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:KY3F10. The peak stimulated-emission cross-section for the 3H4 → 3H5 transition is 0.34×10−20 cm2 at 2345 nm with an emission bandwidth exceeding 50 nm. The excited-state absorption spectra for the 3F4 → 3F2,3 and 3F4 → 3H4 transitions are measured and the cross-relaxation is quantified. A continuous-wave 5 at.% Tm:KY3F10 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:KY3F10 laser with ESA-assisted upconversion pumping (at 1048 nm) is also demonstrated. Due to its broadband emission properties, Tm:KY3F10 is promising for ultrashort pulse generation at ∼2.3-2.4 µm.

The 3 H 4 → 3 H 5 transition of Thulium ions (Tm 3+), which features laser emission at ~2.3 µm is studied in details. We revise the conditions for efficient laser operation using a rateequation model accounting for the ground-state bleaching, crossrelaxation and energy-transfer upconversion (ETU). We show that ETU has a crucial role in reaching more than unity pump quantum efficiency (QY) for ~2.3 µm Tm lasers based on highlydoped crystals. A Ti:Sapphire pumped quasi-continuous-wave 3.5 at.% Tm:LiYF 4 laser generated 0.73 W at 2306 nm with a record-high slope efficiency of 47.3% (versus the absorbed pump power) featuring a QY of 1.27. Diode-pumping of this crystal yielded a peak output power of >2 W. The first 2.3 µm Tm waveguide laser is also reported based on Tm:LiYF 4 epitaxial layers with even higher doping of 6.2 at.% generating 0.23 W with a slope efficiency of 19.8%. The spectroscopic properties of Tm:LiYF 4 relevant for the ~2.3 µm laser operation are revised as well.

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