A New Mechanism For Laser-Frequency Mixing In A Scanning Tunneling Microscope

Two infrared laser frequencies coupled into the tunneling junction of a scanning tunneling microscope (STM) lead to the generation of a difference-frequency signal. In this article two different mechanisms of difference-frequency generation (DFG) in the tunneling junction are presented. Besides DFG due to the nonlinearity of the current-voltage characteristic known from previous experiments, nonlinear optical DFG arising from the breakdown of the inversion symmetry at the metal-vacuum interface is demonstrated. This nonlinear optical DFG may be a step further towards laser spectroscopy with an STM.