Wavelength optimization of quantum-well modulators in smart pixels

We extend our recent general discussion of electroabsorption and refraction in multiple-quantum-well modulators to determine the optimum modulator design for smart-pixel applications. In addition to the optimum operating wavelength shift, from that of the zero-voltage exciton, we determine the optimum number of quantum wells, and we calculate the reflectivity change and the contrast ratio obtainable. This analysis is undertaken for both simple detectors and modulators, meaning that they are antireflection coated, as well as for devices that include Fabry-Perot resonators. The optimization is performed on a figure of merit that is inversely proportional to the incident optical read energy required on a device to switch another, downstream device. We maximize the figure of merit to minimize the optical read energy. An interesting result is that there should be no significant improvement in our smart-pixel circuit figure of merit with the use of Fabry-Perot resonant modulators and detectors. Our results are, of course, material-system specific, but for the 850-nm AlGalAs/GaAs quantum-well system the optimum wavelength shift from the exciton location is approximately 6 nm. The general trends and approach are applicable to other material systems.