Prospects of low-phonon energy Ho:MF ₂ (M?=?Ca, Sr, Ba) crystals for 2-3?µm lasers

We report on a comparative spectroscopic study of Ho 3+ -doped fluorite-type crystals MF 2 (where M stands for Ca, Sr, and Ba), regarding their applications in lasers emitting at 2 to 3 m. The transition intensities of Ho 3+ ions are determined within a modified Judd-Ofelt analysis accounting for configuration interaction. The stimulated-emission cross-sections for the 5 I 7 ? 5 I 8 (at 2.1 m) and 5 I 6 ? 5 I 7 (at 2.9 m) are determined and compared with the reference Ho:LiYF 4 crystal. Effects related to electron-phonon interaction are studied: i) the multiphonon sidebands responsible for long-wave emission far beyond 2 m are observed; ii) the rates of multiphonon non-radiative relaxation from the 5 I 5 , 5 I 6 and 5 I 7 Ho 3+ manifolds are computed and linked to the phonon spectra of MF 2 crystals, and iii) the parameters of phonon-assisted energy-transfer upconversion from the 5 I 7 level are quantified using the Burshtein model. Owing to their low phonon energy behavior and inhomogeneously broadened spectral bands, Ho:MF 2 crystals are promising for broadly tunable and mode-locked lasers.