Organic THz Generators: A Design Strategy for Organic Crystals with Ultralarge Macroscopic Hyperpolarizability

Newly designed halogenated organic quinolinium crystals proposed in this work provide fully optimized molecular ordering for maximizing the optical nonlinearity and high‐performance broadband terahertz (THz) wave generation. The ultralarge diagonal optical nonlinearity (almost 300 × 10−30 esu) of the new halogenated crystals is approximately two times larger than that of state‐of‐the‐art pyridinium‐based crystals. In contrast, nonhalogenated analogous crystals exhibit very low (or vanishing) diagonal optical nonlinearity. This is attributed to halogen‐induced unique interionic interactions and fine‐tuning of the space‐filling characteristics. In addition, the halogenated crystals show a good ability for bulk crystal growth of few millimeters lateral size with plate‐like morphology and high thermal stability that are finally required for real‐world applications. The new halogenated quinolinium crystals exhibit excellent THz wave generation characteristics, significantly surpassing the limit of conversion efficiency and spectral bandwidth of inorganic benchmark crystals. A 0.16 mm thick chlorinated crystal generates a 29‐times larger THz field than 1.0 mm thick inorganic ZnTe crystals at 1500 nm pump wavelength with a flat and broadband spectrum extending up to ≈8 THz. Therefore, introducing halogen substituents is a potential design strategy for designing new organic crystals showing ultralarge macroscopic hyperpolarizability and high‐performance THz wave generation. Halogenated organic quinolinium crystals simultaneously achieve ultralarge macroscopic optical nonlinearity (about two times larger diagonal second‐order optical nonlinearity than that of state‐of‐the‐art pyridinium‐based crystals), plate‐like bulk crystal growing ability with few millimeters lateral size, and high thermal stability. They show a high potential for real‐world photonic applications, such as efficient broadband terahertz wave generation.