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Re-radiation Enhancement in Polarized Surface-Enhanced Resonant Raman Scattering of Randomly Oriented Molecules on Self-Organized Gold Nanowires

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journal contribution
posted on 26.07.2011, 00:00 by Barbara Fazio, Cristiano D’Andrea, Francesco Bonaccorso, Alessia Irrera, Giuseppe Calogero, Cirino Vasi, Pietro Giuseppe Gucciardi, Maria Allegrini, Andrea Toma, Daniele Chiappe, Christian Martella, Francesco Buatier de Mongeot
We explore the effect of re-radiation in surface-enhanced Raman scattering (SERS) through polarization-sensitive experiments on self-organized gold nanowires on which randomly oriented Methylene Blue molecules are adsorbed. We provide the exact laws ruling the polarized, unpolarized, and parallel- and cross-polarized SERS intensity as a function of the field polarizations. We show that SERS is polarized along the wire-to-wire nanocavity axis, independently from the excitation polarization. This proves the selective enhancement of the Raman dipole component parallel to the nanocavity at the single molecule level. Introducing a field enhancement tensor to account for the anisotropic polarization response of the nanowires, we work out a model that correctly predicts the experimental results for any excitation/detection polarization and goes beyond the E4 approximation. We also show how polarization-sensitive SERS experiments permit one to evaluate independently the excitation and the re-radiation enhancement factors accessing the orientation-averaged non-diagonal components of the molecular Raman polarizability tensor.