Energy levels and intensity parameters of Ho³⁺ions in GdLiF₄, YLiF₄and LuLiF₄

While the energy levels in Ho:YLF have been measured previously, they have not been as thoroughly investigated in the isomorphs, Ho:LuLF and Ho:GdLF. We report here the measurement of the energy levels of the trivalent lanthanide Ho 3+ in GdLiF 4 (GdLF), YLiF 4 (YLF), and LuLiF 4 (LuLF). The measurement of the energy levels of Ho:YLF, although they have been measured before, are repeated here for self-consistent comparison to Ho:LuLF and Ho:GdLF. The Stark split levels for the first ten Ho manifolds in these materials have been measured, and the results have been fitted to a free-ion plus crystal-field Hamiltonian to generate a theoretical set of energy levels. Crystal-field parameters were varied to determine the best fit between experimental and theoretical energy levels. The energy levels of Ho:GdLF and Ho:LuLF are seen to be very similar to those in Ho:YLF. However, subtle changes resulting from replacing Y 3+ with Gd 3+ or Lu 3+ in the fluoride crystal YLiF 4 result in shorter transition wavelengths in GdLF and longer transition wavelengths in LuLF. This has implications for Ho lasers operating at ∼2.0 µm. The energy levels for Ho:GdLF and Ho:LuLF determined here indicate that Ho:GdLF will have a larger lower laser level thermal population than Ho:YLF, while Ho:LuLF lasers will have a smaller lower laser level thermal population than Ho:YLF. This is consistent with the larger Stark splitting associated with the larger host ions that Ho substitutes for in these lithium fluoride materials. The intensity parameters are also determined from a Judd–Ofelt analysis and used to calculate radiative lifetimes and branching ratios for the first ten manifolds in Ho:GdLF, Ho:YLF and Ho:LuLF.