Mid-infrared laser operation of Er³⁺-doped BaF₂ and (Sr,Ba)F₂ crystals

We report on the first, to our knowledge, mid-infrared laser operation of two Er³⁺-doped barium-containing fluorite-type crystals, BaF₂ and (Sr,Ba)F₂, featuring a low-phonon energy behavior. A continuous wave 4.9 at.% Er:(Sr,Ba)F₂ laser generated 519 mW at 2.79 µm with a slope efficiency of 25.0% and a laser threshold of 27 mW. The vibronic and spectroscopic properties of these crystals are determined. The phonon energy of (Sr,Ba)F₂ is as low as 267 cm^-1. The Er³⁺ ions in this crystal feature a broadband emission owing to the ⁴I_11/2 → ⁴I_13/2 transition and a long luminescence lifetime of the ⁴I_11/2 level (10.6 ms) making this compound promising for low-threshold, broadly tunable, and pulsed 2.8-µm lasers.