A Non-π-Conjugated Molecular Crystal with Balanced Second-Harmonic Generation, Bandgap, and Birefringence

Traditional nonlinear optical (NLO) crystals are exclusively limited to ionic crystals with π-conjugated groups and it is a great challenge to achieve a subtle balance between second-harmonic generation, bandgap, and birefringence for them, especially in the deep-UV spectrum region (E_g  > 6.20 eV). Herein, a non-π-conjugated molecular crystal, NH₃ BH₃ , which realizes such balance with a large second-harmonic generation response (2.0 × KH₂ PO₄ at 1064 nm, and 0.45 × β-BaB₂ O₄ at 532 nm), deep-UV transparency (E_g > 6.53 eV), and moderate birefringence (Δn = 0.056@550 nm) is reported. As a result, NH₃ BH₃ exhibits a large quality factor of 0.32, which is evidently larger than those of non-π-conjugated sulfate and phosphate ionic crystals. Using an unpolished NH₃ BH₃ crystal, effective second-harmonic generation outputs are observed at different wavelengths. These attributes indicate that NH₃ BH₃ is a promising candidate for deep-UV NLO applications. This work opens up a new door for developing high-performance deep-UV NLO crystals.