Ultrafast Dynamics of Surface-Enhanced Raman Scattering Due to Au Nanostructures

Ultrafast dynamics of surface-enhanced Raman scattering (SEAS) was investigated at cleaved graphite surfaces bearing deposited gold (Au) nanostructures (similar to 10 nm in diameter) by using sensitive pump probe reflectivity spectroscopy with ultrashort (7.5 fs) laser pulses. We observed enhancement of phonon amplitudes (C=C stretching modes) in the femtosecond time domain, considered to be due to the enhanced electromagnetic (EM) field around the Au nanostructures. Finite-difference time-domain (FDTD) calculations confirmed the EM enhancement. The enhancement causes drastic increase of coherent D-mode (40 THz) phonon amplitude and nanostructure-dependent changes in the amplitude and dephasing time of coherent G-mode (47 THz) phonons. This methodology should be suitable to study the basic mechanism of SEES and may also find application in nanofabrication.