Optically addressed ferroelectric liquid crystal spatial light modulator

Abstract

We designed, fabricated, and characterized an optically addressed spatial light modulator. The choices of hydrogenated amorphous silicon (a-Si:H) as photosensor and ferroelectric liquid crystal (FLC) as electrooptic material provide the device with high speed and high resolution. An a-Si:H PIN photodiode and a FLC layer are sandwiched between two glass sheets coated with transparent conducting oxide (TCO). A write image beam from an Ar laser is absorbed in the photodiode resulting in an electric field across the FLC which replicates the pattern of the write beam. The image produced on the FLC is read through cross polarizers with a light beam from a He-Ne laser reflected from the interface between the FLC and the a-Si:H. The device is powered by a square-wave voltage applied to the two TCO layers. During the forward bias part of the cycle, the FLC is erased. Under reverse bias, the FLC is written and read. We have observed the performance of the spatial light modulator including response time, contrast ratio, and resolution. These are closely related to the characteristics of the a-Si:H layer. Therefore, we analyzed in detail the response of the a-Si:H to the frequency, applied voltage, and write light intensity. We found an increase in the contrast with reduced bias voltage, which is explained in terms of the effect of a series capacitance associated with FLC and a-Si:H.