Shaping Ultrafast Pulses for Enhanced Resonant Nonlinear Interactions.

Coherent control with shaped ultrafast pulses is a powerful approach for steering nonlinear light-matter interactions. Prior quantum control studies show that, beyond transform-limited pulses, antisymmetric spectral phases can efficiently drive nonresonant multiphoton transitions. However, resonant multiphoton transitions experience dispersion from the material's spectral-phase response, which lowers nonlinear efficiency. Preshaping the pulse to compensate for this response can restore and enhance nonlinear interactions beyond transform-limited pulses. But is this the only spectral phase to yield such enhancement? Here, we study sub-10 fs single-pulse four-wave mixing in resonant plasmonic nanostructures using arctangent spectral-phase-shaped pulses. We identify two distinct enhancement regimes: one compensates material dispersion, and a counterintuitive regime induces an antisymmetric polarization response that drives constructive multiphoton pathway interference. Our theoretical analysis elucidates both phenomena. Notably, it predicts that both mechanisms exhibit distinct scaling behaviors with harmonic order and quality factor, offering a new pathway for significantly enhancing resonant nonlinear processes.