Photo-induced topological phase transition in graphene studied by exact simulation of pump-probe photoemission spectroscopy

The idea of inducing a nontrivial topological band structure using circularly polarized light was triggered by the observation that in a steady ``Floquet'' state, periodically driven Dirac fermions can be mapped [1] to the Haldane model for a quantum Hall state without Landau levels [2]. A recent observation of Floquet-Bloch states on the surface of a spin-orbit driven topological insulator and a surface state energy gap opened by time-reversal symmetry breaking [3] poses the question how a topological phase transition occurs in real time on ultrashort time scales. We use a well developed Keldysh Green function technique [4] to compute the exact time evolution of tight-binding electrons on the honeycomb lattice coupled to realistic short laser pulses. The time- and angle-resolved photoemission response reveals a photo-induced topological phase transition with energy gaps $>$ 100 meV at the Dirac point that should be observable experimentally. [1] T.~Oka and H.~Aoki, Phys. Rev. B 79, 081406 (2009); T.~Kitagawa et.~al., Phys. Rev. B 82, 235114 (2010); N.~H.~Lindner et.~al., Nature Physics 7, 490-495 (2011). [2] F.~D.~M.~Haldane, Phys. Rev. Lett. 61, 2015-2018 (1988). [3] Y.~H.~Wang et.~al., Science 342, 453 (2013). [4] M.~Sentef et.~al., arXiv:1212.4841 (Phys. Rev. X 2013).