Photoionization processes of rare-earth dopant ions in ionic crystals

Abstract

Photoionization of impurity ions in ionic host crystals has been studied through photocoductivity measurements. Besides the conventional method using blocking electrodes and a vibrating reed electrometer to measure directly the photocurrent, the microwave resonant cavity technique was developed and applied for the first time to dielectric crystals. This technique, based on the detection of the transient responses of the dielectric permittivity under pulsed laser excitation, is very promising since it allows to work on polycrystals including nanocrystals, to study details of the photoconductivity dynamics and to know about the nature of the released electrons (free or bound). A systematic study of the photoionization thresholds was able to provide pertinent information on the location of impurity ion states in comparison with the edge of the conduction band. In some divalent rare-earth doped alkaline earth fluorides, it was demonstrated that the delocalization of the electron in the conduction band may result in the formation of impurity-trapped excitons leading to unexpected luminescence. Models have been proposed to evaluate the photoionization energies and to describe exciton-like states.