Terahertz Spectroscopy of van der Waals Heterostructures

Van der Waals heterostructures, assembled from atomically thin sheets of two-dimensional crystals microns in lateral size, have emerged as a key platform for investigating fundamental physics in solids. To date, the low-energy electrodynamics of these systems remains experimentally underexplored, largely because of technical challenges associated with subwavelength spectroscopy in the corresponding frequency range. In this talk, I will present the study of terahertz electrodynamics of such systems using on-chip waveguide-based techniques. In ultraclean graphene encapsulated in boron nitride, we observe a “quantum-critical” electron-electron scattering rate that scales with temperature near charge neutrality. This fast scattering among massless Dirac electrons, in combination with low disorder and slow electron-phonon scattering, further gives rise to a novel hydrodynamic energy wave whose propagation we directly visualize using a spatial-temporal imaging experiment. Moving beyond graphene, I will also discuss our measurement of terahertz response in transition metal dichalcogenide monolayers.