Optical Spectroscopy of Waveguide Coupled Er ³⁺ Ensembles in CaWO ₄ and YVO ₄

ABSTRACT We present an optical study of near‐surface ensembles in waveguide‐integrated and , investigating how nanophotonic coupling modifies rare‐earth spectroscopy. In particular, we compare bulk excitation with evanescently coupled TE and TM waveguide modes. In , we observe a pronounced polarization‐dependent surface effect. TE‐coupled spectra closely reproduce bulk behavior. In contrast, TM coupling induces strong inhomogeneous broadening and an asymmetric low‐energy shoulder of the site S1 Y1Z1 transition, with linewidths exceeding those of the bulk by more than a factor of four. Temperature‐dependent measurements and surface termination studies indicate that surface charges are the dominant mechanism. remains largely unaffected by mode polarization, and surface termination leads only to minor spectral shifts. These observations suggest that non‐charge‐neutral rare‐earth systems are more susceptible to surface‐induced decoherence sources than charge‐neutral hosts.