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 burning efficiencies on the location of impurity ion energy levels relative to both the host valence band and the conduction band. An ideal system is Cr-doped Li2Ge7O15, where the Cr3+ ions occupy three distinct noncentrosymmetric Ge4+ sites, which differ from one another by the location of their charge-compensating ions. Most important for our investigation is the large (similar 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 locations of the ground and excited states of the three Cr3+ centers relative to the host bands. Two of the centers have their energy levels sufficiently close to the host conduction band such that two-step ionization, and thus persistent spectral hole burning, can occur. However, for one of the centers the ground state lies so close to the valence band that an erasure process 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 evaluating the hole burning potential of doped insulators, especially those with relatively narrow band gaps.