title: Near-normal incidence dark-field microscopy: applications to nanoplasmonic spectroscopy.
abstract: The spectroscopic characterization of individual nanostructures is of fundamental importance to understanding a broad range of physical and chemical processes. One general and powerful technique that addresses this aim is dark-field microscopy, with which the scattered light from an individual structure can be analyzed with minimal background noise. We present the spectroscopic analysis of individual plasmonic nanostructures using dark-field illumination with incidence nearly normal to the substrate. We show that, compared to large incidence angle approaches, the near-normal incidence approach provides significantly higher signal-to-background ratios and reduced retardation field effects. To demonstrate the utility of this technique, we characterize an individual chemically synthesized gold nanoshell and a lithographically defined heptamer exhibiting a pronounced Fano-like resonance. We show that the line shape of the latter strongly depends on the incidence angle. Near-normal incidence dark-field microscopy can be used to characterize a broad range of molecules and nanostructures and can be adapted to most microscopy setups.
authors.affiliation: Beckman Institute, University of Illinois, 405 North Mathews Avenue, Urbana, Illinois 61801, United States. jonfan@illinois.edu
authors.forename: Jonathan A
authors.initials: JA
authors.lastname: Fan
authors.forename: Kui
authors.initials: K
authors.lastname: Bao
authors.forename: J Britt
authors.initials: JB
authors.lastname: Lassiter
authors.forename: Jiming
authors.initials: J
authors.lastname: Bao
authors.forename: Naomi J
authors.initials: NJ
authors.lastname: Halas
authors.forename: Peter
authors.initials: P
authors.lastname: Nordlander
authors.forename: Federico
authors.initials: F
authors.lastname: Capasso
mesh_terms: D008422:Materials Testing / Q000379:methods*; D008853:Microscopy / Q000379:methods*; D049329:Nanostructures / Q000737:chemistry* / Q000648:ultrastructure*; D013057:Spectrum Analysis / Q000379:methods*; D020349:Surface Plasmon Resonance / Q000379:methods*