Many-body phenomena in quantum materials emerge from the interplay among a broad continuum of electronic states and controlling these interactions is critical for engineering new phases. One promising approach exploits light confined within optical cavities to tailor electronic properties1. Here we demonstrate that terahertz cavity photons can mediate attractive interactions in a tunable van der Waals (vdW) material and reorganize a continuum of electron–hole transitions into an exciton-like state. We introduce a broadband, sub-wavelength time-domain microscope that integrates exfoliated, dual-gated 2D quantum materials into a terahertz cavity. This approach enables the spectroscopic measurement of the field-tunable bandgap of bilayer graphene2 (BLG) in the terahertz range and, at resonance, reveals ultrastrong coupling3 (USC) with an effective interaction strength exceeding g/ωc ≈ 40% of the bare photon energy. Crucially, we identify a cavity-induced resonance emerging from the interband continuum that resembles Coulomb-bound excitons and remains stable across a broad temperature range. Our findings propose an experimental platform for designing and investigating hybrid light–matter phases in 2D quantum matter.
