Excited-state absorption of Cr3+ in LiCaAlF6: Effects of asymmetric distortions and intensity selection rules.

Abstract

We have measured the \ensuremath{\pi}-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\ifmmode\times\else\texttimes\fi{}${10}^{\mathrm{\ensuremath{-}}20}$ ${\mathrm{cm}}^{2}$. 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\ifmmode\times\else\texttimes\fi{}${10}^{\mathrm{\ensuremath{-}}20}$ ${\mathrm{cm}}^{2}$ 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 \ensuremath{\pi}-polarized $^{4}\mathrm{A}_{2}$${\mathrm{\ensuremath{-}}}^{4}$${\mathrm{T}}_{2}$ absorption and emission features, while it does not contribute oscillator strength to the ESA band. The $^{4}\mathrm{T}_{2}$${\ensuremath{\rightarrow}}^{4}$${\mathrm{T}}_{1}$a ESA band is found to peak near 10 030 ${\mathrm{cm}}^{\mathrm{\ensuremath{-}}1}$, rather than near 7600 ${\mathrm{cm}}^{\mathrm{\ensuremath{-}}1}$ 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 ${\mathrm{Cr}}^{3+}$.