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A comparative study was conducted to investigate the 3.9 µm mid-IR emission properties of Ho³⁺doped NaYF₄and CsCdCl₃crystals as well as Ho³⁺doped Ga₂Ge₅S₁₃glass. Following optical excitation at ∼890 nm, all the studied materials exhibited broad mid-IR emissions centered at ∼3.9 µm at room temperature. The mid-IR emission at 3.9 µm, originating from the⁵I₅→⁵I₆transition, showed long emission lifetime values of ∼16.5 ms and ∼1.61 ms for Ho³⁺doped CsCdCl₃crystal and Ga₂Ge₅S₁₃glass, respectively. Conversely, the Ho³⁺doped NaYF₄crystal exhibited a relatively short lifetime of ∼120 µs. Temperature dependent decay time measurements were performed for the⁵I₅excited state for all three samples. The results showed that the emission lifetimes of Ho³⁺:CsCdCl₃and Ho³⁺:Ga₂Ge₅S₁₃were nearly temperature independent over the range studied, while significant emission quenching of the⁵I₅level was observed in Ho³⁺:NaYF₄. The temperature dependence of the multi-phonon relaxation rate for 3.9 µm mid-IR emission in Ho³⁺:NaYF₄crystal was determined. The room temperature stimulated emission cross-sections for all three samples were calculated using the Füchtbauer-Landenburg equation. Furthermore, the results of Judd-Ofelt analysis are presented and discussed. 

The mid-IR spectroscopic properties of ${\mathrm{E}\mathrm{r}}^{3+}$doped low-phonon ${\mathrm{C}\mathrm{s}\mathrm{C}\mathrm{d}\mathrm{C}\mathrm{l}}_3$and ${\mathrm{C}\mathrm{s}\mathrm{P}\mathrm{b}\mathrm{C}\mathrm{l}}_3$crystals grown by the Bridgman technique have been investigated. Using optical excitations at $\sim <\#comment/>800\mathrm{n}\mathrm{m}$and $\sim <\#comment/>660\mathrm{n}\mathrm{m}$, both crystals exhibited IR emissions at $\sim <\#comment/>1.55$, $\sim <\#comment/>2.75$, $\sim <\#comment/>3.5$, and $\sim <\#comment/>4.5\text{µ<\#comment/>}\mathrm{m}$at room temperature. The mid-IR emission at 4.5 µm, originating from the ${}^4{\mathrm{I}}_{9/2}\to <\#comment/>{}^4{\mathrm{I}}_{11/2}$transition, showed a long emission lifetime of $\sim <\#comment/>11.6\mathrm{m}\mathrm{s}$for ${\mathrm{E}\mathrm{r}}^{3+}$doped ${\mathrm{C}\mathrm{s}\mathrm{C}\mathrm{d}\mathrm{C}\mathrm{l}}_3$, whereas ${\mathrm{E}\mathrm{r}}^{3+}$doped ${\mathrm{C}\mathrm{s}\mathrm{P}\mathrm{b}\mathrm{C}\mathrm{l}}_3$exhibited a shorter lifetime of $\sim <\#comment/>1.8\mathrm{m}\mathrm{s}$. The measured emission lifetimes of the ${}^4{\mathrm{I}}_{9/2}$state were nearly independent of the temperature, indicating a negligibly small nonradiative decay rate through multiphonon relaxation, as predicted by the energy-gap law for low-maximum-phonon energy hosts. The room temperature stimulated emission cross sections for the ${}^4{\mathrm{I}}_{9/2}\to <\#comment/>{}^4{\mathrm{I}}_{11/2}$transition in ${\mathrm{E}\mathrm{r}}^{3+}$doped ${\mathrm{C}\mathrm{s}\mathrm{C}\mathrm{d}\mathrm{C}\mathrm{l}}_3$and ${\mathrm{C}\mathrm{s}\mathrm{P}\mathrm{b}\mathrm{C}\mathrm{l}}_3$were determined to be $\sim <\#comment/>0.14\times <\#comment/>{10}^{-<\#comment/>20}{\mathrm{c}\mathrm{m}}^2$and $\sim <\#comment/>0.41\times <\#comment/>{10}^{-<\#comment/>20}{\mathrm{c}\mathrm{m}}^2$, respectively. The results of Judd–Ofelt analysis are presented and discussed. 

Evidence is presented that a “three-for-one” process based on two cross-relaxations between Pr³⁺ions efficiently populates the mid-infrared-emitting³H₅manifold in a Pr³⁺-doped low-maximum-phonon-energy host. The concentration dependence of infrared fluorescence spectra and lifetimes of polycrystalline Pr:KPb₂Cl₅initially excited to the³F_3,4manifolds 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 the³F₃and³H₆manifold lifetimes suggests that both of these decay by cross-relaxation processes, resulting in more than one ion excited to³H₅for each ion initially excited to³F₃. Indeed, modeling and accounting for all possible decay paths indicate that, on average, about 2.3 ions are excited to³H₅for 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.