Single Molecule Optical Rectification Spectroscopy and Microscopy

Rectification effect is known as the emergence of DC signal due to the existence of an AC signal. It has been widely observed in the systems that have nonlinear I-V response. Rectification current generated by AC bias modulation of frequency range between kHz and THz has been observed on single molecules study with Scanning Tunneling Microscope (STM). However, the rectification signal of single molecules induced by light from near infrared to visible frequency range has not been observed due to the significant tunneling gap oscillation caused by thermal fluctuation from amplitude modulation of light. We apply synchronized tunneling gap modulation to compensate this thermal fluctuation and measure the rectification spectra of a single pyrrolidine molecule adsorbed on a Cu(001) surface irradiated with 780 nm CW light. Our finding indicates a competition between tip enhanced Raman excitation and inelastic electron tunneling excitation of the vibration modes of single molecules.