The Multi-Scale Paradigm: An Integrated Design Logic for Nonlinear Optical Crystals.

Nonlinear optical (NLO) crystals are essential for laser frequency conversion, yet rational crystal design remains challenging due to intrinsic trade-offs among key performance parameters. This review establishes a multi-scale design paradigm to unify fragmented knowledge of NLO materials by correlating microscopic, mesoscopic, and macroscopic structural features, delivering a chemistry-based framework to recognize cross-scale constraints and direct NLO crystal exploration. At the microscopic level, the intrinsic optical responses of planar π-conjugated units, non-π-conjugated tetrahedral motifs, and distorted polyhedra are systematically analyzed at the atomic scale, with emphasis on the chemical substitution, functional-group modification, and multi-unit assembly strategies that directly manipulate the microscopic electronic structures of these functional building blocks. At the mesoscopic scale, cation-regulated coordination, dimensionality engineering, hydrogen bonding, domain manipulation, and interfacial coupling are discussed for ordered functional unit arrangement and constructive polarization accumulation. At the macroscopic scale, we discuss converting small-scale designs into usable bulk crystals, focusing on mitigating layered growth via strengthened interlayer interactions to improve crystal dimension, quality, and device compatibility. Representative NLO crystal examples validate this coherent logic. This framework clarifies structure-property correlations and supplies guidelines for new NLO materials, advancing this field from empirical screening toward rational, application-driven design.