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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. 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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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. Mid-IR spectroscopy of Er ³⁺ doped low-phonon CsCdCl ₃ and CsPbCl ₃ crystals

   https://doi.org/10.1364/JOSAB.470152  

   Brown, Ei Ei; Fleischman, Zackery D.; McKay, Jason; Hommerich, Uwe; Kabir, Al Amin; Riggins, Jazmine; Trivedi, Sudhir; Dubinskii, Mark (January 2023, Journal of the Optical Society of America B)

   The mid-IR spectroscopic properties of E r 3 + doped low-phonon C s C d C l 3 and C s P b C l 3 crystals grown by the Bridgman technique have been investigated. Using optical excitations at ∼ 800 n m and ∼ 660 n m , both crystals exhibited IR emissions at ∼ 1.55 , ∼ 2.75 , ∼ 3.5 , and ∼ 4.5 µ m at room temperature. The mid-IR emission at 4.5 µm, originating from the 4 I 9 / 2 → 4 I 11 / 2 transition, showed a long emission lifetime of ∼ 11.6 m s for E r 3 + doped C s C d C l 3 , whereas E r 3 + doped C s P b C l 3 exhibited a shorter lifetime of ∼ 1.8 m s . The measured emission lifetimes of the 4 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 I 9 / 2 → 4 I 11 / 2 transition in E r 3 + doped C s C d C l 3 and C s P b C l 3 were determined to be ∼ 0.14 × 10 − 20 c m 2 and ∼ 0.41 × 10 − 20 c m 2 , respectively. The results of Judd–Ofelt analysis are presented and discussed. 

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