Light-induced current in molecular junctions: Local field and non-Markov effects

We consider a two-level system coupled to contacts as a model for a charge pump under external laser pulse. The model represents a charge-transfer molecule in a junction and is a generalization of previously published results [B. D. Fainberg, M. Jouravlev, and A. Nitzan, Phys. Rev. B 76, 245329 (2007)]. Effects of local field for realistic junction geometries and non-Markov response of the molecule are taken into account within finite-difference time-domain and on-the-contour equation-of-motion formulations, respectively. We find that contrary to the symmetric behavior of the pump relative to the chirp sign, the duration of the corresponding local-field pulse does depend on the chirp sign, which results in an asymmetric charge pumping. The most effective charge-pump regime is found at positive bias, contrary to Markov consideration of the previous study.