Collective optical excitation of interacting localized electrons

We investigate theoretically the role of dynamic screening in the intersubb and absorption process of a quasi-two-dimensional (quasi-2D) electron gas w ith in-plane localization caused by a strong disorder potential. Due to a c orrelation effect in the single-particle spectrum this system is equivalent to an array of randomly distributed, localized oscillators, which are mutu ally coupled by the electron-electron Coulomb interaction. In the limit of low-electron density, a broad absorption spectrum reflects the disorder-ind uced distribution of the individual transition energies. For increasing ele ctron filling factor we find a depolarization-type blueshift similar to the case of quasi-2D systems without disorder. Simultaneously a dramatic line narrowing is observed, indicating a collective response of the interacting localized electrons. In this collective mode large clusters of mutually pha se-adapted oscillators are formed in the layer plane. Our results are relev ant for the interpretation of intraband absorption experiments in all kinds of disordered quasi-2D systems and in dense arrays of artificial quantum d ots. A similar effect is expected for inter-Landau-level transitions in mag netically quantized 2D systems. [S0163-1829(99)06539-X].