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 hot spots. We demonstrate repeatable SE-CARS measurements at the {Au film}-molecule-{Si particle} heterojunction, underlining the utility of this experimental geometry. This work paves the way for the development of next-generation chemical and biomolecular sensing assays that can minimize some of the major drawbacks encountered in fragile and lossy all-metal plasmonic systems.