I. Jánossy

Anomalous Optical Freedericksz Transition in an Absorbing Liquid Crystal

Abstract Laser-induced reorientation was studied in absorbing nematic films. The optical Freedericksz threshold occured at an intensity level of 50 W/cm2 in contrast to the normally observed few KW/cm2 value for transparent layers. Thermomechanical coupling is considered as a possible explanation of the observed anomaly.

Low-Power Optical Reorientation in Dyed Nematics

Abstract The optical reorientation process in nematic guest-host mixtures is investigated. It is found that for certain dyes the optical Freedericksz threshold is two orders of magnitude lower as in the pure host. The observations can be interpreted in terms of an enhanced optical torque which increases with the dye concentration.

Optical reorientation in dye-doped liquid crystals

The light-induced reorientation of nematic liquid crystals is discussed with special emphasis on the effect of dye-dopants. A molecular model is presented which accounts for the large increase of the optical torque, observed in absorbing nematics. An analogous process is described for dye-doped isotropic liquids and the role of photoisomerization is considered as well.

Photoisomerization of azo-dyes in nematic liquid crystals

The photoisomerization of azo dyes in nematic liquid crystals is investigated. Using a pump-probe technique, the absorption coefficients of the trans and cis isomers are evaluated separately and th...

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 μm thickness, in the nematic phase. At a homeopolar cell of MBBA (150 μm thickness), at normal incidence a threshold laser power of 45 m W 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.

Light-induced spontaneous pattern formation in nematic liquid crystal cells

Optically induced instabilities were observed in nematic liquid crystals sandwiched between a photosensitive layer and a reference plate. The instabilities occurred when the light entered the cell from the reference plate and its polarization was parallel to the director alignment at the entrance face. Two kinds of patterns were detected, a static and a dynamic one. The underlying mechanism of pattern formation is the coupling between the director orientation on the photosensitive plate and the polarization direction of the light on that plate.

Control of the orientational nonlinearity through photoisomerization in dye doped nematics

Photoinduced reorientation in azo dye doped liquid crystals (NLCs) is investigated with consideration of photoisomerization. A model that describes photoinduced reorientation in the presence of photoisomerization [1] is verified in different experimental ways, extending previous investigations. We show that both the magnitude and the sign of the optical nonlinearity for an extraordinary beam can be controlled by an ordinarily polarized beam. Such a control of the nonlinearity provides a new functionality for optical switching and optically addressable spatial light modulators.

Phase diagrams and electro-optical properties of novel ferroelectric mixtures

Abstract Two binary mixtures were prepared, based on three members of the homologous series of 4-n-alkoxyphenyl esters of the 4′-n-alkoxybenzoic acids. Three structurally different chiral compounds were used to make novel ternary and four-component mixtures with chiral smectic C phase. The phase diagrams of two-component, the ternary and four-component mixtures are presented and the corresponding physical and electro-optical properties such as spontaneous polarization, tilt angle and switching time are discussed. Spontaneous polarization values up to 200nC/cm2 were found and two distinct modes of electro-optical switching, associated with different stages of helix distortion, were observed.

Reorientation of Liquid Crystals by Superposed Optical and Quasistatic Electric Fields

The electric field of a CW laser beam can reorient a nematic liquid crystal. Experiments on the influence of a superposed quasistatic electric field are reported showing that this can reinforce or weaken the laser induced reorientation depending on the substance and geometry.

Laser-induced instabilities in liquid crystal cells with a photosensitive substrate.

Liquid crystal layers sandwiched between a reference plate and a photosensitive substrate were investigated. We focused on the reverse geometry, where the cell was illuminated by a laser beam from the reference side. In planar cells both static and dynamic instabilities occurred, depending on the angle between the laser polarization and the director orientation on the reference plate. In cells where the molecules were aligned along the normal of the reference plate, a dynamic pattern was observed at all angles of polarization. A simple model based on a photoinduced surface torque accounts for the findings. Light scattering studies revealed some basic properties of the instabilities.