On the SERS depolarization ratio

Abstract

According to the E 4 model, [18][19][20][21][22][23][24] in SERS the nanoantenna plays a twofold role: firstly it amplifies the local field (excitation-field enhancement) confining it to nanoscale regions (hot spots), and secondly it magnifies the Raman scattering (re-radiation enhancement). Molecules lying in the hot spots (located at the edges of individual nanoantennas or in the nanocavities between near-field coupled NPs [15,[25][26][27][28][29]) experience an amplified local field and an enhanced re-radiation whenever both the wavelengths of the laser pump (λ L ) and of the induced Raman dipole (λ R ) are close to the LSPR wavelength (λ LSPR ) [24]. It is well known that the enhanced local field is polarization sensitive. 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.A measurable quantity that will be strongly affected from such dependence is the depolarization ratio [39]. It is defined in Raman spectroscopy (for linear excitation polarization) as the ratio between the intensity of the Raman field polarized orthogonal to the laser field (I ⟘ ) and the intensity of the Raman field polarized parallel to the laser field (I � ). The depolarization ratio provides information on the Raman polarizability tensor (α) of the vibration considered and is therefore a very useful tool.It was recognized early on that in SERS the relationship between the depolarization ratio and the components of the Raman polarizability tensor was not straightforward [3]. 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. The polarization of the SERS photons is consequently modified, affecting, in a non trivial way, the measured value of the SERS depolarization ratio. In this article we elaborate a model that describes how the SERS depolarization ratio is influenced by the nanoantenna re-radiation properties, sugg...

Anisotropic Optical Response of Silver Nanorod Arrays: Surface Enhanced Raman Scattering Polarization and Angular Dependences Confronted with Ellipsometric Parameters

“…In a typical Raman experiment, measurements employing combinations of different polarizations (or possibly different scattering angles) allow to compute the Raman depolarization ratio 1–3 which gives information on the orientation-averaged components of the Raman tensor of the analyte and consequently on the symmetry of the vibration involved or on the preferential molecular orientation on smooth surfaces (“surface selection rules”) 4, 5 . However, in SERS, the relationship between the symmetry of the Raman tensor of the analyte and the SERS depolarization ratio is often completely overwhelmed by polarization-dependent coupling of the incident (or scattered) field to a given nanostructure 3, 6, 7 . This effect is probably most pronounced for dimers 8–11 , i.e .…”

Continuous-Wave Coherent Raman Spectroscopy via Plasmonic Enhancement

“…We suspect that the variation of SRG for cross-polarized light is due to slight depolarization of light inside the nanoparticle solution. The depolarization ratio can be higher than 0.75 during the interaction of light beams with NPs 38,39 .…”

Polarization- and Angular-Resolved Optical Response of Molecules on Anisotropic Plasmonic Nanostructures

“…As a result, polarization felt by the molecule can be drastically different from polarization of the incident beam. It causes the depolarization ratio to no longer depend only on the intrinsic properties of the Raman tensor of the analyte, but also (and often mainly) on the geometry of the given nanostructure [ 39 , 40 , 41 ]. This difference occurs even in the simplest nanostructure geometries, such as a small sphere in the dipole approximation, it can be commonly observed in arrays of elongated nanoparticles (NPs) and is most pronounced for molecules in the location of hot-spots which exhibit anisotropic behaviour [ 40 , 42 ].…”

“…Since different polarizations couple to metallic nanostructures with different efficiency, the enhancement factor (EF) can no longer be regarded as a scalar but it takes the form of a tensor. In this case, the polarization and angular dependence of the SERS signal is therefore mostly dictated by the coupling of the laser to the plasmons while symmetry of the Raman tensor of the analyte often plays a minor role [ 39 , 40 , 43 ].…”

“…Using a well-established pyridine molecule, the authors demonstrated that the in-plane (ring stretching) vibrational modes are sensitive to the morphological anisotropy while the out-of-plane (ring deformation) modes are not. A more in-depth approach was developed by P. Gucciardi et al [ 31 , 39 , 43 ], who in his analysis went beyond the commonly used approximation. For a hot-spot represented by a dimer with the axis lying in the direction, the authors used the field enhancement tensor in the form: and the re-radiation enhancement tensor as: where is a small perturbation accounting for the difference between the enhancement for the incident and the scattered radiation.…”