Steric Group Design for Enhancement of Optical Nonlinearity in Isoxazolone-Based Crystals and Terahertz-Wave Generation

Isoxazolone-based crystals are one of the important molecular type nonlinear optical crystals, which can exhibit good output properties in the terahertz region. In this work, by rational steric group design, three novel isoxazolone-based nonlinear optical crystals of C 16 H 11 NO 3 (OH-PYO, Fdd 2), C 13 H 13 NO 3 (OH-IPO, Fdd 2), and C 15 H 17 NO 3 (HD-IPO, Pca 2 1 ) and were successfully synthesized and grown by evaporation method. The optimization of steric group from phenyl (−C 6 H 6 ) to isopropyl [−CH(CH 3 ) 3 ] effectively modified the molecular arrangement style and enhanced the powder second harmonic generation intensities of OH-IPO and HD-IPO to 0.7 and 0.8 times that of benchmark OH1 ( d 33 = 120 pm/V at 1.9 μm) and 2 times higher than unoptimized OH-PYO. Remarkably, OH-IPO exhibited large band gap of 2.38 eV and wide transparency range (522–2150 nm), and its terahertz-wave generation was demonstrated for the first time by difference frequency generation method, indicating OH-IPO is a potential terahertz nonlinear optical material.