Linear and nonlinear optical absorption of 2D monolayer NbOCl2

The in-layer crystal structure of a two-dimensional material is stable, but the interaction force between the atomic layers is much weaker. Niobium oxide dichloride (NbOCl2) not only has high second-order nonlinear coefficients unique to common monolayer two-dimensional materials but also weak electronic coupling between layers. These characteristics make NbOCl2 very suitable as a quantum light source in quantum optical systems. This article mainly studies the linear and nonlinear optical absorption of two-dimensional monolayer NbOCl2. The characterization means include a surface distance projection map and three views, total density of states, photoelectron spectrum, charge density difference, transition dipole moment density, one-photon absorption spectrum, and two-photon absorption spectrum. There is a size effect on the total density of states, photoelectronic energy spectrum, and one-photon absorption spectrum, that is, as the size increases, the total density of states is red-shift, the photoelectronic energy spectrum is blue-shift, and the absorption spectrum has a significant red-shift. The charge density difference indicates that there is charge transfer around the Nb atom of NbOCl2. The transition dipole moment density changes periodically. The study of the two-photon absorption spectrum shows that it is dominated by a one-step transition. In summary, our work provides an in-depth understanding of its optical properties and is of great significance for the miniaturization of quantum light sources.