Cavity electrodynamics of van der Waals heterostructures

Van der Waals (vdW) heterostructures exhibit a wide range of exotic many-body phenomena that can be tuned in situ using electrostatic gates. These gates are typically graphite flakes that naturally form sub-wavelength plasmonic cavities, confining light in standing waves of current density. Their resonances typically lie in the ~meV energy scale characteristic of many quantum electronic effects in the materials they control. This suggests that these built-in cavity modes could be used to sense and tune the low-energy physics of vdW heterostructures. However, capturing this light-matter interaction is difficult as devices are smaller than the diffraction limit, hindering far-field spectroscopic tools. I will discuss the cavity electrodynamics of graphene embedded in a vdW heterostructure microcavity. Using on-chip THz spectroscopy, we observed an avoided crossing between the graphite cavity and graphene plasmon modes, revealing their ultrastrong coupling. Our findings show that intrinsic cavity modes of metallic gates can shape the low-energy electrodynamics of vdW heterostructures. This allows for deeper understanding of emergent phases and new functionality through cavity control.