Hyperspectral Nanoimaging of van der Waals Polaritonic Crystals.

Phonon polaritons (PhPs) in van der Waals (vdW) crystal slabs enable nanoscale infrared light manipulation. Specifically, periodically structured vdW slabs behave as polaritonic crystals (vdW-PCs), where the polaritons form Bloch modes. Because the polariton wavelengths are smaller than that of light, conventional far-field spectroscopy does not allow for a complete characterization of vdW-PCs or for revealing their band structure. Here, we perform hyperspectral infrared nanoimaging and analysis of PhPs in a vdW-PC slab made of h-BN. We demonstrate that infrared spectra recorded at individual spatial positions within the unit cell of the vdW-PC can be associated with its band structure and local density of photonic states (LDOS). We thus introduce hyperspectral infrared nanoimaging as a tool for the comprehensive analysis of polaritonic crystals, which could find applications in the reconstruction of complex polaritonic dispersion surfaces in momentum-frequency space or for exploring exotic electromagnetic modes in topological photonic structures.