Comparative study of rare-earth doped low-phonon fluoride and chloride crystals for mid-IR laser potential

摘要

A comparative study was performed on the mid-infrared emission properties of trivalent erbium (Er~(3+)) and holmium (Ho~(3+)) doped fluorides (BaF_2, NaYF_4) and ternary chloride-based crystals (CsCdCl_3, KPb_2Cl_5,> All crystals were grown by vertical Bridgman technique. Following optical excitation at 800 nm, all Er~(3+) doped fluorides and chlorides exhibited mid-infrared emissions at ~4500 nm at room temperature. The mid-infrared emission at 4500 nm, originating from the ~4I_(9/2) → ~4I_(11/2) transition, showed long emission lifetime values of ~11.6 ms and ~3.2 ms for Er~(3+) doped CsCdCl_3 and KPb_2Cl_5 crystals, respectively. In comparison, Er~(3+) doped and BaF_2 and NaYF_4 demonstrated rather short lifetimes in the microsecond range of ~47 μs and -205 μs, respectively. Temperature dependent decay time measurements were performed for the ~4I_(9/2) excited state for Er~(3+) doped BaF2_, NaYF_4, and CsCdCl_3 crystals. We noticed that the emission lifetimes of Er~(3+):CsCdCl_3 were nearly independent of the temperature, whereas significant emission quenching of ~4I_(9/2) level was observed for both Er~(3+) doped fluoride crystals. The temperature dependence of the multiphonon relaxation rate for 4.5 μm mid-IR emissions was determined for the studied Er~(3+) doped fluorides using the well-known energy-gap law. Using ~890 nm excitation, all studied Ho~(3+) doped fluorides and chlorides exhibited mid-infrared emissions at ~3900 nm originating from the ~5I_5 → ~5I_6 transition. The longest emission lifetime of the ~5I_5 level was determined to be ~14.55 ms from the Ho~(3+):CsCdCl_3 crystal. The room temperature stimulated emission cross-sections for the Er~(3+） ~4I_(9/2) → ~4I_(11/2) and Ho~(3+) ~5I_5 → ~5I_6 transitions were determined using the Fuechtbauer-Landenburg equation. Among the studied crystals, Er~(3+) doped chlorides are more than two orders of magnitude better in terms of emission lifetimes and sigma-tau product than the fluoride crystals.