Observation of superluminal wave-front propagation at the shadow area behind an opaque disk

We demonstrate in theory and experiment superluminal properties of optical field propagation in the shadow area behind an opaque disk (Poisson's spot). The wave front of the field in the shadow zone is initially delayed with respect to the unperturbed field. This phase shift gradually diminishes along the optical path and therefore ensures variation of the phase velocity of the field at the axis. A resulting excess of about cx10(-5) was detected.     

Umbilic point screening in random optical fields

Umbilic points--singular points of curvature characterized by a fractional topological charge q=+/-1/2--are the most numerous of all special points in the landscape of random optical fields (speckle patterns), outnumbering maxima, minima, saddle points, and optical vortices. To the best of our knowledge, we present the first experimental evidence that positive and negative umbilic points screen one another. Theory predicts that in the absence of screening the charge variance in a bounded region is proportional to the area of the region, whereas in the presence of screening the variance is drastically reduced and is proportional to the perimeter. Our data confirm this latter prediction and provide the first estimates of the screening lengths for umbilic points of the intensity and of the amplitude (field modulus).     

Elliptic critical points: C-points, a-lines, and the sign rule

The critical points of generic paraxial ellipse fields consist of singular points of circular polarization, called C -points, and azimuthal stationary points, i.e., maxima, minima, and saddle points. We define these stationary points here and review their properties. The sign rule for ellipse fields requires that the sign of the singularity indices I(C)=+/-1/2 of the C -points on non-self-intersecting lines of constant azimuthal ellipse orientation (modulo pi/2), i.e., a -lines, alternate along the line. We verify this rule experimentally, using a newly developed interferometric technique to measure C -points and a -lines in an elliptically polarized random optical field.     

Noise-induced escape on time scales preceding quasistationarity: New developments in the Kramers problem

Noise-induced escape from the metastable part of a potential is considered on time scales preceding the formation of quasiequilibrium within that part of the potential. It is shown that, counterintuitively, the escape flux may then depend exponentially strongly, and in a complicated manner, on time and friction. (c) 2001 American Institute of Physics.     

Phase-matched light-induced scattering,mirrorless self-oscillation,and generation of nearly retropropagating waves in LiNbO3:Fe in “forbidden” interaction geometry

Two mutually incoherent pump waves impinging symmetrically on a LiNbO₃:Fe crystal from different faces in a plane normal to the C-axis produce chartacteristic patterns of light-induced scattering (lines and rings) and a pair of collimated waves propagating roughly in backward directions with respect to the pump waves. We show that the bright line of scattered light is in fact the mirrorless oscillation due to backward-wave four-wave mixing in the crystal with local (photovoltaic) response, while the retropropagating waves appear only because of high-order diffraction processes.     

Anomalous optical properties of photoactive cholesteric liquid crystal doped with single-walled carbon nanotubes

Optical transmission and selective reflection data are reported for suspensions of single-walled carbon nanotubes (CNTs) in photoactive nematic material ZhK-440 with a mesogenic chiral dopant M5. At small concentrations of CNTs (C ≈ 0.01–0.05%), the preferential localisation of CNTs at oily sticks (cholesteric topological defects) and suppression of the network of oily streaks by CNTs were observed. At the same time, the optical density D was shown to be essentially non-linear and a minimum at certain concentration of CNTs, C ≈ 0.05–0.08%, was observed. This anomalous behaviour was explained by the presence of the structural transition from the loose (ramified) aggregates with highly anisotropic shape (oriented along the anchoring direction on rubbed polyvinyl alcohol) to the compact aggregates with denser packing. The location of this minimum, as well as the selective reflection maximum (helical pitch), was sensitive to partially reversible UV-induced trans–cis–trans isomerisation effects. The UV-controlled helical pitch variation was shown to be only slightly affected by introduction of CNTs.     

Wavefront motion in the vicinity of a phase dislocation: “optical vortex”

In the scalar approximation, an analysis is made of the light field structure in the vicinity of a line of the ring phase dislocation corresponding to the zero value of the field formed by the interference of two uniaxial Gaussian beams. The formation of an “optical vortex” or the toroidal motion of a portion of a light flow around a ring phase dislocation is shown.     

Optical anisotropy of volume phase gratings

A propagation of zero diffraction order through a volume phase grating is analysed. It is shown that a phase shift between S and P polarization components arises, thus providing optical anisotrpy of the grating. Experimental verification of theoretical predictions is presented.