Observing light-induced Floquet band gaps in the longitudinal conductivity of graphene

Subjecting materials to strong, carefully tuned light pulses are an increasingly popular route to realise novel physics and functionalities for solid-state systems, with one such example being Floquet engineering. Here, the authors propose to employ optical longitudinal conductivity to probe Floquet-Bloch bands, and demonstrate optical controllability of the conductivity of graphene. Floquet engineering presents a versatile method of dynamically controlling material properties. The light-induced Floquet-Bloch bands of graphene feature band gaps, which have not yet been observed directly. We propose optical longitudinal conductivity as a realistic observable to detect light-induced Floquet band gaps in graphene. These gaps manifest as resonant features in the conductivity, when resolved with respect to the probing frequency and the driving field strength. The electron distribution follows the light-induced Floquet-Bloch bands, resulting in a natural interpretation as occupations of these bands. Furthermore, we show that there are population inversions of the Floquet-Bloch bands at the band gaps for sufficiently strong driving field strengths. This strongly reduces the conductivity at the corresponding frequencies. Therefore our proposal puts forth not only an unambiguous demonstration of light-induced Floquet-Bloch bands, which advances the field of Floquet engineering in solids, but also points out the control of transport properties via light, that derives from the electron distribution on these bands.