Uv-Induced Color Centers and Gain Measurements in Trivalent Cerium Ion Doped Lithium Yttrium Fluoride.

The origin of the laser performance limitation characteristics of Ce('3+) : LiYF(,4) tunable solid laser material have been identified and studied in this thesis. Thermal lensing effects suggested before as an origin of the poor lasing problems turned out to be negligible. Instead, UV-induced color centers are produced following the optical pumping at 308 nm of the lowest 4f (--->) 5d transition of the trivalent cerium ions in LiYF(,4). They consist of transient and stable color centers and have strong effects on the laser performance of this crystal. Experimental data from the measured pump power dependence and growth kinetics of the absorption indicate that a resonant two-photon absorption from the cerium ion is responsible for their creation. The photoionized electrons could be trapped temporarily at other cerium ion sites resulting in transient color centers or permanently at pre-existing fluorine ion vacancies resulting in stable color centers. Creation and bleaching from these traps compete with each other during the laser pumping. The recombination of these two processes is observed in the pump power dependence of the induced absorption and is modeled by simple rate equations. The transient color centers show a wavelength dependent optical absorption and several decay time constants which are well separated. These time constants span over ten decades of time scales at temperatures between 77 K and 300 K and two thermal trap depths were estimated from them. The stable color centers show a strong polarization dependent absorption spectra which are very similar to those found in (gamma)-ray or electron beam irradiated LiYF(,4). These spectra show a dominant band which is due to F centers and overlap the cerium emission band of 310-340 nm. To investigate the effects of the two kinds of UV-induced color centers the single pass gain was measured as a function of both the pump repetition rate and the sample temperature. Data indicated that transient color centers are responsible for the repetition rate limitation at 0.5 Hz and both type of color centers are the origin of the low gain and large threshold energy in the Ce:YLF laser. Possible methods to overcome these UV-induced color centers is discussed.