Optical transitions in Mn3+-doped garnets

The optical spectra of Mn3+-doped garnet crystals reveal a large Jahn-Teller stabilization energy of about 1900 cm(-1) for the E-5 ground state, and smaller Jahn-Teller stabilization energies for the excited states, i.e., approximate to 325 cm(-1) for the T-5(2), and approximate to 180 cm(-1) for the T-1(2) level. The absorption spectra are dominated by the spin-allowed E-5 --> T-5(2) transition. At low temperatures, the emission occurs from the T-1(2) level to the Jahn-Teller-split ground state and the T-3(1) intermediate level. With increasing temperature the T-5(2) level becomes thermally populated and the emission spectrum is dominated by the spin-allowed T-5(2) --> E-5 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 approximate to 6 ms for all crystals, while at roam temperature it is between 1.1 ms for Mn3+:Y3Al5O12 and < 0.5 mu s for Mn3+:Gd3Sc2Ga3O12 The radiative lifetimes of the T-1(2) and T-5(2) levels were determined to be about 6 ms and 16 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 T-5(2) level. The nonradiative decay is more pronounced for lower crystal-field strengths, because of the smaller T-5(2)-T-3(1) energy gap and the higher population of the T-5(2) level. Excited-state absorption transitions arising from the energetically lower T-1(2) in higher lying singlet levels cover the entire spectral range of the emission; therefore laser oscillation at room temperature is unlikely in Mn3+-doped garnets.