Observation of Floquet-induced gap in graphene.

Floquet engineering provides a powerful pathway for creating non-equilibrium phases of matter with tailored electronic structures and properties through time-periodic driving. As the original theoretical prototype, graphene established the framework in which the Floquet topological insulator with the light-induced anomalous Hall effect was proposed. However, the defining spectroscopic signature of Floquet engineering in graphene, light-induced hybridization (avoided-crossing) gap at Floquet band crossings, has remained experimentally elusive. Here we report the direct observation of a Floquet-induced hybridization gap in monolayer graphene under resonant driving by a strong light field. Time- and angle-resolved photoemission spectroscopy reveals a gap opening at Floquet band crossings, accompanied by coherent Floquet sidebands. The gap exhibits pronounced momentum anisotropy, featuring two Dirac nodes protected by spatiotemporal symmetry and tunable by light polarization. These results provide the long-sought experimental demonstration of Floquet band engineering in graphene, opening up opportunities for light-field-engineered quantum phases in graphene and related materials.