GaAs Etalons and Waveguides: Bulk versus Multiple-Quantum-Well Material

Nonlinear absorption spectra of bulk GaAs and 299-, 152-, and 76-A GaAs/AlGaAs multiple-quantum-well (MQW) samples are reported. Nonlinear index spectra are calculated by Kramers-Kronig transformations of the absorption spectra. It is concluded that the principal nonlinear refractive mechanisms for optical bistability in bulk GaAs etalons are band filling and reduction of the Coulomb enhancement of continuum states; exciton saturation and band-gap renormalization occur at lower carrier densities, but their contributions are of opposite sign and almost cancel. Exciton saturation nonlinearities dominate for bistability in MQW samples with <100-A wells.