Nonlinear Optical Benzoindolium-Based Single Crystals: H-Contacts Induced Noncentrosymmetric Alignment Approaching Efficient Terahertz-Wave Generation

Abstract

Ionic-type nonlinear optical (NLO) crystals, which belong to noncentrosymmetric (NCS) space groups, have been the core device materials in the field of laser technology, especially in the field of terahertz-wave generation. In this study, three iodide benzoindolium-based crystals, namely, MBI (C24H24NOI, Cc), m-OHBI (C23H22NOI, P21), and p-OHBI (C23H22NOI, P21/c), with different substitution sites of oxygen-containing donor groups have been successfully synthesized and grown. Acentric MBI and m-OHBI crystals exhibit transparent edges below 639 nm, good environmental stabilities, and a strong second-harmonic generation (SHG) response close to typical OH1 (d33 = 120 pm/V at 1.9 μm), and the oxygen-containing donor groups are analyzed to be effective for inducing noncentrosymmetric alignment of the functional building units. Notably, for the first time, a widely tunable terahertz wave has been realized through an MBI prototype device by the difference frequency generation method. This work provides a feasible strategy for a new NLO crystal design to achieve strong SHG intensity and efficient terahertz-wave generation.