Keynote Talk

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Prof. Dr. Lukas Eng: "Infrared and THz spectroscopy down to the 1-nm length scale"

Lukas M. Eng ¹,²

¹ Institute of Applied Physics, TU Dresden, Nöthnitzerstr. 61, 01187 Dresden, Germany
² ct.qmat: Würzburg-Dresden Cluster of Excellence - EXC 2147, TU Dresden, Germany

Contact Email lukas.eng@tu-dresden.de

Scattering-type near-field optical microscopy (s-SNOM) relies on tapping or non-contact scanning force microscopy (SFM) where illuminating the cantilever tip with appropriate laser light induces a tip dipole. That dipole in turn, induces an image dipole into the sample under test, with the overall interaction then being scattered to the detector. Both elastic and inelastic interactions thus can be read and quantified hereafter. We have extended this concept to very low energies of ~1 meV only, needing special THz laser light sources, i.e. the using the Free-electron laser at the Helmholtz Center Dresden-Rossendorf (HZDR).

The reliable probing of nanoscale optical properties by s-SNOM, however, is often obscured by a manifold of local artefacts, with local electronic potential variations playing the major role into that game. In this contribution, we illustrate how to compensate for 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 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 pure 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].

[1]   U. Zerweck et al., Phys. Rev. B 71, 125424 (2005); https://doi.org/10.1103/PhysRevB.71.125424.

[2]   T. Nörenberg et al., APL Photon. 6, 036102 (2021); https://doi.org/10.1063/5.0031395.

[3]   J. Döring et al., Nanoscale 10, 18074 (2018); https://doi.org/10.1039/C8NR04081H.

[4]   L. Wehmeier et al., Phys. Rev. B 100, 035444 (2019); https://doi.org/10.1103/PhysRevB.100.035444.

[5]   D. Lang et al., Rev. Sci. Instrum. 89, 033702 (2018); https://doi.org/10.1063/1.5016281.

[6]   J. Barnett et al, Nano Lett. 21, 9012 (2021); https://doi.org/10.1021/acs.nanolett.1c02353.

[7]   S.C. Kehr et al., Synch. Rad. News 30, 31 (2017); https://doi.org/10.1080/08940886.2017.1338421.