Infrared spectroscopy of Dysprosium doped KPb$_{2}$Br$_{5}$ and KPb$_{2}$Cl$_{5}$.

The infrared optical properties of rare earth doped crystals with narrow phonon spectrum ($<$ 300 cm$^{-1})$ remains of current interest for applications in IR solid-state gain media. The maximum phonon energies of KPb$_{2}$Cl$_{5}$ and KPb$_{2}$Br$_{5}$ are only $\sim $200 cm$^{-1}$ and $\sim $150 cm$^{-1}$, which reduce non-radiative decay through multi-phonon relaxations. In this work, we present spectroscopic results of Dy: KPb$_{2}$Cl$_{5}$ and Dy: KPb$_{2}$Br$_{5}$ for possible applications in mid-infrared gain media. The investigated materials were grown by horizontal and vertical Bridgman technique. Dy: KPb$_{2}$Cl$_{5}$ and Dy: KPb$_{2}$Br$_{5}$ exhibited characteristic Dy$^{3+}$ absorption bands in the visible and infrared regions. Optical excitation at $\sim $800 nm resulted in the observation of a broad 4-5 $\mu $m mid-IR emission ($^{6}$H$_{11/2}$ $\to \quad ^{6}$H$_{13/2})$ at room temperature. The mid-IR emission lifetime was measured to be $\sim $5.5 ms for Dy: KPb$_{2}$Cl$_{5}$ and $\sim $3.8 ms for Dy: KPb$_{2}$Br$_{5}$, respectively. Based on temperature dependent lifetime studies and Judd-Ofelt calculations, the emission quantum efficiencies for the 4-5 $\mu $m bands were estimated to be near unity. More details on the IR optical properties of Dy: KPb$_{2}$Cl$_{5}$ and Dy: KPb$_{2}$Br$_{5 }$will be presented at the conference.