Silicon (Si) nanoparticles (NPs) and self-organized high spatial frequency laser (HSFL) induced periodic surface structures were fabricated by means of femtosecond ablation of bulk Si target in acetone. The ablation was performed with ∼40 fs (fwhm) pulses and different input energies of ∼500, ∼200, ∼150, ∼100, ∼50, and ∼10 μJ. Fabricated NPs and nanostructures (NSs) were characterized by UV–visible absorption spectroscopy, photoluminescence (PL) spectroscopy, Raman spectroscopy, transmission electron microscopy, and field emission scanning electron microscopy. The average sizes of the NPs were estimated to be in the 4–135 nm range. From the PL studies of Si NPs of different sizes, we have observed a size-dependent shift toward blue spectral region. We could tune the observed PL peak in the spectral range of 440–515 nm. The crystalline and amorphous nature of the Si nanoparticles and nanostructures was investigated using selected area electron diffraction and Raman spectra. Complex refractive index, conduction band electron density of the Si NPs, estimated by measuring the effective spot size corresponding to each input energies, were observed to play a crucial role in determining the periodicity of HSFL induced periodic surface structures. Experimentally measured periodicity of gratings was in good agreement with the theory.

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Three-dimensional silver nanoparticles decorated vertically aligned Si nanowires (Si NWs) are effective surface-enhanced Raman spectroscopy (SERS) substrates for molecular detection at low concentration levels. The length of Si NWs prepared by silver assisted electroless etching is increased with an increase in etching time, which resulted in the reduced optical reflection in the visible region. These substrates were tested and optimized by measuring the Raman spectrum of standard dye Rhodamine 6G (R6G) of 10 nM concentration. Further, effective SERS enhancements of $10 5 and $10 4 were observed for the cytosine protein (concentration of 50 lM) and ammonium perchlorate (oxidizer used in explosives composition with a concentration of 10 lM), respectively. It is established that these three-dimensional SERS substrates yielded considerably higher enhancement factors for the detection of R6G when compared to previous reports. The sensitivity can further be increased and optimized since the Raman enhancement was found to increase with an increase in the density of silver nanoparticles decorated on the walls of Si NWs.

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