Tip-enhanced Raman spectroscopy with high contrast

Tip-enhanced Raman spectroscopy (TERS) is emerging as a promising spectroscopic tool for nanoscale characterization of chemical composition, structure, stresses and conformational states. However, its widespread application requires optimization of the technique to reproducibly achieve sufficiently high contrast between near-field and far-field signals. We present a TERS spectrometer, based on side illumination geometry, which demonstrates reproducible enhancements of the Raman signal of the order of 10 ³-10 ⁴ for a variety of molecular, polymeric and semi-conducting samples using both silver- and gold-coated tips. We estimate the localization of the Raman signal enhancement to be ∼20 nm. For thick samples, the contrast is limited by a strong far-field signal (from the laser illuminated spot) that overpowers the near-field signal (enhanced in the vicinity of the tip). Optimizing the polarization geometry and the incident angle, we have achieved a contrast between near-field and far-field signal of 12 times on (100) Si - a level that makes this technique attractive for characterization of silicon nanostructures.