Fig. 1. 5×5 supercell of ML-BN and schematic of defect structures. To simplify the graph, three defect structures are sketched in the same supercell. In the calculations, the 5×5 supercell contains only one defect structure

Fig. 2. Convergence tests of |χxxx(2)(ω)| of V_N with respect to k-mesh and number of IPA empty bands included in the sum-over-states calculation (1 a.u.=24.4 pm/V). (a)(c) Pure interband transition; (b)(d) mixed transition

Fig. 3. Theoretical and experimental OPA spectra of ML-BN. FWHM is the full width at half maximum

Fig. 4. Energy band structure and OPA spectra for V_N. (a) Energy band structure; (b) OPA spectra; (c) calculation and tracing results of sum-over-states performed on the first characteristic absorption peak in the OPA spectra at spin-up; (d) calculation and tracing results of sum-over-states performed on the first characteristic absorption peak in the OPA spectra at spin-down

Fig. 5. Energy band structure and OPA spectra for V_B. (a) Energy band structure; (b) OPA spectra; (c) calculation and tracing results of sum-over-states performed on the first characteristic absorption peak in the OPA spectra at spin-up; (d) calculation and tracing results of sum-over-states performed on the first characteristic absorption peak in the OPA spectra at spin-down

Fig. 6. Energy band structure and OPA spectra for V_BN. (a) Energy band structure; (b) OPA spectra; (c)(d) calculation and tracing results of sum-over-states performed on the first and second characteristic absorption peaks in the OPA spectra

Fig. 7. Dispersion curves of real and imaginary parts of χxxx(2)(ω) for V_N based on the decomposition of different contributions. (a)(c) χinter(2); (b)(d) χmix(2); (e)(g) ω; (f)(h) 2ω

Fig. 8. Dispersion curves of real and imaginary parts of χxxx(2)(ω) for V_B based on the decomposition of different contributions. (a)(c) χinter(2); (b)(d) χmix(2); (e)(g) ω; (f)(h) 2ω

Fig. 9. Dispersion curves of real and imaginary parts of χxxx(2)(ω) for V_BN based on the decomposition of different contributions. (a)(c) χinter(2); (b)(d) χmix(2); (e)(g) ω; (f)(h) 2ω

Fig. 10. SHG coefficients of pristine ML-BN and three defect structures