Organic Terahertz Crystals with Unusual Chlorinated Electron Donors

Abstract Organic terahertz (THz) crystals with large optical nonlinearity and low phonon absorption are highly desirable for diverse THz applications. In this work, a novel molecular design strategy is introduced for developing high‐performance organic THz salt crystals by introducing highly polar chlorinated substituent into the electron donor (ED) units of nonlinear optical cationic chromophores, a modification typically avoided due to concerns over reduced microscopic optical nonlinearity, for the first time. Compared to crystals based on non‐chlorinated EDs, the newly designed chlorinated ED‐based crystals exhibit significantly improved structural properties including reduced void volume, higher crystal density, and enhanced multiple‐interionic interactions induced by chlorine substituents. These features lead to remarkably narrow and weak phonon absorption across the 0.5–2.2 THz range. Importantly, these crystals exhibit large microscopic and macroscopic optical nonlinearities, comparable to or exceeding those of non‐chlorinated analogs. As a result, they deliver outstanding THz wave generation performance with a 0.32 mm thick 3Cl‐OHQ‐T crystal achieving a peak‐to‐peak THz electric field approximately 16 times higher than that of a 1.0 mm thick ZnTe crystal under identical excitation condition with 130 fs pump pulses at 1140 nm. Furthermore, the THz generators based on chlorinated ED‐based crystals enable successful racemic detection of representative compounds.