Observation of Floquet–Bloch states in monolayer graphene

Floquet engineering enables the manipulation of quantum phases of matter through periodic driving. It has been implemented across different platforms, ranging from photonic systems to optical lattices of ultracold atoms. In solids, coherent light–matter interaction induced by periodic driving leads to hybridization of Bloch electrons with photons, resulting in the formation of replica bands known as Floquet–Bloch states. These states have been observed in several materials, and their properties have been linked to a range of predicted phase transitions. However, direct energy and momentum-resolved observation of these states remains limited to a few. Here we report the direct observation of Floquet–Bloch states in monolayer epitaxial graphene. By using time-resolved and angle-resolved photoemission spectroscopy with mid-infrared pump excitation, we detected replicas of the Dirac cone. The dependence of these replica bands on pump polarization shows that they originate from the scattering between Floquet–Bloch states and photon-dressed free-electron-like photoemission final states, known as Volkov states. Our method can potentially be used to directly observe Floquet–Bloch states at large momenta in other quantum materials.