Crystal growth, upconversion, and infrared emission properties of Er3+-doped KPb2Br5

We report on the material preparation and optical properties of Er3+-doped KPb2Br5 (KPB). KPB has a maximum phonon energy of only 138 cm-1 and is non-hygroscopic, which makes KPB an attractive candidate for solid-state laser applications. The preparation of Er doped KPB was based on a careful purification of starting materials followed by self-seeded Bridgman crystal growth. Under 975 nm diode laser pumping, Er:KPB revealed intense blue upconversion emission. For comparison, Er-doped KPb2Cl5 (KPC), which has a maximum phonon energy of 203 cm-1, exhibited a dominant green Er3+ upconversion emission. The blue upconversion from Er:KPB can be attributed to emission from the 4F7/2 excited state of Er3+, which is quenched in most solid hosts due to strong multiphonon non-radiative decay. Due to the small phonon energy of KPB, the 4F7/2 level becomes highly radiative with a room-temperature lifetime of ∼85 μs and an estimated quantum efficiency of ∼94%. For comparison, the 4F7/2 decay time in Er:KPC was only ∼11 μs at room temperature and the radiative quantum efficiency was estimated to be ∼9%. Infrared (IR) emission bands were observed at 1.5 μm (4I13/2 →4I15/2), 1.7 μm (4I9/2→4I13/2), 2.0 μm (4F9/2→4I13/2), 2.7 μm (4I11/2→4I13/2), 3.6 μm (4F9/2→4I9/2), and 4.5 μm (4I9/2→4I11/2), indicating the potential of Er:KPB for IR laser applications. The absorption and IR emission properties of Er:KPB were investigated in terms of transition linestrengths, branching ratios, radiative decay rates, and emission cross-sections using the Judd-Ofelt method