Watt-Level Thulium Laser Operating on the 3H4 → 3 H5 Transition with ~70% Slope Efficiency

A Thulium laser operating on the 3H4→3H5 transition in high-phonon-energy KLu(WO4)2 generates 1.12 W at 2.22&2.29 μm with a record-high slope efficiency of 69.2% representing a two-for-one pump process through efficient energy-transfer upconversion.