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

Abstract

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 E(4) 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.

On the SERS depolarization ratio

“…According to the E 4 model, if both the excitation and Raman photon energies are within the plasmonic resonance of the nanoantenna, the antenna enhances both the local excitation and the re-radiated fields. We can model SERS as a three step phenomenon: [34,38] 1. enhancement of the excitation field at wavelength λ L and the generation of hot spots (6) 2. generation of a molecular Raman dipolar field at λ R wavelength (7) 3. amplification of the SERS field at wavelength for molecules located at hot spots (8) where we have introduced the excitation field enhancement tensor, , and the re-radiation enhancement tensor, , to describe the amplification of the excitation and the Raman field and which are, in principle, wavelength-dependent. Using Notably ρ SERS does not depend only on the Raman polarizability but also on ϴ, the excitation polarization angle and, in turn, on the coupling between the optical fields and the nanoantenna.…”

“…The only component of the incident field yielding the local field amplification is the one capable of exciting the LSPR of the antenna [30][31][32][33]. On the other hand, in near-field coupled nanoantennas the re-radiation effect yields a selective enhancement of the Raman dipole component parallel to the nanocavity axis at the single molecule level, linearizing the polarization of the Raman field [34][35][36][37][38].The re-radiation effect, therefore, causes a strong modification of the polarization of the SERS field, whose components will be altered with respect to what would have been measured in normal Raman spectroscopy in absence of the nanoantennas. In addition, such an alteration is dependent on the orientation of the antenna.…”

“…One striking example is provided by Fazio and coworkers, [38] where the polarized SERS intensity is found to be at a maximum in the direction orthogonal to the excitation field, whereas in Raman spectroscopy Abstract: The Raman depolarization ratio is a quantity that can be easily measured experimentally and offers unique information on the Raman polarizability tensor of molecular vibrations. In Surface Enhanced Raman Scattering (SERS), molecules are near-field coupled with optical nanoantennas and their scattering properties are strongly affected by the radiation patterns of the nanoantenna.…”

Plasmonic Color-Graded Nanosystems with Achromatic Subwavelength Architectures for Light Filtering and Advanced SERS Detection

“…We believe the most interesting application may be found addressing resonance Raman effects 32,33 in aptly-chosen molecules. LSP-graded substrates might in this case provide superior performances 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 14 with respect to conventional platforms, allowing more freedom in matching the spectral overlap between the sources of resonance.…”

Nanoparticles and Nanostructures for Biophotonic Applications

“…Metal nanoparticles present specific optical properties that depend on their size and geometry. While a symmetrical shape allows for polarization-independent plasmonic excitation, a dichroic absorption is expected in elongated NPs (Toma et al, 2008;Fazio et al, 2011). Beside this, the optoplasmonic response can be additionally tailored by acting on the nanoparticle distribution and mutual coupling (Fischer & Martin, 2008).…”