Design considerations for three-terminal optically addressed MQW spatial light modulators

Abstract

Although current developments in multiple quantum well (MQW) device technology show great promise, problems remain when considered in the larger framework of processing element (PE) arrays. Spatial light modulation requires many small modulator cells (25 × 25 μm2) with low switching energy (<20 fJ/μm2), low driving voltage (<20 V), good dynamic range (> 10 dB), and integrable into large arrays (>100 × 100 PEs) with a technology compatible with detector fabrication. For digital optical processors to become viable, spatial light modulators with at least 104 processing elements operating at 100-MHz frame rate are required. Assuming ideal detection efficiency, a unity power gain, and operation near thermal limits, this frame rate results in an impractical input light power density of 100 W/cm2 for a modulator array consuming 10 fJ/μm2. To ease the optical power requirements a power gain of several hundred must be achieved. For an acceptable dynamic range the processing elements should be three-terminal devices consisting of physically separated detectors and MQW modulators and should be biased at lower energies than the energy corresponding to the excitonic absorption peak. This is in contrast to two-terminal optical bistable devices using the self-electroabsorption effect and operable only with low dynamic range. A detector with high gain is also essential for weak signals to produce larger photocurrents than the currents generated in the MQW modulator. GaAs/GaAIAs heterojunction bipolar phototransistors may be good candidates to perform the required detection functions, because of their compatibility with GaAs/GaAIAs MQW modulator technology.