Stimulated-Emission, Phonon Sidebands and Energy-Transfer Upconversion in Ho³⁺:MF₂ Crystals for Lasers at 2-3 μm

Low-phonon-energy materials (crystals and glasses) doped with Holmium ions (Ho<sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3+</sup>) have been in the spotlight of research in recent years for the development of solid-state lasers emitting in the near-to mid-infrared, at 2 to 4 μm [1], [2], Fig. 1(a). Fluorite-type crystals M<sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2+</sup>F<inf xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</inf> (where M stands for Ca, Sr, or Ba), are suitable for rare-earth doping due to a combination of good thermal properties, low phonon energies, and a strong ion clustering leading to “glassy-like” emission / gain profiles supporting femtosecond pulse generation [3]. We report on the stimulated-emission properties, and quantification of electron-phonon interaction and energy-transfer upconversion (ETU) in Ho3+-doped fluorite-type crystals for developing power-scalable and ultrafast lasers operating at <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">$2-3 \mu \mathrm{m}$</tex>.