It remains a challenge to develop UV nonlinear optical (NLO) crystals that can achieve a desirable trade-off on UV absorption edge, second harmonic generation (SHG), and birefringence. Here we report a thiosulfate UV NLO crystal of a 2D van der Waals layered structure, [C(NH₂)₃]₂SO₃S. Remarkably, this thiosulfate realizes the desired trade-off, with a short absorption edge of 254 nm, a strong SHG response of approximately 2.8 times that of the benchmark KH₂PO₄, and a sufficient birefringence of 0.073 at the wavelength of 546 nm. In addition, it exhibits strong in-plane anisotropy of the SHG intensity. According to the first-principles calculations, the non-π-conjugated [SO₃S]^2– anion is the dominant SHG functional gene, while the π-conjugated [C(NH₂)₃]⁺ cation serves as the functional gene of birefringence. This is different from common UV NLO materials whose functionals of SHG and birefringence are the same. These findings indicate that combining different function genes may be an effective strategy to develop outstanding NLO materials with the desirable property trade-off.

中文翻译：

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二维范德华分层 [C(NH2)3]2SO3S 表现出理想的紫外非线性光学权衡

开发可以在紫外线吸收边缘、二次谐波产生 (SHG) 和双折射方面取得理想平衡的紫外线非线性光学 (NLO) 晶体仍然是一个挑战。在这里，我们报告了一种 2D 范德华层状结构的硫代硫酸盐 UV NLO 晶体，[C(NH ₂ ) ₃ ] ₂ SO ₃ S。值得注意的是，这种硫代硫酸盐实现了所需的权衡，具有 254 nm 的短吸收边，强倍频响应约为基准 KH ₂ PO _{4 的}2.8 倍，并且在 546 nm 波长处具有足够的双折射 0.073。此外，它表现出很强的SHG强度的面内各向异性。根据第一性原理计算，非π共轭[SO ₃ S] ^2–阴离子是显性的SHG功能基因，而π共轭的[C(NH ₂ ) ₃ ] ⁺阳离子是双折射的功能基因。这不同于普通的紫外非线性光学材料，其SHG和双折射的功能是相同的。这些发现表明，结合不同的功能基因可能是开发具有理想属性权衡的优秀 NLO 材料的有效策略。