Collective Intersubband Excitations in Quantum Wells: Coulomb Interaction versus Subband Dispersion

The dependence of the intersubband absorption line shape on the subband dispersion and the Coulomb interaction between electrons is studied using the Hartree-Fock semiconductor Bloch equations. For subbands with the same effective masses, we show that the absorption/emission line shape is solely determined by the single-electron properties. For very different effective masses, the Coulomb interaction results in a strong redistribution of the oscillator strength. The line shape in this limit is determined by a Fermi-edge collective mode induced by a repelling electron-hole pair.