Calibration method for complex permittivity measurements using s-SNOM combining multiple probe tapping harmonics.

Scattering-type scanning near-field optical microscopy (s-SNOM) enables sub-diffraction spectroscopy, featuring high sensitivity to small spatial permittivity variations of the sample surface. However, due to the complexity of the near-field probe-sample interaction, the quantitative extraction of the complex permittivity leads to a computationally demanding inverse problem, requiring further approximation of the system to an invertible model. Black-box calibration methods, similar to those applied to microwave vector network analyzers, allow the extraction of the permittivity without detailed electromagnetic modeling of the probe-sample interaction.