Light-Induced Valleytronics In Pristine Graphene

Graphene, the first monolayer material, has achieved significant attraction in both applied and fundamental sciences in the past decade [1]. The charge carriers in graphene, also known as Dirac fermions, have peculiar properties from the linear dispersion and zero bandgap of the material.

One of the most exciting features of graphene and gapped graphene materials is the electrons extra degree of freedom, the valley pseudospin, associated with populating the local minima K and K in the lowest conduction band of the Brillouin zone. This extra degree of freedom can encode, process and store quantum information, opening the field of valleytronics [2]. In gapped graphene materials, valley selectivity is achieved by a pump pulse resonant with the bandgap and with matching helicity to the Berry curvature of the material [3]. The vanishing bandgap makes graphene unsuited for such resonant valley-selective excitations - a disappointing conclusion given its exceptional transport properties.

In the present work, we show that valley-selective excitation in graphene can be achieved in an all-optical- means [4]. This non-resonant valley polarization mechanism uses a combination of two counter-rotating circularly polarized fields, the fundamental and its second harmonic. Controlling the relative phase between the two colours allows us to select the valleys where the electron-hole pairs and higher-order harmonics are generated. The tailored field allows one to both break the symmetry between the adjacent carbon atoms and also exploit the anisotropic regions in the valleys, taking advantage of the fact that the energy landscape of the valleys are mirror images of each other. Our proposal offers an all-optical route to valleytronics in pristine graphene.