Normalized Descriptor for Unbiased Screening of Second-Order Nonlinear Optical Materials

Second-order nonlinear optical materials enable frequency doubling of light (second-harmonic generation, SHG), which is essential for optoelectronic applications ranging from materials characterization to quantum technologies. However, comparing SHG performance across materials remains challenging as the second-order nonlinear susceptibility, χ(2), spans several orders of magnitude and strongly depends on the band gap, Eg. To address this, we empirically validate a theoretical upper bound on χ(2) using new databases of ab initio-computed nonlinear optical (NLO) properties. We then formulate a normalized descriptor, d̂, which expresses the NLO response of a material relative to the band gap-dependent physical limit. We show that d̂ exhibits a similar distribution across a wide range of band gap energies. We also demonstrate that the formalism used to compute χ(2) leads to a competition with methodological uncertainty driven by band gap predictions to systematically alter the prediction of the second-order nonlinear response. The universality of d̂ supports its use as a robust, generalizable metric for chemistry-informed machine-learning models and quantification of NLO performance, enabling accelerated materials discovery and optimization across broad application frequencies.