Engineering nanostructures for giant optical fields

SERS spectra obtained from two nanostructures are reported. In the first, silver nanoparticles are assembled using a dithiol molecular linker into small clusters appropriately structured to produce strong local electromagnetic (em) fields in the interstitial regions between neighboring nanoparticles, precisely where the molecular linker resides. The second consists of bifurcated silver nanowires, predicted to show strong enhancements in the cleft of the bifurcation when excited by light polarized across the cleft. The results suggest that the giant SERS enhancements needed for near-single-molecule sensitivity largely result from extraordinarily intense local em fileds at geometrically favorable locations in nanostructures.