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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. 

Expanded CAG/CTG repeats are sites of DNA damage, leading to changes in repeat length. To determine how ssDNA gap filling affects repeat instability, we inserted (CAG)70 or (CTG)70 repeats into a single-strand annealing (SSA) assay system such that resection and filling in the ssDNA gap would occur across the repeat tract. After resection, when the CTG sequence was the single-stranded template for fill-in synthesis, repeat contractions were elevated and the ssDNA created a fragile site that led to large deletions involving flanking homologous sequences. In contrast, resection was inhibited when CTG was on the resected strand, resulting in repeat expansions. Deleting Rad9, the ortholog of 53BP1, rescued repeat instability and lost viability by increasing resection and fill-in speed. Deletion of Rad51 increased CTG contractions and decreased survival, implicating Rad51 in protecting ssDNA during gap filling. Taken together, DNA sequence within a single-stranded gap determines repair kinetics, fragility, and repeat instability. 

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. 

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.