Ultra-low maximum-phonon energy of the ternary-halide crystalline hosts and multi-millisecond-long 3H5 level lifetimes of Pr3+ dopant make them promising gain media for mid-infrared lasers in the 4.6-5.1 µm spectral domain with “3-for-1” enhanced laser efficiency.

In this work, we present the results of our recent spectroscopic investigation on Dy3+-doped Ga2Ge5S13 (Dy:GGS) glass, aiming to explore its potential for mid-infrared (3-5 µm) laser applications. Under 910 nm excitation, the studied Dy:GGS glass displayed broad emission bands centered at ~2.9 µm and ~4.35 µm corresponding to 6H13/2 --> 6H15/2 and 6H11/2 --> 6H13/2, respectively. The measured fluorescence decay time of the 6H11/2 manifold (upper laser level for 4.35 µm laser transition) was found to be in the millisecond range, demonstrating similarity to other sulfide glasses doped with Dy3+. Spectroscopic results and data modeling including the temperature dependent emission and decay dynamics, concentration dependent studies, Judd-Ofelt analysis, and transition cross-sections, will be presented.

We have developed a simple approach to deriving the efficiency of Q-switched four-level lasers, valid even for arbitrarily long lower laser level lifetimes. By eliminating time dependence from the calculation, numerical solutions can be obtained very rapidly. Its threshold and limiting slope efficiency values provide useful estimates for free-running pulsed four-level lasers as well as Q-switched.

All material types are being considered, from crystals to ceramics and glasses, with focus on those RE3+ hosts with low maximum energy. In this work, a comparative study was performed on the mid-IR (3-5 um) spectroscopic properties of erbium doped in low-phonon fluoride (BaF2) and chloride (CsCdCl3) crystals as well as sulphide (Ga2Ge5S13) glasses. Among the studied materials, Er3+:CsCdCl3 showed the longest 4I9/2 emission lifetime of ~11 ms whereas the ~ 46 us observed from Er3+:BaF2 was the shortest 4I9/2 lifetime. These results reflect the reduced nonradiative rates through multiphonon relaxation in chloride crystals. Spectroscopic results and data modeling including the temperature dependent emission and decay dynamics, Judd-Ofelt analysis, and transition cross-sections will be presented.

The comparative analysis of Er3+- and Dy3+-doped low-phonon laser gain materials aiming to identify the best dopant for directly diode-pumped mid-infrared lasers operating in the 4.1-4.8 µm spectral domain has been performed for the first time.

With the goal of developing new mid-IR laser sources, Rare-earth doped low-phonon crystals and sulfide-based chalcogenide glasses are being explored. Low maximum phonon energy materials are necessary to minimize competing non-radiative decay processes such as multi-phonon relaxation (MPR). This work presents the results of a comprehensive mid-IR spectroscopic study on Dy3+ doped sulfide-based chalcogenide glasses, as well as comparative results from similarly doped chloride and fluoride crystals. Spectroscopic results will focus on absorption, fluorescence, and decay characteristics. From these measurements, laser relevant parameters such as cross sections and radiative lifetimes are calculated.

Development of new solid-state mid-infrared (mid-IR) laser sources for wide range of applications in remote sensing, free-space communications, materials processing, and medicine remains to be a challenge, and demands advanced laser material development. RE 3+ ions possess promising emission transitions in the mid-IR spectral region but require host materials with low maximum phonon energies to circumvent competing nonradiative multi-phonon relaxation (MPR), thus preserving high efficiency of mid-IR emission [1–3]. RE 3+ doped low-phonon fluoride $(300-450\ \text{cm}^{-1})$ and ternary chloride crystals $(200-250\ \text{cm}^{-1})$ are well studied as laser materials for mid-IR. Recently, interest in chalcogenide glasses increased significantly due to their chemical and mechanical durability, wide mid-IR transparency, sufficiently low phonon energies. Among RE 3+ ions, trivalent dysprosium Dy 3+ , holmium Ho 3+ , and erbium Er 3+ are known to be the most common laser active ions in solid host materials capable of producing laser output in the 3–5 $\upmu\mathrm{m}$ spectral range [1].

We have developed a simple approach to deriving the efficiency of Q-switched four- level lasers, valid for long lower laser level lifetimes. Its threshold and limiting slope efficiency provide useful estimates for free-running four-level lasers as well.

