I7 Non-linear optical effects and solitons in liquid crystals Kristiaan Neyts, Jeroen Beeckman, Pieter Vanbrabant 1 ELIS Department, Ghent University, Sint-Pietersnieuwstraat 41, 9000 Gent, Belgium, kneyts@elis.ugent.be Keywords: liquid crystals, non-linearities, solitons, waveguides 1. Introduction The success of liquid crystals is to a large extent related to the extremely low amounts of energy that are required to manipulate the optical properties. In electrically driven liquid crystal displays, the low operation voltage and current enable battery operation of large area high quality displays. For the same reason, liquid crystals are - compared to other materials - also very susceptible to light, leading to non-linear effects for relatively low light power. This makes liquid crystal an interesting material for use in devices based on all-optical signal handling. In nematic liquid crystals many different kinds of non-linearities can be distinguished. Increasing the intensity of a light beam may increase the temperature and change the optical properties, but this is only important if there is some absorbing species present (ITO electrodes or dye molecules). Typically a temperature increase leads to a reduction of the order parameter and the birefringence of the liquid crystal or the transition to the isotropic state. The geometry of the heat source and heat flow is very important in this case. The light beam may generate electrical charges in the (doped) liquid crystal or in the alignment layers, and these may have an important influence on the electric field and the director orientation in the liquid crystal. The electrical field of a light beam -just like the electric field that is applied over a liquid crystal device- exerts a torque on the liquid crystal molecules and can reorient the director. It turns out that the torque, director reorientation and light propagation can be described rather accurately, based on a simple set of equations. 2. Torque from light in an anisotropic material Light in an anisotropic medium exerts a torque on the anisotropic medium that tries to align the director n parallel to the electric field (for the case that ). The expression for the (time-averaged) torque is: r 0 e n n > ( )( ) 2 2 * 1 0 0 2 ( ) Re e n n nE n E τ ε ⎡ ⎤ = − ⋅ × ⎣ ⎦ r r r r r [1]. The ordinary wave ( ) does not influence the director, while the extra-ordinary wave tries to tilt the director away from the propagation vector E n ⊥ r r k r . In general the torque is determined by the intensity, the propagation direction and the polarization of the light beam, and involves the simultaneous action of both linearly polarized states. The existence of a torque on an anisotropic medium is illustrated by the Beth experiment, in which a half wavelength plate is rotated by an incident circularly polarized light beam. The energy for the rotation is delivered by a (small) red-shift of the light beam and the torque arises from a change in the angular momentum of the light (for example from − h to for every photon) [2]. h 20