Q-Factor Matching for Femtosecond Near-Field Enhancement in Hybrid Metal-Dielectric Metasurfaces.

Near-field enhancement in nanocavities governs the efficiency of nonlinear optical processes and ultrafast light-matter interactions. However, maximizing the cavity quality factor does not necessarily maximize the response under femtosecond excitation. Here, we experimentally demonstrate that hybrid metal-dielectric metasurfaces provide a practical platform for optimizing this trade-off. By coupling a low-Q localized surface plasmon resonance of Au nanodisks to a high-Q dielectric mode of a TiO_{2} metasurface, we continuously tune the Q factors of the hybrid modes over a broad range while preserving the plasmonic hotspot geometry. Using four-photon photoemission electron microscopy under 100-fs excitation, we map the nonlinear near-field response and correlate it with spectrally extracted Q factors and ultrafast dynamics measured by time-resolved photoemission electron microscopy. The response varies nonmonotonically with Q and reaches a maximum at Q≈20, where the photoemission yield is enhanced approximately 15-fold relative to the uncoupled metasurface. These results identify pulse-cavity Q-factor matching between the cavity and the driving pulse as a key design principle for pulsed-laser nanophotonics.