The role of focused laser plasmonics in shaping SERS spectra of molecules on nanostructured surfaces

TL;DR

This study reveals that the axial position of the laser focus significantly influences SERS spectra on nanostructured surfaces, affecting signal intensity and spectral ratios, with implications for quantitative analysis.

Contribution

It demonstrates the focus-dependent variation of SERS spectra and attributes these effects to plasmonic near-field responses through FDTD simulations.

Findings

SERS signal intensity peaks above the sample surface following a Lorentzian profile.

Spectral band ratios vary non-monotonically with focus position.

Plasmonic near-field responses explain the observed spectral distortions.

Abstract

The dependence of surface-enhanced Raman scattering (SERS) spectra on the precise axial position of the laser focus relative to a solid nanostructured substrate has received little to no attention in the literature. Here we show this dependence is both real and physically meaningful. Through vertical (Z-axis) scans varying the distance between the laser focus and a planar SERS substrate, we find that the SERS signal intensity follows a Lorentzian axial profile that peaks consistently above the physical sample surface. More significantly, the relative intensities of different spectral regions, i.e. SERS bands and background, vary non-monotonically and non-uniformly along the Z axis, meaning that band intensity ratios are focus-dependent. Finite-Difference Time-Domain (FDTD) simulations attribute these effects to plasmonic near-field responses specific to the focused and defocused beam interacting with the nanostructured metal surface. These findings reveal a previously overlooked source of spectral distortion in solid-substrate SERS measurements, with direct implications for the design and interpretation of quantitative assays based on band intensity ratios.