Polarization properties of the SERS radiation scattered by linear nanoantennas with two distinct localized plasmon resonances

Playing with the field polarization permits to gain important insight on the near-field interaction between molecules and nanoantennas, as well as to develop new concepts of broadband plasmonic sensors (Fazio B. et al. ACS Nano 5(7):5945–59562, 2011; D’Andrea C. et al. J Phys Chem C 118(16):8571–8580, 2014; D’Andrea C. et al. ACS Nano 7(4):3522–3531, 2013; Foti A. et al. Nanospectroscopy 1:26–32, 2015). By studying the polarization state of the Surface Enhanced Raman Scattering it is, in particular, possible to better understand the re-radiation effects in linear nanoantennas featuring spectrally distinct localized plasmon resonances (LSPR). Here we show that, through the selective excitation of the short- or the long- axis LSPR, it is possible to control and totally rotate (90°) the polarization of the SERS photons of randomly oriented molecules adsorbed on linear nanoantennas. By a proper design of the nanoantenna, this can be accomplished switching the excitation wavelength from the visible to the NIR, while using orthogonal fields polarizations, so to match the two distinct LSPRs. Modeling the nanoantenna enhancement with wavelength-dependent field enhancement tensors we recover the relations ruling the parallel-polarized and unpolarized- SERS intensity dependence found experimentally when selectively driving the two LSPRs. Our results support a physical picture in which the strongly coupled molecule-nanoantenna system behaves as an hybrid structure whose energy levels are determined by the molecule while the far-field radiation properties are determined by the antenna modes.