Up-converted luminescence and excited-state excitation spectroscopy of ${\mathrm{Cr}}^{4+}$ ions in forsterite

The electronic structure of ${\mathrm{Cr}}^{4+}$-doped forsterite is studied using up-converted luminescence and excited-state excitation spectroscopy. Two metastable states with energies higher than the energy of the ${}_{}{}^3{}_{}{}^{}$${\mathit{T}}_2$ lasing level are identified. The first, higher-lying state was identified as the ${}_{}{}^3{}_{}{}^{}$${\mathit{T}}_1$(${\mathit{t}}_2$e) electronic state. Three lines were resolved at 26 424, 26 529, and 26 625 ${\mathrm{cm}}^{\mathrm{-}1}$, corresponding to the spin-orbit-split zero-phonon line of the ${}_{}{}^3{}_{}{}^{}$${\mathit{T}}_1$(${\mathit{t}}_2$e) state. The lifetime of this state was measured to be 395 ps. The second, lower-lying state is located at 12 847±90 ${\mathrm{cm}}^{\mathrm{-}1}$ and is assigned to the ${}_{}{}^3{}_{}{}^{}$${\mathit{T}}_1$(${\mathit{t}}_2^2$) state. Its lifetime is estimated to be of the order of a few hundred nanoseconds.