Femtosecond laser-nanostructured substrates for surface enhanced Raman scattering

We present a new substrate for efficient surface enhanced Raman scattering (SERS). Using a train of focused frequency-doubled femtosecond laser pulses from a regeneratively amplified Ti:Sapphire laser, we fabricate submicron surface structures on a silicon wafer. After irradiating the silicon wafer with 400nm, 100fs laser pulses in a cuvette of water, we observe the formation of an array of spikes, each approximately 500nm tall and 200nm wide. The wafer is scanned across the beam to form an arbitrarily-sized nanostructured area. When covered with a thin film of a noble metal, the structured surface exhibits a strong enhancement of the Raman signal as measured using micro-Raman spectroscopy. To quantify the surface enhancement factor of the device, we cover the surface with a self-assembled monolayer of benzenethiol. The Raman surface enhancement factor of the structured area is measured to be approximately 10^10. These inexpensive, reproducible, and efficient SERS substrates show great promise for use in chemical and biological sensing, as well as demonstrate yet another application for ultrafast laser technology.