Optical activity has long been an area of interest in Raman scattering spectroscopy as it is a powerful tool for elucidating molecular chirality. Here, I provide details from an investigation into plasmonic (rather than molecular) optical activity in surface-enhanced Raman spectroscopy. The sample used is the prototypical dumbbell nano-antenna (nantenna) consisting of two gold nanospheres functionalized with bipyridyl ethylene molecular reporters. Previous analysis of the linear dichroism reveal that the structures scatter as dipolar antennas, with the molecular vibrations following the polarization patterns of the dumbbell on which they reside. Current investigations into circular optical activity of the nantenna reveal two notable observations. First, the Raman optical activity is at least two orders of magnitude larger than values typically observed for molecular species. Second, the observed handedness of the dumbbell is entirely dependent on its orientation in the plane perpendicular to excitation.

We attribute this counter-intuitive effect to the multipolar response of the plasmonic nantenna, which arises from its large size with respect to the excitation wavelength (~λ/2). Here, the long-wave approximation breaks down and retardation effects can no longer be ignored, giving rise to the inclusion of the electric quadrupole and magnetic dipole modes in addition to the zeroth-order electric dipole. The observed ODH can be understood in terms of the electric and magnetic dipole terms, which appear to undergo a simultaneous time reversal operation upon in-plane rotation. These results are explained in the framework of Jones calculus and have important implications in the treatment of bi-isotropic media.

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.