S. G. Rautian

Optics of metal nanoparticle aggregates with light induced motion

Light-induced forces between metal nanoparticles change the geometry of the aggregates and affect their optical properties. Light absorption, scattering and scattering of a probe beam are numerically studied with Newtons equations and the coupled dipole equations for penta-particle aggregates. The relative changes in optical responses are large compared with the linear, low-intensity limit and relatively fast with nanosecond characteristic times. Time and intensity dependencies are shown to be sensitive to the initial potential of the aggregation forces.

Universal Asymptotic Profile of a Spectral Line under a Small Doppler Broadening

The general form of the asymptotic profile of a separate spectral line under a small Doppler broadening is found. All the known factors that have an effect on the line profile in a gas are taken into account.

Optical bistability driven by the light-induced forces between metal nanoparticles

A motion-induced optical bistability is shown for a metal nanoparticle dimer that is plasmon coupled and bound with dispersion (colloidal) forces. The effect does not require any material nonlinearity.

Polarization Effects in Nanoaggregates of Silver Caused by Local and Nonlocal Nonlinear-Optical Responses

A theoretical and experimental study is made into the combined manifestation of local and nonlocal optical responses in a cubic nonlinear isotropic medium such as an aggregated colloidal silver solution. The phenomenological treatment of polarization effects is performed for the general case with due regard for the frequency dispersion of both local and nonlocal nonlinearities and for the noncollinear propagation of pump and probe light waves. The inverse Faraday effect, the optical Kerr effect, and the self-rotation of the polarization ellipse in a fractal-disordered nonlinear medium are observed for the first time. The tensor components of the local and nonlocal cubic nonlinearities of colloidal silver solutions are measured for different degrees of aggregation. It is demonstrated that, as the size of silver aggregate increases, the nonlocal nonlinear response increases much more strongly than the local one. An inference is made that the mechanical motion of metal nanoparticles because of their dynamic interaction with the light wave field can contribute to the nonlinear polarization effects.

Giant nonlinear optical activity in an aggregated silver nanocomposite

Nonlinear optical activity due to spatial dispersion is observed in a colloidal solution of silver. It is shown experimentally that the effect is substantially enhanced (by a factor of ∼102) when the silver particles aggregate into fractal clusters. The self-rotation angle of the plane of polarization is 2 mrad at an intensity of 2 MW/cm2 for λ=0.532 μm and a pulse duration of 11 ns. A method of separating the contributions of the local and nonlocal effects to the rotation of the plane of polarization is proposed and implemented.

Nonlinear Optical Effects and Selective Photomodification of Colloidal Silver Aggregates

Colloidal silver aggregates of nanoparticles were studied experimentally using optical spectroscopy, electron microscopy, near-field optics, and nonlinear optics. Changes in absorption spectra, local structure, and near-field optical response after the irradiation of fractal colloidal aggregates with a laser pulse (selective photomodification) were studied. The diameters of the selectively photomodified domains decreased as the laser wavelength increased, in accordance with the theory of the optics of fractal clusters. Giant enhancements of nonlinear optical responses were found for aggregated nanocomposites compared with nonaggregated. The enhancements are due to excitation of the collective plasmon modes in the aggregates. The plasmon modes are anisotropic and chiral. Nonlinear effects governed by local and nonlocal responses (degenerate four-wave mixing, nonlinear absorption, refraction and gyrotropy, inverse Faraday effect, and ellipse self-rotation) were studied.

Saturation and self-saturation effects in the field of intense laser waves: The J = 1 − J = 1 transition

The shape of the resonance of a saturated absorption is calculated at an arbitrary intensity of a probe field. It is revealed that, in the case of orthogonal polarizations of strong and probe fields, an increase in the intensity of the probe field can invert the shape of the nonlinear resonance of clearing of the medium, changing it to the resonance of absorption. The saturation of optical transitions by intrinsic spontaneous radiation increases the contrast of the inverted resonance of saturation and narrows its width. In the case of parallel polarizations of the optical fields, an absorption peak can arise in the shape of the nonlinear resonance upon splitting of the lower state.

Contours and widths of the self-alignment magnetooptical resonances in the presence of an intrinsic-emission-induced magnetic coherence exchange between levels

Self-alignment magnetooptical resonances were investigated theoretically for magnetic coherence exchange between levels, which was induced by a discharge self-radiation or by electron collisions. The structure of the resonances was determined and the nonlinear pressure dependence of the resonance width in the presence of such exchange was established.

Physical optics of photonic crystals

A one-dimensional harmonic model of a photonic crystal is considered using the methods of physical optics. The theoretical formalism is based on the notion of the Fresnel volume reflection and the system of two first-order differential equations, which are equivalent to the wave equation. Using the Rayleigh layer as an example, it is shown that the volume reflection plays a role of the friction, similar to the friction in oscillations of a pendulum, and, in a strongly inhomogeneous medium, can suppress field oscillations and turn the group velocity to zero. In the approximation of small modulation factor, the models of two, four, and six waves are considered. In the two-wave model, the dispersion relation contains a zone of inhomogeneous waves, whose width is determined by the Fresnel reflection coefficient from one period. The refinement in terms of the four-wave or six-wave model yields only a small correction to the position of the zone, retaining its width unchanged. The wavenumber as a function of frequency is described by a circle inside the zone of inhomogeneous waves and by a hyperbola outside this zone. Mathematically, the method used is significantly simpler than those based on the application of the Floquet theorem to the wave equation. It is shown that the notion of forbidden zones is inconsistent with respect to photonic crystals, and the term zones of inhomogeneous waves is proposed instead.

Probe-field spectroscopy under conditions of self-saturation

Nonlinear absorption of a probe field is analyzed taking into account intrinsic spontaneous radiation (the self-saturation effect). Systems of two and three nondegenerate levels, as well as a system of two levels with the angular momenta equal to unity, are considered. Changes produced by the self-saturation in the population, polarization, and nonlinear interference effects, as well as in the field splitting of lines, are determined. In certain situations, incoherent spontaneous radiation enhances the interference phenomena.