Optical Spectroscopy of Waveguide coupled Er$^{3+}$ ensembles in CaWO$_4$ and YVO$_4$

TL;DR

This study investigates how nanophotonic coupling affects the optical properties of Er$^{3+}$ ensembles in CaWO$_4$ and YVO$_4$, revealing polarization-dependent surface effects and surface-induced decoherence mechanisms.

Contribution

It provides the first detailed comparison of surface effects on Er$^{3+}$ spectroscopy in CaWO$_4$ and YVO$_4$ under waveguide coupling, highlighting the role of surface charges.

Findings

TE coupling reproduces bulk spectra in CaWO$_4$

TM coupling causes significant inhomogeneous broadening in CaWO$_4$

YVO$_4$ spectra are minimally affected by surface and mode polarization

Abstract

We present an optical study of near-surface Er$^{3+}$ ensembles in waveguide-integrated CaWO$_4$ and YVO$_4$, investigating how nanophotonic coupling modifies rare-earth spectroscopy. In particular, we compare bulk excitation with evanescently coupled TE and TM waveguide modes. In Er$^{3+}$:CaWO$_4$, 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. Er$^{3+}$:YVO$_4$ 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.