Condensed Matter/Materials & Biological Physics Seminar with Hope Bretscher on Cavity electrodynamics of van der Waals heterostructures

Van der Waals (vdW) heterostructures exhibit a wide range of exotic many-body quantum phenomena that can be tuned in situ using electrostatic gates. These gates are typically graphite flakes that, due to their subwavelength size, naturally form plasmonic cavities, confining light in standing waves of current density. Their resonances typically lie in the GHz – THz range range, corresponding to the same µeV-meV energy scale characteristic of many quantum electronic effects in the materials they control. This raises the possibility that these built-in cavity modes could be used to sense and manipulate the low-energy physics of vdW heterostructures. However, capturing this light-matter interaction remains challenging as devices are significantly smaller than the diffraction limit at these wavelengths, hindering far-field spectroscopic tools. In this talk, I will discuss 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 shape 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.

This lecture was made possible by the William C. Ferguson Fund.