Continuous-wave, fourfold upconversion laser

Abstract
We report a new Er3+:LiYF4 cryogenic upconversion laser pumped by a fourfold upconversion process. Excitation at 1.5 μm results in laser emission at 701.5 nm on a transition with an upper state at nearly four times the pump photon energy. Mechanisms and the concept of cooperative upconversion enhancement are examined.