Anisotropic electron-hole excitation and giant optical linear dichroism in two-dimensional NbOX2 (X=I,Br,Cl) monolayer

Anisotropic optical response in two-dimensional (2D) black phosphorene (BP) has attracted intense interest in condensed matter physics and materials science. The strong contrast in the optical response between lights polarized along the in-plane two orthogonal directions gives rise to wide applications in optoelectronics, such as optical polarizers and photodetectors. Expanding the family of anisotropic 2D materials toward a wider spectrum is, therefore, highly desirable. Here, based on a many-body perturbation method, we have investigated the quasiparticle band structure, exciton states, and optical properties in three niobium oxide dihalides $\mathrm{Nb}\mathrm{O}{X}_{2}$ ($X=\mathrm{I},\mathrm{Br},\mathrm{Cl}$). Similar to 2D transition-metal dichalcogenides, e.g., ${\mathrm{Mo}\mathrm{S}}_{2}$, the strong exciton effect has been identified in these 2D systems. However, in addition to the enhanced electron-hole interaction from unusual 2D screening, the flat bands revealed in the three 2D systems make the electron-hole excitation closer to Frenkel type. In the meantime, the anisotropic 2D crystal structure makes the exciton excitation orientationally preferred and consequently giant linear dichroism. Compared with anisotropic BP, violet phosphorene (VP), monochalcogenides $AB$ ($A=\mathrm{Ge}$ and $\mathrm{Sn}$; $B=\mathrm{S}$ and $\mathrm{Se}$), and magnetic $\mathrm{Cr}\mathrm{SBr}$, these niobium oxide dihalides exhibit much stronger optical anisotropy. In particular, for incident light at the wavelength of near ultraviolet, ultraviolet, and deep ultraviolet, the polarization rotation angle for reflected light exceeds ${9}^{\ensuremath{\circ}}$ and hyperbolic exciton-polaritons could be realized, guaranteeing their potential applications in 2D anisotropic optoelectronics.