Material purification, crystal growth, and spectroscopy of Tm-doped KPb2Cl5 and KPb2Br5 for 2 µm photonic applications

Results of the material purification, crystal growth, and optical characterization of Tm-doped KPb2Cl5 (KPC) and KPb2Br5 (KPB) for ~2 μm photonic applications are presented. Undoped KPC and KPB were synthesized from ultra dry starting materials and purified through multi-pass translation in a zone-refinement system. 1 wt% of ultra-dry Tm-halides (TmCl3 or TmBr3) were added to the purified host materials and molten under bubbling of HCl or HBr gas, respectively. Tm: KPC and Tm: KPB crystals were subsequently grown using the Bridgman technique. The good optical quality of the resulting crystals was indicated by infrared transmissions as high as ~70-75%. Under optical pumping, both crystals exhibited broad infrared emission bands centered at ~1.82 µm with nearly exponential decay times ranging from ~2-3 ms. Judd-Ofelt intensity parameters were calculated to determine the radiative lifetimes of the 3F4→3H6 Tm3+ transition. The optical absorption and emission cross-sections of Tm: KPC and Tm: KPB were determined using the McCumber theory. The investigated crystals were also evaluated as potential solid-state materials for optical cooling applications.