Fluorescence-enhancing plasmonic silver nanostructures using azopolymer lithography

The performance of fluorescence-based measurement techniques is fundamentally limited by the intrinsic quantum yield of a fluorophore. Radiative decay engineering using metallic nanostructures has significant potential to improve the fluorescence emission of a fluorophore. Especially, periodic arrays of metallic nanostructures have the advantage of exhibiting a strong optical response, which is beneficial for improving sensitivity in surface enhanced spectroscopic techniques. In this work, we present a cost-effective and large-scale fabrication scheme for creating periodic plasmonic nanoparticle arrays for fluorescence enhancement. The fabrication process uses an azopolymer mask that forms a two-dimensional surface relief grating when illuminated with two orthogonal exposures, which can directly be used as a soft etching mask to define the nanoparticle array. This approach allows the creation of periodic arrays of symmetrical metallic nanostructures that exhibit good long-range order. Furthermore, the dimensions of the array and the structures can be tuned by changing the exposure or process parameters. The plasmonic behaviour of the fabricated structures was studied both experimentally and by numerical simulations. The fluorescence enhancement performance for the blue and green wavelength regions was verified by using Rhodamine 6G and Cascade Blue as fluorophores. A significant 14-fold fluorescence intensity increase for Rhodamine 6G was observed, whereas the fluorescence intensity for Cascade Blue was roughly doubled. In addition, time-resolved measurements displayed a shortening of the fluorescence lifetime for both of the fluorophores when deposited on the nanoparticle grating. We expect that this approach could be advantageous for other application areas of plasmonics, such as SERS or sensing.