We report an improved method to directly and accurately fabricate plasmonic nanostructures with ultrasmall gaps. The fabrication is based on high-resolution focused ion beam milling with closely packed nanoring patterns. With fine and precise adjustment of the ion beam, elegant plasmonic nanostructures with ultrasmall dimensions down to 10 nm are achieved. We also show that the gap dimensions have a strong effect on the optical reflectance and transmittance of the plasmonic nanostructures. Measured results show reasonable agreement with finite-difference time-domain calculations. Our approach could find promising applications in plasmon-assisted sensing and surface-enhanced spectroscopy.

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We show the fabrication of well-aligned gold and silver nanopillars with various array parameters via interference lithography followed by ion beam milling and compare the etching rates of these two metallic materials. Silver is suitable for fabricating ultrafine arrays with ultrasmall separations due to high milling rates. The optical properties of the fabricated nanopillars are specifically characterized from both normal incidence and oblique incident angles. Tunable surface plasmon resonances are achieved with varying structural parameters. Strong coupling effects are enabled when the separation between adjacent nanopillars is dramatically reduced, leading to useful applications in sensing and waveguiding.

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