Symmetry effect on the mechanism of the optical absorption of phosphorene quantum dots

Based on a rectangular phosphorene quantum dot (PQD) we perform a systematical investigation into the mechanism of the optical absorption of the PQD. It is found that it is the symmetries of the related initial and final states of the PQD, together with the symmetry of the PQD structure, that determines whether the optical absorption can occur, as well as the absorption strength. More fundamentally speaking, the optical absorption spectra are determined by the symmetry of the confined PQD structure, the symmetry of phosphorene, and the chemical property of phosphorus atoms. The universality of the above conclusion is further verified in other three typical PQDs of different symmetries including the triangular, romboidal, and hexagonal PQDs. We want to emphasize that our conclusion should be also applicable in understanding the optical absorption of the quantum dots formed in other materials.