A comparative study was conducted to investigate the 3.9 µm mid-IR emission properties of Ho3+ doped NaYF4 and CsCdCl3 crystals as well as Ho3+ doped Ga2Ge5S13 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 5I5 → 5I6 transition, showed long emission lifetime values of ∼16.5 ms and ∼1.61 ms for Ho3+ doped CsCdCl3 crystal and Ga2Ge5S13 glass, respectively. Conversely, the Ho3+ doped NaYF4 crystal exhibited a relatively short lifetime of ∼120 µs. Temperature dependent decay time measurements were performed for the 5I5 excited state for all three samples. The results showed that the emission lifetimes of Ho3+:CsCdCl3 and Ho3+:Ga2Ge5S13 were nearly temperature independent over the range studied, while significant emission quenching of the 5I5 level was observed in Ho3+:NaYF4. The temperature dependence of the multi-phonon relaxation rate for 3.9 µm mid-IR emission in Ho3+:NaYF4 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.

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.

In recent years, ceramic sesquioxide materials have emerged as a promising alternative to crystalline laser hosts for near- and mid-infrared laser applications. Ceramics offer a number of manufacturing advantages over crystals including lower fabrication temperatures and the amenability to forming much larger size samples. In this work, a number of RE ions, doped into multiple sesquioxide hosts, are spectroscopically characterized in order to assess their potential for near- and mid-infrared laser applications. Characterization methods included absorption and fluorescence spectroscopy as well as decay dynamics, all measured as functions of temperature. The results are analyzed in order to determine the best laser gain media in the near- and mid-infrared spectral regions.

The development of mid-IR lasers faces some unique challenges when striving to achieve the highest efficiency and power output. Most critical among these challenges is luminescence quenching of the relatively closely-spaced upper and lower laser levels, usually occurring through the process of multi-phonon relaxation. This quenching can be mitigated by using gain materials with small maximum phonon energies. In this work, mid-IR spectroscopic characterization of RE3+ doped cesium cadmium chloride (CsCdCl3) crystals was performed. The transition probabilities of RE3+ ions using Judd-Ofelt analysis as well as the multiphonon non-radiative transition rates in RE3+:CsCdCl3 were estimated. Obtained experimental results, inclusive of temperature dependent absorption and fluorescence studies, transition cross-sections, and fluorescence dynamics, were interpreted from the standpoint of optimization for diode-pumped ~3-5 um laser development.

The mid-IR spectroscopic properties ofEr3+doped low-phononCsCdCl3andCsPbCl3crystals grown by the Bridgman technique have been investigated. Using optical excitations at∼800nmand∼660nm, both crystals exhibited IR emissions at∼1.55,∼2.75,∼3.5, and∼4.5µmat room temperature. The mid-IR emission at 4.5 µm, originating from the4I9/2→4I11/2transition, showed a long emission lifetime of∼11.6msforEr3+dopedCsCdCl3, whereasEr3+dopedCsPbCl3exhibited a shorter lifetime of∼1.8ms. The measured emission lifetimes of the4I9/2state 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 the4I9/2→4I11/2transition inEr3+dopedCsCdCl3andCsPbCl3were determined to be∼0.14×10−20cm2and∼0.41×10−20cm2, respectively. The results of Judd–Ofelt analysis are presented and discussed.

A large number of rare-earth (RE) activated materials have been investigated to develop new solid state infrared (IR) laser sources for potential applications in atmospheric sensing, material processing, laser remote sensing, medicine, and free space communications. RE3+-doped low-phonon chalcogenide glasses have shown efficient mid-IR emission as well as lasing at room temperature. In this work, we report the results of a comparative study of mid-IR spectroscopic properties of RE3+ doped chalcogenide glasses (e.g. GaGeX (X= S, Se)) aimed at exploring their potential for efficient mid-IR laser operation.

Extending solid-state laser technology to longer wavelengths is difficult because the transitions that lead to mid-IR emission experience strong competition from the multiphonon-relaxation (MPR) which reduces the emission quantum efficiency. In this work, a comparative study of mid-IR (3-5 μm) spectroscopic properties on RE3+ ions doped in various low-phonon fluoride and chloride based crystals were explored. Obtained experimental results, including temperature dependent absorption and fluorescence, transition cross-sections, and fluorescence dynamics are discussed and the potential for efficient laser operation is evaluated. Ultimately, the chloride materials demonstrated more favorable laser parameters than the fluoride materials, including significantly longer upper laser level lifetimes.

The increased interest in lasers operating in the mid-infrared spectral region has prompted the development of new gain materials with low maximum phonon energy. Fluorites (calcium fluoride [CaF2], strontium fluoride [SrF2], and barium fluoride [BaF2]) have emerged as promising laser host crystals due to their low phonon energies, high thermal conductivities, and ability to incorporate RE dopants. Dy3+ has been studied in CaF2 and SrF2 but its spectroscopic properties are largely unexplored in BaF2. In this work, dysprosium-doped barium fluoride was explored for its mid-infrared laser potential in the 3-μm spectral region. Results of absorption and fluorescence measurements were used to generate stimulated-emission cross sections, and the gain characteristics were determined at both room temperature and 77 K.

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.