Various super-resolution imaging techniques have been established to overcome the diffraction limit of light. However, most of these techniques are limited to resonance imaging in the visible regime, while mid- to far-infrared (IR) approaches are rare. The prime tool for IR super-resolution microscopy is scattering-type scanning near-field optical microscopy (s-SNOM). In nanophotonics, s-SNOM is often employed to study phonon polaritons (PhPs), quasi-particles which enable light confinement to length scales far below the diffraction limit. Although s-SNOM has provided great insight to PhPs, the technique often suffers from long image acquisition times, due to the point-scanning approach, and the limitation for the applied laser power to avoid laser induced damage. This thesis introduces a novel approach to IR sub-diffractional imaging of phonon polaritons by wide-field sum-frequency generation (SFG) microscopy. Here, the IR electric fields of phonon polaritons are upconverted with a visible laser, resulting in a non-linear SFG signal. This provides IR resonance information and a spatial resolution of 1.4 µm ((~λ_IR/9)) that is well below the IR-diffraction limit. By using a tunable IR free-electron laser (FEL), polaritons can be excited across the whole mid- to far-IR range. In this work, the newly-developed SFG microscope is first employed for interferometric imaging of propagating surface PhPs on an AlN substrate. A detailed analysis gives insight into the anisotropy- and polarization-dependent dispersion of the propagating modes. Structuring substrates with microresonators leads to confinement on the µm-scale and the observation of localized PhPs. By employing SiC 1D resonator chains with lengths from one to several resonators, the formation of a collective response is detected, originating from coupling between the individual localized polaritons. The thesis continues with the observation of hybridization of localized and propagating polaritons in a 2D metasurface consisting of SiC microresonators on a SiC substrate. Spectro-microscopy allows for two complementary approaches to measure the hybridized polariton dispersion, namely angle-dependent resonance imaging and polariton interferometry. The analysis reveals the effect of strong coupling on the localization of polaritons, as well as the formation of edge states. Moving from cylindrical to elongated rod-like resonators, standing waves inside the rods are observed despite losses to the substrate. SFG imaging reveals tunable resonances by changing the rod's length which is in good agreement with s-SNOM experiments and simulations. Overall, this work introduces a novel technique for IR sub-diffractional imaging, which is extensively employed to study PhPs. The findings form a basis for future applications and investigations of PhPs in nanophotonic devices.
