Mechanisms of current rectification in an STM tunnel junction and the measurement of an operational tunneling time

The effects of material, thermal, and geometrical asymmetries on the nonlinearity of the I-V characteristics of a scanning tunneling microscope (STM) tunneling junction are investigated theoretically and experimentally. As a consequence of the nonlinearity, an STM junction can rectify an incident laser beam to produce both DC and AC tunneling current components. Using the dependence of the DC rectified current on the tip-sample distance s and the laser frequency v, an operational definition of a tunneling time is proposed. An experiment using a laser-irradiated STM junction is also performed to measure the rectified current as a function of s, for a fixed v. From the preliminary results of this experiment, a tunneling time on the order of 1.8 fs is determined. This time is in reasonable agreement with those obtained in other experimental and theoretical studies of tunneling junctions of comparable dimensions and energies. It is also shown that the AC current contains components at the laser frequency and its higher harmonics. This implies that the STM junction can be used as a nonlinear device to detect, generate, and measure laser frequency harmonics.