Photoionization spectral hole burning and its erasure in Li2Ge7O15 : Cr3+:Effects of site-dependent energy-level shifts within the band gap

We report on investigations of the dependence of ionization spectral hole b urning efficiencies on the location of impurity ion energy levels relative to both the host valence band and the conduction band. An ideal system is C r-doped Li2Ge7O15, where the Cr3+ ions occupy three distinct noncentrosymme tric Ge4+ sites, which differ from one another by the location of their cha rge-compensating ions. Most important for our investigation is the large (s imilar to eV) and site-specific shift of the Cr3+ energy levels relative to the host valence and conduction bands. As a result, only one of the three sites shows persistent spectral hole burning. It is shown that this result can be understood from an analysis of the differences of the energy-level l ocations of the ground and excited states of the three Cr3+ centers relativ e to the host bands. Two of the centers have their energy levels sufficient ly close to the host conduction band such that two-step ionization, and thu s persistent spectral hole burning, can occur. However, for one of the cent ers the ground state lies so close to the valence band that an erasure proc ess involving promotion of an electron from the valence band to the ionized center leads to the restoration of the initial charge state. This erasure process, clearly demonstrated in this paper, has to be considered when eval uating the hole burning potential of doped insulators, especially those wit h relatively narrow band gaps.