Active propagation and cut-off for low TM modes in a nonlinear nematic waveguide

Liquid crystals are optically active media whose properties may strongly va ry when they an subjected to magnetic or electric fields. This high suscept ibility can have a profound influence on the propagation of light through t he fluid and gives rise to highly nonlinear coupling between the optical fi eld and the reorientation. In the present work, the influence of the reorie ntation of the crystal on the propagation of the TM optical modes when this reorientation is due to the propagating optical field is studied. Furtherm ore, only the TM optical modes which couple to the reorientation are consid ered. The orientation of the nematic in the waveguide is taken to be such t hat the frequency of impinging light of low amplitude is below the cut-off frequency. However, when the impinging light wave is of sufficiently high a mplitude, reorientation occurs, so that the cut-off frequency is lowered an d the mode propagates through the crystal. A front of reorientation then pr opagates into the waveguide, which in turn makes the crystal transparent. S implified equations governing this process are derived from the full equati ons for the optical field and the reorientation by noting the different tim e and space scales involved. These simplified equations are studied qualita tively and the induced transparency phenomenon is thus explained in terms o f the bistable behaviour of the reorientation equation, this bistability be ing induced by the incident wave. The simplified equations are also solved numerically for a range of parameter values susceptible to experimental ver ification. The paper is concluded by a discussion of the advantages and lim itations of the model and its possible application to other situations. (C) 2000 Elsevier Science B.V. All rights reserved.