First-principles second-harmonic generation of reconstructed monolayer Bi2O3 phases

Monolayer Bi2O3 has recently attracted interest as a candidate atomic-thickness p-type oxide semiconductor, but its relaxed monolayer structures and symmetry-dependent nonlinear optical responses remain to be clarified. Here, using density-functional-theory-based first-principles calculations, we revisit the reported monolayer β-Bi2O3 structure and obtain two reconstructed phases through different relaxation routes: a directly optimized R phase and a 3×3-supercell-derived R3×3 phase. The R phase belongs to the non-centrosymmetric C2 point group, whereas the R3×3 phase belongs to the centrosymmetric D2h point group. Consequently, the R phase exhibits a finite electric-dipole second-harmonic generation (SHG) response, while the R3×3 phase is SHG-inactive under the electric-dipole approximation. Band-resolved and k-resolved analyses further reveal that the SHG response of the R phase originates from specific dominant two-band transition channels and projected three-band coupling pathways. These results identify SHG as a low-cost and nondestructive optical fingerprint for distinguishing reconstructed monolayer Bi2O3 phases and for probing the microscopic electronic origins of their nonlinear optical responses.