Anisotropic Magneto–Photonic Interaction of Erbium in WS₂ Flakes at Telecom Wavelengths

Erbium ions (Er³⁺) offer telecom-band optical transitions with pronounced magnetic-dipole character, making them promising for quantum communication and spin–photon interfaces. A central challenge is identifying hosts that suppress decoherence while supporting photonic integration. Here we study Er³⁺ emission in tungsten disulfide (WS₂) flakes, a low-nuclear-spin layered host providing centrosymmetric substitutional sites and narrow telecom emission. We find that modest out-of-plane magnetic fields (smaller than 0.2 T) markedly dim the Er³⁺ photoluminescence, lengthen the excited-state lifetime, and rotate the emission dipole, whereas in-plane fields have little effect. DFT calculations predict changes smaller than observed and fail to reproduce the strong field-orientation dependence; instead, photonic simulations of a dipole embedded in a finite-thickness WS₂ flake—explicitly accounting for the thickness-dependent local density of optical states—seem to better capture the observed trends, indicating a photonic rather than atomic origin. Together, these results reveal a magneto-photonic interaction unique to layered hosts, where Zeeman mixing, combined with an anisotropic and thickness-tunable LDOS, enables external-field control of emission at telecom wavelengths.