Fluorinated Organic Electro-Optic Quinolinium Crystals for THz Wave Generation

Abstract

Fluorinated electro-optic crystals with state-of-the-art second-order nonlinear optical response and excellent characteristics for terahertz (THz) wave generation are reported. The fluorinated organic ionic crystals consist of optically highly nonlinear fluorinated HM6FQ (6-fluoro-2-(4-hydroxy-3-methoxystyryl)-1-methylquinolinium) or HM7FQ (7-fluoro-2-(4-hydroxy-3-methoxystyryl)-1-methylquinolinium) cations and 4-methylbenzenesulfonate (T) counter anions. Compared to benchmark electro-optic crystals based on nonfluorinated HMQ (2-(4-hydroxy-3-methoxystyryl)-1-methylquinolinium) cations, introducing fluorine substituent on HM6FQ cations creates additional hydrogen bonds (Ar Symbol of the Klingon Empire F center dot center dot center dot H Symbol of the Klingon Empire C). Such a molecular engineering leads to an enhanced thermal stability and significant modulations of phonon vibrational modes of crystals in THz frequency region, while excellent pi-pi stacking and space filling characteristics of HM6FQ cations in crystals lead to state-of-the-art diagonal component of the macroscopic nonlinear optical susceptibility, similar to the case of HMQ cations. HM6FQ-based crystals exhibit a very high optical-to-THz conversion efficiency, comparable to benchmark HMQ-based crystals. In addition, due to additional hydrogen bonds induced by fluorine substituents, the spectral shape of the generated THz wave is remarkably modified; e.g., the largest spectral gap is near 1.5 and 2.0 THz for HM6FQ- and HMQ-based crystals, respectively. The fluorinated cationic engineering on nonlinear optical crystals having benchmark nonlinear optical response is, as far as is known, reported for the first time.