Unbiased screening of deep-ultraviolet and mid-infrared nonlinear optical crystals: Long-neglected covalent and mixed-cation motifs

One of the most important nonlinear optical (NLO) effects is the second-harmonic generation (SHG), and crystals with strong SHG effects are called NLO crystals. The traditional anion group theory has guided the early discovery of NLO crystals, but the variety of NLO-active motifs discovered so far is limited. In this study, material-type unbiased high-throughput first-principles calculations are performed to screen thousands of materials in a materials database for NLO crystals at target frequencies. The electronic, linear, and nonlinear optical properties of these materials are calculated. Among them, 40 NLO crystals suitable for mid-infrared (MIR) frequencies and 5 for deep-ultraviolet (DUV) frequencies are identified, from 229 materials to which scissors correction is applied. As an extension, several NLO-active motifs that dominate the SHG response are identified, and they all show good transferability among similar materials. Furthermore, for materials where scissors correction cannot be applied due to the lack of accurate bandgap value in the database, a recommendation list of NLO crystals based on the scaling law of bandgap and SHG susceptibility is presented. The discovery of new NLO crystals and NLO-active motifs beyond traditional methods will greatly accelerate the applications of NLO crystals at DUV and MIR frequencies and enrich our understanding in the search and design of new NLO crystals.