Excitation of the 1S0 state of trivalent praseodymium in insulating crystals

The fluorescence and two-photon absorption properties of the 1S0 state of Pr3+ in LaF3 and several other crystal hosts were investigated. The high-lying (4f2) 1S0 state of Pr3+ is of particular interest because it is the closest of the metastable rare earth levels to the next configuration of opposite parity (the 4f5d band), and this proximity varies from host to host. However, the 1S0 state has received relatively little attention because its energy is beyond the range of dye lasers and frequency doubling crystals. The 1S0 state is excited directly through a two-photon excited state absorption process, and indirectly, through pumping the 4f5d band. In most host materials the 1S0 level of Pr3+ lies above the lowest 4f5d band component, and consequently only 4f5d band emission is observed. In LaF3, however, the 1S0 state lies below the band, and only the 1S0 emission is observed. In Pr3+:CaF2 there are sites with each of these properties, and both 1S0 and 4f5d emission are observed, with a relative strength that is concentration dependent