Optical Addressing of High-Speed Spatial Light Modulators with Hydrogenated Amorphous Silicon
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OPTICAL ADDRESSING OF HIGH-SPEED SPATIAL LIGHT MODULATORS WITH HYDROGENATED AMORPHOUS SILICON G. MODDEL, C. T. KUO, K. M. JOHNSON, AND W. LI University of Colorado, Department of Electrical and Computer Engineering and Center for Optoelectronic Computing Systems, Boulder, Colorado 80309-0425 ABSTRACT We demonstrate the operation of a high-speed optically addressed spatial light modulator utilizing a hydrogenated amorphous silicon photosensor and a ferroelectric liquid crystal modulator. The device has numerous optical parallel processing and interconnect applications. It combines desirable resolution, switching speed, size, and contrast characteristics. The devices are driven by a square-wave voltage, such that read and write operations take place under reverse bias, and an erase operation occurs under forward bias. The capacitance associated with the photosensor plays a critical role in the device performance. INTRODUCTION Optical computing systems offer increased information processing throughput rates by taking advantage of parallel optical architectures. The fundamental components in these architectures are devices which can modulate two-dimensional optical data. These devices, spatial light modulators (SLMs), have many applications including input/output data displays, spatial and matched filters, incoherent-coherent light converters, optical crossbar switches, optical interconnection networks, and neurocomputers. The ideal system requirements placed on SLMs include high resolution (100 lp/mm), 1000 x 1000 pixels, megahertz frame rates, 1000:1 contrast ratio, and low cost [1]. An operating characteristic which often prevents SLMs from achieving these specifications is device power dissipation [2]. In this paper we discuss a new spatial light modulator which marries two commercially driven technologies, hydrogenated amorphous silicon (solar cells) and ferroelectric liquid crystals (high contrast computer displays) with the potential for attaining these system requirements. Optical addressing of SLMs provides a technique for direct addressing of the modulating layers. In a reflection-mode optically addressed SLM (OASLM), shown in Fig. 1, the read and write beams enter from opposite sides of the device. In operation, when the photosensor is illuminated, a voltage applied across the sandwich is shunted to the modulating layer where the write beam image is replicated. OASLMs have been demonstrated using a variety of photosensors and modulating materials, as summarized in Table I. Most of these devices have switching speeds that are limited to milliseconds by the modulating material. This places modest demands on the photosensor. The Hughes liquid crystal light valve, for example, uses a CdS photosensor [3,4] In this device a nematic liquid crystal is switched with a 30 msec cycle time. Other successful photoaddressed SLMs use crystalline silicon to switch nematic liquid crystals [5] and el'ectro-optic crystals [6]. The response times are also on the order of milliseconds, although in the electro-optic crystal a cyc
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