Gap-type metallic nanostructures are widely used in catalytic reactions, sensors, and photonics because the hotspot effect on these nanostructures supports giant local electromagnetic field enhancement. To achieve hotspots, researchers devote themselves to reducing gap distances, even to 1 nm. However, current techniques to fabricate such narrow gaps in large areas are still challenging. Herein, a new coupling way to boost the sub-10 nm plasmonic nanogap array is developed, based on the plasmon-triggered optical waveguide resonance via near-field coupling. This effect leads to an amplified local electromagnetic field within the gap regions equivalent to narrower gaps, which is evidenced experimentally by the surface-enhanced Raman scattering intensity of probed molecules located in the gap and the finite-difference time-domain numerical simulation results. This study provides a universal strategy to promote the performance of the existing hotspot configurations without changing their geometries.

Coupling efficiency between the localized surface plasmons (LSPs) of metal nanoparticles (NPs) and incident light dominates the sensitivities of plasmon-based sensing spectroscopies and imaging techniques, e.g., surface-enhanced Raman scattering (SERS) spectroscopy. Many endogenous features of metal NPs (e.g., size, shape, aggregation form, etc.) that have strong impacts on their LSPs have been discussed in detail in previous studies. Here, the polarization-tuned electromagnetic (EM) field that facilitates the LSP coupling is fully discussed. Numerical analyses on waveguide-based evanescent fields (WEFs) coupled with the LSPs of dispersed silver nanospheres and silver nano-hemispheres are presented and the applicability of the WEF-LSPs to plasmon-enhanced spectroscopy is discussed. Compared with LSPs under direct light excitation that only provide 3–4 times enhancement of the incidence field, the WEF-LSPs can amplify the electric field intensity about 30–90 times (equaling the enhancement factor of 106–108 in SERS intensity), which is comparable to the EM amplification of the SERS “hot spot” effect. Importantly, the strongest region of EM enhancement around silver nanospheres can be modulated from the gap region to the side surface simply by switching the incident polarization from TM to TE, which widely extends its sensing applications in surface analysis of monolayer of molecule and macromolecule detections. This technique provides us a unique way to achieve remarkable signal gains in many plasmon-enhanced spectroscopic systems in which LSPs are involved.