Paper NS+TR-TuM6
Field Amplification in Surface-Enhanced Raman Scattering

Tuesday, November 10, 2009, 9:40 am, Room L

Surface-enhanced Raman scattering (SERS) is experiencing renewed popularity as a result of the present interest in nanostructures and reported observations of SERS from single molecules. However, the detailed reasons for enhancements, which have been reported to be as much as 10¹⁴, remain unclear. We address the issue from a fundamental perspective that has provided insight into the physics of second-harmonic generation and other nonlinear-optical processes, considering metallic spherical inclusions for simplicity. One obvious result, although it contradicts many statements in the literature, is that resonant-plasmonic enhancement of both incident and emitted waves by the same sphere is impossible, since the sphere cannot simultaneously exhibit plasmonic resonances at the frequencies w_o and w_R of the driving and Raman fields, respectively. The presence of an adjacent inclusion nominally resonant at the other frequency does not resolve the issue due to coupling by the longitudinal (Coulomb) interaction. However, under certain circumstances amplification can occur at a second frequency by means of a resonance activated by a renormalization of the effective mass of the bonding electrons of the molecule due to their longitudinal interactions with the sphere. Local configurations in deposited thin films where this could occur would likely be extremely rare, but cannot be ruled out completely.