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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.

We present broadband terahertz wave generation based on benzothiazolium crystals at a 100 MHz repetition rate. Among various benzothiazolium crystals, we chose PMB-4TFS (2-(4-(4-(hydroxymethyl)piperidin-1-yl)styryl)-3-methylbenzothiazol-3-ium 4-(trifluorome-thyl)benzenesulfonate) due to its large macroscopic optical nonlinearity, good crystal characteristics, and suppressed molecular phonon vibrations. When pumped at the telecommunication wavelength of 1560 nm, the generated THz spectrum covers up to ∼15 THz and the corresponding THz amplitude for a 0.29 mm thick crystal at normal incidence is by more than one order of magnitude higher compared to the inorganic standard, 1.0 mm thick ZnTe generator crystal.

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.

This work reports a series of new organic electro-optic salt crystals possessing unusual orthogonal dipole-coupling assembly that act as dimple-free THz generators. In the new crystals, optically anisotropic aromatic anions introducing highly electronegative fluorinated substituents are orthogonally assembled with optically anisotropic nonlinear optical benzothiazolium cations. These orthogonal cation–anion assembled crystals exhibit strong suppression of intrinsic THz phonon modes along the polar axis, which results in a wide THz absorption-free range from 1.5 to 4.0 THz without substantial absorption peaks. This is sharply in contrast to previous benchmark crystals with parallel cation–anion assembly; for example, in analogous crystals, several strong absorption peaks appear in this frequency range. This absence of strong absorption peaks is attributed to the orthogonal cation–anion dipole coupling assembly, the formation of fluorinate-induced strong interionic interactions, and a high packing ability of uniaxial anions. With top-level macroscopic optical nonlinearity, new orthogonal-dipole assembled crystals make dimple-free THz wave generation possible in the range of about 1.5–4.0 THz, which is very beneficial for characterizing signature peaks related to molecular phonon motions of analytes in THz spectroscopy. Therefore, orthogonal molecular assembly of optically highly anisotropic molecules provides an efficient way to substantially reduce and control intrinsic THz phonon modes.

The terahertz range of the electromagnetic spectrum reveals important insights when studying material properties. An ultra-broadband terahertz time-domain spectroscopy system based on state-of-the-art, high-stability organic nonlinear optical crystals used as both THz wave generator and detector is presented. In transmission geometry, a broad spectrum exceeding 20 THz and a high dynamic range of more than 80 dB is achieved using a compact 100 MHz femtosecond laser working at telecom wavelength 1560 nm. In the normal-incidence reflection geometry, a similar bandwidth with a dynamic range surpassing 60 dB is reported. The experimental results are supported by a complete theoretical model, which includes the pump pulse duration, THz phonon/vibrational modes of the organic crystals and optical/THz beam path optimizations. The effectiveness of the newly developed system is demonstrated by measuring pharmaceutical samples with distinct THz features in the ultra-broadband THz range and by measuring narrow water vapor lines at a spectral resolution of 2.7 GHz (0.090 cm−1), resulting in an excellent accuracy with a frequency deviation of less than 0.05% from the reference values.

Organic π-conjugated crystals with second-order optical nonlinearity are considerably attractive materials for diverse terahertz (THz) wave photonics. An overview of the research of organic nonlinear optical (NLO) crystals for THz wave generation, detection, and applications is provided here. First, the status of organic NLO crystals compared to other alternative THz materials is described. Second, the basic theory and requirements for organic THz generators and detectors are explained. Third, with a primary focus on the organic NLO crystals that have been developed during the last decade, various molecular engineering approaches are discussed from the perspective of THz photonics. Finally, various types of high-power broadband coherent THz sources and their applications in THz nonlinear optics are discussed.

We demonstrate 1.4 milliwatt average power, broadband terahertz generation extending up to 6 THz at 10 MHz repetition rate through optical rectification in the organic crystal HMQ-TMS. The pump source is based on a simple external pulse compression scheme for the generation of ultra-short pulses using compact Yb:fiber laser technology.

