Two-dimensional graphene-like BeO sheet: a promising deep-ultraviolet nonlinear optical material with strong and highly tunable second harmonic generation

• • Ultrawide band gap of 6.86 eV exceeds the 6.2 eV threshold for DUV transparency, enabling direct SHG output below 200 nm without phase-matching constraints. • • Strong SHG coefficient χ22(2)(2D) = 6.81 Å pm/V in the BeO monolayer, competitive with bulk DUV NLO crystals, supports efficient frequency doubling in ultrathin devices. • • Out-of-plane χ15(2)(2D) and χ33(2)(2D) exhibit a 30% change under in-plane biaxial strain from −2% to 5%, offering extreme tunability for strain-engineered NLO switches. • • In-plane χ22(2)(2D) remains robust against large strains, ensuring stable SHG performance under mechanical deformation, critical for flexible integrated photonics.