Abstract

We present here the first observation of quasi steady-state optical nonlinearities associated with absorptive and dispersive changes in GaAs/AlAs "Type-Il" superlattices. The nonlinearity arises from space-charge layers (holes at the Γ-point in GaAs and electrons at the X-point in AlAs layers). Using pump/probe spectroscopy we have observed a deep saturation, >80% (Δa = -50,000 cm⁻¹), and blue shift >20 meV, of the heavy-hole exciton at low temperature (10 K). The basic underlying many-body effects are a) microscopic space-charge potential and b) dynamical correlations within each plasma, the latter being the dominant nonlinearity. A numerical calculation of the appropriate Bethe-Salpeter equation for the optical polarization within the screened Hartree-Fock approximation yields good agreement with the experiment data. The dominant sources of the heavy-hole shift are found to be phase-space effects, whereas the dynamical screening contributions are less important in the relevant density range. Correponding nonlinear-dispersive changes are computed by Kramers-Kronig transformation. In the transparency region of the superlattice, a large index change of Δn~−0.2 for 2000 Å of active material and a few milliwatts excitation is observed. Potential applications of the observed nonlinearity and a new class of electroabsorption modulators based upon our observations of the above phenomena are discussed.

© 1989 Optical Society of America