Polarized SERS on linear arrays of silver half-shells: SERS re-radiation modulated by local density of optical states

This work investigates polarization effects in surface-enhanced Raman scattering (SERS) on a particular kind of hybrid colloidal plasmonic-photonic crystal consisting in linear arrays of polystyrene colloids coated by a silver film forming caps (half-shells). The polarization of Raman scattering of adsorbed molecules is found to be imposed by the linear morphology of the plasmonic nanostructures, independent of the incident polarization. Specifically, it is demonstrated that the electric field component parallel to the linear plasmonic nanostructures brings the main contribution to the SERS enhancement for both parallel and perpendicular excitation. The stronger plasmon resonance, polarized along the chain of metal half-shells, is the one that determines the polarization of the SERS scattering, even if this resonance is not directly excited by the incident laser, but only re-radiated photons couple to it. The observed dependences indicate a strong contribution of the second part of the E4 enhancement, related to the enhancement at the frequency of the Raman scattered photons. FDTD electromagnetic simulations, taking into account both the excitation enhancement at the location of the scatterer and the enhancement of the scattering (re-radiation) by local density of optical states (LDOS) effects, support the experimental results. The presented results emphasize the practical importance of knowing and controlling the excitation detection conditions in SERS experiments with nanostructures possessing well-defined and oriented morphological features. The LDOS approach employed here for the theoretical analysis, despite not being common for this purpose, proves its potential for becoming a useful ingredient in the analysis of plasmon-enhanced spectroscopies.