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Ferroelectrics

Optical bistability of a nematic liquid crystal with photorefractively amplified feedback

M. Ingold, J. Keller, Gilles Pauliat, Peter Günter

Published Apr 1, 1989• 5 citations
Publisher

Abstract

Optical bistability has been observed in a new configuration consisting of a nematic liquid crystal (ROTN 403) and a photorefractive crystal (KNbO3). The nematic liquid crystal (NLC), placed between crossed polarizers, serves as an optically controlled gate. The transmitted signal beam is amplified by photorefractive two-wave mixing and sent back onto the NLC in such a way that it changes the transmission of the gate. Bistability of the optical signal in the feedback loop with a large hysteresis is found. The bistable hysteresis curve depends on the nonlinearities of the NLC and of the photorefractive crystal, on the intensity of the storage beam and on the losses in the feedback loop.

DOI
10.1080/00150198908211336

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Optical bistability has been observed in a new configuration consisting of a nematic liquid crystal (ROTN 403) and a photorefractive crystal (KNbO3). The nematic liquid crystal (NLC), placed between crossed polarizers, serves as an optically controlled gate. The transmitted signal beam is amplified by photorefractive two-wave mixing and sent back onto the NLC in such a way that it changes the transmission of the gate. Bistability of the optical signal in the feedback loop with a large hysteresis is found. The bistable hysteresis curve depends on the nonlinearities of the NLC and of the photorefractive crystal, on the intensity of the storage beam and on the losses in the feedback loop.
Several configurations for the optical bistability of a nematic liquid crystal in a nonresonant cavity are investigated. These bistabilities result from optical feedback and occur below the Freedericksz transition. An electric-field-induced optical switching with hysteresis is observed as well. Photorefractive two-wave mixing in a KNbO3 crystal is used for amplification of the feedback beam, leading to an optical hysteresis with the pump beam of the photorefractive amplifier as the control beam. In this configuration competition between two beams injected into the nonresonant cavity (optical thresholding) is demonstrated: The stronger beam is identified and discriminated from the weaker one by bistable switching.
The properties of intrinsic optical bistability in nematic liquid crystals were studied. This kind of optical bistability is without resonant optical cavity and electric feedback. We find that the nematic ROTN-200 film can exhibit bistability when applying a constant ac electric field (≊560 kHz) that is greater than a tricritical voltage VTc=2.614 V. The bistable interval is measured and the hysteresis loop is characterized at each different voltage. The splay and bent elastic constants k11 and k33 are obtained to calculate the numerical result. It is found that the experimental results are in good agreement with theory. The experimental setup with which we can easily observe the bistable phenomena is illustrated. Under the consideration of the finite size of the laser beam, the thermal effect and finite beam size effect are discussed. In addition, some new interesting phenomena about the diffraction ring structure of nematic liquid crystal are reported.
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.