Manifold enhancement in the near-field and SERS efficiency of all-sputter grown Ag-nanoparticles on Al-film based mirror structures

•Ag-NPs and Al film based nanoparticles-on-mirror system for manifold SERS enhancement.•All sputtering-grown Ag-NPs and Al film separated by thin dielectric overlayer of AlOx or PTFE.•A huge reduction in reflectance after growth of Ag-NPs on dielectric films coated Al films.•Optical model of spectroscopic ellipsometry exhibits strong plasmonic coupling.•Manifold SERS enhancement from 15 to 26 times for Ag-NPs and Al film based nanoparticles-on-mirror system.•An analytical enhancement factor up to 109 with a limit of detection up to 10−13 M.•FDTD simulations validate the gap mode plasmonic coupling of Ag-NPs with Al films.