Verification of Floquet states in graphene using ultrafast momentum microscopy

Recent advances in the field of condensed-matter physics have unlocked the potential to realize and control emergent material phases that do not exist in thermal equilibrium. One of the most powerful approaches to study such non-equilibrium phases is femtosecond time- and angle-resolved photoelectron spectroscopy. In this regard, we recently developed a combined microARPES electronic structure and real-space photoelectron system that we use to access electronic and excitonic dynamics in space and time [1,2,3]. We further employ this method to test one of the most promising concepts to create light-matter coupled phases, namely Floquet engineering, which is the coherent dressing of matter via time-periodic perturbations. In this field of research, the broad applicability of Floquet engineering to quantum materials is still unclear, because, for the paradigmatic case of monolayer graphene, the theoretically predicted Floquet-induced effects have been put into question. We overcome this problem by using our advanced photoemission setup to provide direct experimental evidence of Floquet engineering in graphene. We report light-matter dressed Dirac bands by measuring the contribution of Floquet sidebands, Volkov sidebands, and their quantum path interference to graphene's photoemission spectral function [4]. Our results finally demonstrate that Floquet engineering in graphene is possible, paving the way for the experimental realization of the many theoretical proposals on Floquet-engineered band structures and topological phases.

[1] Schmitt et al., Nature 608, 499 (2022)

[2] Bange et al., Science Adv. 10, eadi1323 (2024)

[3] Schmitt et al., Nature Photonics, in press (2024)

[4] Merboldt et al, arXiv:2404.12791 (2024)