Thin-film photosensor design for liquid crystal spatial light modulators Pierre R. Barbier* Li Wang Garret Moddel University of Colorado Electrical and Computer Engineering Department and Optoeleôtronic Computing Systems Center Boulder, Colorado 80309-0425 E-mail: moddel@ boulder.colorado.edu 1 Introduction Optically addressed spatial light modulators (OASLMs) are devices that alter the properties of a readout light beam in response to an input write image. They are key elements of optical processing. They can perform wavelength conversion, coherent-to-incoherent image conversion, and intensity am- plification of 2-D images. OASLMs are formed from a sand- wich of a photosensor layer, which absorbs a write-light im- age to create a spatially varying electric field, and a modulating layer, which responds to the electric field to mod- ulate a read light that passes through it. High-performance OASLMs have been fabricated by as- sembling light-modulating liquid crystals (LCs) with thin- film photosensors.' Critical parameters in OASLM photo- sensors are photosensor thickness, high lateral resistivity, and good photoresponse. Although LC OASLMs are 2-D imaging devices, they may not include patterned pixels. In this case their spatial resolution depends only on the LC and photo- sensor material properties and may be high compared to pat- terned devices. In this paper we review thin-film photosensors that have been incorporated into LC OASLMs. We discuss the optoelectronic and physical properties that the photo- sensor must fulfill in such OASLMs and present experimental characteristics of different types of hydrogenated amorphous silicon photosensors. sCunent affiliation: Laboratory for Physical Sciences, 8050 Greenmead Drive, College Park, MD 20740. Paper 10083 received Aug. 19, 1993; accepted for publication Oct. 25, 1993. 1994 Society of Photo-Optical Instrumentation Engineers. 009 l-3286/94/$6.00. 1322 / OPTICAL ENGINEERING / April 1994 / Vol. 33 No. 4 Abstract. Liquid crystal (LC) spatial light modulators (SLMs) are ad- dressed optically with semiconductor thin-film photosensors incorporated into the devices. Nematic LCs, which are insensitive to the polarity of the applied voltage, are addressed by optically modifying the effective resistance of the photosensors to be much smaller than or much larger than a threshold value. Much faster ferroelectric LCs, which are polarity sensitive, are addressed by supplying sufficient photogenerated charge. Because the spatial resolution of the devices decreases rapidly with in- creasing mobility of carriers at the photosensor/LC interface, very low mobilities, less than 1 cm2 V s1, are required. Photodiodes of hydro- genated amorphous silicon in p-i-n, Schottky, and metal insulator semi- conductor configurations form practical photosensors for optically ad- dressed SLMs. Thin-film photoconductors, having nonblocking contacts, cannot be used in many cases because of their large dark currents. 2 Liquid Crystals An LC is a birefringent medium whose optical axes can rotate under the influence of an externally applied electric field, modifying the polarization ofa light beam that passes through it. By an appropriate orientation of polarizers and crossed analyzers, this polarization change may be transformed into an intensity modulation. Two types of LCs have been com- monly incorporated into OASLMs: nematic liquid crystals (NLCs) and ferroelectric liquid crystals (FLCs).2 Both types of LCs can be inserted between two transparent electrodes whose surfaces have been previously coated with an align- ment layer to orient the LC molecules. The resulting LC cells are driven by applying a voltage waveform across the two transparent electrodes. Although they both produce a rotation of the polarization of transmitted light, NLC and FLC cells respond differently to an applied voltage, and thus require different photosensor configurations. NLCs respond to the magnitude of the external electric field independent of its polarity. As a consequence, NLCs are usually driven with dc-balanced voltage waveforms such as sine waves. Several types of NLC cell structures have been used to make OASLMs. In the birefringent mode, the NLC molecules are parallel to each other and to the substrates in the absence of an applied voltage.3 When a sufficiently large voltage is applied to the cell, the molecules rotate perpen- dicularly to the substrates. The transfer characteristic of a birefringent-mode NLC cell is illustrated in Fig. 1 where the optical response is plotted versus the rms value of the applied voltage. Other NLC alignment modes that have been used in OASLMs are the twisted nematic mode and the hybrid field mode, discussed elsewhere.4 Subject terms: photosensors; photodiodes; photoconductors; hydrogenated amor- phous silicon; spatial light modulators; liquid crystals; ferroelectric liquid crystals; nematic liquid crystals. Optical Engineering 33(4), 1322—1329 (April 1994).