Enhancing Third‐Harmonic Generation via Broadband Plasmonic Nanocavity

ABSTRACT Efficient third‐harmonic generation (THG) in plasmonic nanocavities requires simultaneous field enhancement at the fundamental and harmonic wavelengths within the same nanoscale volume. Here, we construct a triple‐symmetry nested plasmonic nanocavity consisting of an Au nanoparticle coupled to three Ag‐coated polystyrene nanoparticles(PS@AgNPs). The structure supports a broadband multiresonant response and highly overlapped local‐field distributions at the pump and TH wavelengths. Under femtosecond excitation, the TH intensity from the bare nested nanocavity is approximately 45 times that from the PS@AgNPs array. Loading H 2 TBPP molecules provides an additional nonlinear contribution, reducing the detectable TH excitation threshold from 210 to 86 µW and yielding a sevenfold increase in TH intensity at the same pump power. At higher excitation powers, the molecule‐loaded nanocavity exhibits a power‐dependent blueshift and spectral distortion, which are consistent with an optical‐Stark‐type field‐induced perturbation together with molecular absorption and the associated complex dielectric response. These results establish broadband, spatially overlapped plasmonic nanocavities as an efficient platform for low‐power nonlinear frequency conversion and nanoscale spectral engineering.