Prospects of low-phonon energy Ho:MF2 (M = Ca, Sr, Ba) crystals for 2-3 µm lasers

Abstract

We report on a comparative spectroscopic study of Ho3+-doped fluorite-type crystals MF2 (where M stands for Ca, Sr, and Ba), regarding their applications in lasers emitting at 2 to 3 μm. The transition intensities of Ho3+ ions are determined within a modified Judd-Ofelt analysis accounting for configuration interaction. The stimulated-emission cross-sections for the 5I7 →5I8 (at 2.1 μm) and 5I6 →5I7 (at 2.9 μm) are determined and compared with the reference Ho:LiYF4 crystal. Effects related to electron-phonon interaction are studied: i) the multiphonon sidebands responsible for long-wave emission far beyond 2 μm are observed; ii) the rates of multiphonon non-radiative relaxation from the 5I5, 5I6 and 5I7 Ho3+ manifolds are computed and linked to the phonon spectra of MF2 crystals, and iii) the parameters of phonon-assisted energy-transfer upconversion from the 5I7 level are quantified using the Burshtein model. Owing to their low phonon energy behavior and inhomogeneously broadened spectral bands, Ho:MF2 crystals are promising for broadly tunable and mode-locked lasers.