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1. 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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2. 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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3. Spectroscopic Evaluation of Er: Ga2Ge5S13 Glass for Mid-IR Laser Applications

   E. Brown; Z. D. Fleischman; J. McKay; U. Hommerich; W. Palosz; S. Trivedi; and M. Dubinskii (July 2022, Optical materials express)

   We present mid-IR spectroscopic characterization of the low-phonon chalcogenide glass, Ga2Ge5S13 (GGS) doped with Er3+ ions. Under the excitation at ∼800 nm, Er3+:GGS exhibited broad mid-IR emission bands centered at ∼2.7, ∼3.5, and ∼4.5 µm at room temperature. The emission lifetime of the 4I9/2 level of Er3+ ions in GGS glass was found to be millisecond-long at room temperature. The measured fluorescence lifetimes were nearly independent of temperature, indicating negligibly small nonradiative decay rate for the 4I9/2 state, as can be expected for a low-maximum-phonon energy host. The transition line-strengths, radiative lifetimes, fluorescence branching ratios were calculated by using the Judd-Ofelt method. The peak stimulated emission cross-section of the 4I9/2 → 4I11/2 transition of Er3+ ion was determined to be ∼0.10×10−20 cm2 at room temperature. 

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4. 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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5. Comparative study on upconversion and downconversion luminescence of Er3+, Yb3+ co-doped BaY0.67Mo0.33O3 phosphors for optical thermometry

   https://doi.org/10.1117/12.3078751  

   Talewar, Rupesh A; Williams, Marcus; Reddy, B Rami (March 2026, SPIE)

   Digonnet, Michel J; Jiang, Shibin (Ed.)

   This study presents a comparative analysis of upconversion and downconversion luminescence of Er3+,Yb3+ co-doped BaY0.67Mo0.33O3 phosphors for potential application in optical thermometry. The phosphors were synthesized via a conventional solid-state reaction method and characterized using photoluminescence spectroscopy and temperaturedependent emission studies. Under near-infrared excitation (980 nm), intense green and red upconversion emissions were observed. An efficient energy transfer from Yb3+ to Er3+ ions. Simultaneously, near-infrared downconversion emissions were obtained under 980 nm excitation. The intensity ratio of thermally coupled levels 2H11/2(4S3/2)→4I15/2 exhibited strong temperature sensitivity, enabling precise optical temperature readout. The results demonstrate that BaY0.67Mo0.33O3:Er3+,Yb3+ is a promising dual-mode luminescent material for advanced non-contact temperature sensing applications. 

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