ZnSe:(Cr,Fe) laser crystal matrices: Challenges related to doping

We investigate co-doped ZnSe: (Cr, Fe) laser crystals with an approach that includes both optical spectroscopy and theoretical modelling. We found that concentrations of chromium and iron up to 10¹⁸ cm⁻³ are optimal for obtaining a homogeneous solid solution in designing laser crystals. Higher dopant concentrations lead to the formation of clusters with spinel structure in the ZnSe matrix. The optimal effectiveness of ZnSe laser media with active Cr²⁺ and Fe²⁺ elements is achieved at temperatures of around 100 K. Heating induces charge transfers like Fe²⁺→Fe³⁺ and Cr²⁺→Cr³⁺, while cooling results in phonon freezing. A theoretical model has been developed to explain the observed temperature evolution of the absorption spectrum. The types and values of distortions of the Cr- and Fe-based coordination complexes are determined. We have found that the Jahn-Teller distortions are crucial for modelling the temperature-dependent changes of the absorption spectra.