Deep‐ultraviolet nonlinear optical (DUV NLO) materials are attracting increasing attention because of their structural diversity and complexity. Using the two‐dimensional (2D) crystal structure prediction method combined with the first‐principles calculations, here we propose layered 18‐membered‐ring (18MR) boron oxide B2O3 polymorphs as high‐performance NLO materials. 18MR‐B2O3 with the AA and AB stackings are potential DUV NLO materials. The superior performing 18MR‐B2O3AB has an unprecedentedly high second harmonic generation coefficient of 1.63 pm V−1, the largest among the DUV NLO materials, three times larger than that of the advanced DUV NLO material KBe2BO3F2 and comparable to that of β‐BaB2O4. Its unusually large birefringence of 0.196 at 400 nm guarantees the phase‐matching wavelength λPM to reach this material's extreme absorption edge of ≈154 nm.

Deep‐ultraviolet nonlinear optical (DUV NLO) materials are attracting increasing attention because of their structural diversity and complexity. Using the two‐dimensional (2D) crystal structure prediction method combined with the first‐principles calculations, here we propose layered 18‐membered‐ring (18MR) boron oxide B2O3 polymorphs as high‐performance NLO materials. 18MR‐B2O3 with the AA and AB stackings are potential DUV NLO materials. The superior performing 18MR‐B2O3AB has an unprecedentedly high second harmonic generation coefficient of 1.63 pm V−1, the largest among the DUV NLO materials, three times larger than that of the advanced DUV NLO material KBe2BO3F2 and comparable to that of β‐BaB2O4. Its unusually large birefringence of 0.196 at 400 nm guarantees the phase‐matching wavelength λPM to reach this material's extreme absorption edge of ≈154 nm.

Cation substitution has been proven to be an effective strategy to explore novel infrared (IR) nonlinear optical (NLO) materials. In this research, a class of alkali-metal chalcogenides with different A-site cations, AInS2 (A = Na, K, Rb, and Cs), have been successfully obtained using the first-principles calculation method by the A-site cation substitution of the excellent NLO material LiInS2. Interestingly, only NaInS2 maintains the same structure with the benchmark LiInS2. AIIInS2 (AII = K, Rb, and Cs), in spite of keeping the same space group Pna21, are distinct from LiInS2 due to the difference in the coordination number of cations (from 4 to 6). All compounds exhibit strong second harmonic generation (SHG) responses, and among them, CsInS2 has a comparable SHG response to that of AgGaS2. In addition, the contribution of microstructure motif to SHG responses has been determined by the method of band-resolved χ(2) and its integral value, as well as SHG density. The results reveal that the InS4 tetrahedra play a major role in the SHG effect. Remarkably, there is a correlation between the volume, band gap, and the largest effective SHG coefficient d 32, which suggests that the variation of d 32 and band gap could be a volume effect. Therefore, volume modulation would be an effective way to tune the two crucial NLO properties of band gap and SHG coefficient in the exploration of new IR NLO materials.