Triply resonant raman scattering by LO phonons in a Wannier-Stark ladder

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 muliples 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 when 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.