Spectroscopy and dynamics of ${\mathrm{Cr}}^{4+}$:${\mathrm{Y}}_3$${\mathrm{Al}}_5$${\mathrm{O}}_{12}$

We present a spectroscopic overview of ${\mathrm{Cr}}^{4+}$-doped ${\mathrm{Y}}_3$ ${\mathrm{Al}}_5$ ${\mathrm{O}}_{12}$ (YAG), a new laser material for the near-infrared region (NIR). It is found that only 2% of the Cr ions are tetrahedrally coordinated ${\mathrm{Cr}}^{4+}$. However, the optical spectra are dominated by this ion. The missing inversion symmetry for the tetrahedral site causes higher cross sections than are common for octahedrally coordinated ions. The experimental data include a survey of the absorption and emission features. Lifetime measurements, as well as piezospectroscopic experiments yield additional spectroscopic information. Our data suggest an energy-level assignment that is different from the one previously used to describe the spectrum. Instead of ascribing the strong NIR absorption band, centered at around 1000 nm, to the ${}_{}{}^3{}_{}{}^{}$ ${\mathit{B}}_1$ ${(}^3$ ${\mathit{A}}_2$)${\to }^3$E${(}^3$ ${\mathit{T}}_2$) transition, we assign it to the ${}_{}{}^3{}_{}{}^{}$ ${\mathit{B}}_1$ ${(}^3$ ${\mathit{A}}_2$)${\to }^3$ ${\mathit{A}}_2$ ${(}^3$ ${\mathit{T}}_1$) transition. This assignment is able to describe the polarization-dependent spectra, the piezospectroscopic results, and the results from earlier polarization-dependent saturation experiments. An analysis of the luminescence decay rate yields quantum efficiencies of around 49% at 10 K and 15% at room temperature, resulting in a peak emission cross section of 3.5×${10}^{\mathrm{-}19}$ ${\mathrm{cm}}^2$.