CW THz rectification spectroscopy of single molecules in STM tunneling junction*

We report rectification spectra obtained for single molecules in the STM junction induced by a frequency-tunable, continuous-wave (CW) THz laser. By focusing the THz beam into the tip-substrate junction of an 8K-STM, we observed rectification current at DC biases where the I-V curve exhibits nonlinearity. The rectification spectra over a single molecule exhibit line shape similar to those obtained by the inelastic electron tunneling spectroscopy (IETS) and provide an alternative way of measuring vibrational modes in a single molecule. Two different methods are proposed for the calibration of THz nearfield at the tip-substrate junction. Variation of the voltage drop induced by the CW THz field across the STM junction was not observed in the measured range of the tip-substrate gap.