Zeeman Spectroscopy of Vacancy-Charge-Compensated Er3+ Sites in CaWO4 under Vector Magnetic Fields

TL;DR

CaWO$_4$ doped with Er$^{3+}$ hosts telecom-relevant emission but exhibits multiple charge-compensated Er$^{3+}$ environments formed by Ca$^{2+}$ vacancies. Using polarization-resolved Zeeman spectroscopy under vector magnetic fields, the study maps Zeeman-split transitions for three Er$^{3+}$-related sites, extracting effective $g$-factors and validating them against EPR data to support a model with several inequivalent vacancy configurations. Site 1 remains axial with S$_4$ symmetry, while sites 2 and 3 show rhombic/C$_1$-like symmetry, consistent with near-Ca vacancy perturbations; anti-crossings and polarization dependence reveal two ground-state manifolds and distinct excited-state couplings. These results provide a detailed spectroscopic and symmetry-based characterization of defect-engineered Er$^{3+}$ centers in CaWO$_4$, with implications for quantum information applications leveraging telecom-compatible transitions.

Abstract

We present polarization-resolved optical absorption measurements on Er3+ ions in CaWO4 under vector magnetic fields, focusing on charge-compensated sites arising from local Ca2+ vacancies. While the known axial Er3+ site displays a single symmetric Zeeman-split transition pattern consistent with S4 symmetry, two additional sites exhibit more complex spectral behavior, including sets of transitions that interchange under 90° crystal rotations-evidence of reduced, rhombic-like 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 Ca2+ vacancies around Er3+ sites in the host lattice. This detailed characterization contributes to understanding defect-engineered rare-earth sites for quantum information applications.

Figures (9)

• Figure 1: a: Energy levels of site 1 - 3 with highlighted investigated Z1Y1 transition. (energies taken from Becker.2025) b: Absorption coefficients of sites 1 - 3 in $\alpha$-polarization with Lorentzian fit. c: CaWO$_4$ lattice presented in ionic radii surrounding an Er$^{3+}$ (violet) incorporated at Ca$^{2+}$ (reddish) site. Oxygen ions are excluded for visibility, and W$^{6+}$ ions are presented in grey. Ca$^{2+}$ is color-coded from dark red (nearest neighbor set) to bright red (5th nearest neighbor set)
• Figure 2: Zeeman splitting of a Z1Y1 transition a: Zeeman splitted Z1Y1 transition of site 1 at $1.76K$ in $\alpha$-polarization with B$\parallel$b for different magnetic field magnitudes. b: Schematic Zeeman splitting of Z1Y1 transitions using the convention from Bottger.2009.
• Figure 3: Symmetry of Zeeman splitted transitions of site 1 - 3 in the b-c plane measured in $\alpha$-polarization. Site 1 is measured at $0.6T$ at $1.76K$, site 2 and site 3 are measured at $0.5T$ at $10K$.
• Figure 4: Symmetry of Zeeman split transitions of site 1 - 3 in the a-b plane measured in $\sigma$ ($-180°$ to $0°$, a, c, e) and $\pi$ ($0°$ to $180°$, b, d, f) - polarization. Site 1 is measured at $0.6T$ at $1.76K$, site 2 and site 3 are measured at $0.5T$ at $10K$.
• Figure 5: Symmetry of Zeeman split transitions of site 2 (a) and 3 (b) in the a-b plane measured in $\sigma$-polarization ($-90°$ to $0°$) at $0.5T$ at $1.76K$.