Quasienergy spectra of graphene dots in intense ac fields: Field anisotropy and photon-dressed quantum rings

A graphene quantum dot under intense ac field and static low magnetic field is investigated. From a tight-binding perspective, applying a Fourier-Floquet transformation and renormalization process, we observe that graphene- intrinsically anisotropic-reveals field polarization signatures in the quasidensity of states. For the ac field polarized along the armchair direction, the dressed electronic structure shows an emergent property: an ac-field-induced quantum ring. This is inferred by the orientation-dependent formation of a miniband of energy states periodically modulated with increasing magnetic field, exactly analogous to the behavior of a quantum-ring spectrum. © 2009 The American Physical Society.