Intense anomalous high harmonics in graphene quantum dots caused by disorder or vacancies

This article aims to study the linear and nonlinear optical response of inversion symmetric graphene quantum dots (GQDs) in the presence of on-site disorder or vacancies. The presence of disorder or vacancy breaks the special inversion symmetry, leading to the emergence of intense Hall-type anomalous harmonics. This phenomenon is attributed to the intrinsic time-reversal symmetry breaking in quantum dots with a pseudo-relativistic Hamiltonian, even in the absence of an external magnetic field. We demonstrate that the effects induced by disorder or vacancy have a distinct impact on the optical response of GQDs. In the linear response we observe significant Hall conductivity. In the presence of an intense laser field, we observe the radiation of strong anomalous odd- and even-order harmonics already for relatively small levels of disorder or monovacancy. Both the disorder and vacancy lift the degeneracy of states, thereby creating new channels for interband transitions and enhancing the emission of near-cutoff high-harmonic signals.