The electric field at the apex of a near-field probe: implications for nanoRaman spectroscopy

Near-field or nano-Raman spectroscopy is different from its far-field counterpart in several important respects. We present a unified view of nano-Raman that accounts for these differences, which include surface enhancement, propagation differences, the presence of z-polarized light, as well as electric field gradients that give rise to new spectroscopic selection rules. We also discuss some of the recent advances in near-field scanning optical microscopy (NSOM) and their positive impact on nano-Raman spectroscopy. In particular progress has been made in fabricating better probes, both apertured and apertureless, for NSOM resulting in larger signal levels important for Raman spectroscopy. Larger signals result in shorter imaging times, the ability to achieve higher resolution, and broader applicability of the technique.