Van der Waals (vdW) heterostructures host rich many-body quantum phenomena that are highly tunable via electrostatic gating. Intriguingly, the gates and 2D materials in these structures inherently form plasmonic self-cavities, confining light in standing waves of current density due to finite-size effects. The plasmonic resonances of typical graphite gates fall within the GHz-THz range, aligning with the μeV- meV energy scale of the phenomena they electrically control in vdW heterostructures. This suggests that the built-in cavity modes of graphite gates may play a significant role in shaping the low-energy physics of vdW heterostructures. However, probing these cavity-coupled electrodynamics is challenging due to the sub-wavelength scale of the devices relative to the diffraction limit. 

In this talk, I will present our recent results using advanced on-chip THz spectroscopy to probe the intrinsic cavity conductivity of gate-tunable graphene heterostructures. By tuning the carrier density, we reveal ultrastrong coupling and hybridization between graphene and graphite plasmonic cavity modes, accompanied by significant spectral weight transfer. To interpret these findings, we introduce an analytical framework and outline design principles for future cavity-engineered vdW systems.

Our study demonstrates that intrinsic cavity effects are essential for understanding the low-energy electrodynamics of vdW heterostructures and offer new opportunities for functionality through cavity control.