Effects of nonparabolicity on collective intersubband excitations

We have used inelastic light scattering to study the collective charge- and spin-density excitations associated with electronic intersubband transitions in GaSb-AlSb superlattices. The density of the photoexcited two-dimensional electron-hole plasma was controlled by varying the incident laser power. A comparison of experiment with detailed calculations of the response functions shows that the noparabolicity of the conduction band must be properly included in order to yield quantitative agreement. It is found that nonparabolicity, in addition to decreasing the resonance energies of the collective excitations, provides a mechanism for observing single-particle excitations at q=0.