Laser Induced Reorientation of Nematic Liquid Crystals

Abstract

A significant increase of the divergence and appearance of a fringe system has been observed by illuminating of MBBA (p-n-methoxybenzilidene-p-butylaniline) and OCB (octyl-cyano-biphenyl) nematic liquid crystalline samples with a collimated beam of an argon ion laser. The dependence of this effect on laser power. beam polarization, and angle of incidence has been studied in homeopolar and planar sandwich-like cells of 50-150 pm thickness, in the nematic phase. At a homeopolar cell of MBBA (150 pm thickness), at normal incidence a threshold laser power of 45 mW was found. The phenomena can be explained as deformation of the orientation by the Fredericks effect due to light fields. The observations are in good agreement with the theoretical predictions including an estimation of the laser power threshold.

The Influence of the Finite Size of the Light Spot on the Laser Induced Reorientation of Liquid Crystals

“…As it was reported by us previously [1], by illuminating a nematic liquid crystalline film with a focused laser beam, an increase in the beam divergence and a ring system appearing in it can be detected. This effect could be explained as an orientational deformation due to a Fredericks transition in optical fields.…”

“…The factor ne /nQ comes from the coupling between the light propagation and the deformation field. The importance of this coupling has been emphasized in a previous publication [1].…”

“…experiment a homeotrop nematic MBBA cell of 150 pm thickness was il luminated with an argon ion laser beam of 65 pm diameter /FWHM/ [1].At 21,5 °C we got for the threshold 45 + 10 mW, while the theoretically expected value was 35 mW, which we found to be satisfactory, taking into account the simplifying assumptions and experimental uncertainities.In the second, more accurate and detailed investigation a homeotropic OCB sandwich-like cell of 150 pm thickness was studied. The aim of this work was to investigate the dependence of the threshold laser power first on the spot size of the focused larer beam in the middle of the nematic temperature range /36,5 °C/ and then its dependence on temperature in the nematic range as well as its dependence on the wavelength in the visible region.The experimental setup was similar to that described earlier[1].The linearly polarized beam of an argon ion laser, oscillating at 515 nm in the fundamental transversal /ТЕМ / mode, was focused with lenses of oo different focal lengths to a small spot onto the NLC sample. The polariza tion plane of the incoming beam could be changed with the aid of a po larization rotator /Spectra Physics Model 310-21/.…”

Laser-induced optical anisotropy in self-supporting amorphous GeSe2films

“…It is interesting to note that this type of reorientation which passes through an isotropic state is contrary to the laser induced orientation ob served in liquid crystals (Csillag et al 1982). As we have seen the x and у axes remain the principal axes during the reorientation process, consequently it is sufficient to use the quantity Дп = пх~Пу for describing the kinetics.…”

Introduction

“…A systematic description of the problern was first given by Csillag et al 30 They pointed out that in the adiabatic approximation of light propagation reorientation cannot be caused by an o-polarized input. In the o-ray the electric field is everywhere perpendicular to the director, hence the optical torque is zero (Eq.…”