Backscattering of s-polarized light from a cloud of small particles above a dielectric substrate

The scattering of s-polarized electromagnetic waves by a cloud of small particles above an interface is numerically studied, accounting for multiple scattering. The backscattering effect is first observed without the interface and the influence of the index of the particles is shown. In the case of a dilute system of particles with a low index such that no backscattering peak appears without the interface, it is shown that the introduction of an interface restores the backscattering effect. A physical mechanism responsible for this peak is described. In conclusion, a comparison of this model with scattering by rough surfaces is made.     

Backscattering of light from a dielectric layer on a reflecting substrate

Scattering of light from rough dielectric films is studied experimentally. It is shown that the interference pattern of the scattered field depends critically on the power spectrum of the roughness, especially on its long-scale component. When the height of roughness is small compared with the wavelength, the backscattering peak (if it exists) is due to the interference of the singly scattered fields; hence the properties of the backscattered peak are rather unusual. In particular, the width of the peak is determined by the thickness of the film and is independent of the parameters of disorder. The intensity of the peak increases with an increase of the rms height of the surface roughness and becomes independent of the rms when the roughness is of the order of the wavelength.     

Scattering by small thin dielectric particles

In the analysis of electromagnetic scattering by distributions of small dielectric particles an approximation to the scattered field can be obtained by representing the electrical interaction of the particles in terms of the dipole moments of the individual particles. The calculation of the moments necessitates the solution of certain static scattering problems, and this becomes numerically difficult when the particles are thin. An integral equation formulation of the static scattering problem specialized to the case of thin planar dielectric plates is presented, along with an efficient numerical routine. Dipole moments are obtained over a range of permittivities for plates with several thicknesses and a variety of cross-sectional shapes, and the shape dependence is discussed.     

Surface modes and optical absorption of a small sphere above a substrate

An exact derivation of the electrostatic surface modes of a sphere above a substrate is presented. The absorption spectrum of the sphere is calculated and compared with that of an isolated sphere. The interaction with the substrate causes the main absorption peak to shift to the low frequency side, and also gives rise to additional absorption peaks. The deficiencies of the dipole approximation, which is often used, are discussed.     

Backscattering of laser light from colloidal silica

Light-scattering experiments gained prominence as potential applications of quantum optics, non-linear optics, and photon localization. The possibility of the realization of lasing action in random media has created much interest in the study of the coherent structure of the backscattered light from disordered media. Backscattering (BS) studies are carried out to analyze the possibilities of photon localization in colloidal silica. The scattering enhancement is best associated with the density of the scatterers. The width of the BS cone and, hence, the mean-free path is related to the concentration of the medium. The dependence of the photon wavelength on the possible characteristics of the scattering is presented.     

Anomalous light scattering by small particles

Light scattering by a small spherical particle with a low dissipation rate is discussed based upon the Mie theory. It is shown that if close to the plasmon (polariton) resonance frequencies the radiative damping prevails over dissipative losses, sharp giant resonances with very unusual properties may be observed. In particular, the resonance extinction cross section increases with an increase in the order of the resonance (dipole, quadrupole, etc.); the characteristic values of electric and magnetic near fields for the scattered light are singular in the particle size, while energy circulation in the near field is rather complicated, so that the Poynting vector field includes singular points whose number, types, and positions are very sensitive to fine changes in the incident light frequency. The results may provide new opportunities for a giant, controlled, highly frequency-sensitive enhancement and variation of electromagnetic field at nanoscales.     

Reflection of light by a slab containing electrically small dielectric spheres

Theoriginal and theenhanced Maxwell-Garnett estimates for the permittivity of a particulate medium are applied to the reflection of light by a composite dielectric slab. The reflection coefficients for incident s and p polarizations are calculated and some curves are plotted and discussed.     

Polarization of light backscattered by small particles

We study scattering of light by wavelength-scale spherical, cubic, and spheroidal particles as well as clusters of spherical particles for equal-volume-sphere size parameters 4≤x≤10 and refractive indices 1.1≤m≤2.0. Such particles exhibit three specific features in the regime of backscattering: first, the intensity shows a backscattering peak; second, the degree of linear polarization for unpolarized incident light is negative; and, third, the depolarization ratio is double-lobed. We find that the overall characteristics of the scattering-matrix elements can be explained by an internal field composed of waves propagating in opposite directions near the particle perimeter and forming standing waves, as well as a wave propagating forward with the wavelength of the internal medium. When moving from the central axis of the particle toward its perimeter, the internal field changes from a forward-propagating wave with a wavelength dictated by the particle refractive index toward a standing wave with an apparent wavelength of the surrounding medium. The mapping of the internal field to the scattered far field is like an interference dial where rotation of the dial by a quarter of a wavelength on the particle perimeter results in a change from a destructive to constructive interference feature in the angular patterns (or vice versa). The dial is a manifestation of a well-known rule of thumb: the number of maxima or minima in the scattering-matrix elements is given by the size parameter. We explain the backscattering peak as deriving from the backward-propagating internal wave near the particle perimeter. Negative polarization follows from the spatial asymmetry of the internal fields: inside the particle, the fields are amplified near the central plane perpendicular to the polarization state of incident light, resulting in more pronounced interference effects for the perpendicular polarization than for the parallel polarization. The double-lobe feature in the depolarization results from the same internal-field structure with leading cross-polarized fields located slightly different from the copolarized fields. We discuss practical implications of these findings for the retrieval of particle sizes, shapes, and refractive indices from observations and laboratory experiments.     

