Coherent sum-frequency generation via continuous-wave laser excitation within plasmonic nanogap arrays.

Mid-infrared (MIR) vibrational spectroscopy offers rich chemical specificity but is limited by detector noise, thermal background, and reliance on high-peak-power or phase-matched nonlinear optics. We show that plasmonic multilayer aggregates (MLaggs), robust, disordered Au nanoparticle films with reproducible nanogaps, enable continuous-wave coherent sum-frequency generation (SFG) between MIR and near-infrared (NIR) light, achieving nonlinear upconversion efficiencies typically associated with ultrafast lasers. Co-localised MIR and NIR confinement produces clear resonant vibrational signatures and a non-resonant plasmonic background. Spatial SFG mapping reveals that MIR and NIR hotspots only partially overlap, showing that their field localisation may be structurally decoupled. Time-resolved measurements confirm that MLaggs preserve ultrafast vibrational coherence. These results unify surface-enhanced IR absorption and Raman spectroscopy (SEIRA/SERS) field enhancement with low-power nonlinear vibrational spectroscopy, removing the need for phase matching and high peak intensities. MLaggs thus provide a path toward compact MIR-to-visible transducers for chemical sensing and integrated photonics.