Multiwavelength Surface‐Enhanced Raman Spectroscopy Using Rainbow Trapping in Width‐Graded Plasmonic Gratings

Abstract Plasmonic gratings of rectangular groove profile with gradient in the groove width perform as unique surface‐enhanced Raman spectroscopy (SERS) substrates by simultaneously confining multiple laser wavelengths proximally and inside their rectangular nanotrenches. These gratings consist of a metal–insulator–metal (MIM) groove of 40 nm width at the center, surrounded by grooves with widths increasing in 10 nm steps to a maximum of 180 nm. It is experimentally shown and theoretically confirmed that upon illumination a maximally enhanced electromagnetic field is generated at the center of these gratings as a result of plasmonic light trapping as well as waveguiding produced by the surrounding grooves. SERS enhancement factors of 10⁶–10⁷ are demonstrated for 20 μL min⁻¹ flow of 1 × 10⁻³m aqueous phospholipid solution using 532, 638, and 785 nm laser illumination of the gratings integrated within microfluidic devices. These robust multiwavelength SERS substrates offer highly reproducible plasmonic field enhancement that can be tuned to cover broad wavelength ranges within the visible and near‐infrared regime and are ideal for static and dynamic characterization of low concentration species. Further, the multispectral characteristic of these gratings facilitates multiplexing through various laser wavelengths thereby making it possible to readily access weak or silent Raman modes.