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Annual Meeting Optical Society of America

Theory and experiment on the nonlocal radial dependence of laser-induced molecular reorientation in a nematic film

I. C. Khoo, Ping Yan, T. H. Liu, G. M. Finn, R. R. Michael

Published Jan 1, 1986• 0 citations
Publisher

Abstract

We present a quantitative theoretical and experimental study of the transverse dependence of nematic liquid crystal axis reorientation by a laser beam of beam sizes that are comparable to the liquid crystal film thickness. As a result of the interplay between the boundary torques and the torques exerted by the molecules outside the laser beam on the molecules within the laser beam, interesting nonlocal effects arise. The natures of the torque balance equations and the solutions for the reorientation profile are very different for laser polarizations that are normal, or obliquely inclined, to the director axis. If the laser polarization is perpendicular to the director axis there is a threshold intensity for reorientation to occur. The relative width of the reorientation profile (with respect to the laser beam waist) can be larger (or smaller) than unity depending on whether the laser beam size is smaller (or larger) than the film thickness. On the other hand, if the beam polarization is not normal to the director axis, the width of the response is always larger than the laser beam waist. These nonlocal behaviors have profound effects on transverse optical bistability switching behaviors and switching intensities.

DOI
10.1364/oam.1986.mcc10

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We present a quantitative theoretical and experimental study of the transverse dependence of nematic liquid crystal axis reorientation by a laser beam of beam sizes that are comparable to the liquid crystal film thickness. As a result of the interplay between the boundary torques and the torques exerted by the molecules outside the laser beam on the molecules within the laser beam, interesting nonlocal effects arise. The natures of the torque balance equations and the solutions for the reorientation profile are very different for laser polarizations that are normal, or obliquely inclined, to the director axis. If the laser polarization is perpendicular to the director axis there is a threshold intensity for reorientation to occur. The relative width of the reorientation profile (with respect to the laser beam waist) can be larger (or smaller) than unity depending on whether the laser beam size is smaller (or larger) than the film thickness. On the other hand, if the beam polarization is not normal to the director axis, the width of the response is always larger than the laser beam waist. These nonlocal behaviors have profound effects on transverse optical bistability switching behaviors and switching intensities.
Abstract Molecular reorientation and laser heating induced by an optical field can yield significant changes in the refractive indices in a nematic liquid crystal. A c.w. laser beam is intense enough to induce a phase retardation much larger than 2pi in a nematic film less than 100 pm thick. Optical bistability in such a film sandwiched between mirrors can be readily observed. Coupling between the two mechanisms for induced refractive indices can lead to interesting results in the bistable operation.
Recently, a new class of optical bistability, involving transverse self-phase modulation effect with a single reflection feedback, has been demonstrated. We present here a detailed theoretical calculation of the effect of saturation and nonlocality on the switching behavior and power. The technique employed is similar to that given in a recent paper but generalized to the case where the nonlinear refractive-index change contains a saturation parameter. It is found that both the switching powers and the hysteresis loops are considerably changed. Multiple close-by switching loops occur. Because of the extreme sensitivity of the transverse bistability switching on the width of the transverse nonlinear phase shift, we have also investigated the effect due to nonlocal response of the thin film subjected to a Gaussian laser beam. Specific detailed calculations were performed for nonlinear thin film where such nonlocal behaviors are expected (e.g., liquid crystal reorientation and diffusive processes in semiconductors). The most prominent result is that a more accurate prediction of the switching intensity is obtained if the nonlocal effects are included.
We have studied theoretically and experimentally the nonlinear Fabry-Perot transmission of a cavity with a thin nematic liquid-crystal film. Various modes of operation have been observed, using a moderate-power cw laser. In the limit of small reorientation angles, the optically induced molecular reorientation of the birefringent crystal and the nonlinear phase shift is solved for a homeotropically aligned crystal. Multiple-valued Fabry-Perot transmission is obtained by solving the nonlinear equations that couple the optical phase shift with the cavity transmission. The time-independent nature of the process and the explicit quantitative expressions involving the nematic liquid-crystal parameters allow a direct comparison between the thoery and experimental results. Good quantitative agreement is demonstrated.
A theory is presented for the transverse intensity distribution bistability of a Gaussian optical beam after its passage through a nonlinear thin film. The equations governing the intensity distribution are cast in the form analogous to optical bistability in a longitudinal cavity (a Fabry-Perot interferometer), i.e., into two coupled transcendental equations from which multiple solutions are obtained. This formalism allows one to examine various physical approximations in obtaining the equations, and to improve on these approximations. It also lucidly illustrates the mechanisms of transverse intensity distribution bistability. The theoretical predictions are verified with quantitative experimental results on thin films of nematic liquid crystals.
Light-controlling-light is one of the most advanced paradigms in optical signal processing, including light-induced waveguides as well as all-optical switching and routing. Other fundamental aspects of all-optical processing are optical memories and sequential elements, which require responses depending on the evolution history of the system, such as the hysteresis stemming from optical multistability. Hereby, we report on optical bistability and hysteresis in cavityless geometries and in the presence of self-localized beams, i.e., spatial optical solitons, exploiting the nonlocal reorientational nonlinearity of nematic liquid crystals (NLC). When the optic axis of NLC is initially orthogonal to the applied electric field, the molecular dipoles start to rotate above a threshold named after the Fréedericksz transition, the latter being usually of the second order. Here, we show that such transition become of the first order via the intrinsic feedback provided by self-focusing, in turn leading to the appearance of a hysteresis loop between diffracting and self-confined beams. We report on hysteresis of the beam size versus input power, as well as hysteresis versus applied voltage at a fixed beam power. Our findings introduce a novel kind of cavity-less optical bistability with propagating light beams and disclose a novel approach to information storage based on light self-localization.