Combining THz and Infrared Light to Control Valley Charge and Current in Gapless Graphene

The absence of a selection rule coupling oppositely circularly polarized light to conjugate valleys, the basis of "lightwave valleytronics", would appear to curtail the possibilities of graphene as a valleytronic platform. Here we show that combining linearly polarized THz light and circularly polarized infrared light allows not only near perfect valley charge polarization but also complete light control over the associated valley current. The underlying mechanism involves a THz induced momentum space shift of valley uncontrasted charge generated by the circularly polarized component, yielding charge at the K and Γ points. Ab initio simulation reveals an ultrafast de-excitation of this latter charge, resulting in perfect valley polarization. We argue that this pulse design will allow control of the valley current and charge in a broad range of gapless systems, including the Xene family and various few layer graphene systems, opening the way to potentially utilize these materials in valleytronics applications.