Periodic structures induced by scattered light in a photosensitive film on a dielectric plane-parallel substrate

Periodic structures appearing in a thin AgCl-Ag film on a plane-parallel substrate irradiated with a normally incident, focused, linearly polarized Gaussian laser beam λ=633 nm) with a waist spot size of about 0.01 mm have been studied. The structures are formed in the beam spot and its nearest vicinity (with a radius of up to 0.2 mm) as a result of the interference of the incident beam and the waveguide TE modes excited due to the Rayleigh scattering in the film. In the vicinity of the waist spot, the periodic structures develop under the action of a coherent recurrent scattering from back side of the substrate. In addition, periodic structures of ring symmetry have been revealed far (up to 10 mm) from the irradiated spot. It is demonstrated that these structures are due to the interference of radiation scattered from the film and back side of the substrate and reflected from the two boundaries. Interference models are proposed that provide for a good agreement of the interference period, calculated as a function of the distance from the beam center, with the experimental values.     

Backscattering and negative polarization of agglomerate particles

We used the discrete dipole approximation to study the backscattering of agglomerate particles consisting of oblong monomers. We varied the aspect ratio of the monomers from approximately 1 (sphere) to 4, while we kept the total particle volume equivalent to that of an x = 10 sphere for m = 1.59 + i0 and 1.50 + i0 and considered two values of agglomerate packing density: rho = 0.25 and rho = 0.1. We found that these particles do not display a prominent brightness opposition effect but do produce significant negative polarization over a range of near-backscattering angles. Increasing the monomers' aspect ratio can make the negative polarization much more prominent. We have noted also that decreasing m and p can reduce the amplitude of the negative polarization for these particles.     

Backscattering properties of poly-dispersed small spheroid particles with their rotary axis orientations in normal distribution

The study investigates the backscattering cross section of small spheroid particles in air with their rotary axis orientation in a normal distribution. Backscattering cross section as a function of distribution parameters (mean and variance) and wavelength has been numerically calculated and analyzed in order to provide scientific basis for atmospheric microwave transfer and remote sensing, and especially for improvement of quantitatively rainfall measuring accuracy with Doppler weather radar.     

Quantum size effects in the elastic scattering of light from small metal particles

A quantum mechanical calculation of the differential elastic scattering cross-section of light from a metal microparticle is presented. The scattering intensity is found to exhibit oscillations as a function of the frequency due to the discreteness of the electron energy levels. The magnitudes of the oscillations have a sensitive dependence on the size of the electron mean free path relative to the diameter of the particle.     

On scattering of light by small axially symmetric particles

Light scattering by small dielectric particles of an arbitrary axially symmetric shape is analyzed. A simple approximate expression that governs the polarizability of the particle is found under the assumption of field homogeneity inside of these particles. The expression includes four relatively simple one-dimensional integrals that can be calculated analytically for some types of particles (except for spheroids). A comparison with the numerical data obtained for various Chebyshev particles and finite cylinders showed that the obtained approximation yields acceptable results, even when the shape of scatterers is significantly different from spheroidal. For spheroids, our approximation coincides with the Rayleigh one.     

Quantitative analysis of depolarization of backscattered light by stochastically inhomogeneous dielectric particles

We determine the relationship between the depolarization properties of inhomogeneous particles and the statistical parameters of their internal refractive-index distributions. Our analysis demonstrates that the linear depolarization ratio of backscattered light by an inhomogeneous particle is approximately proportional to both the squared standard deviation and the squared correlation length of the particle's internal refractive-index distribution. We verify this result by conducting rigorous numerical studies using the finite-difference time-domain method. This improved understanding of light depolarization by inhomogeneous structures may enhance polarization-based biomedical optical imaging techniques.     

Backscattering of light ions from the surfaces of inhomogeneous multicomponent amorphous materials

An analytic theory proposed in Ref. 1 [V. S. Sukhomlinov and é. N. Fafurina, Zh. Tekh. Fiz 65(4), 9 (1995); Tech. Phys. 40, 295 (1995)] for the backscattering of light ions by a solid surface is developed for the cases of multicomponent materials and materials in which the concentrations of some of its components vary with depth. Zh. Tekh. Fiz. 67, 17–21 (January 1997)