Optical mixing of CO2-laser radiation in a scanning tunneling microscope

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Abstract
Two infrared laser beams coupled into the tunneling junction of a scanning tunneling microscope lead to the generation of a signal at the difference frequency. In this article it is described that two different frequency mixing mechanisms are responsible for this process. One part of the signal is generated through a mixing process owing to the nonlinearity in the static current‐voltage characteristic. Another part has its origin in a nonlinear susceptibility at the surface; it therefore corresponds to frequency mixing in nonlinear optics. It will be shown that the difference‐frequency signals generated by the two processes can be separated owing to their different dependence on the tip‐sample distance.