A Infrared-Sensitive Optically Addressed Spatial Light Modulator

Optically addressed spatial light modulators (OASLMs) find many applications in optical image processing and optical computing. In particular, correction for atmosphere turbulence in a free-space laser communication system demands high performance OASLMs that are sensitive at a wavelength of 1.55-mum. The development of such an OASLM is described in this thesis. Also investigated were effects of charge spreading on the resolution of OASLMs. A metal-insulator-semiconductor (MIS) model was developed first for better understanding the operation of OASLMs having a sandwich structure. As an example, the model was applied to OASLMs consisting of hydrogenated amorphous silicon (a-Si:H) photosensors. Based on the simulation with the MIS model, an OASLM consisting of a ferroelectric liquid crystal layer was fabricated with an InGaAs/InP pin photodiode, rather than a simple intrinsic InGaAs layer. InGaAs/InP was chosen as the photosensor material because of its low leakage current and high sensitivity at 1.55- μm wavelength. The OASLM was demonstrated at frame rates up to a few kilohertz, and had a sensitivity of ~1 mW/cm ^2 to 1.55-μm write light. However, the OASLM had a small contrast ratio and a very poor resolution. Its flatness also needs to be improved for practical applications. A transient charge transport model was developed to investigate resolution-degrading mechanisms associated with charge spreading in the photosensor bulk and at the interface between the photosensor and the liquid crystal. It was found that effects of bulk drift and diffusion are largely independent of the carrier mobility, but are inversely proportional to the thickness of the semiconductor photosensor. Charge spreading at the interface is often the most significant resolution-degrading mechanism in OASLMs. Low mobility and a short trapping time at the interface help preserve the resolution. Based on the resolution model, a multiple-heterojunction pin photodiode, consisting of InGaAs(P)/InP photosensitive layers and an a-Si:H charge-confining layer, was proposed to improve the resolution of the infrared-sensitive OASLM. Techniques to improve the flatness of the OASLM were also explored, including chip-bonding and epitaxial lift-off.