Three-step asymmetric coupled quantum wells have unique excitonic properties, particularly under bias. We demonstrate these properties through the absorption changes in quantum well optical modulators. The samples consisted of p-i-n diodes with an active region of 20 coupled wells, each coupled well containing a 50 Å GaAs well and a 20 Å In0.2Ga0.8As well separated by a 10 Å Al0.33Ga0.67As barrier. Analysis of the structure shows that field-induced enhancement and suppression of electron and hole envelope wave function overlap can be observed through a corresponding increase or decrease in exciton absorption peaks. Our devices showed suppressed absorption with bias for the electron-heavy hole 1 exciton and enhanced absorption with bias for the electron-heavy hole 2 exciton. Stress-related effects on the electron-light hole 1 exciton are also observed. Absorption change per applied bias is five times lower than at the zero-field exciton wavelength in quantum well devices utilizing the conventional quantum-confined Stark effect (QCSE). At higher bias, the QCSE becomes dominant, producing absorptive bistability. Our devices exhibit lower chirp and lower-voltage operation than single-well devices and the flexibility of design allows for further optimization of absorption changes.
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Trezza et al. (1993) studied this question.