Polarization-dependent near-field phonon nanoscopy of oxides: SrTiO₃, LiNbO₃, and PbZr₀.₂Ti₀.₈O₃

Resonant infrared near-field optical spectroscopy provides a highly material-specific response with sub-wavelength lateral resolution of about 10 nm. Here, we provide the near-field response of selected paraelectric and ferroelectric materials, i.e. SrTiO3, LiNbO3, and PbZr0:2Ti0:8O3, showing resonances in the wavelength range from 13.0 to 15.8 µm. We investigate these materials using scattering scanning near-field optical microscopy (s-SNOM) in combination with a tunable midinfrared free-electron laser (FEL). Fundamentally, we demonstrate that phonon-induced resonant near-field excitation surprisingly is possible for both p- and s-polarized incident light, a fact that is of particular interest for the nanoscopic investigation of anisotropic and hyperbolic materials. Moreover, we show that near-field spectroscopy, as compared to far-field techniques, bears substantial advantages such as lower penetration depths, stronger confinement, and a high spatial resolution. The latter permits the investigation of minute material volumes, e.g. with nanoscale changes in crystallographic structure, which we prove here via near-field imaging of ferroelectric domain structures in PbZr0.2Ti0.8O3 thin film.