Blue, green and yellow upconversion lasing in Er:YLiF₄ using 1.5µm pumping

The authors report blue, green and yellow upconversion laser action in Er³⁺: YLiF₄ following pumping at 1.500µm below 80 K. For the blue transition the ratio of the output frequency to the input frequency is 3.3, the highest yet reported for an upconversion laser. In spite of the multiple upconversion steps, the thresholds for laser action were less than 80 mW. It is proposed that the nonlinear upconversion pumping proceeds via a sequence of several pairwise crossrelaxation steps.