We numerically investigate the optical field enhancement based on gap surface plasmon polaritons (GSPPs) that are enhanced by propagating surface waves launched by a circular slit at a metal–dielectric interface. The optical field enhancement originates not only from multiple scattering and coupling of GSPPs in the spacer region between two metal layers but also from propagating surface plasmon polaritons (SPPs) launched by a circular plasmonic lens. We find that the combination of the GSPPs and the propagating SPPs launched by the plasmonic lens can achieve extremely strong field confinement, and we find that the surface-enhanced Raman scattering (SERS) enhancement factor can be up to at the tip of the equilateral triangular nanostructures. The structure proposed here is expected to find promising applications where strong field enhancement is desired, such as optical sensing with the SERS effect.
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