| Popis: |
We have investigated optical switching and limiting by electrooptically controlled light scattering in a medium consisting of nematic liquid crystal (NLC) droplets embedded in a thin polymer matrix sandwiched between transparent electrodes. With no applied field, the medium is highly scattering. At sufficiently high voltages, the medium becomes optically clear. In our experiments, an unpolarized 0.5-mW He-Ne laser irradiates the composite film. A bias voltage is applied to select an initial film transmittance. The transmitted light is detected, amplified, and added to the bias voltage. With positive feedback, this device exhibits optical bistability. The hysteresis curves are characterized by very sharp thresholds for both on and off switching and a high contrast between these states. At higher bias voltages, the device exhibits self-pulsations, which we believe are related to noise-induced switching. The frequency of the self-pulsations decreases with increasing optical power. A distinct optical bounce appears in the switch oFF portion of the pulsations, which may be related to electrical polarization or NLC shear flow effects. When the device is configured with negative feedback, it functions as an optical power limiter. Limiting powers are functions of bias voltage and feedback gain. |
