Excited-state absorption ofCr3+inLiCaAlF6: Effects of asymmetric distortions and intensity selection rules

Abstract

We have measured the π-polarized excited-state absorption (ESA) spectra of the new laser material, ${\mathrm{LiCaAlF}}_6$:${\mathrm{Cr}}^{3+}$ (hereafter, ${\mathrm{Cr}}^{3+}$:LiCAF). We have found that the peak cross section of the ESA band is 0.17×${10}^{\mathrm{-}20}$ ${\mathrm{cm}}^2$. We are therefore able to explain the high efficiency previously observed for the ${\mathrm{Cr}}^{3+}$:LiCAF laser by noting that the peak emission cross section of 1.3×${10}^{\mathrm{-}20}$ ${\mathrm{cm}}^2$ is much larger than this ESA cross section. As a result, the ESA does not diminish the value of the effective stimulated-emission cross section, in spite of its tendency to overlap the emission band in most ${\mathrm{Cr}}^{3+}$-doped crystals. It is then shown that the ESA transition is weak relative to the emission band because the $t_{2u}$ distortion present at the ${\mathrm{Al}}^{3+}$ site where the ${\mathrm{Cr}}^{3+}$ substitutes adds transition strength to the π-polarized ${}_{}{}^4{}_{}{}^{}\mathrm{A}_2^{}{}_{}{}^{}$ ${\mathrm{-}}^4$ ${\mathrm{T}}_2$ absorption and emission features, while it does not contribute oscillator strength to the ESA band. The ${}_{}{}^4{}_{}{}^{}\mathrm{T}_2^{}{}_{}{}^{}$ ${\to }^4$ ${\mathrm{T}}_1$a ESA band is found to peak near 10 030 ${\mathrm{cm}}^{\mathrm{-}1}$, rather than near 7600 ${\mathrm{cm}}^{\mathrm{-}1}$ as expected on the basis of simple crystal-field theory. This is explained as being due to the influence of non-totally-symmetric distortions in the excited states of ${\mathrm{Cr}}^{3+}$.