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

TL;DR

This paper introduces an improved calibration method for s-SNOM that accounts for probe tapping, enabling more accurate complex permittivity measurements in the mid-infrared range without detailed electromagnetic modeling.

Contribution

The proposed method explicitly incorporates probe tapping effects into black-box calibration, simplifying permittivity extraction in s-SNOM measurements.

Findings

Validated on silicon microstructures with known doping levels

Achieved accurate permittivity measurements without detailed electromagnetic models

Enhanced calibration method suitable for slowly modulated probe-sample distances

Abstract

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 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 analysers, allow the extraction of the permittivity without detailed electromagnetic modelling of the probe-sample interaction. These methods, however, are typically designed for stationary setups. In contrast, the distance between the sample and the probe tip of the s-SNOM is slowly modulated, which is required for the lock-in detection used to extract the near-field interaction buried in the far-field background. Here we propose an improved calibration method that explicitly takes probe tapping into account. We validate our method for an s-SNOM operating in a mid-infrared spectral range by applying it to measurements of silicon microstructures of different but well characterised doping.