Near-field infrared nanospectroscopy of insulating and metallic SrTiO₃

Scattering-type, scanning near-field infrared nanospectroscopy is a powerful tool for exploring nanoscale properties of materials that are not easily accessible by conventional far-field techniques. For example, one can resolve optical constants of thin surface layers and probe light-matter excitations such as propagating surface-phonon polaritons (SPhPs). Experimental spectra in the infrared and terahertz range have been obtained on the surface metallic layer of insulating strontium titanate (SrTiO₃) crystals. The surface metallicity is due to oxygen deficiencies induced by vacuum annealing. The spectra are modeled with full-wave numerical simulations taking into account the full geometry of the experimental probe. Hence, we extract the dynamical properties of the free charge carriers in the metallic layer. Additionally, we experimentally obtain hyperspectral line scans mapping the interference of SPhPs launched from the probe tip and from a gold edge on the insulating surface of pristine SrTiO₃. Our full-wave numerical method is used for modeling of the experimental data to obtain the properties of the SPhPs.