Absorption of in-plane polarized light in quasi-2D systems enabled by strong potential fluctuations

We investigate theoretically the absorption of infrared radiation by quasi-2D conduction band electrons in strongly doped semiconductor quantum wells. Due to the break-down of the k-conservation rule in such disordered systems, absorption is possible not only with light polarized in growth direction, but also in the case of in-plane polarization. We start with realistic single particle states, localized in the layer plane by the potential fluctuations of the random impurity distribution. The absorption spectrum for both polarization modes is then calculated on the basis of these electron states, using Fermiʹs golden rule. For perpendicular polarization, our theory yields the usual resonance line at the intersubband energy separation ℏωres=ε1−ε0, yet asymmetrically broadened by the disorder. The same model system shows for in-plane polarized light a broader absorption band in the low-energy range 0⩽ℏω<ℏωmax, which is due to non-k-conserving intra-subband transitions between localized states. With increasing electron density, this spectrum becomes narrower and its peak is red shifted more and more. When the disorder is reduced by modulation doping, intra-subband absorption can be observed only in the range of very small photon energies, e.g. ℏωmax→0. Both effects can be explained by considering the properties of the disordered electron states.