Abstract

Nonperturbative electron dynamics inside of solids under strong optical fields has recently found particular attention. Light-field driven effects such as high-harmonic generation and sub-optical-cycle interband population transfer have been reported for dielectrics and semiconductors. However, much less is known about strong-field phenomena in conducting materials. Graphene is an ideal playground for studying strong-field phenomena in a conductor because of its broadband optical response and its much weaker screening compared to conventional metals. Here we report a transition from a perturbative to a non-perturbative mechanism in generating optical-phase sensitive currents in graphene illuminated with few-cycle laser pulses [1]. The electric-field waveform of such an ultrashort laser pulse is characterized by the carrier-envelope-phase (CEP), which determines when the maximum peak of the field is found inside of the pulse envelope (Fig. 1a). We observed a CEP-dependent current in graphene stripes excited with linearly polarized laser pulses parallel to the graphene stripe with a peak field of up to 3 V/nm (Fig. 1a,b). The amplitude of the current scales nonlinearly as a function of the field strength (Fig. 1c). The CEP dependent current changes its sign at around 1.8 V/nm with increasing the field strength (Fig. 1d). The main experimental features, especially the tell-tale change in current direction as a function of field strength, are well reproduced by numerical simulations (Fig. 1e,f).

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