We studied the effect of concentration on the optical spectra in the
visible and FIR regions of ensembles of semiconductor quantum dots
(QDs).
Semiconductor QDs possess discrete exciton and phonon spectra, which
determine their dielectric response in the visible and FIR regions,
respectively.
We found experimentally that, when the QDs concentration increases,
both exciton and phonon related peaks in the absorption shift to the
lower energy side and broaden.
This is explained by the increasing dipole-dipole interaction between
the QDs, polarized by the electromagnetic wave. We performed
calculations based on the couple dipoles equations, which confirm the
observed effects experimentally.