The Optics Laboratory

Gradient-Field-Raman Spectroscopy (GFR)

Raman spectroscopy enables the study of vibrations in molecules and solids through the interaction of light with the vibrations. In normal Raman spectroscopy, the coupling between the light and the molecule is brought about by a change in polarizability (charge separation along a bond when it is placed in an electric field) as the molecule vibrates along that bond. We derived GFR to describe lines observed in a nano-Raman spectroscopy apparatus that could not be explained by the usual Raman excitation modes. The coupling between the light and molecule in GFR is moderated by a strong electric field gradient that shifts the potential energy of the atoms as they move during the vibration. The field gradient is due to the light itself, and exists nearby and perpendicular to a metal surface.