Recent measurements of dc current-voltage characteristics of scanning-tunneling-microscope (STN) junctions have confirmed their expected high rectification property. We propose to exploit this property (i) to study rectification at infrared and optical frequencies and (ii) to arrive at a procedural definition of an electron tunneling time. We envisage focusing a laser beam of linearly polarized light onto a STM junction and detecting the dc bias induced across the junction by the alternating, asymmetrical tunnel current. The asymmetry may be of geometrical, material, and/or thermal origin. The dc rectified bias should vanish at sufficiently high laser frequency for fixed tip-to-surface distance or when withdrawing the tip away from the surface for fixed laser frequency. The average electron tunneling time is then one half of the inverse cutoff frequency observed in such a laser-rectification experiment. Similar experiments have already been conducted successfully with metal-whisker diodes and have resulted in the recent redefinition of the meter. The new opportunity offered by STM is the controllability of the junction and its microscopic size, entailing high responsivity. Additional possible laser-STM studies are proposed.

• Received 9 February 1987

DOI:https://doi.org/10.1103/PhysRevB.35.7774