Theory of pump-probe photoemission in graphene: Ultrafast tuning of Floquet bands and local pseudospin textures
The control of physical properties of solids with short laser pulses is an intriguing prospect of ultrafast materials science. Continuous-wave high-frequency laser driving with circular polarization was predicted to induce a light-matter coupled new state possessing a quasi-static band structure with an energy gap and a quantum Hall effect, coined "Floquet topological insulator". Whereas the envisioned Floquet topological insulator requires well separated Floquet bands and therefore high-frequency pumping, a natural follow-up question regards the creation of Floquet-like states in graphene with realistic pump laser pulses. Here we predict that with short low-frequency laser pulses attainable in pump-probe experiments, states with local spectral gaps at the Dirac points and novel pseudospin textures can be achieved in graphene using circular light polarization. We demonstrate that time- and angle-resolved photoemission spectroscopy can track these states by measuring sizeable energy gaps and quasi-Floquet energy bands that form on femtosecond time scales. By analyzing Floquet energy level crossings and snapshots of pseudospin textures near the Dirac points, we identify transitions to new states with optically induced nontrivial changes of sublattice mixing that can lead to Berry curvature corrections of electrical transport and magnetization.