Optical Spectroscopy of Waveguide coupled Er³⁺ ensembles in CaWO₄ and YVO₄

We present an optical study of near-surface Er³⁺ ensembles in waveguide-integrated CaWO₄ and YVO₄, investigating how nanophotonic coupling modifies rare-earth spectroscopy. In particular, we compare bulk excitation with evanescently coupled TE and TM waveguide modes. In Er³⁺:CaWO₄, 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. Er³⁺:YVO₄ 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.