We report on Raman scattering by longitudinal-optical phonons in a strongly coupled GaAs/AlAs superlattice. Applying an electric field along the growth direction, the Stark-ladder energy is adjusted to multiples of the longitudinal-optical-phonon energy in order to achieve multiple resonance conditions for the scattering processes. Double and triple resonances are investigated by excitation at suitable laser wavelengths. The triply resonant process is observed both for the case in which the second intermediate state is at k≠0 in the same Stark-ladder level as the third intermediate state as well as when the second intermediate state is located at k=0 in a separate Stark-ladder level.

• Received 28 August 1990

DOI:https://doi.org/10.1103/PhysRevB.42.11430