Tips versus Holes: ×10 Higher Scattering in FIB-made Plasmonic Nanoscale Arrays for Spectral Imaging

Plasmonic nanostructure arrays, designed for performance as pixels in an advanced SERS imaging device, were fabricated by gallium focused ion beam (FIB). Though the FIB is best suited for etching holes and negative structures, our previously reported simulations favor protrusions. Herein, we report on the FIB methodology to “sculpt” positive structures by “ion-blasting” away the surrounding material. Nanoprotrusions and nanoholes with different aspect ratios are compared experimentally with depth and height controlled by the dwell time. The amplitude and spectra of optical absorption and scattering from the two species are compared as a function of structure height. Measurements were performed using ASI’s model Rainbow hyperspectral camera, demonstrating the utility of hyperspectral microscopy for plasmonic imaging applications. Both the scattered and absorbed radiation display the broad peak qualitatively similar to the localized surface plasmon (LSP) scattering spectrum of gold nanospheroids. The intensity of the scattered light from the protrusions─measured in dark-field─was observed to be an order of magnitude higher than that from the nanoholes, consistent with simulation predictions. Poor contrast against bright background specular reflection is inherent in bright-field reflection mode; image-processing succeeded in detecting patterns indiscernible to the eye. To extract the absorption the full hyperspectral image field was exploited, allowing the sample to serve as its own reference. The resulting spectra display a plasmonic resonance which grows stronger and increasingly red-shifted at increasing heights, corroborating visual observations of changes in sample hue.