Abstract

Crystals of ${\mathrm{BaY}}_{1.75}{\mathrm{Yb}}_{0.24}{\mathrm{Tm}}_{0.01}{\mathrm{F}}_8,$ ${\mathrm{BaY}}_{1.50}{\mathrm{Yb}}_{0.49}{\mathrm{Tm}}_{0.01}{\mathrm{F}}_8,$ and ${\mathrm{BaY}}_{1.00}{\mathrm{Yb}}_{0.99}{\mathrm{Tm}}_{0.01}{\mathrm{F}}_8$ 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 ${\mathrm{BaY}}_{1.75}{\mathrm{Yb}}_{0.20}{\mathrm{Tm}}_{0.05}{\mathrm{F}}_8$ is considered as representative of the upconversion laser media. Stark levels for the ${}^3{H}_6,$ ${}^3{F}_4,$ ${}^3{H}_5,$ ${}^3{H}_4,$ ${}^3{F}_3,$ ${}^3{F}_2,$ ${}^1{G}_4,$ ${}^1{D}_2,$ and ${}^1{I}_6$ 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.

© 1998 Optical Society of America

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