FROHLICH EXCITON-PHONON INTERACTION IN QUANTUM WELLS: RESONANT RAMAN SPECTROSCOPY UNDER ELECTRIC FIELDS

We compare measurements and calculations of the dependence of the resonant Raman intensity of LO phonons confined in GaAs/AlAs quantum wells upon the laser energy and an electric field applied perpendicular to the layers. We find our experimental results refute the simplest model which considers scattering by the LO phonon alone. However, good agreement is found for a higher-order process involving elastic scattering by interface roughness as well as the phonon. This dominates because the phonon can have finite in-plane k-vector, so that (i) the electron and hole contributions no longer cancel, due to their different in-plane spatial extent and (ii) the phonon can scatter between different excitonic states, allowing a triply resonant process involving the continuum and 1s. The relaxation of k-vector conservation also explains the presence of interface and TO features in the Raman spectra.