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Electronics and Communications in Japan (Part II Electronics)

Hybrid optical bistability in ferroelectric liquid crystal and its dynamic characteristics

Akira Tagawa, Hiroshi Moritake, Masanori Ozaki, Katsumi Yoshino

Published Jan 1, 1992• 2 citations
Publisher

Abstract

Abstract In the area of optical logic and optical information processing, such nonlinear optical devices as optical Instability, optical amplifier, and optical limiter are important basic elements. In this paper, the hybrid optical bistability and optical limiter characteristics are realized by means of an electrical linear feedback to a ferroelectric liquid crystal cell (SSFLC cell). If the feedback gain is positive, the optical bistability is obtained while the optical limiter characteristics are obtained if the gain is negative. Also, it was demonstrated that by instantaneously increasing or cutting the incident optical intensity in a pulsed state in the optical bistability region, the transmission light intensity can be switched from the low‐transmission state to the high‐transmission state or from the high‐ to low‐transmission state. The minimum pulsewidth needed for switching is 70 μs for the low‐transmission state to the high‐transmission state and 50 μs for the high‐transmission state to the low‐transmission state. Since the speed is very fast, the high‐speed response of the ferroelectric liquid crystal is used effectively.

DOI
10.1002/ecjb.4420750604

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Optical Logic Element Using Ferroelectric Liquid Crystal with Electrical Feedback

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Ferroelectrics•Apr 1, 1989•5 citations
Abstract In the area of optical logic and optical information processing, such nonlinear optical devices as optical Instability, optical amplifier, and optical limiter are important basic elements. In this paper, the hybrid optical bistability and optical limiter characteristics are realized by means of an electrical linear feedback to a ferroelectric liquid crystal cell (SSFLC cell). If the feedback gain is positive, the optical bistability is obtained while the optical limiter characteristics are obtained if the gain is negative. Also, it was demonstrated that by instantaneously increasing or cutting the incident optical intensity in a pulsed state in the optical bistability region, the transmission light intensity can be switched from the low‐transmission state to the high‐transmission state or from the high‐ to low‐transmission state. The minimum pulsewidth needed for switching is 70 μs for the low‐transmission state to the high‐transmission state and 50 μs for the high‐transmission state to the low‐transmission state. Since the speed is very fast, the high‐speed response of the ferroelectric liquid crystal is used effectively.
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.
In the last chapter, we examined the effect of cubic nonlinearity on selfrefraction of an optical beam. We have also seen that the presence of a quadratic nonlinearity can also contribute to change in the profile of a Gaussian beam in a crystal. In this chapter we will see how the effect of nonlinearity, alongwith the proper feedback, can give rise to optical bistability and hysteresis. This is not unexpected as hysteresis and bistability are also observed in nonlinear electronic circuits with feedback, such as in the Schmitt trigger, as well as in hybrid optical devices, such as an acoustooptic device with feedback (Banerjee and Poon, 1991). In what follows, we will examine two different types of optically bistable devices based on optical nonlinearities. The first is a nonlinear ring resonator comprising a two-level gain medium as the nonlinear element. We will treat both absorptive and dispersive bistability. Absorptive bistability is the case when the incident optical frequency is close to or equal to the transition frequency of the atoms from one level to another. In this case, the absorption coefficient becomes a nonlinear function of the incident intensity. On the other hand, if the frequencies are far apart, the medium behaves like a Kerr-type material and the system exhibits what is called dispersive bistability. In this case the material can be modeled by an effective n2 or v (3), meaning that the refractive index becomes a nonlinear function of the optical intensity. The second configurationwewill examine is a linear-nonlinear interface. In this case, the interface provides the feedback due to refractive index mismatch. In fact, the reflection coefficient (as well as the transmission coefficient) is modified due to the nonlinearity, which, as we shall see later, is responsible for demonstrating optical switching and bistability.
Ferroelectric oxides with perovskite structure such as PbLaTiO3 (PLT), BaTiO3, PbTiO3, SrBi2Ta2O9 and LiNbO3 are very attractive class of materials which possess numerous useful properties such as high dielectric constant, large spontaneous polarization, and remarkable optical nonlinearity. Potential applications of these materials include real-time holography, correlation filtering and various novelty filter applications (Sutherland 1996 and Eaton 1991). They are also popular materials for the fabrication of nonvolatile memories (Ramesh 2001). Over the past few years, ferroelectric oxides have been widely investigated for various nonlinear optical applications (Shi 2006; Xuan 1998; Zhang 1999; Zhao 1996) especially for optical switches. Optical switches are devices invented to perform multiplexing at very fast speeds and with less delay than the customary switches works with electronic signals. A ferroelectric optical switch is expected to allow the processing of millions of signals at a speed of terahertz. To obtain optical bistability phenomenon, two ingredients are necessary, a nonlinear process and a feedback mechanism (Gibbs, 1977, 1979, and 1985). In all optical systems, the feedback can be “distributed”, “extrinsic” or “intrinsic”. In multilayer systems with alternating nonlinear materials, the feedback is “distributed”; it arises from the interaction of the propagating wave with many cross-sections of a nonlinear medium. In a Fabry-Perot (FP) resonator, the feedback is “extrinsic”; it arises as a result of reflection from the mirrors placed at its interface. In a single nonlinear layer, the feedback can be “intrinsic” or “mirrorless”; it arises in each elementary oscillator due to the strong local nonlinear response of an individual atom or molecule. In the usual or the standard analysis to study the optical bistability (Marburger 1978, Gupta 1987, Biran 1990, Danckaert 1989, Shen 1984, and Haelterman 1989) in nonlinear optics, the governing equation for optical propagation within the nonlinear medium is a nonlinear wave equation in the electric field derived from Maxwell’s equations. The usual constitutive relation between the nonlinear polarization and the electric field is then obtained by expanding the nonlinear polarization as a Taylor series in the electric field. The usefulness of this constitutive relation is that the polarization is a natural source term in the Maxwell’s
When the output parameter of an optical device could be a multivalued function of the input parameter, this device can be working in an optical bistable status. The most typical optical bistable device is a Fabry–Perot cavity containing a nonlinear optical medium, the refractive index of which depends on the light intensity. Upon the action of an intense input optical beam, the transmitted (or reflected) intensity of this device may exhibit a nonlinear response to the incident light intensity variation and, especially under appropriate working conditions, the characteristic output/input curve may manifest a single (or multiple) hysteresis loop(s). Such type of optical bistable device can be used as an optical differential amplifier, optical switch, optical limiter, optical clipper, optical discriminator, or an optical memory element, depending on the chosen nonlinear medium and operation conditions.
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.