New organic THz generators are designed herein by molecular engineering of the refractive index, phonon mode, and spatial asymmetry. These benzothiazolium crystals simultaneously satisfy the crucial requirements for efficient THz wave generation, including having nonlinear optical chromophores with parallel alignment that provide large optical nonlinearity; good phase matching for enhancing the THz generation efficiency in the near‐infrared region; strong intermolecular interactions that provide restraining THz self‐absorption; high solubility that promotes good crystal growth ability; and a plate‐like crystal morphology with excellent optical quality. Consequently, the as‐grown benzothiazolium crystals exhibit excellent characteristics for THz wave generation, particularly at near‐infrared pump wavelengths around 1100 nm, which is very promising given the availability of femtosecond laser sources at this wavelength, where current conventional THz generators deliver relatively low optical‐to‐THz conversion efficiencies. Compared to a 1.0‐mm‐thick ZnTe crystal as an inorganic benchmark, the 0.28‐mm‐thick benzothiazolium crystal yields a 19 times higher peak‐to‐peak THz electric field with a broader spectral bandwidth (>6.5 THz) when pumped at 1140 nm. The present work provides a valuable approach toward realizing organic crystals that can be pumped by near‐infrared sources for efficient THz wave generation., Organic crystals, which are designed through molecular engineering of the refractive index, phonon mode, and spatial asymmetry, simultaneously satisfy the crucial requirements for efficient and broadband THz wave generation, particularly at near‐infrared pump wavelengths near 1100 nm.

We report new organic nonlinear optical salt crystals comprising bis(head-to-tail) complementary cation–anion assembly that results in extremely efficient THz wave generation. In the new salt crystals, each of the ends of the molecular anion, 4-(trifluoromethyl)benzenesulfonate, possesses bis(hydrogen-bond acceptors), while each of the ends of the nonlinear optical molecular cationic chromophore, 2-(4-(4-(hydroxymethyl)piperidin-1-yl)styryl)-3-methylbenzothiazol-3-ium, possesses bis(hydrogen-bond donors). The resulting assembly fulfills the requirements for efficient broadband THz wave generation, namely, perfectly parallel alignment of the nonlinear optical cationic chromophores for maximizing the optical nonlinearity as well as strong interionic binding interactions for reducing self-absorption of the generated THz waves. The new benzothiazolium crystals provide extremely high optical-to-THz conversion efficiency with a broad THz spectral bandwidth of 8 THz, where the peak-to-peak THz electric field amplitude is 36 times higher than that of the benchmark inorganic 1.0 mm-thick ZnTe crystal when pumped at 1300 nm and also notably higher than those of benchmark organic analogous crystals.

We demonstrate the first megahertz (MHz) repetition-rate, broadband terahertz (THz) source based on optical rectification in the organic crystal HMQ-TMS driven by a femtosecond Yb:fibre laser. Pumping at 1035 nm with 30 fs pulses, we achieve few-cycle THz emission with a smooth multi-octave spectrum that extends up to 6 THz at -30 dB, with conversion efficiencies reaching 10−4 and an average output power of up to 0.38 mW. We assess the thermal damage limit of the crystal and conclude a maximum fluence of ∼1.8 mJ·cm−2 at 10 MHz with a 1/e2 pump beam diameter of 0.10 mm. We compare the performance of HMQ-TMS with the prototypical inorganic crystal gallium phosphide (GaP), yielding a tenfold electric field increase with a peak on-axis field strength of 7 kV·cm−1 and almost double the THz bandwidth. Our results further demonstrate the suitability of organic crystals in combination with fibre lasers for repetition-rate scaling of broadband, high-power THz sources for time-domain spectroscopic applications.

Various electro-optic nonlinear organic crystals have been recently developed and successfully used as very efficient materials for the generation and detection of broadband terahertz (THz) waves due to their large second-order optical nonlinearities and excellent phase matching characteristics. In the present talk, some of newly developed highly nonlinear organic crystals, which can be applied for efficient ultra-broadband THz wave generation up to ~10 THz by near-infrared pumping, are introduced. Additionally, two approaches for suppressing phonon-mode absorption, leading to strong modulations of the THz spectra generated in most organic crystals, are discussed.

Efficient broadband organic terahertz (THz) generators using X‐shaped alignment of the nonlinear optical chromophores, as an alternative to the parallel alignment of chromophores in benchmark organic crystals, are reported. All the developed six organic benzothiazolium crystals exhibit an isomorphic X‐shaped alignment of chromophores, resulting in an unprecedentedly large off‐diagonal optical nonlinearity (>100 × 10−30 esu), which presents one of the largest off‐diagonal optical nonlinearity of organic crystals. The benzothiazolium crystals exhibit efficient broadband THz wave generation employing the off‐diagonal optical nonlinearity, in contrast to the present state‐of‐the‐art organic THz generators that mostly utilize diagonal optical nonlinearity. For using off‐diagonal and diagonal optical nonlinearities, the polarization of the optical pump is perpendicular and parallel, respectively, to the polar axis of crystals. In addition to a large THz wave generation efficiency with one order of magnitude higher peak‐to‐peak THz electric field than that generated in a 1.0‐mm‐thick inorganic benchmark ZnTe crystal, the benzothiazolium crystals generate broadband THz spectra with an upper cut‐off frequency of near 8 THz, and the absence of strong absorption dimples in the range of 0.5−4 THz. Therefore, the X‐shaped alignment of chromophores presents an interesting potential alternative for efficient broadband organic THz generators.

