Fluorinated molecular engineering of nonlinear optical cationic chromophores in organic salt crystals for ultrabroadband terahertz wave generation

Compact table-top terahertz time-domain spectroscopy systems capable of achieving ultrabroad bandwidths in the range of 1–12 THz, extending beyond the reach of conventional THz and infrared spectroscopy, are highly desirable for diverse spectroscopic applications. In this work, we report fluorinated molecular engineering of nonlinear optical cationic chromophores in organic salt crystals that efficiently generate ultrabroadband THz waves via optical rectification of femtosecond laser pulses. The newly designed fluorinated cationic chromophore, 7-fluoro-2-(4-hydroxystyryl)-1-methylquinolin-1-ium (OH7FQ), is incorporated with the 4-methoxybenzenesulfonate (MBS) counter anion in crystals. OH7FQ-MBS crystals are structural analogues of previously reported crystals that differ in the position (or presence) of the fluoro substituent on the electron acceptor. OH7FQ-MBS crystals simultaneously satisfy key requirements for nonlinear optical and THz applications, including large macroscopic optical nonlinearity, plate-like morphology with parallel surfaces, low void volume, and high thermal stability. Unlike previously reported analogous crystals with an out-of-plane polar axis, as-grown OH7FQ-MBS crystals exhibit an in-plane polar axis, which is optimal for simple collinear THz wave generation. When pumped by a compact, relatively low-cost, and low-energy pulsed femtosecond fiber laser with a repetition rate of 100 MHz, a wavelength of 1560 nm, a pulse duration of 38 fs, and a pulse energy of 1.9 nJ, these crystals generate THz waves with an ultrabroad bandwidth extending up to 15 THz. Using this OH7FQ-MBS THz source, we demonstrate clear spectral features and successful discrimination of representative chemicals, namely, the biologically and industrially important α-D-glucose anhydrous and α-D-glucose monohydrate.