Ultrafast Third-Harmonic Spectral Modulation and Self-Action in Resonant Nonlocal Metasurfaces

Quasi-bound states in the continuum enable exceptional field confinement, strongly reducing the pump intensity threshold for nonlinear light-matter interaction in dielectric metasurfaces. As a result, nonlinear self-action effects, often elusive in bulk nonlinear media, emerge at moderate excitation intensities. Here, we show how pulse duration and resonant coupling govern the nonlinear self-action mechanism in resonantly enhanced third-harmonic (TH) generation from a nonlocal metasurface. We identify two excitation regimes that interact differently with the resonant mode, revealing complementary intensity-dependent responses. Under spectrally narrow picosecond excitation, resonance-enhanced TH generation shows pronounced deviations from cubic scaling at high intensities. In contrast, broadband femtosecond excitation transiently drives the resonance, encoding the nonlinear response in the spectral reshaping and broadening of the TH signal. Simulations reproduce both regimes: continuous-wave modeling captures picosecond power scaling and higher-harmonic interactions, while time-domain simulations resolve femtosecond dynamics. These results clarify nonlinear self-action in metasurfaces featuring quasi-bound states in the continuum, linking strong field confinement to conversion efficiency, scaling behavior, and distinct spectral dynamics under different excitation conditions. This work sheds light on the interplay between pulse duration, bandwidth, and resonant coupling in high-Q nonlocal dielectric metasurfaces, advancing their use in ultrafast and nonlinear nanophotonics.