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.