Coherent Fano resonances in a plasmonic nanocluster enhance optical four-wave mixing.

Plasmonic nanoclusters, an ordered assembly of coupled metallic nanoparticles, support unique spectral features known as Fano resonances due to the coupling between their subradiant and superradiant plasmon modes. Within the Fano resonance, absorption is significantly enhanced, giving rise to highly localized, intense near fields with the potential to enhance nonlinear optical processes. Here, we report a structure supporting the coherent oscillation of two distinct Fano resonances within an individual plasmonic nanocluster. We show how this coherence enhances the optical four-wave mixing process in comparison with other double-resonant plasmonic clusters that lack this property. A model that explains the observed four-wave mixing features is proposed, which is generally applicable to any third-order process in plasmonic nanostructures. With a larger effective susceptibility χ(3) relative to existing nonlinear optical materials, this coherent double-resonant nanocluster offers a strategy for designing high-performance third-order nonlinear optical media.