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

Traditional nonlinear optical (NLO) crystals are exclusively limited to ionic crystals with pi-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-pi-conjugated molecular crystal, NH3BH3, which realizes such balance with a large second-harmonic generation response (2.0 x KH2PO4 at 1064 nm, and 0.45 x beta-BaB2O4 at 532 nm), deep-UV transparency (E-g > 6.53 eV), and moderate birefringence (Delta n = 0.056@550 nm) is reported. As a result, NH3BH3 exhibits a large quality factor of 0.32, which is evidently larger than those of non-pi-conjugated sulfate and phosphate ionic crystals. Using an unpolished NH3BH3 crystal, effective second-harmonic generation outputs are observed at different wavelengths. These attributes indicate that NH3BH3 is a promising candidate for deep-UV NLO applications. This work opens up a new door for developing high-performance deep-UV NLO crystals.