New Organic Nonlinear Optical Crystals with High Crystal-Density for THz Wave Generation

Abstract

Organic Nonlinear optical (NLO) crystals hold promise for efficient terahertz (THz) generation, applicable in security screening and manufacturing quality inspection. Existing NLO organic crystals face THz self-absorption issues due to phonon vibration, limiting efficiency. To address this, we propose a novel design strategy introducing additional intermolecular interactions. In this research, we designed and synthesized a series of new organic NLO crystals based on OHQ (2-(4-hydroxystyryl)-1-methylquinolinium) chromophores by introducing fluorine. Ionic crystals with high density are developed by selecting various counter anions with different space-filling characteristics, resulting in diverse molecular ordering. Fluorinated OH-6FQ (6-fluoro-2-(4-hydroxystyryl)-1-methylquinolinium) cations creates additional hydrogen bonds, suppressing phonon vibrations and self_x0002_absorption of the generated THz waves. Notably, new OH-6FQ-TFO crystals containing TFO (4-(trifluoromethoxy)benzenesulfonate) anions which is fluorinated anions based on highly electronegative trifluoromethoxy groups, exhibit F···F intermolecular interactions. Moreover, new fluorinated OH-6FQ-based crystals show higher crystal-density than benchmark crystals. High crystal-density might result in reducing phonon vibration and low absorption coefficient in the THz frequency range. We also developed OH-nXQ-based crystals with halogenated CBS (4-chlorobenzenesulfonate) anions and non-halogenated MBS (4-methoxybenzenesulfonate) anions, which could be expected exhibit high crystal_x0002_density. In addition, OH-nXQ-based crystals from additional hydrogen bonds induced by halogen substituents, it can be expected as high-efficiency THz wave generators. Therefore, molecular engineering through halogen introduction and _x000D_ <br>control of space-filling characteristics may prove a promising strategy for effective THz wave generators, showcasing reduced self-absorption and higher microscopic optical nonlinearity compared to benchmark OHQ-based crystals.