Valley polarization in graphene quantum rings by ultrafast optical pulse

We study theoretically electron dynamics in graphene quantum rings placed in an ultrashort and strong optical pulse. We describe the graphene quantum rings within an effective model with an infinite mass boundaries. For the optical pulse with the duration of just a few femtoseconds the electron dynamics is coherent and is described by the time-dependent Schrodinger equation. If the optical pulse is circularly polarized then two valleys of graphene nano ring, K and K’, are populated differently after the pulse resulting in finite valley polarization of the system. This is a unique property of graphene nanoscale systems, while for graphene monolayer the circularly polarized pulse does not produce any valley polarization. The valley polarization of the graphene nano ring depends on the parameters of the system, such as inner and outer radii. For small field amplitudes, we interpolate this dependence and obtain an analytical expression for the valley polarization of any graphene quantum ring with sizes up to 50 nm. This expression could be used to design a system, which can produce a given valley polarization. Our findings are useful in nanostructure designs for information storage and processing.