The model of the rotational diffusion of the azo-dye molecules under the action of polarized uv light was used to explain the formation of the photoinduced order in azo-dye layers. We consider both the approximations of negligible and strong molecular interaction during the process of the reorientation under the field of a polarized light. We constructed an experimental setup, based on a photoelastic modulator, that allows accurate in situ measurements of the phase retardation ␦ of thin film as a function of the exposure time t exp and exposure power W ͑W/cm 2 ͒. A good agreement with experiment was observed. Fitting the experimental curves ␦͑t exp ͒ for different power values W, we can estimate the coefficient of rotational diffusion D, azo-dye order parameter S͑t exp ͒, and other parameters of the rotational diffusion model.

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Total solution of flexoelectric instability problem is given for finite thickness sample and real elastic moduli

A method of visualizing the 18O"-domain structure in TGS type crystals by means of nematic liquid crystals (NLC) is described. The possibility of visualization is due to the different orientation of NLC molecules on the domains with opposite orientation. The method enables the observation of static domain structure on cleaved surfaces of TGS and domain dynamics on polished and etched surfaces. The temperature and DC-field dependences of domain motion are similar to those previously revealed by indirect methods. The AC-dependences of domain nucleation and sideward motion of domain walls are shown to be quite different in TGS crystals grown in the paraelectric and ferroelectric phases. In the latter the existence of a row structure of centres of nucleation was observed which do not coincide with growth layers.