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 similar to 10 nm. Here, we report on the study of 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 mu m. We investigate these materials using scattering scanning near-field optical microscopy in combination with a tunable mid-infrared free-electron laser. Fundamentally, we demonstrate that phonon-induced resonant near-field excitation 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 exploit 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 films.