Excitation of the S-1(0) State of Trivalent Praseodymium in Insulating Crystals
In this dissertation we present an investigation of the fluorescence and two-photon absorption properties of the ('1)S(,0) state of Pr('3+) in LaF(,3) and several other crystal hosts. The high-lying (4f('2)) ('1)S(,0) state of Pr('3+) is of particular interest because it is the closest of the meta-stable rare earth levels to the next configuration of opposite parity (the 4f5d band), and this proximity varies from host to host. However, the ('1)S(,0) state has received relatively little attention because its energy is beyond the range of dye lasers and frequency doubling crystals. We excite the ('1)S(,0) state directly, through a two-photon excited state absorption process, and indirectly, through pumping the 4f5d band. In most host materials the ('1)S(,0) level of Pr('3+) lies above the lowest 4f5d band component, and consequently only 4f5d band emission is observed. In LaF(,3), however, the ('1)S(,0) state lies below the band, and only the ('1)S(,0) emission is observed. In Pr('3+):CaF(,2) there are sites with each of these properties, and both ('1)S(,0) and 4f5d emission are observed, with a relative strength that is concentration dependent. Two-photon absorption from the ('3)H(,4) ground state into the ('1)S(,0) state is doubly forbidden by the two-photon spin and J selection rules ((DELTA)S = 0 and (DELTA)J <= 2), and we have not observed this transition. However, we do observe a two-photon absorption into the ('1)S(,0) state from each of the lowest two Stark components of the meta-stable ('1)D(,2) state. These transitions are allowed by all two-photon selection rules, though not for all polarizations, and we observe a strong polarization dependence. We also observe line narrowing of this transition as a smaller subset of the ('1)D(,2) population is excited, and we see homogeneous broadening of the transition as the temperature is raised to 77K. The ('1)S(,0) lifetime is much shorter than that predicted by the Judd-Ofelt theory, and is only weakly concentration dependent. We account for the short lifetime with a modification of the theory to make it more appropriate for transitions involving one state which is very close to the next opposite parity configuration.
- Pub Date:
- Physics: Condensed Matter