OPTICAL TRANSITIONS IN MN3+-DOPED GARNETS

The optical spectra of ${\mathrm{Mn}}^{3+}$-doped garnet crystals reveal a large Jahn-Teller stabilization energy of about $1900{\mathrm{cm}}^{\mathrm{\ensuremath{-}}1}$ for the ${}^{5}E$ ground state, and smaller Jahn-Teller stabilization energies for the excited states, i.e., $\ensuremath{\approx}325{\mathrm{cm}}^{\mathrm{\ensuremath{-}}1}$ for the ${}^{5}{T}_{2},$ and $\ensuremath{\approx}180{\mathrm{cm}}^{\mathrm{\ensuremath{-}}1}$ for the ${}^{1}{T}_{2}$ level. The absorption spectra are dominated by the spin-allowed ${}^{5}{E}^{5}{T}_{2}$ transition. At low temperatures, the emission occurs from the ${}^{1}{T}_{2}$ level to the Jahn-Teller-split ground state and the ${}^{3}{T}_{1}$ intermediate level. With increasing temperature the ${}^{5}{T}_{2}$ level becomes thermally populated and the emission spectrum is dominated by the spin-allowed ${}^{5}{\stackrel{\ensuremath{\rightarrow}}{{T}_{2}}}^{5}E$ transition. The emission lifetime is nearly independent of the detection wavelength, but strongly dependent of the temperature and the host lattice. At 12 K the lifetime is $\ensuremath{\approx}6\mathrm{ms}$ for all crystals, while at room temperature it is between 1.1 ms for ${\mathrm{Mn}}^{3+}{:\mathrm{Y}}_{3}{\mathrm{Al}}_{5}{\mathrm{O}}_{12}$ and $l0.5\ensuremath{\mu}\mathrm{s}$ for ${\mathrm{Mn}}^{3+}{:\mathrm{G}\mathrm{d}}_{3}{\mathrm{Sc}}_{2}{\mathrm{Ga}}_{3}{\mathrm{O}}_{12}.$ The radiative lifetimes of the ${}^{1}{T}_{2}$ and ${}^{5}{T}_{2}$ levels were determined to be about 6 ms and 16 \ensuremath{\mu}s, respectively. Both the radiative and the nonradiative rate are temperature dependent due to the coupling of odd-parity and totally-symmetric phonons, and the thermalization of the ${}^{5}{T}_{2}$ level. The nonradiative decay is more pronounced for lower crystal-field strengths, because of the smaller ${}^{5}{T}_{2}{\ensuremath{-}}^{3}{T}_{1}$ energy gap and the higher population of the ${}^{5}{T}_{2}$ level. Excited-state absorption transitions arising from the energetically lower ${}^{1}{T}_{2}$ in higher lying singlet levels cover the entire spectral range of the emission; therefore laser oscillation at room temperature is unlikely in ${\mathrm{Mn}}^{3+}$-doped garnets.