Nano-optics is a unique tool to characterize materials at the smallest length scales and to strongly enhance light-matter interaction. Central to this field are polaritons, hybrid light-matter quasiparticles, that emerge from dipolar excitations, including phonons, excitons, and plasmons. The ability to tailor this interaction using 2D materials, cavities, and artificially structured metasurfaces has made polaritons a rapidly growing research field over the past decade. This opens new pathways for extreme confinement of light, novel optoelectronic devices, and photonic engineering of materials.

In the first part of my talk, I will show how polaritons in different material platforms intrinsically reach the ultrastrong coupling regime, typically explored with complex cavity setups. This coupling is particularly pronounced for phonon polaritons. I will demonstrate how ultrastrong coupling of hyperbolic phonon polaritons in Hf-based van der Waals materials enables extreme confinement of THz light down to UV-wavelength scales.

In the second part, I will introduce advanced nonlinear optical characterization techniques to image polaritons and characterize 2D materials. Nonlinear mixing of mid-infrared and visible lasers via sum-frequency generation enables sub-diffractional imaging of polaritons in SiC metasurfaces, as well as visualization of crystal orientation and stacking in typically invisible hexagonal boron nitride layers.

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