Cavity electrodynamics of van der Waals heterostructures

TL;DR

This paper demonstrates ultrastrong coupling between cavity modes of metallic gates and graphene plasmons in van der Waals heterostructures, revealing new ways to control low-energy quantum phenomena.

Contribution

It provides the first experimental observation of intrinsic cavity modes influencing the electrodynamics of vdW heterostructures using on-chip THz spectroscopy.

Findings

Spectral weight transfer observed between cavity and graphene modes

Avoided crossing indicating ultrastrong coupling

Cavity modes can manipulate low-energy physics of vdW heterostructures

Abstract

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.