Atomic-Scale Coupling of Photons to Single-Molecule Junctions

The study of optical phenomena at the atomic scale is expected to provide new understanding of molecules and their chemical dynamics. The combination of lasers with a low temperature scanning tunneling microscope (STM) has led to the observation of photo-induced resonant tunneling with sub-molecular spatial resolution for single molecules adsorbed on a solid surface [\textit{Science} \textbf{312}, 1362, (2006)]. Furthermore, the irradiation of femtosecond laser pulses into this single-molecule junction defined by STM results in non-linear coupling by two-photon excitation, rather than the single-photon coupling in the case of continuous wave (CW) lasers. These experiments lead to new opportunities by tapping into the unique properties of lasers and the STM.