High-Q dielectric metagrating as broadband Raman enhancement substrate enabled by quasi-guided modes

All-dielectric nonlocal metasurfaces can sustain high-Q resonances and enhanced local electric fields, offering an efficient alternative to surface plasmon-based substrates for surface-enhanced Raman scattering (SERS) applications. We demonstrate significantly enhanced broadband SERS by incorporating the period-doubling design into a grating structure to effectively excite quasi-guided modes (QGMs). Their intrinsically robust Q-factor supported across a wide range of Raman shifts allows the simultaneous enhancement of multiple Raman signals directed toward various output angles using a single structure. We fabricated the period-doubled metagrating composed of PMMA ridges on a thin silicon nitride (Si₃N₄) slab. Using a single high-numerical-aperture objective to collect Raman signals from various angles, we achieved a maximum enhancement factor of 138 times for Raman scattering signals from Si₃N₄ slab and 300 times from PMMA molecules, over a broad Raman shift of 3000 cm^-1. Furthermore, we successfully demonstrated the efficacy of such metagrating as efficient SERS substrates for the detection of dye molecules, achieving a limit of detection down to 10^-7 M. This work highlights the potential of QGMs for high-sensitivity chemical sensing applications.