Optical power limiting and bistability in polymer-dispersed liquid-crystal films with linear feedback

Abstract

Polymer-dispersed liquid-crystal films are composite materials consisting of a liquid crystal dispersed in the form of micron-sized droplets in a polymer binder. External fields may be used to control the alignment of the liquid crystal in the inclusions. If the refractive indices of the polymer and the liquid crystal are appropriately matched, the film may be switched from an opaque scattering state to a clear transparent state by externally applied fields. We have studied the behavior of polymer-dispersed liquid-crystal films in feedback systems where the applied voltage depends on the intensity of light transmitted by the film. Linear feedback of the photodetector voltage gives rise to an optical transmittance which is a nonlinear function of incident power. This results in optical power limiting behavior in the case of negative feedback, and in optical bistability when the feedback is positive.