High- Q guided-mode resonances on thin-film LiNbO 3 via SiO2 overlay gratings

Guided-mode resonances (GMRs) in periodically perturbed dielectric slabs enable ultra-narrow spectral features with small footprints. Still, achieving high-Q operation on thin-film lithium niobate (TFLN) without degrading material quality remains challenging. Here, we numerically study a silicon dioxide overlay grating on an unetched LiNbO3 slab that implements an etchless GMR metasurface compatible with wafer-scale lithium niobate-on-insulator (LNOI) processing. Using a reproducible single-cell finite-element workflow, we map geometry to performance through hierarchical sweeps of period (P), overlay thickness (hg), and slab thickness (hwg). The best-performing device (P=820nm, hg=80nm, hwg=320nm) exhibits a near-unity reflectance peak around 1.5µm with full width at half maximum of ∼0.11nm, corresponding to Q≳104. The etchless architecture preserves the LN surface, avoids plasma-induced loss, and supports electro-optic tuning via the Pockels effect. These results provide design rules for narrowband filters and label-free refractometric sensors on TFLN and establish a scalable route to ultra-narrow, voltage-tunable metasurfaces.