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Photorefractive Effects, Materials, and Devices

Optical Bistability of a Nematic Liquid Crystal with Photorefractively Amplified Feedback

M. Ingold, K. Totland, Gilles Pauliat, Peter Günter, M. Schadt

Published Jan 1, 1990• 0 citations
Publisher

Abstract

Light induced changes of the refractive index in a nematic liquid crystal [1] and in a photorefractive crystal (KNbO3) [2] have been used to design an optically bistable device [3]. In the nematic liquid crystal (NLC) light can reorient biréfringent molecules. Therefore the NLC, placed between crossed polarizers, acts as an optically controlled gate. In the KNbO3 crystal photorefractive two-beam coupling is used for light amplification.

DOI
10.1364/pmed.1990.e6

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Light induced changes of the refractive index in a nematic liquid crystal [1] and in a photorefractive crystal (KNbO3) [2] have been used to design an optically bistable device [3]. In the nematic liquid crystal (NLC) light can reorient biréfringent molecules. Therefore the NLC, placed between crossed polarizers, acts as an optically controlled gate. In the KNbO3 crystal photorefractive two-beam coupling is used for light amplification.
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.
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.
It is known that two-wave coupling in photorefractive crystal may result in an intensity-dependent phase shift as well as energy transfer between the two waves without phase crosstalk. In a sense, this kind of phase shift is similar to the intensity-dependent refractive-index effects in Kerr media. Consequently, optical bistability can be established in an optical cavity with a photorefractive crystal. We formulate the model and get the expected bistable transmission with numerical simulations. Several cases, including that of the codirectional coupling and the contradirectional coupling, are considered. The effects of the photorefractive crystal’s distribution absorption and the spatial phase shift between the index grating and the light interference pattern in the crystal on the nonlinear transmission characteristics have also been studied. It is interesting to note that the output of the pumping wave is dependent on the cavity intensity and thus shows bistable behavior with respect to the cavity input as well.
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.