We observed the intrinsic optical bistabily, multistability in the Fabry - Perot interference filter and also the hybrid optical bistability and multistability in the 90℃ twisted nematic liquid crystal. In ZnS Fabry - Perot interference filter, when the initial detunning was about full width at half maximum band width, the switch-on power and time were measured to be 120 mW and 7 msec, respectively. $Ar^+$-laser with λ = 5145Å was used.
The beam was focused onto the filter by using the lens whose focal length is 15cm. In order to remove the switch - on power drift, the filter was covered with a slide glass using optical epoxy, where we observed the optical multistability.
To find the origin of multi - stability, experiment were performed with Fabry-Perot interference filter with optical epoxy spacer, and the nonlinear transmission in the epoxy filter was found. The ZnS interference filter in this experiment was made to have a structure of $(HL)^4(H^1H^1)^4(LH)^4S$, where $H^1$, H represent quarter wave layer of ZnS and $PbCl_2$, respectively. And S represents slide glass substrate.
The filter had full width at half maximum band width 4.5nm and peak transmission of about 50% at 518.5nm for normal incident beam. The beam spot diameter on the filter was measured to be 27㎛ by using a knife edge scanning method. The twisted nematic liquid crystal had 30㎛ thickness and 90° twist angle. For the various angle between the electrode surface liquid crystal orientation and incident beam polarization, we observed the various transmission curves of twisted nematic liquid crystal about applied voltage. At this experiment, for 0°,25°,45°,75° angles, we observed the transmission curves and hybrid optical bistability, multistability at each curves. Switch-on power and time were about 10 mW, 100msec, respectively.
