Spatial Light Modulator Using A CDS Thin Film Photocapacitor
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SPATIAL LIGHT MODULATOR USING A CDS THIN FILM PHOTOCAPACITOR JOSEPH REICHMAN Grumman Corporate Research Center, Mail Stop A02-26, BethpageN.Y. 11714 ABSTRACT A brief summary is given of the characterization of an optically addressed Spatial Light Modulator (SLM) that uses a thin film of CdS as the hhotosensor and a liquid crystal as the electro-optic material. The required ghdark impedance of the CdS thin film is achieved by a depletion region created by surface acceptor states. Analysis of the SLM complex impedance as a function of frequency and illumination indicates that the voltage switching is due to the reduction in impedance with illumination caused by The impedance the increased capacitance of the CdS thin film. measurements as well as photoluminescence data were consistent with an acceptor surface state model accounting for the depletion region of the CdS thin fim. Additionally the SLM is shown to exhibit photoelectrochemical cell characteristics such as dark rectification, and a photocurrent and The SLM photovoltage proportional to the illumination intensity. performance characteristics of good resolution and sensitivity is attributed to the surface state induced depletion region. INTRODUCTION Spatial Light Modulators (SLMs) have numerous applications in a variety of optical computing and image processing systems[1,21. We have developed a transmissive mode optically addressed SLM that employs a photosensitive layer consisting of a thin film of CdS and a liquid crystal as the electro-optic layer. A schematic cross section of the SLM is shown in Fig. 1. The photosensitive layer is a thin fim of CdS 2 to 4 microns thick deposited by vacuum evaporation onto an Indium Tin Oxide (ITO) coated A.C. VOLTAGE SUPPLY ALIGNMENT
LAYERS
TRANSPARENT CONDUCTOR
TRANSPARENT CONDUCTOR
READ LASER LIGHT
GLASS PLATE-
-INCOHERENT WRITE LIGHT
IL
GLASS PLATE
ILLIQUID CRYSTAL
PHOTOCONDUCTOR
Fig. 1 Grumman Spatial Light Modular (SLM) incident laser light is modulated by spatially varying incoherent while light signal. Mat. Res. Soc. Symp. Proc. Vol. 161. '11990 Materials Research Society
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glass substrate. The liquid crystal is a cyanobiphenyl which has a positive dielectric anisotropy. It is aligned in a 90 degree twisted nematic configuration by using rubbed polyimide layers on the CdS and ITO surfaces. An AC voltage of 4 to 8 volts rms at 1 to 2 kHz is applied across the ITO electrodes. When no write light is incident on the photoactive layer, the voltage drop across the liquid crystal is below its switching threshold. Incoming polarized light with photon energy greater than the 2.4 eV bandgap of the CdS is then transmitted with its plane of polarization rotated 90 degrees by the liquid crystal layer. When the CdS is illuminated by write light greater than the band gap energy, its impedance decreases. The voltage drop across the liquid crystal then increases above its threshold value. This results in reorientation of the twist configuration of the liquid crystal molecules. Incoming linearly polarized light in the
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