Polarization characteristics of surface-enhanced Raman scattering from a small number of molecules at the gap of a metal nano-dimer

Abstract

Polarized SERS was measured at the substrate with an Ag nano-dimer array immersed in 4,4'-bipyridine solution. The orientation of the molecule at the gap of the dimer changed the polarization of the scattering photons.

On the SERS depolarization ratio

“…Nevertheless, it is often observed that the minimum σ SERS (ϴ= π/2) measured is higher than -1 without any clear explanation [35][36][37]. Notably, if we include the terms beyond the E 4 approximation, assuming for simplicity equal enhancement factors (ε=0), we find that the SERS degree of polarization becomes (19) On one hand, we retrieve the information on the molecular depolarization ratio, as expected, although encoded by the molecule-nanoantenna coupling parameters. On the other hand, we observe that when the terms beyond the E 4 approximation are taken into account in the calculation of σ SERS (ϴ) (Figure 2d, red line), the minimum gets higher than -1 and the curve is slightly shifted with respect to the cos 2ϴ trend expected in the E 4 approximation (blue line), possibly explaining the experimental results.…”

“…

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].

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Coherent anti-Stokes Raman scattering with single-molecule sensitivity using a plasmonic Fano resonance

“…The applied laser power was too low to affect the stability of the molecule, thus the output spectra were stable during the B10-min acquisition time. Interestingly, the CARS spectrum of p-MA showed quadrumer-to-quadrumer variations, characteristic of a depolarization behaviour with random fluctuations, likely due to variable orientations of molecules within the gap 31,32 . Figure 3c shows the reconstructed SECARS spectrum of p-MA, obtained by averaging three output spectra at different pumpStokes delays ( Supplementary Fig.…”

Nanoscale Insights into Enhanced Raman Spectroscopy

“…Combined studies using cryogenic, polarized UHV-TERS and nanolithographically fabricated model nanostructures, supported by the state-of-the-art calculations to determine the Raman polarizability tensor components of a molecule-metal can lead to the formulation of TERS surface selection rules [46,63,64]. Home built STM-TERS systems in the Duyene and the Wang group are making first steps in this direction.…”