Material and laser characterizations of intermediate compositions of Ce:LiSr x Ca 1-x AlF 6

The development of solid state tunable lasers in the ultraviolet region would have a wide range ofapplications, includingatmospheric remote sensing, atmospheric spectroscopy and pollution monitoring. The first work with Ce:LiCAF waspublished in 1993, with subsequent reports on favorable results with Ce:LiSAF. While the optical quality ofCe:LiSAF issuperior, it suffers from solarization. In contrast, Ce:LiCAF shows minima! solarization and higher efficiencies, even withthe typically high-scatter materials available. This presentation will summarize the crystal growth and materialcharacterization ofa series ofcolquiriite crystals with various Sr/Ca ratios. Based on these results, in addition to lasermeasurements, an optimum composition ofCe:LiSrCa1AlF6 will be determined. 1. INTRODUCTION There are two primary private-sector commercial markets for the cerium-doped colquiriites. The first will be as a new add-on component for research scientists who already have fourth harmonic (266 nm) laser systems in their laboratories. Severallaser companies have introduced diode-pumped 266 nm lasers into the marketplace, and there are over 2000 lasers in the fieldwith the capability ofoperating at 266 nm. The second commercial market that would have an interest in a well-developedCe-doped colquiriite laser material is the environmental monitoring industry. There are many chemical species ofinterest inthe UV that could be accessed ifa highly efficient, broadly tunable, all-solid-state system were available. For this market,laser manufacturers anticipate sales ofcomplete units, which would include the 266 nm pump source and the Ce:colquiriiteadd-on component. Through the appropriate selection of an optimized Ce-doped laser host and laser resonator component forfrequency tuning, the Ce:colquiriite crystals could generate output to probe the absorption ofatmospheric pollutants,contaminants, and aerosol species, such as ozone (280-320 nm). Several other species of interest in the UV include 502, 03,toluene, benzene, N0, ClO, and xylene. This market, albeit somewhat smaller than the market for direct harmonicgeneration at a fixed UV wavelength, is expected to grow rapidly in the next decade if sources and crystals become readilyavailable. Additionally, there are several government-based markets for tunable Ce-doped colquinite solid state lasers.Some ofthe non-classified applications include anti-ship missile defense devices, shipboard threat warning systems, short-range urban communication systems in the solar blind region ofthe ultraviolet, and covert communications systems to beutilized during in-flight fueling by Navy and Air Force pilots. There is also a need for a moderate energy source (lO-lOOmJ,10Hz, 280-320 nm) for standoffdetection ofbiological warfare monitoring and detection. Tryptophan, which is an agenttypically used to carry biological agents in the battlefield, has a broad absorption form 280-310 nm. 290 nm sources wouldbe useful for ground-based line-of-sight monitoring ofchemical pollutants from refineries, power plants, and smokestacks.Previous work with the growth ofthe cerium-doped colquiriites resulted in the selection ofa modified Czochralski furnacefor the growth ofthese crystals. Attempts at improving dopant distribution prompted growth runs using Na, Mg2, and Zn2as co-dopants. The use ofNa as a codopant resulted in crystals with the least amount ofcracking, more uniformity in Ce3doping, and a lower tendency for solarization.Cr:LiSr08CaAlF6was first reported as a potential tunable laser in November 1992.' In this report, it was noted that LiSAFexhibits a weaker crystal field strength, resulting in a larger cross section than Cr:LiCAF. The LiCAF host, however, hasmore favorable mechanical properties. The mixed crystal reportedly had spectroscopic and lasmg properties similar toLiSAF and mechanical properties more typical of LiCAF. In the case of cerium doping of the colquiriites, Ce:LiSA.F grows