The reliable probing of nanoscale optical properties by scattering-type near-field optical microscopy (s-SNOM) is often obscured by a manifold of local artefacts, with local electronic potential variations playing the major role into that game. As a consequence, the tip-sample interaction unavoidably is affected by a multitude of different force interactions that then need to be disentangled one by one. The local-scale ‘optical’ signal in s-SNOM hence may always be erroneous or even totally wrong, leading to overinterpretation of the recorded optical near-field data and deducing wrong consequences.
In this contribution, we illustrate how to compensate for such electrostatic artefacts in-situ, by elegantly combining s-SNOM with the capabilities of Kelvin-Probe Force Microscopy (KPFM) operated in frequency-modulation (FM) mode (1). Not only are we then be able to monitor nearly electronic-artefact-free near-field signals at any of the different higher harmonics demodulated in s-SNOM, but furthermore, also to gather quantitative local information on the sample surface electrostatic conditions quasi for free (2,3). We will introduce into this technical merger (2) by demonstrating its necessity with a manifold of different s-SNOM examples, i.e. s-SNOM data recorded on bare metals (Au), semiconducting (Si) and dielectric (SiO2) samples, on different ferroelectric surfaces (2,3,4) and multiferroics (5) both at ambient and liquid-helium temperatures (4), but equally when investigating phase-change materials (6). Notably, we show both resonant and non-resonant optical sample excitations in these experiments, hence demonstrating the great benefits of our s-SNOM/KPFM combinations at FELBE and TELBE when performing investigations over the broad wavelength range from VIS down to 1 THz (7).
References:
1 U. Zerweck et al., Phys. Rev. B 71, 125424 (2005);
2 T. Nörenberg et al., APL Photon. 6, 036102 (2021);
3 J. Döring et al., Nanoscale 10, 18074 (2018);
4 L. Wehmeier et al., Phys. Rev. B 100, 035444 (2019);
5 D. Lang et al., Rev. Sci. Instrum. 89, 033702 (2018);
6 J. Barnett et al, Nano Lett. 21, 9012 (2021);
7 S.C. Kehr et al., Synch. Rad. News 30, 31 (2017);
