Probing the Potential Profile of Twisted Bilayer Graphene via Fabry-Pérot Interference

We use Fabry-P\'erot interference to probe the internal potential profile of large-angle twisted bilayer graphene. We trace anomalous resistance oscillations in the nominally unipolar regime to a hidden cavity formed by unintentional local doping in our device. By analyzing zero-field interference patterns, we determine the location and size of this inhomogeneity. Magnetotransport measurements support the model, distinguishing local cavity modes from global resonances through their different magnetic dependence. Simulations using our extracted profile reproduce the experimental features. Our results highlight interference spectroscopy as a simple, non-invasive probe for identifying local defects and internal potential barriers in ballistic devices.