Ultrasensitive SERS platform of Si ripples substrates fabricated by femtosecond laser

High quality, low spatially frequency ripples on silicon (Si) surface were directly fabricated by femtosecond (fs) laser irradiation in air and decorated with Au nanoparticles (NPs) developing large-area, low-cost substrates for surface-enhanced Raman spectroscopy (SERS). Rippled subwavelength structures exhibit a significant SERS response thanks to both an electromagnetic (EM) field enhancement, originating from the narrow gaps between adjacent ripples, and a plasmonic coupling among the Au NPs subsequently deposited via magnetron sputtering. SERS mapping shows a good uniformity, with ±8 % deviation over a 15 × 20 μm2 area, and reveals a large disparity in the signal strength with respect to that displayed by the grooves with micron-sized period produced at higher laser fluence, for which a two order of magnitude lower SERS signal is achieved. The SERS substrates with low spatial frequency ripples are able to detect a Raman analyte at a minimal concentration of 10−12 M for Rhodamine B (RhB) and 10−11 M for 4-hydroxybenzoic acid (4-MBA), respectively. Furthermore, a good agreement is attained between the values of the Raman enhancement factors (EFs) obtained experimentally and by simulations through Finite Elements Method calculations. Our findings demonstrate that the proposed approach based on low spatial frequency laser induced periodic surface structures (LSFL) on Si decorated with Au NPs by magnetron sputtering can provide a feasible and efficient method for the fabrication of SERS substrates with high Raman detection capability of analytes in trace amounts.