Infrared laser radiation is coupled via the tip into the tunneling junction of a scanning tunneling microscope. Rectification and difference frequency generation of the laser light caused by the nonlinearity in the current‐voltage characteristic of the junction are observed. Images with atomic resolution are obtained either by recording the rectified signal or by measuring the intensity at the difference frequency when the tip is scanned across the surface. These images are compared with others obtained in the conventional way by observing the tunneling current.

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