A mode-locked laser focused on the tunneling junction of a scanning tunneling microscope (STM) superimposes a microwave frequency comb with hundreds of harmonics on the DC tunneling current. Each harmonic, at an integer multiple of the laser pulse repetition frequency, sets the present state-of-the-art for narrow linewidth at its frequency to enable low-noise measurements at an average laser power of several milliwatts. Measurements of the attenuation of the harmonics, which is caused by the spreading resistance, may be used to determine the resistivity of the sample. In Scanning Frequency Comb Microscopy (SFCM) feedback control of the tip-sample distance is based on the power at the harmonics. No DC bias voltage or DC tunneling current is required and the data rate is much higher than that with an STM. Simulations of the spatial distribution of the power dissipated in the sample show the feasibility of non-destructive true sub-nm resolution in the carrier profiling of semiconductors. With no DC bias voltage and no DC tunneling current band-bending and other changes to semiconductor samples in an STM are mitigated and there is a possibility for in vivo microscopy in biology and medicine.
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