Nonlinear optical (NLO) crystals, especially those can realize the fourth (266 nm) and six (177.3 nm) harmonic generation from the most common 1064 nm laser source, are versatile materials and thus an ever-growing emphasis is being placed on functionalizing these crystals into optical devices for laser-driven applications. Second-order nonlinearity is the foremost parameter and can be expressed as second-order susceptibility tensor χ⁽²⁾, depending on the polarizability of microscopic unit and its arrangement in the lattice. In order to discover crystals with large second-order nonlinearities, the NLO active units used mainly include: i) π-conjugated units, ii) non-π-conjugated tetrahedra; iii) polar-chalcogenide tetrahedra, iv) distorted polyhedra with d⁰ or d¹⁰ cations, and v) lone pair cations with stereoactivity. The introduction of these units is beneficial for the local formation of dipolar moments in the lattice. However, the association of NLO units in a non-centrosymmetric framework in optimally aligned configurations is not straightforward. In fact, especially for inorganic crystals, the homogeneity of bond strengths does not allow to single-out a specific group of NLO active units and hence to state that their use can guarantee crystals with large enough second-order nonlinearities.

Recently, Prof. Shilie Pan (Xinjiang Technical Institute of Physics & Chemistry, Chinese Academy of Science) and Prof. Kenneth R. Poeppelmeier (Northwestern University) have successfully obtained a new borate CaZn₂(BO₃)₂ with Sr₂Be₂B₂O₇ (SBBO) double-layered like configuration. This crystal not only preserves the structural merits but also eliminates the limitations of SBBO crystal. CaZn₂(BO₃)₂ shows a large PM second harmonic generation (SHG) reponse of 3.8×KDP, which is 38 times higher than its barium analogue. This enhancement originates from the ¹[Zn₂O₆]_∞ polar chains with large net dipole moment and [BO₃] units with high NLO active density. Our findings show the great significance of [ZnO₄] tetrahedra introduced strategy to design beryllium-free SBBO-type NLO crystals and also verify the feasibility of using simple non-isomorphic substitution to induce giant second-order nonlinearity enhancement.

The paper was published in Angew. Chem. Int. Ed. with the title of “Strong Nonlinearity Induced by Coaxial Alignment of Polar Chain and Dense [BO₃] Units in CaZn₂(BO₃)₂”. This work was financially supported by National Natural Science Foundation of China, National Key Research Project, and Xinjiang Key Research and Development Program.

https://onlinelibrary.wiley.com/doi/10.1002/anie.202202096

Structural comparison between CZBO and SBBO