Dipole-Dipole Interaction Effect on the Optical Response of Quantum Dot Ensembles
Semiconductors quantum dots (QDs) possess discrete excitonic and phonon spectra. For a spherical QD of radius R, the excitonic spectrum in the effective mass approximation (EMA) is well known . Quantization of dipole-active optical phonons in very small crystals has received much less attention. Usually it is assumed that, for a spherical QD, there is only one far-infrared (FIR) active mode corresponding to a uniform polarization of the sphere, called the Fröhlich mode. Its frequency does not depend on the sphere’s radius. This is approximately correct if the QD radius exceeds 2-3 ran for II-VI materials like CdS and CdSe. For smaller QDs, the quantization of phonon modes is important. As it has been shown theoretically and experimentally in , FIR absorption spectrum of such a QD is constituted by a set of (homogeneously broadened) peaks at certain frequencies within the reststrahlen band of the bulk material. These phonon frequencies in function of the QD size can be calculated within a continuum approach similar to the EMA  or numerically.
KeywordsRefraction LaF3 Oxyfluoride