Two-photon excitation of Ho3+ in the CaF2, SrF2, and CdF2 lattices

The observation of two-photon absorption by a very efficient up-converted emission is reported for Ho3+ in CaF2, SrF2, and CdF2 with both cw- and pulse-laser excitations. The two-photon excitation spectra are found to be highly structured and consist of two bands: The band A is excited by the dye-laser photons of frequencies 15 400-15 700 cm-1: The band B is excited by the dye-laser photons of frequencies 15 800-16 400 cm-1. No one-photon absorption is observed for Ho3+ in the above lattices at twice these frequencies. Studies of concentration dependence, temperature dependence, and dependence on the pulse repetition rate are used for the assignment of the band. The investigation shows that the two bands A and B arise from two different modes of two-photon excitation. The band A is assigned to arise from a sequential (stepwise) two-photon excitation I85 F55 D25. The band B is assigned to arise from the direct two-photon absorption I85 L83. Our results are in agreement with the prediction of the Judd-Ofelt theory of lanthanide transition intensities.