LARGE, LOW-VOLTAGE ABSORPTION CHANGES AND ABSORPTION BISTABILITY IN GAAS ALGAAS/INGAAS ASYMMETRIC QUANTUM-WELLS/

Three-step asymmetric coupled quantum wells have unique excitonic prop erties, particularly under bias. We demonstrate these properties throu gh the absorption changes in quantum well optical modulators. The samp les consisted of p-i-n diodes with an active region of 20 coupled well s, each coupled well containing a 50 angstrom GaAs well and a 20 angst rom In0.2Ga0.8As well separated by a 10 angstrom Al0.33Ga0.67As barrie r. 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 abso rption peaks. Our devices showed suppressed absorption with bias for t he electron-heavy hole 1 exciton and enhanced absorption with bias for the electron-heavy hole 2 exciton. Stress-related effects on the elec tron-light hole 1 exciton are also observed. Absorption change per app lied bias is five times lower than at the zero-field exciton wavelengt h in quantum well devices utilizing the conventional quantum-confined Stark effect (QCSE). At higher bias, the QCSE becomes dominant, produc ing absorptive bistability. Our devices exhibit lower chirp and lower- voltage operation than single-well devices and the flexibility of desi gn allows for further optimization of absorption changes.