  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.
