Cavity-driven attractive interactions in quantum materials

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/ω_c ≈ 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.