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

Over fifty years have passed since the groundbreaking discovery of Surface Enhanced Raman Scattering (SERS), yet many aspects of this phenomenon remain elusive. In this study, we unveil novel observations concerning the spatial variation of SERS signal profiles through vertical (<i>Z</i> axis) scans, performed by varying the distance between the laser focus and a solid, planar, nanostructured SERS substrate. The signal strength profile manifests a Lorentzian shape during axial scans along the <i>Z</i> direction, consistently peaking above the actual sample surface. More intriguingly, the intensity ratio of various spectral regions-including SERS bands and background-exhibits significant non-constancy along the <i>Z</i> axis. Finite-Difference Time-Domain (FDTD) simulations suggest that these variations can be attributed to specific plasmonic near-field responses induced by the focused/defocused beam at the SERS substrate. This research highlights the critical need to consider that focus imprecision can alter spectral profiles in SERS analyses on solid nanostructured SERS substrates, particularly when devising quantitative assays based on band intensity ratios.