We have implemented an optical homodyne interferometer to measure the tip oscillation amplitude in apertureless near-field optical microscopy. The setup is fully embedded in the microscope’s design, avoiding the presence of external arms. Our method is based on the synchronous detection of the interference between the fields reflected by the tip and a glass sample surface, while scanning the tip–sample distance over a few wavelengths. With the help of a simple model, we show how the different interference terms arising at frequencies multiple of the tip oscillation can be exploited to easily achieve sub-Ångstrom resolution.

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