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We theoretically study the kinetics of photoinduced reordering triggered bylinearly polarized (LP) reorienting light in thin azo-dye films that wereinitially illuminated with LP ultraviolet (UV) pumping beam. The process ofreordering is treated as a rotational diffusion of molecules in the lightintensity-dependent mean-field potential. The two dimensional diffusion modelwhich is based on the free energy rotational Fokker-Planck equation anddescribes the regime of in-plane reorientation is generalized to analyze thedynamics of the azo-dye order parameter tensor at varying polarization azimuthof the reorienting light. It is found that, in the photosteady state, theintensity of LP reorienting light determines the scalar order parameter (thelargest eigenvalue of the order parameter tensor), whereas the steady stateorientation of the corresponding eigenvector (the in-plane principal axis)depends solely on the polarization azimuth. We show that, under certainconditions, reorientation takes place only if the reorienting light intensityexceeds its critical value. Such threshold behavior is predicted to occur inthe bistability region provided that the initial principal axis lies in thepolarization plane of reorienting light. The model is used to interpret theexperimental data on the light-induced azimuthal gliding of liquid-crystal easyaxis on photoaligned azo-dye substrates.

Photoinduced reordering in thin azo-dye films and light-induced reorientation dynamics of nematic liquid-crystal easy axis