Optical heterodyne detection at a silver scanning tunneling microscope junction

Abstract

The 435 MHz beat signal between two modes of a He–Ne laser (633 nm) has been detected in the current of a scanning tunneling microscope (STM), using a silver probe to enhance the sensitivity using its plasmon resonance. The difference frequency between this optical beat and a radio frequency voltage applied to the junction has also been detected in the STM current. The difference frequency and heterodyne signals, as well as the direct current (dc) probe expansion, fit a calculation of modulated and dc thermal expansion of the probe tip, once one assumes that a surprisingly large fraction of the laser beam is absorbed at a small structure at the probe end. The beat signal due to current rectification is not observed for the experimental conditions, but the potential for utilizing the plasmon resonance to obtain such a signal using a silver probe is evaluated for other, feasible conditions where it might provide exceptional sensitivity.