X. Guo, K. Bertling and A. D. Rakića
Applied Physics Letters 118, 041103 (2021)
Scattering-type scanning near-field optical microscopy (s-SNOM) allows for the characterization of optical properties of samples at the nanoscale, well below the diffraction limit of the interrogating wavelength. Typically, it relies on a model for the probe-sample interaction to extract complex optical constants of the sample. Here, we propose an s-SNOM calibration method that allows for the extraction of these constants without prior knowledge of the probe geometry nor the details of the probe-sample interactions. We illustrate the technique using terahertz time-domain spectroscopy-based s-SNOM to extract the optical properties of several organic and inorganic materials and differently doped regions of a standard silicon random access memory sample. The accuracy of the technique is comparable to that of conventional far-field techniques while additionally providing spatial distribution of optical constants at the nanoscale. The source-independent nature of the proposed technique makes it directly applicable for s-SNOM measurements in other spectral ranges.