Highly efficient 2.3 µm thulium lasers based on a high-phonon-energy crystal: evidence of vibronic-assisted emissions

We report on highly efficient and power-scalable laser operation in a thulium-doped high-phonon-energy crystal [monoclinic double tungstate, KLu(WO4)2] on the 3H4→3H5Tm3+ transition giving rise to the short-wave infrared emission at ∼2.3µm. A 3 at. % Tm-doped crystal generated a maximum continuous-wave output power of 1.12 W at ∼2.22 and 2.29 µm with a record-high slope efficiency of 69.2% (versus the absorbed pump power), a slightly multimode beam (Mx,y2=2.2 and 2.6), and a linear laser polarization. The ∼2.3µm laser outperformed the one operating on the conventional 3F4→3H6 transition (at ∼1.95µm). The effect of the Tm concentration on the ∼2.3µm laser performance indicates a gradually increasing pump quantum efficiency for the 3H4 upper laser level with the Tm doping. For the 3 at. % Tm-doped crystal, it reached 1.8±0.1 (almost two-for-one pump process), which is attributed to efficient energy-transfer upconversion. We discuss the physical nature of the laser emissions occurring at intermediate wavelengths between the electronic 3H4→3H5 and 3F4→3H6 transitions and highlight the role of electron-phonon coupling (vibronic processes) in the appearance of such laser lines. This allowed us to better understand the near- and mid-infrared emission from thulium ions, which can be used in broadly tunable and fs mode-locked 2–2.3 µm lasers.