INTERACTION BETWEEN EXCITONS AND RARE-EARTH IONS

Energy transfer between rare-earth (RE) ions and excitons in semicondu ctors has already been known for some time. In this paper we would lik e to direct your attention to the interaction between excitons and RE ions in ionic crystals as used, e.g. for laser applications. A suitabl e model substance to study these effects is RE-doped CsCdBr3. The reas ons are: (i) the low phonon frequencies due to the heavy ion masses (( h) over bar omega(max) < 200 cm(-1)); (ii) metastable excitonic states in the visible spectral range; (iii) strong exciton-ion interaction d ue to covalent overlap of the wavefunctions. Due to the low phonon fre quencies, multiphonon processes of the RE ions are reduced drastically and ion-exciton processes can be studied in more detail. The followin g processes were found: (i) exciton-mediated nonradiative relaxations. These processes can bridge much larger energy gaps than direct multip honon relaxation and are more efficient. The reason is that the nonrad iative multiphonon relaxation of the RE ion is dominated by a fast mul tistep one-phonon relaxation of the exciton by energy transfer; (ii) e xciton-mediated quantum upconversion. This effect is based on a cooper ative energy transfer from two excited RE ions to an exciton and a sub sequent back transfer to a single ion. This process is much faster tha n upconversion by phonon-assisted cross relaxation between two excited RE ions; (iii) exciton-induced changes in the crystal-held splitting of RE ions. Energy levels of RE ions in resonance with the excitons sh ow crystal-held splittings which cannot be described by the parameters suitable for the other levels. We propose an increased covalent overl ap between the wavefunction of the RE ion and of the exciton-forming l igands due to hybridization as an explanation for this effect. (C) 199 8 Elsevier Science B.V. All rights reserved.