Mid-infrared emission properties of Er3+,Dy3+-codoped CaF2 crystals

We present a comprehensive spectroscopic investigation of singly Dy3+-doped and Er3+,Dy3+-co-doped calcium fluoride (CaF2) crystals for mid-infrared laser applications around 3 μm and 4.4 μm. The f–f transition probabilities of Dy3+ were evaluated using a modified Judd–Ofelt formalism that accounts for configuration interaction effects. The stimulated-emission cross section (σSE) reaches 0.25 × 10−20 cm2 at 2.93 μm for the 6H13/2 → 6H15/2 transition, with an emission bandwidth of ∼350 nm. This transition exhibits a pronounced phonon sideband extending up to 3.9 μm. The luminescence lifetime of the 6H13/2 level decreases from 2.04 to 0.52 ms as the Dy3+ concentration raises from 0.1 to 0.5 at.%. Energy transfer from Er3+ to Dy3+ in codoped crystals is quantified, demonstrating an efficient pathway for sensitized pumping. For the first time, 4.4 μm emission from Dy3+:CaF2 is observed at room temperature. The interplay between radiative and non-radiative transition rates is analyzed, highlighting the critical role of phonon-assisted relaxation in mid-IR emission. Finally, the potential of other Dy3+-doped fluorite-type hosts, such as SrF2 and BaF2, is evaluated.