Photo-isomerization fronts in dye-doped nematic liquid crystals

Abstract

An experimental study of the photo-isomerization dynamics in dye-doped nematic crystals is reported, which shows that, when the sample is illuminated by a Gaussian beam, and for high enough input power, a transition from the nematic to the isotropic phase takes place in the illuminated area. The two phases are spatially connected via a front propagating outward from the center of the beam and following the local intensity profile and thus inducing a photo-controlled optical aperture. The optical intensity and temperature fields on the sample follow the same dynamical profile. The front dynamics is described by a phenomenological bi-stable model with an inhomogeneous control parameter, directly related to the beam intensity profile.

Dissipative structures induced by photoisomerization in a dye-doped nematic liquid crystal layer

“…The previous dynamical behaviour can be described universally by means of a bistable model with inhomogeneous parameters [17,18]. In order to reconcile our description with this universal one, we consider the limit for which the light beam varies slowly (w 1) and the decay rate related to the transition from cis to trans state is much faster than the variation rate of the order parameter (λ A).…”

“…The nature of the nematic-isotropic transition is of first-order type, and the system exhibits a region of coexistence between two stable states [17]. Figure 3a shows the experimental bifurcation…”

“…where the bifurcation parameter A + C 0 (x, y, I 0 , w) is controlled by the light intensity. By means of a simple variable translation with the objective of eliminating the quadratic term, one retrieves the universal model used to describe the front propagation in inhomogeneous media [17,18]. Note that by means of pulsed light, one can induce molecular disorder and molecular reorientation of pure liquid crystals [21].…”

Optical wall dynamics induced by coexistence of monostable and bistable spatial regions

“…Inspired by interface dynamics reported in optical experiments [34][35][36][37], where two spatially inhomogeneous states coexist, we consider a phenomenological one-dimensional stochastic bistable model with a diffusion effect and inhomogeneous media [34][35][36][37]…”

“…In the liquid crystal light valve with optical feedback [34,35], u(x,t) accounts for molecular orientation around an equilibrium state, while η is associated with the intensity of the optical feedback. In the nematic-isotropic transition induced by photoisomerization [37], u(x,t) stands for molecular order, where u ≈ 1 and u ≈ −1, respectively, accounts for the nematic and the isotropic phase, while η accounts for the injection field amplitude. Notice that for constant η, Eq.…”