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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 (January 2023, Optical Materials Express)

   A comparative study was conducted to investigate the 3.9 µm mid-IR emission properties of Ho 3+ doped NaYF 4 and CsCdCl 3 crystals as well as Ho 3+ doped Ga 2 Ge 5 S 13 glass. 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 the 5 I 5 → 5 I 6 transition, showed long emission lifetime values of ∼16.5 ms and ∼1.61 ms for Ho 3+ doped CsCdCl 3 crystal and Ga 2 Ge 5 S 13 glass, respectively. Conversely, the Ho 3+ doped NaYF 4 crystal exhibited a relatively short lifetime of ∼120 µs. Temperature dependent decay time measurements were performed for the 5 I 5 excited state for all three samples. The results showed that the emission lifetimes of Ho 3+ :CsCdCl 3 and Ho 3+ :Ga 2 Ge 5 S 13 were nearly temperature independent over the range studied, while significant emission quenching of the 5 I 5 level was observed in Ho 3+ :NaYF 4 . The temperature dependence of the multi-phonon relaxation rate for 3.9 µm mid-IR emission in Ho 3+ :NaYF 4 crystal 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. 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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4. 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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5. Evaluating mid-IR laser potential of Er ³⁺ vs. Dy ³⁺ in low-maximum-phonon-energy hosts for 4.1-4.8 µm applications

   https://doi.org/10.1364/OME.572481  

   Brown, Ei Ei; Fleischman, Zackery D; Merkle, Larry; Hommerich, Uwe; McKay, Jason; Palosz, Witold; Trivedi, Sudhir; Dubinskii, Mark (January 2025, Optical Materials Express)

   Andrea, A (Ed.)

   A comparative spectroscopic investigation was conducted to evaluate the potential of Dy3+ and Er3+ doped Ga2Ge5S13 chalcogenide glass and CsCdCl3 crystal for suitability as gain media for mid-infrared lasers operating within the 4.1-4.8 um spectral range. Both rare-earth ions exhibited broad mid-IR emissions at room temperature centered at ∼4.4-4.5 µm, attributed to the Dy3+: 6H11/2→H13/2 and Er3+:4I9/2→4I11/2 transitions. The fluorescence lifetimes of these mid-IR transitions varied significantly between these two host materials. Dy3+and Er3+ doped CsCdCl3 crystals exhibited long lifetimes of ∼9-12 ms, while the same ions in Ga2Ge5S13 glass yielded lifetimes in the range of ∼1 ms. Temperature-dependent lifetime measurements further confirmed the role of multiphonon relaxation in Dy3+: Ga2Ge5S13, although fitting with the temperature dependence of the multiphonon energy-gap law indicated the presence of additional nonradiative decay channels, likely involving impurity interactions. In contrast, Er3+ emissions exhibited minimal temperature dependence across both hosts, with measured lifetimes closely matching calculated radiative values. Judd-Ofelt analyses revealed radiative properties and transition probabilities comparable to other low-maximum-phonon-energy rare-earth-doped hosts, with the studied Dy3+ doped host materials standing out by offering sigma-tau products several times higher than those of Er3+in the same hosts. 

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