In a recent experiment, Schiffrin et al. [Nature 493, 70 (2013); 507, 386 (2014)] demonstrated how a strong (I~1013-1014 W cm-2) non-resonant 4 fs laser pulse can be used to induce currents along gold-silica-gold nanojunctions. Phenomenologically, these currents arise due to the nonlinear interaction of the junction with a laser pulse that has a low temporal symmetry. By varying the carrier envelope phase it is possible to vary the degree of time asymmetry of the incident radiation, and thus the direction and magnitude of the photoinduced current. The experiment marks a new frontier in the experimental laser control of electronic dynamics and is the fastest existing method for the generation of currents. However, since the photo-generated current depends on a highly non-equilibrium state of matter that has never been observed before, the microscopic origin of this rather spectacular effect is unclear and the subject of recent controversy. In this contribution, we assess the feasibility of different mechanisms that have been proposed to explain the experimental observations by performing state-of-the-art nonequilibrium Green's function computations of the laser-induced transport along the nanojunction. The simulations take into account the effect of the gold electrodes in the wideband limit, and accurately describe the electronic structure of the silica via a first-principle based generalized tight binding model. We investigate the effect of changing junction size, position of the Fermi level of leads, and the laser’s intensity, carrier envelope phase and polarization direction on the photoinduced currents. The computations quantitatively agree with the experiment, and clarify the microscopic origin of the effect.

Laser-induced currents along nanoscale junctions

251st National Meeting (2016)

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