Mid-IR emission characteristics of low-phonon erbium-doped ternary chloride-based single crystals

The mid-infrared fluorescence properties of erbium (Er) doped low-phonon ternary chloride-based crystals (KPb₂Cl₅, Cs₂HfCl₆, CsPbCl₃, CsCdCl₃) have been investigated. All crystals were grown by vertical Bridgman technique. Following optical excitations at 805 nm and 660 nm, all Er³⁺ doped chlorides exhibited infrared emissions at ~2750, ~3500, and ~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 ~7.8 ms and ~11.6 ms for Er³⁺ doped Cs₂HfCl₆ and CsCdCl₃ crystals, respectively. In comparison, Er³⁺ doped KPb₂Cl₅ and CsPbCl₃ demonstrated shorter lifetimes of ~3 ms and ~1.8 ms, respectively. The temperature dependence of the ⁴I_9/2 decay times was performed for Er³⁺ doped CsPbCl₃ and CsCdCl₃ crystals. We observed that the fluorescence lifetimes were nearly independent of the temperature, indicating a negligibly small non-radiative decay rate through multiphonon relaxation, as predicted by the energy gap law for low phonon energy hosts. The room temperature stimulated emission cross-sections for the ⁴I_9/2 → ⁴I_11/2 transition were determined to be in a range of ~0.14-0.54 x 10^-20 cm² for the studied Er doped chloride crystals.