Spectroscopy and Imaging

Raman spectroscopy is a modern and attractive optical tool in chemical and biological applications [1][2]. The detection limit of this method down to lowest concentrations is given by the inherent small Raman cross section. In order to increase the Raman signal by several orders of magnitude surface enhanced Raman spectroscopy (SERS) can be applied. The base for highest surface enhancement are suitable substrates, which will allow the detection of analyte molecules in lowest concentration down to the single molecule level. Within this contribution we introduce microfabricated and surface-metalized polymer gratings as highly homogenous and highly sensitive substrates for surface enhanced Raman spectroscopy (SERS). The field-enhancement is based on the periodical grating structures which couple the light into the metallic surface, the generation of propagated surface plasmon polaritons (SPP) and whose enhancement at the edges of the dot-like pattern. The gratings, made from PMMA, are two-dimensional pattern with periods down to 200 nm and a height of 70 nm. Prefabricated on waferscale (4”), single chips with a dimension of 5x10mm² contain several gratings with a size of 200x200µm² for the SERS-investigation and 1x1mm² gratings for the spectroscopic characterization in the UV/VIS-wavelength range. On top of these PMMA-gratings, which are made by using electron-beam- or imprint-lithography, a 10 nm thin alumina-film as protection layer followed by a 20 to 40 nm thick metallization layer is deposited. As metal films gold, silver and aluminum are suitable. Compared to gold, silver yields larger surface enhancement of Raman scattering in the visible spectral range and aluminum is more appropriate for measurements in the UV-range. Used in Raman-setups our SERS-substrates show highly sensitive and highly homogenous plasmonic surfaces. Covered with a thin silver film, enhancement factors in the range of 105 to 106 were achieved. Regarding to the goal of an establishment of SERS as a routinely applied analytical tool in bio sensing and material science the proposed fabrication method and the obtained SERS-substrates fulfil the need for low-cost and easy-to-handle SERS approaches. [1] M. Schmitt, J. Popp, Journal of Raman Spectroscopy, 37 (2006) 20-28. [2] A. Kudelski, Talanta, 76 (2008) 1-8.