  We theoretically study the kinetics of photoinduced reordering triggered by
linearly polarized (LP) reorienting light in thin azo-dye films that were
initially illuminated with LP ultraviolet (UV) pumping beam. The process of
reordering is treated as a rotational diffusion of molecules in the light
intensity-dependent mean-field potential. The two dimensional diffusion model
which is based on the free energy rotational Fokker-Planck equation and
describes the regime of in-plane reorientation is generalized to analyze the
dynamics of the azo-dye order parameter tensor at varying polarization azimuth
of the reorienting light. It is found that, in the photosteady state, the
intensity of LP reorienting light determines the scalar order parameter (the
largest eigenvalue of the order parameter tensor), whereas the steady state
orientation of the corresponding eigenvector (the in-plane principal axis)
depends solely on the polarization azimuth. We show that, under certain
conditions, reorientation takes place only if the reorienting light intensity
exceeds its critical value. Such threshold behavior is predicted to occur in
the bistability region provided that the initial principal axis lies in the
polarization plane of reorienting light. The model is used to interpret the
experimental data on the light-induced azimuthal gliding of liquid-crystal easy
axis on photoaligned azo-dye substrates.
