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Optical memory with twisted nematic liquid crystal (TNLC) devices

José Antonio Martín Pereda, Miguel A. Muriel

Published Jan 1, 1983• 4 citations
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Abstract

A design for obtaining memory in optical bistability with liquid crystals is reported. This design uses optical feedback on a twisted nematie liquid crystal ( TNLC ) through an optoelectronic system. A constant input light is the read-out and its value depends on the desired initial working point, usually at the bottom of the T(V) vs. V curve. Light levels depend on the feedback. An input light pulse change the working point to the top of the transmission curve. When this pulse vanishes, the working point remains at the upper part of the curve. Hence a memory function is obtained. Minimum pulse width needed was 1msec. ON-OPF ratio was 100:3.

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Optical memory with TNLC devices (1982)

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IEEE Journal of Quantum Electronics•Dec 21, 2016•1 citation
A design for obtaining memory in optical bistability with liquid crystals is reported. This design uses optical feedback on a twisted nematie liquid crystal ( TNLC ) through an optoelectronic system. A constant input light is the read-out and its value depends on the desired initial working point, usually at the bottom of the T(V) vs. V curve. Light levels depend on the feedback.
An input light pulse change the working point to the top of the transmission curve. When this pulse vanishes, the working point remains at the upper part of the curve. Hence a memory function is obtained. Minimum pulse width needed was 1msec. ON-OPF ratio was 100:3.
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.
Optical bistability in surface-stabilized ferroelectric liquid crystal (SSFLC) with an electrical feedback has been studied. The hysteresis in the transmission-voltage curve which is characteristic of SSFLC is found to contribute to the expansion of the optical bistable region. Dynamic characteristics of the switching between optical bistable states by an incident light pulse have also been studied. A new multilogic element with both OR and AND functions which can be selected by controlling the duration of the reset light pulse can be realized.
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.