Laser‐frequency mixing in the junction of a scanning tunneling microscope

The frequency response and the frequency‐mixing properties of the tunneling junction of a scanning tunneling microscope (STM) were investigated using the radiation of two CO2 lasers. Difference frequencies of up to 90 MHz were detected over a wide range of bias voltages and tunneling currents. At tunneling currents typical of STM operation the beat signal decreases as the bias voltage is increased. However, this dependence is reversed at higher tunneling currents, where the STM begins to resemble a point‐contact metal‐insulator‐metal diode. For low tunneling currents the generation of the beat signal is attributed mainly to thermally assisted tunneling.

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