Sum frequency generation (SFG) allows the study of surfaces and inversion broken systems. In a new approach we implemented a wide field sum-frequency microscope combining the FHI infrared free electron laser (IR FEL) as excitation source with visible upconversion. The IR FEL provides a powerful, narrow band, and tunable light source [1]. By direct imaging of the SFG light with a microscope in a wide field scheme without scanning the sample or the focus [2], we achieve a spatial resolution well beyond the infrared diffraction limit.
I will show first micrsoscope images of phonon polariton resonances in subdiffractional SiC structures. Full spectral mapping of the structures allows spectroscopic identification of the various polariton resonances. Additionally, the high spatial resolution of the microscope (< 1  µm) resolves the modal structure of polaritons within the different structures. We follow the evolution of the polariton modes by varying the geometrical parameters.
[1] Schöllkopf et al., Proc. of SPIE (2015)
[2] Kiessling et al., ACS Photonics, (2019)

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