How laser wavelength and polarization shape the molecular Raman response enhanced by capped Ag nanoparticle chains.

Abstract

Surface-enhanced Raman spectroscopy (SERS) sensors using capped Ag nanoparticle (NP) chains provide long-term stability and reusability. However this configuration distances the molecules from the plasmonic hot spots, thus decreasing the signal enhancement. This paper systematically investigates the SERS response of 2,2'-bipyridine (Bpy) molecules deposited on self-organized Ag NP chains capped with 5 nm-thick Al2O3 or SiOx films, fabricated via ion-beam sputtering. The substrates exhibit optical dichroism with two polarization-dependent localized surface plasmon resonances (LSPRs). SERS spectra, acquired under five excitation wavelengths (458-633 nm) and transverse/longitudinal polarization, reveal two distinct contributions: (i) a continuous background from the Ag NPs and dielectric host, and (ii) Raman peaks from Bpy vibrational modes. While the background intensity and shape depend on both wavelength and polarization, the Bpy peak magnitudes are primarily governed by the excitation wavelength, with a unique molecular fingerprint for each wavelength. finite-difference time-domain simulations confirm that the Raman signal is amplified by electromagnetic field enhancement driven by both LSPRs, regardless of incident polarization, due to the spatial and directional variability of the local electric field at the capping layer/air interface.

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