Depolarization shift of the in-plane polarized interlevel resonance in a dense array of quantum dots

We have investigated experimentally the midinfrared normal-incidence response of holes confined in an array of Ge/Si self-assembled quantum dots. The dots have a lateral size of about 15 nm and a density $3\times {10}^{11}{\mathrm{cm}}^{-2}.$ An in-plane polarized absorption in the 70–90 meV energy range is observed and attributed to the transition between the first two states in the dots. As the hole concentration in the dot ground state is increased, the absorption peak shifts to higher energies, its linewidth is reduced, and the line shape is changed from asymmetric to symmetric. We attribute all features to a depolarization-type effect similar to the case of two-dimensional systems. We believe that our results provide experimental evidence for the dynamic screening of an external field by in-plane polarized interlevel collective excitations.