Strain-tunable linear dichroism and second-harmonic generation response in TaOX₂ (X = F, Cl, Br) monolayers: a...

Two-dimensional (2D) materials with intrinsic optical anisotropy and nonlinear optical response are highly desirable for next-generation optoelectronic applications. Here, we systematically investigate the structural, electronic, and optical properties of TaOX₂ (X = F, Cl, Br) monolayers using first-principles calculations combined with many-body perturbation theory. Our results reveal that TaOX₂ monolayers possess a non-centrosymmetric structure, stabilized by Peierls distortion and spontaneous polarization. These monolayers are indirect-bandgap semiconductors with significant in-plane anisotropy. The inclusion of electron-hole interactions leads to strongly bound excitons with large binding energies (1.25-1.83 eV). Remarkably, TaOX₂ exhibits pronounced linear dichroism (LD) exceeding 80% in the infrared region (1.1-1.6 eV). Furthermore, strain engineering is demonstrated to be an effective strategy for tailoring the optical properties: applying tensile strain along the y-direction significantly broadens the high-LD region to the ultraviolet range (2.5-4.5 eV) and modulates the second-harmonic generation (SHG) response. Specifically, a 2% tensile strain enhances the SHG coefficients of TaOCl₂ (about 130 pm V^-1) to values comparable to NbOCl₂, while compressive strain quenches the SHG due to symmetry restoration. Our findings highlight the potential of TaOX₂ monolayers for anisotropic optoelectronic and nonlinear optical applications.