Radiative and non-radiative decays from the excited state of Ti³⁺ ions in oxide crystals

The fluorescence spectra of Ti³⁺ in Y₃Al₅O₁₂ (YAG), Al₂O₃ (sapphire), YAlO₃ (YAP) observed at 10 K are composed of zero-phonon lines accompanied by the broad vibronic sidebands. The temperature dependence of the fluorescence lifetime and of the total intensity of the broadband measured in YAG and Al₂O₃ indicate that the radiative decay times from the excited states are nearly constant in the range 10-300 K. This demonstrates that the broadband radiative emissions in Ti³⁺:YAG and Ti³⁺:Al₂O₃ are due to magnetic dipole transitions or to electric dipole transitions induced by static odd-parity distortion, respectively. The decrease of the fluorescence lifetime with increasing temperature in Ti³⁺:YAG and Ti³⁺:Al₂O₃ is due to non-radiative decay from the excited state which occurs through phonon-assisted tunnelling between the excited and ground states. The radiative decay of Ti³⁺:YAP is enhanced with increasing temperature, indicating that radiative decay rate contains a term associated with odd-parity phonons. Nevertheless, a non-radiative decay rate of 3.6 × 10⁴ s^-1 observed in the temperature range 10-300 K is due to excited state absorption, which depopulates the excited state and quenches the fluorescence at the laser wavelength.