Nanoscale chemical mapping using three-dimensional adiabatic compression of surface plasmon polaritons

The interest in knowing the spectroscopic signature of nanostructured materials is wide and spans from physics, chemistry and biology, where efficient single-molecule spectroscopy is highly desirable. In this work we report on the design and fabrication of a novel nano-optical device for sensing of a few nanometric entities, and we demonstrate its detection capabilities on a single CdSe quantum dot and on monolayers of organic compounds.1 In all cases, the number of molecules involved covers a range between 10 and 200. The working principle combines the light harvesting capabilities of a dielectric photonic crystal cavity with the extraordinary confining properties of a plasmonic nanowaveguide. This leads to efficient optical excitation of target samples through surface plasmon polariton modes localized at nanoscale, as demonstrated by Raman scattering measurements, in confocal configuration, and confirmed by numerical calculations. Furthermore, by fabricating a tapered metallic waveguide directly on atomic-force microscopy cantilever, we demonstrate the capability of performing simultaneous topographic and Raman scattering mapping of silicon nanostructures with a spatial resolution of 7 nm.2 The present results, demonstrating label-free detection of single nano-sized entities in subwavelength regime and in far-field configuration, opens up new perspectives toward efficient spectroscopic characterizations at the nanoscale level in different areas of research.