Role of dynamic depolarization for the intersubband resonance in the localization regime

We investigate the electronic intraband absorption in quantum wells with a strong lateral random potential, realized for example by modulation doping with a thin spacer layer. In such systems, electrons become in-plane localized in isolated potential minima and behave like an inhomogeneous array of natural quantum dots. When excited with a coherent light field, the dots respond as individual oscillators, which are however coupled by dynamic dipole–dipole interactions. The absorption spectrum is then determined by the interplay of the single dot properties (related to the disorder potential) and the many-particle Coulomb interactions. Using a simple model for the single-particle states, we calculate the absorption spectrum as a function of the electron density. In the case of light polarized perpendicular to the layer, we find with increasing density a dramatic line narrowing (associated with a collective excitation of the electrons) and a depolarization blue shift. For in-plane polarized light, the peak is shifted to the red. Our theory also applies to far-infrared absorption experiments in artificial quantum dot arrays.