U3+ new activator ion in various fluoride matrices for infrared laser: synthesis and optical properties

Recently, there has been considerable interest in infrared diode pumped solid state lasers. U3+ in fluoride matrices is a good candidate, however the trivalent state is not the most stable valence state. We show that γ-irradiation can reduce 100% of U4+ in matrices such as LiYF4, LiYbF4 and β-YF3, but it fails mostly in the fluoride glasses where tetravalent sites are present. A dose rate of about 500 kGy is necessary for reducing 1000 ppm of uranium in the single crystals. Then the conversion rate decreases with the increase of uranium concentration. The large crystal-field in LiYF4 and β-YF3 induces a large ground multiplet splitting, allowing a four level scheme laser at room temperature for the 4I112 → 4I92 transitions terminating at the highest energy found multiplet. These transitions occur at 2.5, 2.83 and 2.97 μm for LiYF4 and 2.29, 2.5 and 2.66 μm in β-YF3. Moreover, the emission cross sections for these transitions are at least 10 times larger than the ones for 4f—4f transitions such as the emission at 2.7 μm of Er3+ for example in LiF4.