Zeeman spectroscopy of vacancy-charge-compensated Er 3 + sites in CaWO 4 under vector magnetic fields

We present polarization-resolved optical absorption measurements on Er 3 + ions in CaWO 4 under vector magnetic fields, focusing on charge-compensated sites arising from local Ca 2 + vacancies. While the known axial Er 3 + site displays a single symmetric Zeeman-split transition pattern consistent with S 4 symmetry, two additional sites exhibit more complex spectral behavior, including sets of transitions that interchange under 90 ∘ crystal rotations-evidence of reduced, rhombiclike symmetry. From these polarization- and temperature-dependent spectra, we extract effective g factors. Our findings are corroborated by electron paramagnetic resonance measurements and support a model of multiple inequivalent Ca 2 + vacancies around Er 3 + sites in the host lattice. This detailed characterization contributes to understanding defect-engineered rare-earth sites for quantum information applications.