Surface-enhanced CARS from plasmonic gold island films fabricated by simple thermal annealing

Thermally engineered plasmonic films offer a powerful route to tune nanoscale optical fields, yet their use in coherent Raman enhancement remains underexplored. Here, we demonstrate that controlled annealing of evaporated gold nanofilms produces plasmonic island morphologies capable of supporting surface-enhanced coherent anti-Stokes Raman scattering (SE-CARS) of Raman-labelled molecules. Gold films of 6, 8, and 10 nm thickness were fabricated by thermal evaporation and annealed at 450 °C to induce nanoscale restructuring, yielding a systematic red-shift in the localised surface plasmon resonance from 558 nm to 610 nm. These structural and optical changes were correlated with order-of-magnitude increases in Raman enhancement factors for three model reporters-BPT, BPE, and IR820-reaching up to 1.3 × 10⁷ for IR820 on 10 nm films. Combining spontaneous SERS and SE-CARS under identical excitation conditions revealed that annealing-induced plasmonic islands support coherent vibrational amplification, with no evidence of photobleaching or spectral distortion under the measurement conditions employed. The SE-CARS spectra of BPT exhibited distinct red-shifts consistent with molecule-surface charge transfer, confirming plasmonic field confinement at the metal-molecule interface. This simple thermal tuning strategy converts continuous gold films into optically active nanostructures, providing proof-of-concept for an accessible fabrication route to plasmon-enhanced nonlinear spectroscopy substrates.