Absorption of far-infrared (FIR) radiation in very small CdSe crystallites (quantum dots, QDs) via excitation of confined dipolar vibrational modes was studied, both theoretically and experimentally. Spatial quantisation of optical phonons was analysed in the framework of a continuum model, which couples the phonon amplitude to the electrostatic potential via the Fröhlich interaction. The frequencies and oscillator strengths of the dipole-active optical vibration modes are presented calculated for a spherical QD, and for the first time for II–VI QDs these quantised modes were observed experimentally. Another effect studied in this work is the influence of the QD concentration on the FIR absorption of their ensemble. It is shown that the dipole–dipole interaction becomes very important in dense QD ensembles and leads to smearing of the structure in the CdSe phonon band as the QD concentration increases.
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