Terahertz nanoscopy of quantum-geometric photovoltaics in bilayer graphene

This preprint develops cryogenic, polarization-sensitive terahertz microscopy with deep-subwavelength spatial resolution and uses it to study the bulk photovoltaic effect in dual-gated AB-stacked bilayer graphene. When the gate-tunable band gap is brought close to resonance with the THz photon energy, the optical conductivity is enhanced and the photovoltaic response can be mapped in space. Polarization-resolved measurements show a twofold angular symmetry and a distinct shift-vector direction. The dependence on carrier density and temperature reveals trigonal warping and electron–hole asymmetry, and the authors report agreement with theory linking the effect to the quantum geometry of the electronic bands.