Dispersion of collective intersubband excitation in semiconductor superlattices.

We report the observation, by inelastic light scattering, of the dispersion of collective intersubband excitations of electrons confined in GaAs quantum well superlattices. The ability to probe plasmons with large in plane wavevectors allow the unique possibility to study excitations in the two-dimensional limit. Intrasubband plasmons in this regime are also observed for the first time. The results are interpreted with current theories which include estimates of many body effects.