Organic Terahertz Generators with Wide Entire-Molecular Phonon-Free Range and Their Application in Broadband Terahertz Spectroscopy

Abstract

New organic nonlinear optical crystals with a broad range free from strong molecular phonon vibrations have been developed for dimple-free THz wave generation. The newly designed 7-fluoro-2-(4-hydroxy-3-methylstyryl)-1-methylquinolin-1-ium (OM7FQ) crystals exhibiting an optimal order parameter feature a unique orthogonal cation-anion dipole coupling, in contrast to the parallel cation-anion dipole coupling found in benchmark organic crystals. The introduction of a fluoro substituent on the cationic electron acceptor, compared to nonfluorinated analogs, results in the additional formation of stronger cation-anion and cation-cation interactions, leading to increased crystal density and reduced void volume. OM7FQ single crystals exhibit a broad phonon-free range from 0.9 to 2.3 THz, defined by an absorption coefficient <= 15 mm-1. This leads to efficient, dimple-free THz wave generation with a dimple-free flat spectral band spanning 0.5-2.7 THz when pumped at the technically significant wavelength of 800 nm. Additionally, OM7FQ crystals produce THz electric fields 3.6 times higher than analogous nonfluorinated benchmark crystals with parallel cation-anion dipole coupling. The application of OM7FQ crystals in broadband THz spectroscopy has been successfully demonstrated for sensing biologically important lactose in commercial infant formulas.