A new class of light-induced parametric scattering, not included in the conventional model, has been discovered in photorefractive Sr(0.61)Ba(0.39)Nb(2)O(6):Cr illuminated by two coherent beams. A novel model of multiwave mixing of coherent noise and transmitted light is developed to explain the new scattering phenomena. The model includes all known types and predicts a multitude of new types of parametric scattering. Generalized phase-matching conditions for parametric scattering in photorefractive crystals are proposed.

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A new photorefractive approach using the temperature evolution of beam fanning to study the polar structure of the ferroelectric relaxor strontium-barium-niobate (SBN) undergoing the ferroelectric-paraelectric phase transition is proposed. A strong temperature dispersion of the decay of polar structures of different sizes is observed. We compare our results with those obtained by other measurement methods to show their dependence on the size distribution of the polar structures.

The dynamics of optical hexagon excitation is studied experimentally in photorefractive barium titanate. The hard mode of the oscillation onset is discovered as well as a pronounced hysteresis loop in the dependence of the hexagon intensity on the crystal coupling strength. The analogy of excitation threshold of optical hexagon and first order optical phase transition is confirmed. PACS numbers: 42.65.Hw, 42.65.Pc, 42.65.Sf, 47.54. + r Starting from publication of Haken [1] it has become clear that the onset of oscillation in lasers can be considered as a kind of nonequilibrium optical phase transition. Below the threshold an active medium emits spontaneous radiation with a rather broad spectrum; the angular distribution of the emitted light is uniform in a 4p steradian solid angle. Above the threshold considerable narrowing is observed for both the temporal and the spatial spectra of radiation.The control parameter of phase transition in lasers is the pump intensity; the normalized laser intensity may be considered an order parameter. For a free-running solid state laser the oscillation intensity is zero exactly at the threshold, i.e., the order parameter changes continuously from zero to finite value in the vicinity of transition. This behavior is typical for the second order phase transition.A critical slowing down of fluctuations has been observed near the threshold, and critical indices (close to unity) were evaluated, also pointing to the second order phase transition [2]. At the same time a laser with the bleachable dye in the cavity (Q switched) exhibits the hard onset of oscillation, with a discontinuous jump of intensity to a certain finite level at threshold; this behavior is analogous to the first order transition.Recently the analogy with phase transition was discussed also for coherent oscillators based on nonlinear wave mixing in photorefractive crystals [3,4]. In photorefractive coherent oscillators the temporal frequency of radiation is nearly the same as the frequency of the pump wave. The changes in the spatial distribution of radiation, however, are as large as in usual lasers: A highly collimated light beam is generated above the threshold, while wide-angle light-induced scattering is observed below the threshold. Above the threshold a single high-amplitude 3D refractive index grating replaces a multitude of smallamplitude, arbitrarily oriented "noisy" gratings existing below threshold.For photorefractive oscillators the control parameter is the coupling strength (coupling coefficient g times interaction length ᐉ). With the order parameter, one can choose either the diffraction efficiency of the developing grating or the phase conjugate reflectivity (both quantities are zero below the threshold and saturate to unity for lossless optimized cavity, with increasing gᐉ).A number of possible optical configurations of the photorefractive coherent oscillators [5,6] are much larger than that of usual lasers: Their optical cavities can be open; the oscillation may occur without any cavity. This res...

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