Design of a High-Performance Infrared Nonlinear Optical Crystal via a Multiple Flexible-Group Synergistic Polarization Strategy

The development of high-performance infrared (IR) nonlinear optical (NLO) crystals is fundamentally challenged by the conflicting requirements for a large NLO coefficient, a high laser damage threshold (LDT), and a broad IR transparency range. We establish a structure–property relationship governing nonlinear optical response in diamond-like compounds, namely, a sixth-power scaling relation between the NLO coefficient d ijk and average flexibility index F, i.e., d ijk ∝ F 6 . Based on this relation, a multiple flexible-group synergistic polarization strategy is proposed, which successfully guided the discovery of an exceptional IR NLO crystal, Cd 2 In 3 Si 2 P 7 (CISP). CISP exhibits the largest recorded SHG effect (8.8 × AgGaS 2 (AGS) and 2.5 × ZnGeP 2 (ZGP) @ 2050 nm) among reported pnictide NLO crystals, high NLO coefficients ( d 22 and d 23 = 137.6 and 89.3 pm/V @ 1500 nm, respectively), a high LDT (10.3 × AGS), a moderate birefringence (0.098 @ 2050 nm), and a broad IR transmission range (0.62–18.0 μm). The outstanding comprehensive performances underscore its significant potential as a promising IR NLO material. This work not only provides a strategy for the design of IR NLO crystals but also introduces a straightforward yet powerful descriptor for understanding the structure–property correlation in polarizable functional materials.