Theory of stimulated concentration scattering of light in a liquid suspension of transparent microspheres

A theory of stimulated light scattering in a nonlinear liquid suspension of transparent microspheres—an artificially created medium whose nonlinearity is caused by modulation of the concentration of microspheres by gradient forces in a field of spatially inhomogeneous laser radiation—is constructed. The threshold, angular, and spectral characteristics of the scattering are studied in the diffusion-limit approximation based on the solution of the system of wave equations in combination with the Planck-Nernst two-dimensional equation for the concentration of microspheres. The transient regime of scattering in the field of a specified step-like pump pulse is considered. A sharp angular dependence of the scattering efficiency on the microsphere radius is predicted and proposed for use in optical diagnostics of liquid suspensions of dielectric microspheres—highly efficient wideband nonlinear media.     

Experimental method to detect chiral asymmetry in specular light scattering from a naturally optically active medium

An experimental method is described for measuring chiral asymmetries in specular light scattering from a naturally optically active medium with a sensitivity on the order of one part in 10⁶. The chiral asymmetries of interest are the differential reflection of left and right circularly polarised light (circular differential reflection) and the deviation from mutual orthogonality of reflected transverse electric and transverse magnetic polarised light (skewness). The measurement of these chiral asymmetries would permit one to test the recently predicted Fresnel amplitudes for reflection from isotropic optically active transparent and absorving media and thereby to resolved controversial fundamental issues concerning the electrodynamics of chiral media.     

Ray-wave approximation for the calculation of laser light scattering by transparent dielectric particles,mimicking red blood cells or their aggregates

A theoretical model is presented for the fast calculation of light scattering by transparent dielectric particles with sizes much larger than the wavelength. In this model, the light incident onto the particle and the light propagating inside the particle are represented by sets of rays while the scattered light is represented by a set of spherical waves emitted by different elements of the particle surface (ray-wave approximation, RWA). It is shown that RWA is comparable in precision with the discrete-dipole approximation but significantly exceeds the latter in the calculation rate for the particles with size parameters higher than 50.     

光由光密介质射向光疏介质时半波损失的条件

本文通过对反射波和入射波振幅的比值关系进行详细解析推导、作图呈现和分析讨论,给出反射光和入射光相位差在折射率-入射角空间的详细分布图,并指明半波损失发生的各种条件,其中包括光由光密介质射向光疏介质时反射光发生半波损失的条件.研究表明当光束由光密介质射向光疏介质时反射波也可能发生半波损失,而当光束由光疏介质射向光密介质时反射波也不是一定会发生半波损失.澄清和补充了一些现有教科书中不很明确的结论.     

Measurement of soot morphology by integrated LII and elastic light scattering

A compact experimental setup that integrates laser-induced incandescence (LII) and one-angle elastic light scattering (1A-ELS) to measure the size of polydisperse soot aggregates is described. A 532 nm laser and a detection angle of 35 degrees were employed, which provided sensitivity for aggregate radius of gyrations (R _g) of R _g≤200 nm. Both lognormal and self-preserving distribution functions are compared with width parameters derived from both aggregation theory and transmission electron microscopy (TEM) measurements. Using these distributions, mean aggregate sizes derived from the scattering measurements are compared. The LII+1A-ELS technique is validated with a two-angle elastic light scattering (2A-ELS) approach with an additional detection angle at 145 deg. Unlike LII+1A-ELS, the 2A-ELS technique has the advantage of not requiring knowledge of soot optical properties. Good agreement is found between the two techniques for a given distribution. A fundamental discrepancy exists between distributions derived from TEM and those according to aggregation theory, limiting the accuracy of both 2A-ELS and LII+1A-ELS. The dependence of both techniques on laser fluence and hence soot temperature is examined and discussed.     

Negative shift of a light beam reflected from the interface between optically transparent and resonant media

The lateral shift of a light beam reflected from the interface between an optically transparent dielectric and a medium for which the frequency dependences of the real and imaginary parts of the permittivity have a resonant form is investigated. At certain radiation frequencies and angles of incidence, a negative displacement of the reflected beam along the interface takes place.     

Attenuation of a light signal in a scattering medium

A quantitative assessment of effects due to the interference of waves scattered by an ensemble of particles is given. It is shown that in the propagation of a collimated beam of light in a turbid medium, interference effects give rise to an additional stretching of the scattering indicatrix for a particle system, as compared with the scattering indicatrix for a single particle. The interference tongue obtained is concentrated in a very small angle in the direction of the light beam incident on the particle system. (Over a wide range of experimental conditions this angle is of the order of a few minutes.)Formulas are obtained for the instrumental scattering coefficient in the case of light scattering by volumes in the form of a sphere, disk, or rod. The analytical form of the formulas obtained indicates the possibility of concentration effects in experiments in a turbid medium.     

