96 Journal of Non-Crystalline Solids 115 (1989) 96-98 North-Holland A THREE-TERMINAL SPATIAL LIGHT MODULATOR OPTICALLY ADDRESSED BY AN a-Si:H PHOTOSENSOR R. A. RICE, G. MODDEL, I. ABDULHALIM, and C. M. WALKER Department of Electrical and Computer Engineering and Center for Optoelectronic Computing Systems, University of Colorado, Boulder, CO 80309-0425 USA We have developed a new, three-terminal optically addressed spatial light modulator (OASLM) which utilizes a hydro- genated amorphous silicon (a-Si:H) p-i-n photodiode as the photosensor and a ferroelectric liquid crystal (FLC) as the modulator. A patterned metal layer deposited between the a-Si:H and FLC forms the third terminal. This structure allows for dc operation and has the potential for a variable threshold response to an optical input signal. 1. INTRODUCTION The structure, operation and applications of a standard two-terminal OASLM are described in detail elsewhere, le The device replicates an optical write image in a modulat- ing material, which is subsequently retrieved by a read beam. Although the standard configuration has many applications in image processing and optical computing, the addition of a patterned metal layer between the a-Si:H and the FLC greatly increases the versatility of the OASLM. One function of the metal grid is to act as a mirror, allowing the use of a much higher read-light intensity in reflection mode operation. In the standard configuration,lea square-wave voltage drives the device such that the read and write operations occur under reverse bias, and the erase operation occurs under forward bias. Because the FLC must be switched ON by an electric field, and switched OFF by an opposite polarity field, both polarities must be available during operation. Whereas in the standard device the polarities must be temporally separated, in the three-terminal device they may be spatially separated. Using the grid to provide a positive offset voltage across the FLC allows for dc mode operation. Additionally, with a dc or ac electrical drive the grid is expected to give the OASLM a thresholding capability, in that the write-light intensity threshold for turn ON is controlled by adjusting the grid voltage. The dc bias mode of operation permits applications of the OASLM in unsychronizable systems. For example, it is useful when the input signal frequency is not constant, but varies with time. The potential for thresholding yields an application of this device in optical image filtering by time- sequential threshold decompositions and in a method of optical symbolic computing based on mathematical resolution.4 2. DEVICE DESCRIPTION The three-terminal OASLM incorporates a patterned metal electrode structure between the a-Si:H photosensor and the FLC modulator. In our demonstration device the pixels consist of 200 Ixm diameter Cr dots on 400 ~tm centers, each surrounded by a 5 I.tm non-metalized region, as shown in Fig. 1. The metalized region between the pixels forms the grid electrode, which is maintained at a uniform voltage, and varied with respect to the ITO voltage. A two-dimensional resistive network controls the voltage on the pixels, which form the active area of the three-terminal OASLM. A schematic diagram for one element of this network is shown in Fig. 2. The pixel voltage depends on the transverse resistance (Rt) through the a-Si:H n-layer to the grid electrode, as well as on the 0022-3093/89/$03.50 © Elsevier Science Publishers B.V. (North-Holland)