FIR absorption in CdSe quantum dot ensembles

Absorption of far-infrared (FIR) radiation in very small CdSe crystallites (quantum dots, QDs) via excitation of confined dipolar vibrational modes wa s studied, both theoretically and experimentally. Spatial quantisation of o ptical phonons was analysed in the framework of a continuum model, which co uples the phonon amplitude to the electrostatic potential via the Frohlich interaction. The frequencies and oscillator strengths of the dipole-active optical vibration modes are presented calculated for a spherical QD, and fo r the first time for II-VI QDs these quantised modes were observed experime ntally. Another effect studied in this work is the influence of the QD conc entration on the FIR absorption of their ensemble. It is shown that the dip ole-dipole interaction becomes very important in dense QD ensembles and lea ds to smearing of the structure in the CdSe phonon band as the QD concentra tion increases.