Nano-structured surfaces by laser interference lithography and fs-laser direct writing as substrates for surface-enhanced Raman spectroscopy

Nano-structured surfaces were generated by laser interference lithography and femtosecond-laser direct writing of photo resists that subsequently were metallized by electroless plating or sputter deposition of silver. Laser lithography was performed with a 405 nm coherent diode laser in AZ9260, using two-beam interference with double illumination by 90° rotating of the substrate, leading to 2D periodic surface patterns with smallest features of the order of 200 nm. With fs-laser direct writing using a Ti-sapphire oscillator of 800 nm and 15 fs pulse length, feature sizes down to 100 nm were realized in SU8, even with aspect ratios much larger than 1. Metallization with electroless plating delivered either grainy silver coatings with a grain size around 100 nm or needle-like silver coatings with a needle length around 100 nm and a width of around 10 nm. The metallized substrates were exposed to aqueous solutions of Rhodamine 6G (Rh6G) of different concentrations and the corresponding Raman signals were recorded with a Raman micro-probe spectrometer. The nano-structured surfaces lead to formation of Raman bands attributable to Rh6G. In case of the grainy silver coatings, surfaces without nano-structures did not show Raman activity, indicating that grating-coupled surface plasmons play the dominant role for Raman enhancement. In case of substrates coated with the needle-shaped silver crystallites, Raman activity was also seen in regions without laser-generated nano-structures, indicating that localized particle plasmons play the dominant role for Raman enhancement. A comparison with Raman spectra measured with conventional Raman spectrometer showed that the enhancement factor achieved by the laser-generated nano-structures themself, is of the order of 6×10 4 . Raman intensity as a function of Rh6G concentration revealed a regular behaviour, as expected from a Langmuir isotherm.