Physical origin of planar linear dichroism in van der Waals semiconductors using main group elements

The polarization of light can provide abundant information regarding the polarization degree, phase shift, and Jones vector, which is important in light communication, environmental scanning, quality inspection, etc. Recently, two-dimensional (2D) semiconductors have provided an ideal platform for detecting polarized light due to their remarkable and tunable linear dichroism (LD). However, the physical mechanism of the in-plane LD in 2D semiconductors has not been systematically investigated, limiting the further exploration of the 2D anisotropic semiconductors and the directionality of experiments on polarization photodetection. In this study, the in-plane LD of 100 types of 2D semiconductors composed of main group elements is investigated via first-principles theory combined with the decision tree algorithm and experimental measurement. The in-plane asymmetry of the lattice and band edge wavefunctions are the main origins of the in-plane LD. 2D semiconductors with in-plane orthorhombic and monoclinic lattices tend to have considerable in-plane LD, while their hexagonal counterparts are optically isotropic. Specifically, orthorhombic 2D semiconductors possess larger in-plane LD because their intrinsic mirror planes in the lattice induce in-plane parity of the wavefunctions at the band edges. The decision tree algorithm further reveals that in-plane LD is also related to the difference for the a and b lattice constants and the electronegativity difference between the cation and anion. In addition, heterostructures formed from these 2D semiconductors exhibit high light absorption, strong in-plane LD, and various types of band alignment. The result of our study can promote the application and development of 2D semiconductors in polarization optoelectronics.