Photoinduced reordering in thin azo-dye films and light-induced reorientation dynamics of the nematic 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 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 the liquid-crystal easy axis on photoaligned azo-dye substrates.

Electric and magnetic fields in photoalignment of liquid crystals

“…The obtained value of the relaxation time (τ e = 17.5 min) is typical for lightinduced gliding. 8 The measurements of the polarized light transmission trough the planar cell was also used to define the angle α at different doses D p after the fixed duration of an exposure. The obtained values at t exp = 60 min are equal to 36.6°, 34.5°, and 26.7°for doses D p = 0.35, 0.7, 1.05 J/ cm 2 , respectively.…”

“…At the same time, the phenomenological model fails to describe such experimentally registered phenomena as local ordering and disordering of surface orientation because of light-induced gliding assisted by electric fields. The existing microscopic approach, based on the Fokker-Planck equation, 8 turned out to be useful only for light-induced gliding. So, the additional experimental and theoretical investigations of gliding phenomena under combined action of light and electromagnetic fields are of interest both for basic science and applications.…”

Modeling Reorientation Dynamics of Electrically Assisted Light-Induced Gliding of Nematic Liquid-Crystal Easy Axis

“…In the PA method, the easy axis is determined by the polarization azimuth of the pumping LPUV light, whereas the azimuthal and polar anchoring strengths may depend on a number of governing parameters such as the wavelength and the irradiation dose [8,9].…”

“…So, in an NLC cell with the initially irradiated layer, subsequent illumination with reorienting light which polarization differs from the one used to prepare the layer can trigger the light-induced reorientation of the easy axis which is governed by the photoinduced reordering of azo-dye molecules [9] and may be of considerable interest for applications such as LC rewritable devices [10]. Note that slow reorientation of the easy axis-the so-called electric-field-induced easy axis gliding-on the photosensitive layers prepared using the PA technique was originally observed on poly-(vinyl)-alcohol (PVA) coatings with embedded azo-dye molecules [11].…”

“…In previous studies [13,14], the reorienting light was linearly polarized along the initial easy axis. The theoretical considerations of [9] are based on the diffusion model describing the light-induced reordering in azo-dye films as rotational Brownian motion of azo-dye molecules governed by the lightintensity dependent mean-field potential. These thus deal with the case of the purely light-induced reorientation.…”