R of the boundary separating the scattering and external media. Pis’ma Zh. éksp. Teor. Fiz. 70, No. 3, 187–191 (10 August 1999)     

Resonances and dichroism in the scattering of an electron in an intense laser field by the Coulomb potential

We use a new type of Hamiltonian representing the electron-proton interaction in an intense laser field for an ab initio calculation of the differential scattering cross section. We give the diagrams of the results of calculations of the shapes and widths of the resonances in the cross section of electron scattering by the “field-dressed” Coulomb potential. The resonances emerge because of re-emission of photons by the electron. We also give the angular distribution of the scattered electrons as a function of circular dichroism for different values of the laser field strength and frequency. Zh. éksp. Teor. Fiz. 116, 1241–1249 (October 1999)     

Ponderomotive action of light in the problem of multiple scattering of light in a randomly inhomogeneous medium

The time correlation function of light reflected diffusely from a semi-infinite randomly inhomogeneous medium is calculated with allowance for the acceleration of the scatterers in the field of the laser beam incident on the medium. An analytical expression is found for the characteristic coherence time due to the ponderomotive action of light. It is shown that even with laser radiation power densities of the order of 1–10 W laser-acceleration effects substantially alter the character of the time autocorrelation function of the scattered light and must be taken into account in theoretical calculations. Pis’ma Zh. éksp. Teor. Fiz. 67, No. 9, 611–615 (10 May 1998)     

Polar magneto-optic Kerr effect in the near-light field of a small nonmagnetic particle

A theory of elastic light scattering by a small resonantly polarizing particle located near a flat surface of a magnetic medium has been developed. The effective polarizability of the particle was calculated self-consistently taking account of the dynamic “image forces” in all orders of perturbation theory in the interaction of the particle with the demagnetized ferromagnet, and the magneto-optic perturbation was determined to first order in the magnetization. In the case of a ferromagnet magnetized perpendicular to the surface, the light conversion factors and the magneto-optic corrections to the transverse cross sections of all processes in which the scattering of light by a particle and the polar magneto-optic Kerr effect are elementary events, have been calculated. The results, including an analysis of the near-field contribution to light scattering, comprise the physical foundation for constructing a theory of near-field magneto-optic spectroscopy of ferromagnets and magnetic structures. Fiz. Tverd. Tela (St. Petersburg) 39, 560–567 (March 1997)     

Propagation of circularly polarized light in media with large-scale inhomogeneities

Small-angle multiple scattering of circularly polarized waves in disordered systems composed of large (larger than the light wavelength) spherical particles is discussed. The equation for Stokes’s fourth parameter V — the difference between the intensities of the left-and right-hand polarized light — is shown to have the form similar to that of the scalar transport equation for intensity I, the only difference being the presence of an additional “non-small-angle” term responsible for depolarization. In the case of small-angle scattering, depolarizing collisions are relatively rare and, in contrast to the scalar case, the problem contains an additional spatial scale, namely the depolarization depth. The polarization degree and helicity of the scattered light are calculated for the case of purely elastic scattering and in the presence of absorption in the medium. For strong absorption, depolarization is shown to follow the transition to the asymptotic regime of wave propagation. The features appearing in strong (non-Born) single scattering are also discussed. Zh. éksp. Teor. Fiz. 115, 769–790 (March 1999)     

Raman scattering of light by optical phonons in Si-Ge-Si structures with quantum dots

Raman scattering of light by optical phonons in Si-Ge-Si structures with pseudomorphic germanium quantum dots has been investigated. Resonance amplification of the scattering intensity on E ₀ (Γ₇−Γ₈) transitions has been observed. It is shown that as a result of the formation of the layer of germanium quantum dots, the resonance energy is ∼0.3 eV higher than in the two-dimensional case. Pis’ma Zh. éksp. Teor. Fiz. 64, No. 3, 203–207 (10 August 1996)     

Panoramic measurements of electron beam densities by Thomson scattering of laser radiation

The possibility of using a panoramic detector with a television signal-recording system in an apparatus to observe Thomson scattering of laser radiation by a nonrelativistic electron beam is discussed. Panoramas of Thomson and Rayleigh scattering and of the electron beam luminescence are presented. Estimates are given of the sensitivity and spatial resolution of the apparatus. Results of panoramic and single-point methods of investigation are compared. Possibilities for extending the range of the Thomson scattering method to measure the density distribution in nonrelativstic electron beams are discussed. Zh. Tekh. Fiz. 69, 80–83 (June 1999)     

Spectral properties of backward stimulated scattering in liquid carbon disulfide

The spectral structure of backward stimulated scattering from a 10 cm-long CS₂-liquid cell is investigated by using Q-switched 10-ns and 532-nm laser pulses with different spectral linewidths. Under a narrow spectral line (∼0.1 cm⁻¹) pump condition, very strong sharp lines near the pump wavelength (λ ₀) position and the first-order stimulated Raman scattering (λ _s1) position can be observed. However, under a wide line (≈1 cm⁻¹) pump condition, only a strong and superbroadening spectral band can be observed mainly in the red-shift side of the pump wavelength. The different spectral features under these two conditions can be explained by a competition between stimulated Brillouin, Raman, and Rayleigh-Kerr scattering. Under both pump conditions, the broadening spectral distributions are not consistent with the predictions given by stimulated Rayleigh-wing scattering theories, but can be interpreted well utilizing the theoretical model of stimulated Rayleigh-Kerr scattering. Zh. éksp. Teor. Fiz. 112, 1563–1573 (November 1997) Published in English in the original Russian journal. Reproduced here with stylistic changes by the Translation Editor.     

Theory of scanning near-field magnetooptical microscopy

An analytical theory of scanning near-field magnetooptical microscopy is developed. The theory is based on the elastic scattering of light by small, resonantly polarizable particles, which are used to scan the plane surface of a nonuniformly magnetized medium. The effective polarizability of the particles is calculated with the effect of dynamic “image forces” taken into account in all orders of perturbation theory with respect to the interaction of the particle with a demagnetized ferromagnet, and the magnetooptical perturbation is calculated to first order in the magnetization. The major contributions to the magnetooptical light scattering for a ferromagnetic structure magnetized perpendicular to the surface are found, including a quasistatic approximation for the near-field particle-magnet interaction. The optical size resolution of a magnetic (dielectric) inhomogeneity is estimated. Zh. Tekh. Fiz. 68, 86–91 (July 1998)