Excited-state absorption in ruby, emerald, and MgO:Cr3+

Abstract

Absorption from the metastable ${}_{}{}^2{}_{}{}^{}E$ and ${}_{}{}^2{}_{}{}^{}T_1^{}{}_{}{}^{}$ states of ${\mathrm{Cr}}^{3+}$ has been measured in ruby with more precision than in previous experiments and in emerald and MgO: ${\mathrm{Cr}}^{3+}$ for the first time. The sharp ${}_{}{}^2{}_{}{}^{}E$, ${}_{}{}^2{}_{}{}^{}T_1^{}{}_{}{}^{}\to {}_{}{}^2{}_{}{}^{}T_2^{}{}_{}{}^{}I$ lines are resolved in these three crystals, as well as the ${}_{}{}^2{}_{}{}^{}E\to {}_{}{}^2{}_{}{}^{}A_1^{}{}_{}{}^{}$ lines in ruby and the ${}_{}{}^2{}_{}{}^{}E\to {}_{}{}^2{}_{}{}^{}A_2^{}{}_{}{}^{}$ lines in ruby and MgO: ${\mathrm{Cr}}^{3+}$. Three excited-state bands are also seen in each crystal. To confirm the assignments of the observed transitions, the energy matrices are diagonalized, and intensity calculations using the closure approximation are made. In particular, the sharp lines in MgO: ${\mathrm{Cr}}^{3+}$ are found to be from the tetragonal vacancy and pair systems rather than the cubic system. The predicted polarization selection rules are experimentally confirmed and, except for the ${}_{}{}^2{}_{}{}^{}E\to {}_{}{}^2{}_{}{}^{}A_2^{}{}_{}{}^{}$ transition, the observed intensities are explained well by theory. This anomalous transition is more than an order of magnitude stronger than predicted by theory in all three crystals.