Spectroscopic properties of CaF2:U4+ as a saturable absorber

The spectroscopic properties of crystals of CaF2:U4+ have been examined in light of recent interest expressed for this material as a saturable absorber in the passive Q switching of near infrared solid state lasers. The detailed crystal-field splitting of the energy levels of U4+(5f2 ) is analyzed for the ion in different charge-compensated sites. Identification of the site is based on laser excited site-selective spectroscopic studies reported earlier. Experimental Stark levels are compared with calculated levels based on lattice-sum models that include charge compensation in the lattice structure. The final set of atomic parameters for U4+ is in good agreement with values reported for the ion in UF4. The predicted non-Kramers’ doublet as the ground-state Stark level for U4+ in C3V sites is in agreement with the spectroscopic analysis and with results reported earlier from electron spin-resonance studies of CaF2:U4+. Using pump–probe methods, we have investigated excited state absorption (ESA) at 1.54 μm. Consistent with the interpretation of the absorption spectra, we find that the measured ESA is quite small in these crystals. In turn, we use this conclusion to model CaF2:U4+ as a passive Q switch, with good results.