Fluorine-doped silica is a key material used in all low-loss and/or radiation-resistant optical fibers. Surprisingly, no fluorine-related radiation-induced point defects have been identified. By using electron paramagnetic resonance, we report the first observation of F-related defects in silica. Their fingerprint is a doublet with 10.5 mT splitting due to hyperfine coupling (hfc) to ${}^{19}\mathrm{F}$ nuclear spins. An additional 44.4 mT hfc to the ${}^{29}\mathrm{S}\mathrm{i}$ nucleus indicates that this defect belongs to the “$E^{\prime }$ center” family and has a structure of a fluorine-modified Si dangling bond: 3-coordinated Si atoms with an unpaired electron in an ${sp}^3$ orbital, bonded to a glass network by 2 bridging oxygen atoms and to a F atom.

• Received 27 July 2023
• Accepted 22 November 2023

DOI:https://doi.org/10.1103/PhysRevLett.131.256903