Energy transfer and controllable colors of upconversion emission in Er3+ and Dy3+ co-doped CaF2 transparent ceramics

x at. % Er3+, 3 at. % Dy3+: CaF2 transparent ceramics (x=1-5) with good transparency were fabricated by hot-pressed sintering. The phase composition of nanoparticles and transparent ceramics, microstructure, in-line transmittance, upconversion spectra and lifetime of transparent ceramics, as well as energy transfer mechanism between Er3+ and Dy3+ were investigated. The mean grain sizes of nanoparticles decreased from 33.0 nm to 26.2 nm with the Er3+ doping concentration increasing from 1 to 5 at.%. The microstructure of ceramic samples presented nearly dense microstructure and EDS analysis indicated Er3+ and Dy3+ were uniformly incorporated into CaF2 lattice. Under 900 nm excitation, the emission intensity for 4F9/2→6H15/2 transition of Dy3+ decreased and for 4S3/2→4I15/2 transition of Er3+ increased, the lifetime for the 4F9/2 level of Dy3+ decreased while the 4F7/2 level of Er3+ increased with the raise of Er3+ doping concentration. The energy transfer mechanism was proved to be the dipole-dipole interaction. The upconversion luminescence color was tuned from orange through yellow to green by changing the Er3+/Dy3+ ratio. In addition, the Vickers hardness, fracture toughness, and the thermal conductivity of Er3+, Dy3+: CaF2 transparent ceramics were discussed. All the results showed the Dy3+ could be used as a sensitizer for Er3+: CaF2 transparent ceramic in the upconversion field.