Multipolar Raman Scattering on Chiral Plasmons

Surface-enhanced Raman spectroscopy (SERS) has been an area of active research since its discovery in 1973. In particular, the dimer nano-antenna SERS geometry, consisting of two gold nanospheres functionalized with molecular reporters, has emerged as a powerful and feasible tool for single molecule studies. Despite its popularity, there is still little consensus regrading the role of the nantenna in dictating the observed SERS response. Until issues such as SERS backgrounds, anomalous molecular anti-Stokes intensities, and polarized responses are fully understood, the full power of single molecule Raman can never be realized.Here, the antenna behavior is clarified via two types of studies: one excitation-dependent study, in which the SERS signal is recorded as a function of increasing laser power, and one polarization-resolved study, in which the SERS response is monitored as a function of changing excitation polarization. Two important discoveries emerge clearly here: first, that the background commonly observed in SERS spectra can be unilaterally assigned to the inelastic light scattering (i.e., Raman) of the plasmon. Second, that the dimer alone dictates the polarization of the scattered response, including a clear demonstration of both linear and circular chirality. With these in hand, novel applications, such as a quantum description of the plasmon and orientation-dependent chiral response are explored