Assessment of sensitivity advances in near-field Raman spectroscopy

Near-field Raman spectroscopy can be used to obtain images with both chemical specificity and the subwavelength spatial resolution of near-field scanning optical microscopy. In the absence of signal intensification factors, such as `surface enhancement' or electronic resonance in the specimen, Raman scattering suffers from a small cross section ((sigma) equals 10 -28 cm 2 to 10 -31 cm 2 ). Since most reports of Raman-NSOM to date involve exploitation of a specimen-specific intensification, an assessment of the general applicability of Raman-NSOM to a wider variety of `un-enhanced' samples is of great interest. We report here on several approaches to increasing the sensitivity of near- field Raman spectroscopy that do not rely on specimen properties. The use of chemically etched aperture probes as an illumination source has been investigated and compared to probes fabricated by the traditional heat and pull method.