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Development of new organic crystals possessing large second‐order optical nonlinearity is very challenging because of strong tendency of centrosymmetric dipole–dipole molecular assembly in crystals. This tendency makes it difficult to develop various analogous crystals that allow fine tuning of optical and physical properties to enhance the device performance. A design approach of an isomorphic crystal library consisting of 11 highly efficient nonlinear optical salt crystals is reported. Analyzing the so‐called isomorphic tolerance space in previously reported mother crystals (PMnXQ chromophores, where PM denotes piperidin‐4‐ylmethanol electron donor, n corresponds to the substituted position of halogen (X) group on the quinolinium (Q) electron acceptor), various substituents are introduced into the PMnXQ crystals at different positions, considering their space‐filling characteristics and interionic interaction ability. All 11 PMnXQ crystals exhibit an isomorphic (or pseudo‐isomorphic) crystal structure, in which the cationic chromophores form a perfectly parallel assembly for maximizing the second‐order nonlinear optical susceptibility. The optical, physical, and crystal characteristics of newly designed, synthesized, and grown isomorphic PMnXQ crystals show both similarities and differences. Excellent THz wave‐generation performance is demonstrated in both kHz‐ and MHz‐repetition optical pump systems with new PMnXQ crystals. Therefore, the design approach using isomorphic tolerance space is very attractive for developing diverse isomorphic analogous organic crystals. [ABSTRACT FROM AUTHOR]

In organic π‐conjugated crystals, enhancing molecular optical nonlinearity of chromophores (e.g., first hyperpolarizability β ≥ 300 × 10−30 esu) in most cases unfortunately results in zero macroscopic optical nonlinearity, which is a bottleneck in organic nonlinear optics. In this study, a new class of nonlinear optical organic crystals introducing a chromophore possessing an extremely large first hyperpolarizability is reported. With newly designed 4‐(4‐(4‐(hydroxymethyl)piperidin‐1‐yl)styryl)‐1‐(pyrimidin‐2‐yl)pyridin‐1‐ium (PMPR) chromophore, incorporating a head‐to‐tail cation‐anion OH⋯O hydrogen‐bonding synthon and an optimal selection of molecular anion into crystals results in extremely large macroscopic optical nonlinearity with effective first hyperpolarizability βiiieff$\beta _{{\rm{iii}}}^{{\rm{eff}}}$ of 335 × 10−30 esu. This is in sharp contrast to zero βiiieff$\beta _{{\rm{iii}}}^{{\rm{eff}}}$ value for previously reported analogous crystals. An ultrathin PMPR crystal with a thickness of ≈10 µm exhibits excellent terahertz (THz) wave generation performance. Both i) broadband THz wave generation with a wide flat‐spectral‐band in the range of 0.7–3.4 THz defined at −3 dB and high upper cut‐off generation frequency of > 7 THz as well as ii) high‐generation efficiency (5 times higher THz amplitude than ZnTe crystal with a mm‐scale thickness) are simultaneously achieved. Therefore, new PMPR crystals are highly promising materials for diverse applications in nonlinear optics and THz photonics. [ABSTRACT FROM AUTHOR]

In this study, an organic nonlinear optical salt crystal, 4‐(4‐methoxystyryl)‐1‐methylpyridinium 4‐bromobenzenesulfonate (MOS‐BBS), is newly reported. MOS‐BBS crystals exhibit perfectly parallel molecular ordering of 4‐(4‐methoxystyryl)‐1‐methylpyridinium (MOS) cationic chromophores with noncentrosymmetric P1 space group symmetry, which provides an optimal configuration for maximizing the macroscopic second‐order optical nonlinearity. The macroscopic optical nonlinearity of the MOS‐BBS crystals is very large, with the diagonal effective first hyperpolarizability of 160 × 10−30 esu. Furthermore, the MOS‐BBS crystals exhibit a high thermal crystal‐phase stability. The lowest phase transition temperature of the MOS‐BBS crystals is 265°C, which is ~90°C higher than that of previously reported MOS‐based crystals. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]

Enhanced terahertz (THz) wave generation is demonstrated in nonlinear organic crystals through refractive index engineering, which improves phase matching characteristics substantially. Unlike conventional low‐bandgap nonlinear organic crystals, the newly designed benzimidazolium‐based HMI (2‐(4‐hydroxy‐3‐methoxystyryl)‐1,3‐dimethyl‐1H‐benzoimidazol‐3‐ium) chromophore possesses a relatively wide bandgap. This reduces the optical group index in the near‐infrared, allowing better phase matching with the generated THz waves, and leads to high optical‐to‐THz conversion. A unique feature of the HMI‐based crystals, compared to conventional wide‐bandgap aniline‐based crystals, is their remarkably larger macroscopic optical nonlinearity, a one order of magnitude higher diagonal component in macroscopic nonlinear susceptibility than NPP ((1‐(4‐nitrophenyl)pyrrolidin‐2‐yl)methanol) crystals. The HMI‐based crystals also exhibit much higher thermal stability, with a melting temperature Tm above 250 °C, versus aniline‐based crystals (116 °C for NPP). With pumping at the technologically important wavelength of 800 nm, the proposed HMI‐based crystals boost high optical‐to‐THz conversion efficiency, comparable to benchmark low‐bandgap quinolinium crystals with state‐of‐the‐art macroscopic nonlinearity. This performance is due to the excellent phase matching enabled by decreasing optical group indices in the near‐infrared through wide‐bandgap chromophores. The proposed wide‐bandgap design is a promising way to control the refractive index of various nonlinear organic materials for enhanced frequency conversion processes. [ABSTRACT FROM AUTHOR]

