A few years ago in photoretractive crystals of LiTaO3:Gu [1] and LlNbO3:Cu [2] a new nonlinear optic phenomenon, called the parametric scattering of “holographic type”, has been observed. As in the case of well-known [3] wide-angel spicies of photoinduced scattering, this process was degenerate. But it occured under condition of phase-matching for four coupling waves: and hence had narrow spatial spectrum, where KL, KS1, KS2 are the wave vectors of the pump and two phase-conjugated scattered waves on fequency ω.
The investigation of spatial spectra, dynamics and statistics of parametric holographic scattering in photorefractive LiNbO3:Me (Me=Fe,Cu,Ti,Mg,Zn,Nd,Sn) crystals has been carried out.
We study experimentally the frictional motion of a one-dimensional array of cylinders. Random experimental fluctuations of the confining stress induce a complex behaviour of each cylinder. We show that the mean motion of the particles can be described as a random superposition of well-ordered rotational steady state patterns.
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