Electronically resonant optical cross relaxation in YAG: Tb3+

Abstract Tb 3+ ions in solids usually show a phonon-assisted optical cross relaxation between the levels 5 D 3 → 5 D 4 and 7 F 6 → 7 F 1,2 . In the Y-Al-garnet (YAG) we encounter the very rare and interesting case of an electronically resonant process within the homogeneous widths of the involved crystal field levels. So direct phonon assistance is not needed. The time-resolved donor and acceptor fluorescences were analyzed by the Inokuti-Hirayama model. At low temperatures a dipole-quadrupole interaction was found to be responsible for the cross relaxation whereas at room temperature dipole-dipole interaction prevails. These findings are verified by the selection rules for the electronic transitions involved.