▎ 摘　　要

In epitaxial graphene irradiated by an off-resonance circularly polarized light, we demonstrate a phase transition taking place between the band insulator and Floquet topological insulator. Considering the competition between staggered sublattice potential and photon dressing, we derive the dynamical energy gap and phase diagram in the tight-binding approximation. It is found that a threshold value of light intensity is necessary to realize a Floquet topological insulator. At the phase boundary, for each set of parameters, there is a special state with only one valley that is Dirac cone gapless, but the other remains gapped; in the band insulating phase, only one valley provides low-energy electrons, and it could be switched to the other by reversing the polarization direction of light. From these results, two electronic devices are designed: one is an optical-sensing np junction, where the photodriven unusual intervalley tunneling exhibits a stronger detectable signal than the intravalley tunneling, and the other is a topological field-effect transistor, where polarized light is used to turn on or turn off a nonequilibrium current.