Realizing ultra-multi-mode high-Q plasmonic resonances by extracting guided modes via metallic lattices

Achieving multiple plasmonic resonances with high quality (Q) factors is highly desirable for compact photonic devices and multi-functional systems, as it enables multi-functional operations, such as biosensing, nonlinear optics, and wavefront modulation, while maintaining structural simplicity and reducing fabrication complexity. Here, based on a period-constrained guided-mode theory, we present a dielectric-metal-dielectric-metal (DMDM) nanostructure that supports ultra-multi-mode high-Q plasmonic resonances, together with large near-field enhancements and fine spatial mode overlaps. In this nanostructure, a metallic lattice embedded in a dielectric waveguide supports bright localized surface plasmon resonances (LSPRs) and enables selective excitation of dark guided modes (GMs). Then, the ultra-multi-mode high-Q plasmonic resonances are formed by the far-field dipole-dipole interactions between the LSPRs through the coupling channels of GMs. Theoretical results show that the DMDM nanostructure sustains over 20 resonance modes with Q-factors exceeding 200, among which more than 10 modes have Q-factors above 800. Experimentally, although affected by fabrication errors and restricted by measurement conditions, the fabricated nanostructure simultaneously exhibits 20 high-Q modes (Q > 180), which is unprecedented in plasmonic systems. Additionally, by introducing structural anisotropy, the structure exhibits distinct polarization-switchable responses, benefiting applications requiring tunable responses. These findings demonstrate a feasible approach to realize multiple high-Q plasmonic resonances, and offer a versatile platform for multi-mode optical applications.