Direct observation of Floquet-Bloch states in monolayer graphene

Floquet engineering is a novel method of manipulating quantum phases of matter via periodic driving 1, 2. It has successfully been utilized in different platforms ranging from photonic systems 3 to optical lattice of ultracold atoms 4, 5. In solids, light can be used as the periodic drive via coherent light-matter interaction. This leads to hybridization of Bloch electrons with photons resulting in replica bands known as Floquet-Bloch states. After the direct observation of Floquet-Bloch states in a topological insulator 6, their manifestations have been seen in a number of other experiments 7-14. By engineering the electronic band structure using Floquet-Bloch states, various exotic phase transitions have been predicted 15-22 to occur. To realize these phases, it is necessary to better understand the nature of Floquet-Bloch states in different materials. However, direct energy and momentum resolved observation of these states is still limited to only few material systems 6, 10, 14, 23, 24. Here, we report direct observation of Floquet-Bloch states in monolayer epitaxial graphene which was the first proposed material platform 15 for Floquet engineering. By using time- and angle-resolved photoemission spectroscopy (trARPES) with mid-infrared (mid-IR) pump excitation, we detected replicas of the Dirac cone. Pump polarization dependence of these replica bands unequivocally shows that they originate from the scattering between Floquet-Bloch states and photon-dressed free-electron-like photoemission final states, called Volkov states. Beyond graphene, our method can potentially be used to directly observe Floquet-Bloch states in other systems paving the way for Floquet engineering in a wide range of quantum materials.