Close look on cubic Tm:KYF crystal for highly efficient lasing on the H → H transition.

We report on Czochralski growth, detailed ground- and excited-state absorption and emission spectroscopy and highly-efficient mid-infrared (∼2.3 µm) laser operation of a cubic potassium yttrium fluoride crystal, Tm:KYF. The peak stimulated-emission cross-section for the H → H transition is 0.34×10 cm at 2345 nm with an emission bandwidth exceeding 50 nm. The excited-state absorption spectra for the F → F and F → H transitions are measured and the cross-relaxation is quantified. A continuous-wave 5 at.% Tm:KYF laser generated 0.84 W at 2331-2346 nm by pumping at 773 nm, with a record-high slope efficiency of 47.7% (versus the incident pump power) owing to the efficient action of energy-transfer upconversion leading to a pump quantum efficiency approaching 2. The first Tm:KYF laser with ESA-assisted upconversion pumping (at 1048 nm) is also demonstrated. Due to its broadband emission properties, Tm:KYF is promising for ultrashort pulse generation at ∼2.3-2.4 µm.