Organic nonlinear optical salt crystals are widely used as efficient broadband THz generators. Although solid state molecular motions of organic crystals can greatly influence THz generation characteristics, their origin and effects on THz photonics are not clearly identified. In this work, the origin of solid‐state molecular motions of the state‐of‐the‐art nonlinear optical organic salt crystals and their effects on THz generation characteristics are theoretically investigated. A model crystal, HMQ‐TMS (2‐(4‐hydroxy‐3‐methoxystyryl)‐1‐methylquinolinium 2,4,6‐trimethylbenzenesulfonate) with large macroscopic optical nonlinearity, which is very attractive for intense broadband and narrowband THz wave generation, is chosen. The solid‐state molecular vibrations of HMQ‐TMS crystals can be classified in three frequency regions: phonon mode region, intramolecular motion region, and their mixing region. For the first time for ionic organic crystalline THz generators, the contributions of cationic chromophores and anionic matchmakers on each of vibrational modes are quantitatively separated. In addition, the influence of solid‐state molecular vibrations of HMQ‐TMS crystals on the generated THz spectra is investigated. These results provide an essential information for design of new organic nonlinear optical salt crystals for THz generators as well as detectors and for optimization of THz generation performance.

Ultra‐broadband THz photonics covering the 0.3–20 THz range provides a very attractive foundation for a wide range of basic research and industrial applications. However, the lack of ultra‐broadband THz devices has yet to be overcome. In this work, high‐density organic electro‐optic crystals are newly developed for efficient THz wave generation in a very broad THz spectral range and are successfully used for a broadband THz time‐domain spectroscopy. The new organic THz generator crystals, namely the OHP‐TFS crystals, have very low void volume, high density, and are shown to cover the ultra‐broadband THz spectrum up to about 15 THz, which cannot be easily accessed with the more widely used inorganic‐based THz generators. In addition to the very favorable broadband properties, the generated THz electric‐field amplitude at the pump wavelength of 1560 nm is about 40 times higher than that generated by a commercial inorganic THz generator (ZnTe crystal). By using the newly developed OHP‐TFS as generation crystal in a compact table‐top all‐organic THz time‐domain spectrometer based on a low‐cost telecom fiber laser, the optical characteristics of a model material are successfully determined in the broad 1.5–12.5 THz range with high accuracy.

to obtain the desired physical properties correlated to electronic and vibrational states. [1][2][3] In particular, alterations in the intermolecular interactions of organic nonlinear optical materials can result in dramatic changes in the molecular ordering of chromophores, macroscopic nonlinear optical responses, and molecular vibrational and phonon modes, [4][5][6][7] which are important material characteristics in diverse nonlinear optical and THz photonic applications. [8][9][10] The macroscopic nonlinear optical response of organic materials is directly proportional to the first-order hyperpolarizability β of chromophores and their alignments. [8] In many nonlinear optical organic crystals, strong intermolecular interactions to π-conjugated chromophores can adversely affect the resulting macroscopic optical nonlinearity. In well-known and commercially available pyridinium-based crystals, [8,11,12] the first-order hyperpolarizability of chromophores in the crystalline state (β crystal ) is considerably less than in solution (β solution ); e.g., in stilbazolium and merocyanine crystals, β crystal is only about 20% and 5%, respectively, of the corresponding β solution . [6] In a more precise analysis of intermolecular interactions of stilbazoliumbased crystals, reducing intermolecular interactions (edge-toface π-π interactions and hydrogen bonds (H··· − O-S) between A new approach for the molecular design of highly efficient nonlinear optical organic crystals is proposed by introducing substituents that form σ-holes on both nonlinear optical cationic chromophores and aromatic anions. Introducing chlorinated substituents, in which a relatively positive σ-hole and a negative belt coexist, provides selective reduction capability of specific π-π intermolecular interactions and simultaneous multiple secondary bonding capabilities. This leads to a crystalline state with enhanced first-order hyperpolarizability β crystal of chromophores that favors parallel chromophore alignment and suppression of molecular vibrations, which are optimal characteristics for electro-optic and nonlinear optical applications, including efficient THz wave generation. Compared to benchmark nonhalogenated and fluorinated analogous crystals with state-of-the-art macroscopic optical nonlinearity, σ-hole containing chloro-quinolinium crystals exhibit up to two times higher macroscopic nonlinear optical response and remarkably different crystal characteristics. As a result, a 0.16 mm thick chloroquinolinium crystal exhibits ≈22 times higher optical-to-THz conversion efficiency than the widely used 1.0 mm thick ZnTe inorganic crystal. Moreover, chloro-quinolinium crystals exhibit very broad THz spectra, up to 8 THz with significantly different THz spectral shape compared to benchmark organic crystals, which is attributed to different phase matching between optical and THz frequencies and molecular vibration motions.

New ionic organic crystals with bis(head-to-tail) complementary cation–anion assembly result in extremely efficient THz wave generation due to strong interionic binding interactions and parallel alignment of nonlinear optical cationic chromophores.