High harmonic generation in graphene quantum dots with vacancy defects

We study theoretically the generation of high-order harmonics in hexagonal and triangular graphene quantum dots (QDs) with vacancy defects in the field of an optical pulse. For intrinsic QDs, some high-order harmonics are suppressed due to symmetry of the systems. Then, the main effect of vacancies is the breaking of the QD's symmetry and the enhancement of the corresponding harmonics. Fora hexagonal QD, for an intrinsic case, even-order harmonics are suppressed fora linearly and circularly polarized pulses, while 3 m-order (m is an integer) harmonics are also suppressed fora circularly polarized pulse, with a monovacancy, all high-order harmonics are strongly enhanced. Suppression and enhancement of high-order harmonics for triangular QDs with armchair edges is similar to the case of hexagonal QDs. For triangular QDs with zigzag edges, all high- order harmonics are visible in the intrinsic case, while the presence of a monovacancy enhances the intensities of all harmonics.