Analysis of co-doped Cr2+Fe2+: ZnSe as gain material for infrared fs-laser: Spectroscopic data and theoretical understanding

Abstract

Within the project, it was investigated co-doped ZnSe: (Cr, Fe) laser crystals with an approach that includes both optical spectroscopy and theoretical modelling. Structural characterization by high-resolution X-ray diffraction (HR-XRD), electron paramagnetic resonance (EPR) and scanning electron microscopy (SEM) has revealed that the samples differ in terms of dopant concentration and intrinsic native defects. It was 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+ ele-ments 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 de-termined. We have found that the Jahn-Teller distortions are crucial for modelling the tempera-ture-dependent changes of the absorption spectra. Analysis of the VIS and near IR photolumi-nescence spectra, based on the modified crystal field theory and DFT calculations has shown that Fe and Cr exist in two charge states (+2/+3) and can be located in both tetrahedral and octahedral positions. In the blue light region, quantum dots (QDs) appear. These represent clusters of three Cr-containing octahedral complexes accompanied by Zn vacancies, anticipat-ing the formation of spinel ZnCr2Se4 inclusions in the host chalcogenide ZnSe matrix.