P. A. Prudkovskii

Scattering Effects in Terahertz Wave Spectroscopy of Granulated Solids

Applications of terahertz spectroscopy methods to granulated solids are substantially constrained by the effects of scattering of the incident terahertz waves by spatial heterogeneities of a sample. The experimental results obtained by means of terahertz time-domain spectroscopy are reported for the samples of granulated 2, 4-dinitrotoluene and alpha-lactose without any special filling. We show that in samples of the same substance, but prepared in a different manner, the overall increase of the background extinction with frequency and appearance of additional extinction maxima can obscure the true absorption features of the material. In samples with large grain size, of order of 400 μm, a complicated irregular spectral structure is observed instead of a fingerprint maxima at 1.37 THz for alpha-lactose and at 1.08 THz for 2,4-dinitrotoluene. At higher frequencies no true spectral fingerprints are clearly seen in the raw experimental spectra of all the samples with the grain sizes of 130 μm and more. The influence of grain size, air-filling factor and the dispersion properties of the materials is studied. Theoretical analysis is made with the use of Mie theory in simplifying approximations and taking into account interference between the waves transmitted through a porous sample. It has been shown that the scattering-induced masking effects can be properly simulated even for the air-filled objects with initially unknown spatial and dispersion parameters. The procedure of eliminating the influence of scattering effects from the raw spectral data is proposed. Detection of true high-frequency spectral fingerprints is demonstrated using this procedure for various grain sizes up to 400 μm.

Study of the kinetics of domain structures in thin ferromagnetic films by the Wang-Landau method

The features of a phase transition between the strict striped domain structure in thin ferromagnetic films with transverse anisotropy and the so-called tetragonal phase, where domains can be twisted and broken into parts, have been analyzed. It has been shown that the phase transition should be thermodynamically treated as a Kosterlitz-Thouless transition, where ends of domains serve as vortices. It has been demonstrated with the Wang-Landau numerical algorithm that the kinetics of this phase transition is strongly slowed down owing to the existence of a large number of metastable states; in this property, the system is partially similar to spin glasses.

Nonlinear response of photorefractive lithium tantalate and niobate at acoustic frequencies

An amplification of the intensity of pump oscillations is observed experimentally at frequencies from 100 Hz to 1 kHz during photoinduced light scattering and holographic-type parametric scattering in photorefractive lithium tantalate and niobate. Possible ways are analyzed for explaining the existence of a photorefractive response in these crystals over times of 10−2–10−3 s, which are five orders of magnitude shorter than the Maxwell time.

Effect of Pump Depletion on the Spectrum of Twin Beams

The effect of pump depletion on the spectrum of twin beams, which appear in the process of collinear parametric down-conversion in a nonlinear crystal with an aperiodic domain structure and have a high degree of simultaneity, has been described. A system of equations describing the interaction of pump with all modes of scattered light simultaneously has been derived. It has been shown that pump depletion results in a noticeable broadening of the spectrum of twin beams for a quadratic-hyperbolic law of variation of the reciprocal lattice vector of the domain structure. The effect of narrowing of the coherent peak in the sum-frequency generation signal from twin beams has been demonstrated. This effect increases as the pump is depleted.

Photoinduced gratings in a Sn2P2S6 ferroelectric crystal with the period depending on the optical pump power

The light scattering in the form of ring structures in Sn2P2S6 crystals at the propagation of intense laser pump radiation along the b crystallographic axis is detected. Radiation passing through a crystal is completely scattered into a cone whose angle increases with the pump power and decreases reversibly with a decrease in this power. The observed effect can be attributed to a spontaneous increase in the amplitude of photoinduced bulk diffraction gratings and to the scattering of light on them in the directions where the phase increments of the photorefractive and diffraction natures compensate each other. A similar type of scattering was observed previously in other photorefractive crystals, but the appearance of gratings with the period easily varied by varying the pump power is demonstrated for the first time.

Phase transition in the generalized Ising-Kosterlitz-Thouless model

The features of the phase transition are considered in a model that is a combination of the Ising and Kosterlitz-Thouless models. It is shown that one or two successive phase transitions may occur in such a system, depending on the relation between parameters. An asymptotic formula is derived for the Ising-type phase transition temperature.

Spatiotemporal self-organization upon two-wave mixing in a photorefractive medium

Nonlinear dynamics of recording volume holographic gratings upon two-wave mixing in an inertial photorefractive media were studied. It is found that the space-time dependence of the grating diffraction efficiency can be quasi-regular, allowing the understanding of the experimentally observed irregular behavior of the diffracted light intensity.

Holographic-type parametric scattering: The universal character of the seed radiation

It is concluded on the basis of experimentally obtained time dependences of the intensity of holographic-type parametric scattering induced by artificially produced seed radiation in a photorefractive lithium niobate crystal that the brightness of the natural seed radiation is independent of the pump intensity. This attests to a quantum origin of the seed radiation.

Correlation of photons in scattered light

Relation between the photon correlations in elastic and non-elastic scattering of light and the statistical properties of fluctuations which initiate this scattering is considered. The cases of scattering by dust particles and by ultrasonic waves are investigated experimentally.