Multiply resonant Raman scattering in Stark ladder superlattices.

We report on Raman scattering by longitudinal optical (LO) phonons in GaAs/AlAs superlattices with AlAs barrier thicknesses of nominally 1, 2, and 3 monolayers (ML). A Stark ladder of two-dimensional subbands equidistant in energy was produced by applying an electric field along the growth direction of these structures. The energy difference separating these states was tuned into resonance with the LO phonon energies. Single, double, and triple resonance conditions for Raman scattering by one and two LO phonons, respectively, have thus been realized. For the 3 ML sample, the triply resonant process is observed both for the case that the second intermediate state is at ksub2 is not equal 0 in the same Stark ladder level as the third intermediate state as well as for the second intermediate state being located at ksub2 is equal to 0 in a separate Stark ladder level. The 2 ML and 1 ML samples show a drastic enhancement of the spatially indirect transitions giving rise to a variety of reso nance effects in the Raman signal. For these samples, however, triple resonance effects are difficult to observe due to the relatively weak 2 LO phonon scattering. This is probably due to lifetime broadening of the subbands giving rise to less pronounced resonant enhancement in the Raman scattering cross section.