RESONANT ENHANCEMENT OF 2-PHOTON TRANSITIONS TO THE 2ND EXCITED 4F 5D STATE IN PR3+-Y3AL5O12

The two-photon absorption cross section for transitions from the 3H4 ground-state manifold to the second excited state of the 4f5d configuration of Pr3+ in Y3Al5O12 is observed to be enhanced by about two orders of magnitude when the excitation frequency happens to be resonant with the 3P(J) (J=0,1,2) and 1I6 intermediate states of the 4f2 configuration. The structure and line shape of the two-photon excitation spectrum spanning the 39600-46950 cm-1 transition-energy range are entirely dominated by these resonantly enhanced transitions, while the contributions from nonresonant transitions appear as a slowly varying background beneath the strong resonance peaks. A two-photon absorption cross section as high as 4 X 10(-50) cm4 s is measured at the peak of the resonantly enhanced transition at 44246 cm-1, which involves a level of the 3P2 manifold as the intermediate state. The spectrally integrated cross section over that manifold is 3.3 X 10(-38) cm4.