Gap plasmons and near-field enhancement in closely packed sub-10 nm gap resonators.

Pairs of metal nanoparticles with a sub-10 nm gap are an efficient way to achieve extreme near-field enhancement for sensing applications. We demonstrate an attractive alternative based on Fabry-Perot type nanogap resonators, where the resonance is defined by the gap width and vertical elongation instead of the particle geometry. We discuss the crucial design parameters for such gap plasmons to produce maximum near-field enhancement for surface-enhanced Raman scattering and show compatibility of the pattern processing with low-cost and low-resolution lithography. We find a minimum critical metal thickness of 80 nm and observe that the mode coupling from the far field increases by tapering the gap opening. We also show the saturation of the Raman signal for nanogap periodicities below 1 μm, demonstrating efficient funneling of light into such nanogap arrays.