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.

二维材料的层内晶体结构是稳定的，但原子层之间的相互作用力要弱得多。二氯化氧化铌（NbOCl ₂）不仅具有普通单层二维材料特有的高二阶非线性系数，而且层间电子耦合也较弱。这些特性使得NbOCl ₂非常适合作为量子光学系统中的量子光源。本文主要研究二维单层NbOCl ₂的线性和非线性光学吸收。表征手段包括表面距离投影图和三视图、总态密度、光电子能谱、电荷密度差、跃迁偶极矩密度、单光子吸收谱、双光子吸收谱。总态密度、光电能谱、单光子吸收谱均存在尺寸效应，即随着尺寸增大，总态密度红移，光电能谱蓝移，且吸收光谱有明显的红移。_{电荷密度差表明NbOCl 2}的Nb原子周围存在电荷转移。跃迁偶极矩密度周期性变化。对双光子吸收光谱的研究表明，它以一步跃迁为主。综上所述，我们的工作深入了解了其光学特性，对于量子光源的小型化具有重要意义。