Mikhail V. Vasnetsov

Singularities in vectoral fields

Novel approach for the analysis of singularities in vector fields has been proposed. The essence of this approach is scalar consideration of the phase vortices at the orthogonal field components. The new type of vortices are introduced, namely the phase-difference vortices. The sign principle for the phase-difference vortices is formulated. An interconnection between the characteristics of the complete system of phase vortices associated with orthogonal field components, the behavior of an azimuth of a linearly polarized electrical field at an s-contour, and the polarization singularities, dislocations is established. The feasibility for comprehensive estimation of the characteristics of temporal singularities based on the measurement of the stationary field parameters is shown both theoretically and experimentally. Some elementary polarization situations are analyzed on the basis of the developed approach. The obtained results are extended to the case of quasi-uniform in polarization fields.

Transverse optical effects in bistable active cavity with nonlinear absorber on bacteriorhodopsin

Spatio-temporal effects of thresholding and switching are investigated theoretically and experimentally in bistable single- and multimode cavities containing two nonlinear elements: photochromic absorber on bacteriorhodospin controlled by bleaching and coloring external beams and amplifier on organic dye pumped by periodic laser pulses.

Nonlinear combinations of gratings in Bi 12 SiO 20 : theory and experiments

Multiple optical interference patterns generate phase gratings in Bi12SiO20 as a result of the photorefractive effect. Because of the nonlinear response of the space-charge field to the optical interference pattern, these primary gratings interact and cause the formation of additional gratings at frequencies that are combinations of the primary grating frequencies. Only the diffusion regime is considered in the present investigation. In our setup, two primary gratings are induced as the result of interference among one reference and two closely situated object beams. The interference between the two object beams leads to a negligible photorefractive grating, because the photorefractive Bi12SiO20 crystal cannot respond to the corresponding large fringe spacing. We show theoretically that nonlinear combinations of gratings are introduced through this third interference pattern even though the corresponding grating itself is absent. We obtain analytical expressions for the fundamental frequency components of the space-charge field in the special case of a weak reference beam. In the general case, the frequency components of the space-charge field are calculated numerically by the use of Fourier transforms and are incorporated into the coupled-wave equations. The optical beam propagation method is then used for solving these equations numerically. The effect is probed in a three-wave mixing experiment that uses a sinusoidal phase modulation on one of the object beams to control the grating strength. With this technique one of the gratings is specifically canceled out, and the relative change of the diffraction efficiency in the remaining grating is measured. It is shown that nonlinear combinations of gratings may cause relative changes of more than 35% in the diffraction efficiency. The theoretical predictions are in excellent agreement with our experimental results and the cross talk observed in dynamic optical interconnects.

Analysis of optical vortex beams with integer and fractional topological charge

This work presents an analysis of optical vortex beams with integer and fractional topological charges, produced by binary computer-generated holograms. In the case of integer topological charge, diffraction of a Gaussian beam by the hologram creates an optical vortex beam with amplitude distribution, which can be described in the far field by the Kummer function. We introduced for such beams the name “Kummer beams”. An analysis of the optical vortex beam with 1/2 fractional topological charge shows that it can be represented in the far field as a superposition of a vortex-free wave and single-charged anisotropic optical vortex.

Dynamic spatial structure of spontaneous beams in photorefractive bismuth silicon oxide

We report the domain structure of spontaneously occurring beams (subharmonics) in photorefractive bismuth silicon oxide with an applied electric field from 1 to 6 kV/cm and a running grating. The subharmonic beams are generated in a pattern of domains that evolve dynamically as they move through the crystal. We find that the domains move as a whole with a speed approximately equal to that of the primary grating, but in the opposite direction. The domains are separated by narrow boundary regions, where the phase of the subharmonic waves changes by π. The domain motion is consistent with the group velocity for running space-charge waves.

Relations between spontaneously occurring beams in bismuth silicon oxide with two frequency-detuned pump beams.

We report strong competition of intensities between spontaneously emerging beams (subharmonics) and other diffracted beams in two-wave mixing experiments with two frequency-detuned pump beams in photorefractive bismuth silicon oxide. The measurements show that the onset of subharmonics strongly affects the strength of the fundamental component of the primary grating, which caused the subharmonics in the first place. Suppression of subharmonics by vibration of the crystal increased the diffraction efficiency of the fundamental grating component by more than an order of magnitude.

Free-space evolution of monochromatic mixed screw-edge wavefront dislocations

We show how to synthesize a wave with monochromatic mixed screw-edge dislocation. We obtained theoretically and experimentally that a mixed screw-edge dislocation embedded in a monochromatic Gaussian beam does not propagate as a self- similar stable object, but transforms into several pairs of opposite-sign optical vortices in near field, and produces single-charge optical vortices in far field.

Phase modulation spectroscopy of space-charge wave resonances in Bi12SiO20

Abstract A new experimental method for the study of resonance effects and space-charge wave excitation in photorefractive Bi 12 SiO 20 crystals by using a combination of frequency detuning and phase modulation technique has been developed. The accuracy of the method allows a detection of resonance peaks of diffraction efficiency within 0.5 Hz. Numerical simulations of the nonlinear differential equations describing the behaviour of the space-charge waves in photorefractive crystals have been performed and found to be in a good agreement with experiment. We have measured the photocurrent through the crystal and revealed its resonance dependence. A minimum of electric current through the sample corresponds to the main resonance detected by phase modulation technique.

Anisotropic photoconductivity and current deflection induced in Bi 12 SiO 20 crystals by a high-contrast interference pattern

We have predicted and observed an anisotropic photocurrent induced in the cubic crystal Bi(12)SiO(20) by a high-contrast interference pattern. The transverse current detected when the interference pattern is tilted is caused by deflection of the direct current generated by an external voltage applied to the crystal.

Diffraction of optical vortices

Computer simulation of optical vortex diffraction on edge of nontransparent screen demonstrates the possibility for optical vortex to restore from the part of beam passing trough aperture. Regeneration of vortex is shown to take place for any part of the beam passing trough aperture, even if it does not contain vortex core.