Orientation-dependent fluorescence studies and spectroscopic analysis of doped barium yttrium fluoride upconversion laser crystals (BaY_2–x–yYb_xTm_yF_8)

Abstract

Crystals of BaY1.75Yb0.24Tm0.01F8,BaY1.50Yb0.49Tm0.01F8, and BaY1.00Yb0.99Tm0.01F8 grown by the gradient-freeze method under a hydrogen fluoride atmosphere operate as lasers at 348, 456, 482, 512, and 649 nm when upconversion pumped at 960 nm. The 649-nm red lasers show a clear orientation dependence; lasing is observed only when propagation is perpendicular to the crystallographic [010] direction. The orientation dependence of the red fluorescence confirms that the laser transition is exclusively polarized parallel to [010]. Ellipsoidal representations of intensities are determined from fluorescence data for several red and blue transitions. A crystal with composition BaY1.75Yb0.20Tm0.05F8 is considered as representative of the upconversion laser media. Stark levels for the 3H6,3F4,3H5,3H4,3F3,3F2,1G4,1D2, and 1I6 states are determined from absorption and fluorescence over a range of temperatures from 10 to 298 K. A Judd–Ofelt analysis that averages data for three orthogonal polarizations is also performed.