  Van der Waals (vdW) heterostructures host many-body quantum phenomena that
can be tuned in situ using electrostatic gates. These gates are often
microstructured graphite flakes that naturally form plasmonic cavities,
confining light in discrete standing waves of current density due to their
finite size. Their resonances typically lie in the GHz - THz range,
corresponding to the same $\mu$eV - meV energy scale characteristic of many
quantum effects in the materials they electrically control. This raises the
possibility that built-in cavity modes could be relevant for shaping the
low-energy physics of vdW heterostructures. However, capturing this
light-matter interaction remains elusive as devices are significantly smaller
than the diffraction limit at these wavelengths, hindering far-field
spectroscopic tools. Here, we report on the sub-wavelength cavity
electrodynamics of graphene embedded in a vdW heterostructure plasmonic
microcavity. Using on-chip THz spectroscopy, we observed spectral weight
transfer and an avoided crossing between the graphite cavity and graphene
plasmon modes as the graphene carrier density was tuned, revealing their
ultrastrong coupling. Our findings show that intrinsic cavity modes of metallic
gates can sense and manipulate the low-energy electrodynamics of vdW
heterostructures. This opens a pathway for deeper understanding of emergent
phases in these materials and new functionality through cavity control.
