Photoinduced Nonperturbative Valley Polarization in Graphene.

We investigate a valleytronic device based on graphene with charge separation at different sublattices and correspondingly at nonequivalent valleys. We characterize the maximality condition of valley polarization and investigate the parameters and conditions upon which we can coherently control the carriers and store data via valley degree of freedom. The valley polarization is controlled by the amplitude as well as the carrier envelope phase of the pulse and the curvature of the electron trajectory in the reciprocal space. When strong field excitation is taken into account, the optical selection rule in perturbative optics is replaced by the nonadiabatic geometric effects. As a result, a nonperturbative valley polarization in two dimensional Dirac materials is induced regardless of having an intrinsic bandgap. Microscopically, such a nonreciprocal response of graphene in the chiral electric field is encoded by the quantum Berry phase, as a (pseudo) magnetoelectric monopole.