Photon-assisted tunneling through single molecules induced by terahertz radiation enhanced in the sub-nm gap electrodes

We have investigated the electron transport in single C₆₀ molecule transistors under the illumination of intense monochromatic terahertz (THz) radiation. By employing an antenna structure with a sub-nm wide gap, we concentrate the THz radiation beyond the diffraction limit and focus it onto a single molecule. The photon-assisted tunneling (PAT) in the single molecule transistors has been observed both in the weak-coupling and Kondo regimes. The THz power dependence of the PAT conductance indicates that, when the incident THz intensity is a few tens mW, the THz field induced at the molecule exceeds 100 kV/cm, which is enhanced by a factor of ~10⁵ from the field in the free space.