Dependencies of optical transmittance and luminescence of ${\mathrm{Y}\mathrm{A}\mathrm{G}:\mathrm{C}\mathrm{r}}^{4+}$ crystals upon intensity of excitation at 1064 nm are investigated, when absorption of tetrahedrally coordinated ${\mathrm{Cr}}^{4+}$ ions $({\mathrm{Cr}}^{4+}$ centers) is saturated. The spectroscopic model of ${\mathrm{Cr}}^{4+}$ center in YAG crystals is proposed. Absorption saturation of ${\mathrm{Y}\mathrm{A}\mathrm{G}:\mathrm{C}\mathrm{r}}^{4+}$ crystals at 1064 nm have to be described by two absorption cross sections ${\sigma }_{0\pi }=(3.9\mathrm{}–5.0)\times {10}^{-18}{\mathrm{cm}}^2$ and ${\sigma }_{0\sigma }=(1.5\mathrm{}–1.9)\times {10}^{-19}{\mathrm{cm}}^2,$ arising from the $D_{2d}$ local symmetry of ${\mathrm{Cr}}^{4+}$ centers. The cross sections slightly vary within the pointed range in dependency of level of the Cr dopant and a type of the ${\mathrm{Me}}^{2+}$ dopant $({\mathrm{Mg}}^{2+}$ or ${\mathrm{Ca}}^{2+}).\mathrm{}$ The absorption band centered at 1000 nm is basically due to ${}^3{B}_1{(}^3{A}_2{)}^3{A}_2{(}^3{T}_1)$ transition. Energy positions of ${}^3{A}_2{(}^3{T}_1)$ and ${}^3{E(}^3{T}_2)$ states are very close to each other, and the absorption band due to ${}^3{B}_1{(}^3{A}_2{)}^3{E(}^3{T}_2)$ transition is masked by the absorption due to ${}^3{B}_1{(}^3{A}_2{)}^3{A}_2{(}^3{T}_1)$ transition. The proposed model allows us to explain the dependence of the extent of luminescence polarization on the intensity of light exciting the ${\mathrm{Y}\mathrm{A}\mathrm{G}:\mathrm{C}\mathrm{r}}^{4+}$ crystal. Absorption from the metastable excited state ${}^3{B}_2{(}^3{T}_2)$ is negligible; its absorption cross section is less than $2\times {10}^{-20}{\mathrm{cm}}^2.$ Linear losses from other centers are found in the crystals with a high level of the chromium dopant and with the calcium dopant.

• Received 1 June 1999

DOI:https://doi.org/10.1103/PhysRevB.61.988