Rational Design and Controlled Synthesis of High-Performance Inorganic Short-Wave UV Nonlinear Optical Materials

ConspectusThe invention of the laser marked a milestone in modern science and technology. Inorganic second-order nonlinear optical (NLO) crystals, with their unique frequency conversion capabilities, play a critical role in extending laser wavelength ranges. These materials are indispensable in laser science, information transmission, and other fields such as the industrial Internet. As Moore's Law drives the demand for shorter wavelengths and higher-precision laser sources, the development of high-performance short-wave ultraviolet (UV) (<300 nm) NLO materials for UV solid-state lasers has become increasingly important. While researchers have synthesized a variety of NLO crystals, their discovery has largely relied on trial-and-error approaches, which are not only time-consuming but also serendipitous rather than based on rational design principles. Moreover, the complexity of designing these materials is compounded by the need to meet several strict functional criteria, including a short UV cutoff edge, a strong second-harmonic generation (SHG) effect, and moderate birefringence, all of which hinder efficient synthesis. The rational design and controlled synthesis of high-performance short-wave UV NLO crystals, therefore, remains a significant scientific challenge.In this Account, we propose a three-step strategy to address this challenge: (1) Rational screening of highly polar functional groups, particularly new NLO-active groups with novel bonding characteristics (π-localized distorted [O₂]^2- anions, highly polarizable <i>d</i>¹⁰ cations such as Zn²⁺, Cd²⁺, and Hg²⁺, and cations containing stereochemically active lone pairs (SCALP) including Ge²⁺, Sn²⁺, Sb³⁺, and Pb²⁺) that exhibit significantly enhanced polarization anisotropy and hyperpolarizability, to replace traditional anionic groups (planar π-conjugated groups such as [BO₃]^3-, [CO₃]^2-, and [NO₃]^-, and non-π-conjugated tetrahedral anions, such as [SO₄]^2- and [PO₄]^3-). (2) Precise regulation of crystal structures to sequentially construct functional groups using two methods: (a) a template-oriented synthesis strategy, which is a method that guides the formation of desired materials through crystal engineering, based on ideal structural models, and (b) a multifunctional primitive module assembly strategy, by identifying and designing multifunctional modules with specific structural configurations to achieve ordered arrangement, which facilitates the creation of high-performance materials. (3) Controlled synthesis of target compounds through synthesis method innovation. These strategies have successfully guided the discovery of several high-performance short-wave UV NLO crystals, including GeHPO₃, ASbX₂SO₄ (A = K, Rb, Cs, NH₄; X = F, Cl), A₂Sb(P₂O₇)F (A = K, Rb), Hg₃O₂SO₄, A₃VO(O₂)₂CO₃ (A = Rb, Cs), Y₈O(OH)₁₅(CO₃)₃Cl, and A₂NO₃(OH)₃ (A = Ba, Sr), among others. Finally, we summarize these strategies and offer perspectives on the future development of high-performance short-wave UV NLO materials, providing insights into their potential to advance this critical field.