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1. Comparative study of rare-earth doped low-phonon fluoride and chloride crystals for mid-IR laser potential

   E. Brown, Z. D. ( April 2021 , Proceedings SPIE 11723, Laser Technology for Defense and Security XVII)

   A comparative study was performed on the mid-infrared emission properties of trivalent erbium (Er3+) and holmium (Ho3+) doped fluorides (BaF2, NaYF4) and ternary chloride-based crystals (CsCdCl3, KPb2Cl5,). All crystals were grown by vertical Bridgman technique. Following optical excitation at 800 nm, all Er3+ doped fluorides and chlorides exhibited mid-infrared emissions at ~4500 nm at room temperature. The mid-infrared emission at 4500 nm, originating from the 4I9/2 -> 4I11/2 transition, showed long emission lifetime values of ~11.6 ms and ~3.2 ms for Er3+ doped CsCdCl3 and KPb2Cl5 crystals, respectively. In comparison, Er3+ doped and BaF2 and NaYF4 demonstrated rather short lifetimes in the microsecond range of ~47 us and ~205 us, respectively. Temperature dependent decay time measurements were performed for the 4I9/2 excited state for Er3+ doped BaF2, NaYF4, and CsCdCl3 crystals. We noticed that the emission lifetimes of Er3+:CsCdCl3 were nearly independent of the temperature, whereas significant emission quenching of 4I9/2 level was observed for both Er3+ doped fluoride crystals. The temperature dependence of the multiphonon relaxation rate for 4.5 um mid-IR emissions was determined for the studied Er3+ doped fluorides using the well-known energy-gap law. Using ~890 nm excitation, all studied Ho3+ doped fluorides and chlorides exhibited mid-infrared emissions at ~3900 nm originating from the 5I5 -> 5I6 transition. The longest emission lifetime of the 5I5 level was determined to be ~14.55 ms from the Ho3+:CsCdCl3 crystal. The room temperature stimulated emission cross-sections for the Er3+ 4I9/2 -> 4I11/2 and Ho3+ 5I5 -> 5I6 transitions were determined using the Füchtbauer-Landenburg equation. Among the studied crystals, Er3+ doped chlorides are more than two orders of magnitude better in terms of emission lifetimes and sigma-tau product than the fluoride crystals. 

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2. Ho 3+ doped low-phonon single crystals and chalcogenide glasses for mid-IR source application

   https://doi.org/10.1364/OME.487911  

   Brown, Ei Ei ; Fleischman, Zackery D. ; McKay, Jason ; Merkle, Larry ; Hommerich, Uwe ; Palosz, Witold ; Trivedi, Sudhir ; Dubinskii, Mark ( April 2023 , Optical Materials Express)

   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.

    

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3. Visible emission properties of melt-gown Dy-doped CsPbCl3 and KPb2Cl5 crystals,

   U. Hommerich, S. Uba ( July 2020 , Optical materials express)

   null (Ed.)

   We report results of the optical properties of Dy-doped CsPbCl3 and KPb2Cl5 bulk crystals for potential applications in yellow solid-state laser development. The crystals were synthesized from purified starting materials and melt-grown by vertical Bridgman technique. Optical transmission measurements revealed characteristic absorption bands from intra-4f transitions of Dy3+ ions. Direct optical excitation at 455 nm (6H15/2 -> 4I15/2) resulted in dominant yellow emission bands at 575 nm from the 4F9/2 excited state of Dy3+ ions. In addition, both crystals exhibited weaker emission lines in the blue (483 nm) and red (670 nm) regions. The peak emission-cross sections for the yellow transition (4F9/2 -> 6H13/2) were determined to be 0.22 x 10^20 cm2 for Dy: CsPbCl3 (lpeak = 576.5 nm) and 0.59 x 10^20 cm2 for Dy: KPb2Cl5 (lpeak = 574.5 nm). The spectral properties and decay dynamics of the 4F9/2 excited state were evaluated within the Judd-Ofelt theory to predict total radiative decay rates, branching ratios, and emission quantum efficiencies. 

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4. Visible emission studies of melt-grown Dy-doped CsPbCl 3 and KPb 2 Cl 5 crystals

   https://doi.org/10.1364/OME.398498  

   Hommerich, Uwe ; Uba, Samuel ; Kabir, A. ; Trivedi, Sudhir B. ; Yang, Clayton ; Brown, Ei Ei ( July 2020 , Optical Materials Express)

   We report results of the optical properties of Dy-doped CsPbCl₃and KPb₂Cl₅bulk crystals for potential applications in yellow solid-state laser development. The crystals were synthesized from purified starting materials and melt-grown by vertical Bridgman technique. Optical transmission measurements revealed characteristic absorption bands from intra-4f transitions of Dy³⁺ions. Direct optical excitation at 455 nm (⁶H_15/2→⁴I_15/2) resulted in dominant yellow emission bands at ∼575 nm from the⁴F_9/2excited state of Dy³⁺ions. In addition, both crystals exhibited weaker emission lines in the blue (∼483 nm) and red (∼670 nm) regions. The peak emission-cross sections for the yellow transition (⁴F_9/2→⁶H_13/2) were determined to be ∼0.22 × 10⁻²⁰cm²for Dy: CsPbCl₃(λ_peak= 576.5 nm) and ∼0.59 × 10⁻²⁰cm²for Dy: KPb₂Cl₅(λ_peak= 574.5 nm). The spectral properties and decay dynamics of the⁴F_9/2excited state were evaluated within the Judd-Ofelt theory to predict total radiative decay rates, branching ratios, and emission quantum efficiencies.

    

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5. Mid-IR Emission Characteristics of Low-Phonon Erbium doped Ternary Chloride-based Single Crystals

   E. Brown, Z. D. ( April 2021 , Proc. SPIE 11682, Optical Components and Materials.)

   null (Ed.)

   The mid-infrared fluorescence properties of erbium (Er) doped low-phonon ternary chloride-based crystals (KPb2Cl5,Cs2HfCl6, CsPbCl3, CsCdCl3) have been investigated. All crystals were grown by vertical Bridgman technique. Following optical excitations at 805 nm and 660 nm, all Er3+ doped chlorides exhibited infrared emissions at ~2750, ~3500, and ~4500 nm at room temperature. The mid-infrared emission at 4500 nm originating from the 4I9/2 -> 4I11/2 transition showed long emission lifetime values of ~7.8 ms and ~11.6 ms for Er3+ doped Cs2HfCl6 and CsCdCl3 crystals, respectively. In comparison, Er3+ doped KPb2Cl5 and CsPbCl3 demonstrated shorter lifetimes of ~3 ms and ~1.8 ms, respectively. The temperature dependence of the 4I9/2 decay times was performed for Er3+ doped CsPbCl3 and CsCdCl3 crystals. We observed that the fluorescence lifetimes were nearly independent of the temperature, indicating a negligibly small non-radiative decay rate through multiphonon relaxation, as predicted by the energy gap law for low phonon energy hosts. The room temperature stimulated emission cross-sections for the 4I9/2 -> 4I11/2 transition were determined to be in a range of ~0.14-0.54 x 10-20 cm2 for the studied Er doped chloride crystals. 

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