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

Plasmonic nanostructure arrays, designed for performance\nas pixels\nin an advanced SERS imaging device, were fabricated by gallium focused\nion beam (FIB). Though the FIB is best suited for etching holes and\nnegative structures, our previously reported simulations favor protrusions.\nHerein, we report on the FIB methodology to “sculpt”\npositive structures by “ion-blasting” away the surrounding\nmaterial. Nanoprotrusions and nanoholes with different aspect ratios\nare compared experimentally with depth and height controlled by the\ndwell time. The amplitude and spectra of optical absorption and scattering\nfrom the two species are compared as a function of structure height.\nMeasurements were performed using ASI’s model Rainbow hyperspectral\ncamera, demonstrating the utility of hyperspectral microscopy for\nplasmonic imaging applications. Both the scattered and absorbed radiation\ndisplay the broad peak qualitatively similar to the localized surface\nplasmon (LSP) scattering spectrum of gold nanospheroids. The intensity\nof the scattered light from the protrusionsmeasured in dark-fieldwas\nobserved to be an order of magnitude higher than that from the nanoholes,\nconsistent with simulation predictions. Poor contrast against bright\nbackground specular reflection is inherent in bright-field reflection\nmode; image-processing succeeded in detecting patterns indiscernible\nto the eye. To extract the absorption the full hyperspectral image\nfield was exploited, allowing the sample to serve as its own reference.\nThe resulting spectra display a plasmonic resonance which grows stronger\nand increasingly red-shifted at increasing heights, corroborating\nvisual observations of changes in sample hue.