Material Preparation and optical properties of Ho-doped KPb2Cl5 and KPb2Br5 for applications as mid-IR gain media

In this work, comparative studies were performed on the material preparation and infrared spectroscopy of Ho³⁺- doped KPb₂Cl₅ (KPC) and KPb₂Br₅ (KPB) for applications as mid-IR gain media. KPC and KPB are non-hygroscopic crystals with narrow phonon spectra not exceeding ~200 and ~140 cm^-1, respectively. The maximum phonon energies of chloride and bromide crystals further reduce non-radiative relaxation compared to fluorides and enhance long wavelength IR emissions from trivalent rare-earth ions. The investigated Ho-doped KPC and KPB materials were synthesized through careful purification of commercial starting materials including directional freezing, zone-refinement, and halogination. The halogination process further removed oxidic impurities and enhanced the quality of the crystals.