We have investigated experimentally the mid-infrared normal-incidence response of holes confined in array of Ge/Si self-assembled quantum dots. The dots have a lateral size of approximately 15 nm and a density of 3 x 10(11) 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 lineshape is changed from an asymmetric to symmetric one. We attribute all features to a depolarization-type effect caused by collective interlevel excitations.