Gold-coated silicon nanoripples achieved via picosecond laser ablation for surface enhanced Raman scattering studies

We report the results from ablation studies of polished single crystalline Si targets (of resistivity 1-10 Ohm -cm) submerged in acetone by employing the focused Airy pattern of the p-polarized laser pulses of duration - 2 ps. The Airy pattern was obtained by inserting a circular aperture with hard edge before the focusing convex lens. The ablation was performed on the Si substrate at diverse scanning speeds of the motorized stage (25-600 mu m/ sec) with a fixed pulse energy (-50 mu J) resulting in high/low spatial frequency laser induced periodic surface structures (HSFLIPSS/ LSFLIPSS). These are believed to be the outcome of varied electric fields and the varied pulse numbers at different scan speeds. The effective number of pulses estimated on the Si target demonstrated the surface structures with different grating periods. At lower and higher number of pulses HSFLIPSS were obtained. At an intermediate number of pulses, LSFLIPSS were observed. Further, the obtained HSFLIPSS (with grating periods -111 nm, 114 nm) were gold coated (25 nm) with magnetron dc sputtering technique and subsequently employed them for surface enhanced Raman scattering (SERS) studies of explosive molecules 5amino, 3 -nitro, 1,3,5-nitrozole (ANTA) at a very low concentration of 100 nM. SERS imaging of ANTA from the mentioned HSFLIPSS illustrated the uniformity in SERS signatures with an intensity variation below 10% at different locations of the SERS active platform.