Intrapulse multimodal four-wave sum mixing in the visible range from high contrast index grating with PMMA layer.

Nonlinear metasurfaces have emerged as powerful platforms for enhancing and controlling light-matter interactions at the nanoscale, enabling versatile and compact design of devices for frequency conversion processes. In this work, we report on the experimental observation and theoretical analysis of intrapulse four-wave sum mixing (FWSM) in a high-index contrast grating (HCG) supporting quasi-bound states in the continuum (q-BIC). By engineering a one-dimensional silicon-based HCG with an additional poly(methyl methacrylate) (PMMA) cladding layer, we achieve the simultaneous excitation of a q-BIC and a guided-mode resonance (GMR), enabling nonlinear coupling between the two modes. Broadband femtosecond excitation reveals multiple distinct spectral peaks in the visible range, attributed to FWSM processes involving different combinations of q-BIC and GMR frequencies. Fourier microscopy measurements further confirm the redistribution of the generated nonlinear signals among diffraction orders. Our results demonstrate a new approach to tailoring nonlinear frequency mixing through metasurfaces, leveraging the interaction of multiple non-local resonances, thus opening new pathways for tunable frequency conversion, all-optical signal processing, and nonlinear photonic devices.