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Electrooptical modulators based on quantum well structures have become an important area of research due to potential applications in high-speed optical modulation and image processing. In this paper, we examine the physics of a quantum well modulator within the generalized Kohn-Luttinger Hamiltonian. Issues of importance for the modulator structure are the excitonic absorption shift, exciton binding energy, line broadening, tunneling rates for electrons and holes in the presence of a transverse electric field, and changes in optical absorption coefficients as a function of electric field. A formalism to study these effects for both lattice matched and nonlattice matched quantum well structures is provided and the potential of material tailoring for specific optical response is discussed. It is shown that the reliability of this technology is critically related to the fabrication of high-quality interfaces and alloys since even a one-monolayer variation in quantum well size can have a substantial effect on the modulation properties.