Influence of Driving Pulse Properties on Third-Harmonic Diffraction from Quasi-BIC Metasurfaces.

Quasi-bound states in the continuum in dielectric metasurfaces support sharp Fano resonances that emerge from the interference between bright and dark modes. We exploit this modal interplay to demonstrate tunable third-harmonic emission, controlled through the driving pulse's wavelength and intensity. Our experiments show imbalances in third-harmonic diffraction patterns and non-Gaussian third-harmonic spectral features that exhibit strong variations near the Fano resonance. We explain the observations via a coupled-oscillator model that captures the interplay between the driving field and the nonlinear response of the modes, explaining our observations and providing a predictive framework for optimizing the third-harmonic diffraction efficiency. These results establish pulse-engineered metasurfaces as a powerful platform for nonlinear wavefront shaping and frequency conversion applications while simultaneously serving as a warning that pulse properties play a vital role in metasurface function design.