High-frequency ultrasound scattering from microspheres and single cells

Assessing the proportion of biological cells in a volume of interest undergoing structural changes, such as cell death, using high-frequency ultrasound (20-100 MHz), requires the development of a theoretical model of scattering by any arbitrary cell ensemble. A prerequisite to building such a model is to know the scattering by a single cell in different states. In this paper, a simple model for the high-frequency acoustic scattering by one cell is proposed. A method for deducing the backscatter transfer function from a single, subresolution scatterer is also devised. Using this method, experimental measurements of backscatter from homogeneous, subresolution polystyrene microspheres and single, viable eukaryotic cells, acquired across a broad, continuous range of frequencies were compared with elastic scattering theory and the proposed cell scattering model, respectively. The resonant features observed in the backscatter transfer function of microspheres were found to correspond accurately to theoretical predictions. Using the spacing of the major spectral peaks in the transfer functions obtained experimentally, it is possible to predict microsphere diameters with less than 4% error. Such good agreement was not seen between the cell model and the measured backscatter from cells. Possible reasons for this discrepancy are discussed.     

Spontaneous emission and elastic scattering of light from quantum-well excitons in a Fabry-Perot microcavity

A theory of spontaneous emission and elastic light scattering by quasi-two-dimensional excitons in a quantum well placed in a Fabry-Perot microcavity is developed. The problem is solved by means of electrodynamic Green’s functions with inclusion of fluctuations of the quantum-well width and cavity wall shape treated as a perturbation. General expressions are found in a zero approximation of perturbation theory (plane interfaces) for the radiative decay rates of quasi-two-dimensional excitons and for their energy shifts in the cavity. The boundary conditions for the electromagnetic field are taken into account through the coefficients of inward light reflection from the cavity walls. Resonance contributions to the scattering cross sections, which differ in the polarizations (p or s) of the incident and scattered waves, are derived in the lowest (Born) approximation in quantum-well width fluctuations. The spectral and angular dependences of elastic light scattering are studied numerically for Gaussian and exponential correlation functions. It is shown that the contribution from quantum-well width fluctuations to light scattering exceeds that due to single interfaces (surfaces) of a heterostructure by two orders of magnitude.     

Resonant control of elastic collisions in an optically trapped fermi gas of atoms

We have loaded an ultracold gas of fermionic atoms into a far-off resonance optical dipole trap and precisely controlled the spin composition of the trapped gas. We have measured a magnetic-field Feshbach resonance between atoms in the two lowest energy spin states, /9/2,-9/2> and /9/2,-7/2>. The resonance peaks at a magnetic field of 201.5+/-1.4 G and has a width of 8.0+/-1.1 G. Using this resonance, we have changed the elastic collision cross section in the gas by nearly 3 orders of magnitude.     

Self-recovery of wavefront in a nonlinear light scattering medium

Both theoretical and experimental studies of wavefront self-recovery of intense light beam propagating through heterogeneous nonlinear scattering medium are described. The self-recovery of the light beam wavefront is shown to be observed in the definite range of the light intensities when a mismatch of the linear refractive indices of the medium hetero components is compensated by nonlinear light-induced changes of refractive indices.     

Theory of stationary light scattering in a fluctuating medium

A theory of light scattering under random perturbations is formulated. The stochastic process considered in this paper is general enough in the sense that the process reduces to a single two-state jump process on the one limit and to the Gaussian process on the other limit. The light scattering rate, which is essentially a four-time correlation function, can be evaluated exactly for this model with the aid of the “partial cumulant expansion method”. Based on this a new method of a generalized master equation approach with memory effect is proposed. It is shown that the imhomogeneous term in a time-convolution expansion formula plays an essential role in the second order optical process, although the term is usually neglected in most problems of the master equation approach. The scattering rate is finally expressed as a superposition of continued fractions. Numerical calculations are performed and detailed discussions on coherence properties of the process are given.     

Multiple scattering of polarized light in a turbid medium

It is shown that multiple scattering of polarized light in a turbid medium can be represented as independent propagation of three basic modes: intensity and linearly and circularly polarized modes. Weak interaction between the basic modes can be described by perturbation theory and gives rise to “overtones” (additional polarization modes). Transport equations for the basic and additional modes are derived from a vector radiative transfer equation. Analytical solutions to these equations are found in the practically important cases of diffusive light propagation and small-angle multiple scattering. The results obtained are in good agreement with experimental and numerical results and provide an explanation for the experimentally observed difference in depolarization between linearly and circularly polarized waves.     

Small-angle X-ray scattering probe of intermolecularinteraction in red blood cells

With high concentrations of hemoglobin (Hb) in red blood cells, self-interactions among these molecules could increase the propensities of their polymerization and aggregation. In the present work, high concentration Hb in solution and red blood cells were analyzed by small-angle X-ray scattering. Calculation of the effective structure factor indicates that the interaction of Hb molecules is the same when they are crowded together in both the cell and physiological saline. The Hb molecules stay individual without the formation of aggregates and clusters in cells.