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

TL;DR

This paper presents a theoretical model for photoinduced reordering in azo-dye films, analyzing how polarized light influences molecular orientation and liquid crystal easy axis reorientation, with predictions matching experimental observations.

Contribution

It introduces a generalized rotational diffusion model for azo-dye reordering, accounting for polarization azimuth and threshold effects, advancing understanding of light-induced liquid crystal alignment.

Findings

Reorientation depends on light intensity and polarization azimuth.

A threshold intensity is necessary for reorientation under certain conditions.

The model explains experimental azimuthal gliding of liquid crystal easy axis.

Abstract

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.