Numerous nanophotonic applications stem from the subwavelength confinement of light energy within nanogaps formed between adjacent plasmonic nanostructures. Among these types of nanostructure configurations, ones with air-filled nanogaps are of great significance because the gap provides an open volume for application-specific nanoscale objects to fit into. The fabrication of such structures on substrates faces numerous challenges, especially when trying to obtain the sub-5 nm gap-widths that are of greatest interest. Most issues arise from the resolution and scale limitations of current leading-edge techniques such as electron-beam lithography. The work presented here introduces a novel fabrication process for forming identically aligned plasmonic gold trimers with air-filled vertical nanogaps having widths as small as 4 nm. The devised process enables greater control over spatial parameters than current fabrication methods, including the size and density of the trimer structures and the width of the nanogaps. Key aspects of the process include (i) a nanogap width that is defined by a sacrificial oxide layer, (ii) a glancing angle deposition that results in trimer alignment, and (iii) a sacrificial antimony layer that is used to regulate the trimer assembly process. By further incorporating nanoimprint lithography and glancing angle ion milling into the process, it becomes possible to define periodic arrays of deterministically positioned gold trimers as well as providing the option of fabricating arrays of gold dimers. Optical characterization of the resulting structures, as measured by polarization-dependent spectroscopy, surface-enhanced Raman scattering, and refractive index sensitivity measurements, show close agreement with simulations. The devised process, therefore, advances the techniques needed to localize plasmonic energy at site-specific locations.

Assembly of plasmonic gold trimers and dimers with air-filled nanogaps

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