Reproducible Ultrahigh SERS Enhancement in Single Deterministic Hotspots Using Nanosphere-Plane Antennas Under Radially Polarized Excitation

Surface enhanced Raman scattering (SERS) in a nanometer size hotspot has empowered the investigation of chemical structures and dynamic behaviors of one and a few molecules. However, further advancement is hindered by lack of large enough yet reproducible enhancement in single deterministic hotspots. To resolve this problem, here we introduce a nanosphere-plane antenna under radially polarized laser excitation experiment, which provides an electromagnetic enhancement of 10 9~10 at the gap of each individual nanosphere-plane antenna and a root-mean-square error down to 10 0.08 between them. The experiment also reveals a nonlinear SERS behavior with less than one plasmon, which is also observed within a single hotspot. The unprecedented simultaneous achievement of ultrahigh enhancement and reproducibility in deterministic individual hotspots is attributed to the combination of a well-controlled hotspot geometry, the efficient coupling between vertical antenna and laser which produces orders of magnitude higher enhancement than previous excitation methods, and low power operation which is critical for high reproducibility. Our method opens a path for systematic studies on single and few molecule SERS and their surface chemistry in an in-situ and well-controlled manner.