Instrumentation and Measurement Physics(DIMP)
Physique des instruments et mesures (DPIM)
Hans HALLEN
North Carolina State University
Electric field effects in nanoscale Raman spectroscopy
The nanoscale metal object locally concentrates the electric field. As these evanescent fields decay on a nanometer length-scale, strong field gradients are produced. These gradients have profound effects on the Raman spectra of samples within them, leading to a “Gradient-Field Raman” (GFR) effect. It leads to new selection rules for surface enhanced Raman spectroscopy (SERS), and also to differences between far-field and near-field Raman spectroscopy measured with a metal-apertured near-field optical microscope (NSOM). We describe how a strong gradient of the electric field can alter the Raman spectra, and investigate its implications on selection rules. Heuristically, the field gradient causes the Coulomb force on a polarized bond to vary during the vibration, providing a new coupling mechanism between the field and the vibration. These selection rules differ markedly from the usual Raman selection rules, and allow Raman-like observation of strong IR (not normally Raman) vibrations. The presence of the metal probe near the Raman-emitting dipole also causes plasmons to be created on the tip. We show evidence of this by tracking Raman emission as a function of NSOM tip-sample distance.