Difference-frequency generation at 9 GHz and laser-light rectification was demonstrated in the tunneling junction of a scanning tunneling microscope (STM) using ${\mathrm{CO}}_2$ laser radiation. Simultaneous measurements of mixing signals and the current-voltage (I-V) characteristics of the STM show that the mixing process occurs at the nonlinearity of the static I-V curve.

• Received 20 February 1990

DOI:https://doi.org/10.1103/PhysRevB.41.10229