Metasurfaces are artificial structures that enable flat optical components and pave the way for on-chip light processing. A promising route to new functionalities is to build metasurfaces out of materials with strong optical resonances. Light then propagates across the metasurface in the form of polaritons – mixed light-matter particles. This recently enabled coherent thermal light sources and directional waveguiding. So far, it has been challenging to characterize metasurfaces because of their small sub-wavelength building blocks, their overall large spatial extent, and their wavelength-dependent properties, requiring complimentary imaging and spectroscopy techniques.

In their experimental work recently published in Advanced Materials, the Lattice Dynamics group introduces a new technique to image infrared metasurfaces with combined sub-wavelength spatial resolution and full spectral information. The authors developed a sum-frequency microscope, where the light of a tunable infrared free-electron laser is mixed with a visible upconversion laser inside of a metasurface. With this nonlinear technique it was possible to visualize how different types of phonon polaritons hybridize and propagate inside a SiC metasurface. By correlating spatial and spectral information, the authors found that strong coupling opens a route to tune light propagation in metasurfaces and even activates new polaritonic edge states, which are important characteristics for on-chip photonic devices and novel light sources.