External Electric Field Induced Second-Order Nonlinear Optical Effects in Hexagonal Graphene Quantum Dots

The second-order nonlinear optical (NLO) effects are usually forbidden in materials with inversion symmetry, which restricts the application of graphene in NLO technologies such as phase-only modulation, second-harmonic generation, and sum/difference frequency generation. Here, we break the centrosymmetry by applying external uniform electrostatic fields across hexagonal graphene quantum dots (GQDs) and induce a second-order NLO response in the centrosyrnmetric GQDs. Ab initio quantum chemistry methods were performed to investigate the external electric field effects on the electronic structure and the electronic first hyperpolarizability of the hexagonal GQDs. Under the action of an external electric field, centrosymmetries of the geometric structure and electron density distribution of the hexagonal GQD are both broken, thereby resulting in nonzero electronic first hyperpolarizability. The external electric field induced electronic first hyperpolarizability shows remarkable anisotropy for different directional fields. Particularly, under the electric field of a certain direction, the field dependence of electronic first hyperpolarizability shows a nonmonotonic trend. The electron density redistribution induced by a strong electric field can significantly reduce the frontier orbital energy gap of the quantum dot, thereby evidently enhancing the electronic first hyperpolarizability. This study provides an important theoretical guidance for the experimental realization of an electrically tunable second-order NLO response in GQDs.