Shift current driven by ultrashort light pulses in insulating and metallic inversion asymmetric systems: Time-dependent non-equilibrium Green's function approach

Using the formalism of time dependent non-equilibrium Green's functions (TD-NEGF) we investigate the transport properties of Rice-Mele model when irradiated with ultrashort light pulse in the absence of any bias voltage. The model consists of a chain in which there are two kinds of atoms leading to different onsite energies along with hopping potential that changes from one atom to other. We then attach leads made out of a simple 1D chain and illuminate the Rice-Mele chain with an ultrashort (i.e. femtosecond duration) light pulse whose central frequency is equal to the energy gap of Rice-Mele model. While the time-dependent current oscillates following the pulse, there is an average current and a total charge is injected into the leads. Such a shift current exists even when Rice-Mele chain is short enough that evanescent wave functions from the leads fill its gap and make the whole device metallic because the whole process can be viewed as non-adiabatic charge pumping where the key requirement is broken left-right spatial symmetry. Finally, we quantify the velocity of propagation of shift current generated in the center of clear or disordered Rice-Mele chain and compare with recent experiments [Ref : M. Nakamura et al., Nature Commun. 8, 281(2017)].