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 βcrystal 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 chloro‐quinolinium 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. [ABSTRACT FROM AUTHOR]

A single‐benzene‐based, blue‐emissive diethyl 2,5‐dihydroxyterephthalate (DDT) was prepared by Fischer esterification of 2,5‐dihydroxyterephthalic acid (DHT) and ethanol. The strong fluorescence in both the solution and the solid state from such a simple framework stemmed from the push‐pull structure of the electron‐donating hydroxy groups and the accepting carbonyl groups, as well as structural planarity from intramolecular hydrogen bonds. The strong intermolecular hydrogen bonds enabled DDT to crystallize easily. The color CCD imaging technique showed efficient 1D optical waveguiding with a large optical loss coefficient of 0.15 dB/μm. DDT has potential application in optical sensors, photonic devices, and optoelectronic communication, because of its highly ordered structure and light‐emitting ability. [ABSTRACT FROM AUTHOR]

Abstract: This study presents newly developed yellow‐colored organic electro‐optic crystals to provide high terahertz (THz) wave generation efficiency. Compared with currently existing red‐ or orange‐colored electro‐optic crystals, which are used for most benchmark organic THz sources, yellow‐colored crystals have additional superior advantages for THz wave generation, e.g., higher transparency in the visible wavelength range with accompanying different phase‐matching possibilities. The new yellow‐colored crystals consist of a highly nonlinear optical 4‐(4‐hydroxystyryl)‐1‐methylpyridinium (OHP) cation, with a relatively short wavelength of maximal absorption at 390 nm in solution, and various halogen‐substituted benzenesulfonate anions, with strong secondary‐bonding ability. OHP 4‐chlorobenzenesulfonate (OHP‐CBS) crystals exhibit large off‐resonant macroscopic optical nonlinearity and high transparency, with a cut‐off wavelength for solid‐state absorption near 490 nm. OHP‐CBS crystals provide excellent THz wave generation characteristics based on optical rectification. A 0.53 mm thick OHP‐CBS crystal delivers ≈27 times higher optical‐to‐THz conversion efficiency and a much broader spectrum bandwidth compared with the standard 1.0 mm thick ZnTe at 1300 nm pumping. Particularly, compared with a benchmark organic quinolinium crystal with a similar thickness of 0.55 mm, OHP‐CBS crystals exhibit 1.7 times higher optical‐to‐THz conversion efficiency, and show a significantly different THz spectral shape. [ABSTRACT FROM AUTHOR]

New cationic chromophore HPR (4-(4-hydroxy-3-methoxystyryl)-1-(pyrimidin-2-yl)pyridin-1-ium), with large first hyperpolarizability of up to 264 × 10−30 esu, exhibits large off-diagonal macroscopic optical nonlinearity, which is approximately four times higher than that of the benchmark stilbazolium crystals. [Display omitted] • New cationic HPR chromophore based on N -pyrimidinyl stilbazolium was developed. • New HPR cationic chromophore exhibits large microscopic optical nonlinearity (264 × 10−30 esu). • HPR-based crystals exhibit large off-diagonal macroscopic optical nonlinearity, approximately four times higher than that of benchmark stilbazolium crystals. We report a new π-conjugated cationic chromophore based on phenolic N -pyrimidinyl stilbazolium and its corresponding non-centrosymmetric crystals. The new cationic chromophore, HPR (4-(4-hydroxy-3-methoxystyryl)-1-(pyrimidin-2-yl)pyridin-1-ium), consists of a strong electron donor, the 4-hydroxy-3-methoxyphenyl group based on two electron-donating groups, and a strong electron acceptor, the N -pyrimidinyl pyridinium group based on two electron-withdrawing groups. The HPR chromophore possesses large molecular optical nonlinearity with first hyperpolarizability of up to 264 × 10−30 esu, which is significantly larger than that of the benchmark stilbazolium chromophores. In solution, the HPR chromophore showed co-existence of phenol- and phenolate-like forms with two absorption bands (∼460 and ∼610 nm, respectively). In the crystalline state, all HPR-based derivatives with three different counter anions, N2S (naphthalene-2-sulfonate), VBS (4-vinylbenzenesulfonate), and NBS (3-nitrobenzenesulfonate), exhibit a macroscopic second-order nonlinear optical response. HPR-N2S crystals exhibit non-centrosymmetric monoclinic Pn space group symmetry and large off-diagonal effective macroscopic optical nonlinearity (94 × 10−30 esu), which is approximately four times higher than that of the benchmark stilbazolium crystals. Therefore, the newly developed HPR-based crystals are highly promising materials for second-order nonlinear optical applications. [ABSTRACT FROM AUTHOR]