Plasmonic Nanoparticle-on-Nanoslit Antenna as Independently Tunable Dual-Resonant Systems for Efficient Frequency Upconversion

Dual-band plasmonic nanoantennas, exhibiting two widely separated user-defined resonances, are essential for studying and optimizing plasmon-enhanced optical phenomena, including photoluminescence, Raman scattering, and nonlinear effects such as harmonic and sum-frequency generation. The nanoparticle-on-slit (NPoS) or nanoparticle-in-groove (NPiG) antenna is a recently introduced dual-band structure with independently tunable resonances at mid-infrared and visible wavelengths. It has been used to enhance sum- and difference-frequency generation from optimally located molecules by an estimated 10¹³-fold. However, theoretical understanding of its eigenmodes remains limited, constraining further optimization and broader application. Here, the quasi-normal modes (QNMs) supported by NPoS structures are investigated, analyzing how both near-field (giant photonic density of states) and far-field (radiation pattern) characteristics influence upconversion. Tuning strategies are identified to adjust visible and mid-infrared resonances independently while maintaining strong near-field mode overlap, which governs the efficiency of nonlinear processes. Additionally, mode analysis reveals a previously unexplored resonance offering greater field enhancement and superior spatial mode overlap with the mid-infrared field, potentially improving upconversion efficiency fivefold compared with the existing results. This work helps to rationalize and optimize the enhancement of nonlinear effects across a wide spectral range using a flexible and experimentally attractive nanoplasmonic platform.