Bistability and thresholding by a new photoconductor-twisted nematic liquid crystal device with optical feedback.

A new design for obtaining optical bistability in two dimensions is reported in this paper. This design uses optical feedback on a photoconductor-twisted nematic liquid crystal (PC-TNLC) device with a new structure. The PC-TNLC device is essentially a 2-D array of optically controlled light values. The device structure with a new arrangement of the photoconductor, the liquid crystal film, and a patterned transparent electrode gives better control over its input-output (I-O) response, which is important in obtaining bistable operation. A theoretical analysis is carried out to determine the conditions under which bistability occurs. A sharp transition from the OFF to ON state in the I-O response suggests that the thresholding operation on gray scale input images can be performed. Experimentally, an 8 x 8 PC-TNLC device with optical feedback was demonstrated to have bistable behavior. The ON:OFF ratio of 50:1 was obtained in the output. This device was also used to perform thresholding operation on a gray scale image with eight levels. The observed value of gamma for the thresholding operation was 8. Such a bistable device was shown to have potential applications in digital optical processing.