Strong Nonlinearity Induced by Coaxial Alignment of Polar Chain and Dense [BO 3 ] Units in CaZn 2 (BO 3 ) 2 .

Discovery of new efficient nonlinear optical (NLO) materials with large second-order nonlinearity for the short-wave ultraviolet spectral region (λ PM ≤266 nm, PM=phase-matching) is still very challenging. Herein, a new beryllium-free borate CaZn 2 (BO 3 ) 2 with Sr 2 Be 2 B 2 O 7 (SBBO) double-layered like configuration was rationally designed, which not only preserves the structural merits but also eliminates the limitations of the SBBO crystal. CaZn 2 (BO 3 ) 2 shows a large PM second harmonic generation (SHG) reponse of 3.8×KDP, which is 38 times higher than that of its barium analogue. This enhancement mainly originates from the 1 [Zn 2 O 6 ] ∞ polar chains with a large net dipole moment and [BO 3 ] units with a high NLO active density. Our findings show the great significance of the [ZnO 4 ] 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.