Spectroscopy of chromium centers in Li Sc Ge O 4 and Li In Ge O 4 single crystals

The detailed spectroscopy of $\mathrm{Cr}:\mathrm{Li}\mathrm{Sc}\mathrm{Ge}{\mathrm{O}}_{4}$ and $\mathrm{Cr}:\mathrm{Li}\mathrm{In}\mathrm{Ge}{\mathrm{O}}_{4}$ is presented. Low temperature absorption, emission, and excitation spectroscopies reveal two types of optical centers: trivalent chromium in the distorted octahedral sites and tetravalent chromium in the tetrahedral sites. Energies of the transitions calculated from the exact atom positions using the angular overlap model are in a satisfactory agreement with the experimental data and describe the unusually low energy position of the ${\mathrm{Cr}}^{3+}$ metastable level. The spin-forbidden $^{2}E$ and $^{2}T_{1}$ transitions of ${\mathrm{Cr}}^{3+}$ appear as a Fano antiresonance in the absorption and excitation spectra. The temperature quenching of ${\mathrm{Cr}}^{3+}$ fluorescence is described in terms of the Struck and Fonger model. ${\mathrm{Cr}}^{3+}$ ions in highly distorted octahedral positions are active centers responsible for recently discovered ${\mathrm{Cr}}^{3+}$ laser action in the near-infrared range in these crystals.