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Phonon polaritons enable waveguiding and localization of infrared light withextreme confinement and low losses. The spatial propagation and spectralresonances of such polaritons are usually probed with complementary techniquessuch as near-field optical microscopy and far-field reflection spectroscopy.Here, we introduce infrared-visible sum-frequency spectro-microscopy as a toolfor spectroscopic imaging of phonon polaritons. The technique simultaneouslyprovides sub-wavelength spatial resolution and highly-resolved spectralresonance information. This is implemented by resonantly exciting polaritonsusing a tunable infrared laser and wide-field microscopic detection of theupconverted light. We employ this technique to image hybridization and strongcoupling of localized and propagating surface phonon polaritons in metasurfacesof SiC micropillars. Spectro-microscopy allows us to measure the polaritondispersion simultaneously in momentum space by angle-dependent resonanceimaging, and in real space by polariton interferometry. Notably, we directlyvisualize how strong coupling affects the spatial localization of polaritons,inaccessible with conventional spectroscopic techniques. We further observe theformation of edge states at excitation frequencies where strong couplingprevents polariton propagation into the metasurface. Our approach is applicableto the wide range of polaritonic materials with broken inversion symmetry andcan be used as a fast and non-perturbative tool to image polaritonhybridization and propagation.

Spectroscopic and Interferometric Sum-Frequency Imaging of Strongly Coupled Phonon Polaritons in SiC Metasurfaces