Observation of light-induced topological state in graphene
Driving quantum materials with light can reshape not only their electronic band structures but also the geometry of their electronic wavefunctions. In graphene, circularly polarized light is predicted to open gaps at the Dirac points and induce a pseudospin texture and Berry curvature characteristic of the Haldane model, realizing a Floquet Chern insulator. Transport and all-optical measurements have provided evidence for a light-induced anomalous Hall response, but cannot resolve the underlying electronic states in both energy and momentum space. Time- and angle-resolved photoemission spectroscopy has revealed Floquet-Bloch sidebands and hybridization gaps at crossings between Floquet bands. However, these experiments used linearly polarized driving, which preserves time-reversal symmetry and thus does not induce a topological phase. Here, we combine circular-dichroic time-resolved extreme-ultraviolet momentum microscopy with ab initio theory to investigate graphene driven by circularly polarized light. We observe a pump-induced photoemission circular dichroism near the Dirac point that reverses sign with pump helicity and across the Dirac energy, in agreement with our calculations. These results link the measured dichroic response to the light-induced pseudospin texture and its associated Berry curvature, providing spectroscopic evidence for the Floquet-engineered topological insulating state in graphene.