Enhanced mid-infrared emission of Pr³⁺ ions in solids through a "3-for-1" excitation process - quantified

Evidence is presented that a "three-for-one" process based on two cross-relaxations between Pr³⁺ ions efficiently populates the mid-infrared-emitting ³H₅ manifold in a Pr³⁺-doped low-maximum-phonon-energy host. The concentration dependence of infrared fluorescence spectra and lifetimes of polycrystalline Pr:KPb₂Cl₅ initially excited to the ³F_3,4 manifolds indicate that the 3500-5500-nm fluorescence becomes strongly favored over shorter-wavelength infrared emission bands in the higher-concentration sample. The strong concentration dependence of the ³F₃ and ³H₆ manifold lifetimes suggests that both of these decay by cross-relaxation processes, resulting in more than one ion excited to ³H₅ for each ion initially excited to ³F₃. Indeed, modeling and accounting for all possible decay paths indicate that, on average, about 2.3 ions are excited to ³H₅ for each initially-excited ion. This confirms that the three-for-one excitation process must occur and contribute significantly to the total excitation efficiency. These results indicate that the two distinct cross-relaxation processes observed between Pr ions result in substantially higher excitation quantum efficiency, 230%, than any ever reported in rare-earth doped materials.