Surface-enhanced coherent anti-Stokes Raman scattering of molecules near metal-dielectric nanojunctions

We discuss an experimental configuration consisting of {Au film}-molecule-{Au particle} or {Au film}-molecule-{Si particle} nanojunctions for performing wide-field surface-enhanced CARS (SE-CARS) measurements in a reproducible and controllable manner. While the allowable illumination dosage in the {Au film}-molecule-{Au particle} case is limited by the strong two-photon background from the gold, we successfully generate a detectable coherent Raman response from a molecular monolayer using the lowest reported average power densities to-date. With a vision to minimize the two-photon background and the intrinsic losses observed in all-metal plasmonic systems, we examine the possibility of using high-index dielectric particles on top of a thin metal film to generate strong nanoscopic hotspots. We demonstrate repeatable SE-CARS measurements at the {Au film}-molecule-{Si particle} heterojunction, underlining the usability of this experimental geometry. This work paves the way for the development of next-generation of chemical and biomolecular sensing assays that can minimize some of the major drawbacks encountered in fragile and lossy all-metal plasmonic systems.