We demonstrate an optical-to-THz energy conversion efficiency of 5.5 ⋅ 10−4, by pumping the organic crystal HMQ-TMS with the externally compressed output of a Yb:fibre laser, enabling several kV/cm field strengths at MHz repetition rates.

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.

Ultrafast terahertz (THz) spectroscopy is a potent tool for studying the fundamental properties of matter. Limitations of current THz sources, however, preclude the technique being applied in certain advanced configurations or in the measurement of e.g. strongly absorbing samples. In response to this problem, here we demonstrate the generation of 1.38 mW broadband THz radiation at 10 megahertz (MHz) repetition rate by combining the highly efficient nonlinear organic crystal HMQ-TMS with ultrafast pump pulses generated using a simple and stable external pulse compression of a high power, near-infrared (NIR) femtosecond ytterbium-doped fibre (Yb:fibre) laser. Utilising spectral broadening in a large core, polarisation maintaining photonic crystal fibre and a pair of SF11 prisms, we achieve a tenfold pulse compression of the Yb:fibre laser, yielding compressed 0.35 µJ pulses with a full-width at half maximum pulse duration of 22 fs, exerting a peak power of 13.8 MW at a repetition rate of 10 MHz. THz generation through optical rectification of the NIR pulses is explored in two distinct thicknesses of the organic crystal, leading to a maximum conversion efficiency of ∼5.5·10−4 , an order of magnitude higher than that achieved with inorganic nonlinear crystals, e.g. gallium phosphide, for similar pump parameters. The focused THz beam has a peak on-axis field strength greater than 6.4 kV·cm−1 in unpurged atmosphere. We believe that our moderately strong-field THz source is well suited to a variety of applications in ultrafast THz spectroscopy, in particular THz-enabled scattering-type near-field, and scanning tunnelling spectroscopy, where multi-MHz repetition rate sources are required.

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 millimetres 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.

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.

New highly efficient nonlinear optical crystals based on phenolic quinolinium with various benzenesulfonate and thiophenesulfonate anions are reported for photonic applications. All phenolic quinolinium crystals exhibit strong second harmonic generation (SHG) efficiency. Among them, OHQ-MBS crystals (2-(4-hydroxystyryl)-1-methylquinolinium 4-methoxybenzenesulfonate) with acentric monoclinic Pn space group symmetry exhibit very large macroscopic second-order optical nonlinearity with diagonal effective hyperpolarizability of about 120 × 10−30 esu, which is comparable to the state-of-the-art organic nonlinear optical crystals. OHQ-MBS single crystals with a large area of over 5 × 5 mm2 and good optical quality were successfully grown using a solution growth method in mixed solvent systems. Interestingly, as-grown OHQ-MBS single crystals exhibit an out-of-plane polar axis, which is very unusual, because the state-of-the-art organic nonlinear optical as-grown crystals commonly exhibit an in-plane polar axis. The out-of-plane polar axis is very beneficial for applying various optical configurations. In addition, OHQ-MBS single crystals with an in-plane polar axis are also successfully prepared using a simple cleaving method. Therefore, OHQ-MBS crystals with easily accessible both out-of-plane and in-plane polar axes, as well as the state-of-the-art optical nonlinearity may open up a range of new possibilities for photonic applications.

DSTMS organic electro-optic crystal is a very promising and efficient THz-wave generation material. It allows for both very high THz electric fields generation, exceeding several GV/m using optical rectification of femtosecond laser pulses, as well as the possibility for extremely broad bandwidth extending well beyond 10 THz [1]. However, the usual laboratory THz systems based on these crystals exploit very bulky and expensive fs laser systems [2], which are not desired for industrial THz imaging and spectroscopy applications [3,4]. Recently, some compact system employing organic crystals were demonstrated [5], however they use a conventional photoconductive antenna as a THz-wave detector. Here we present a compact THz time-domain spectroscopy system based on a relatively low-cost and small-size femtosecond fiber lasers operating at 1560 nm. As the THz generation and detection material, we choose the organic crystal DSTMS (4-N,N-dimethylamino-4‘-N‘-methyl-stilbazolium 2,4,6-trimethylbenzenesulfonate), since it reaches the highest overall efficiency and the flattest spectrum among the presently known state-of-the-art organic crystals (Figure 1).

We demonstrate ultrabroadband THz-wave generation and detection in a compact system based on a femtosecond fiber laser at 1560 nm and optical rectification in organic electro-optic crystals DSTMS. Terahertz time-domain spectroscopy with a spectrum extending up to 20 THz with a maximum bandwidth exceeding 70 dB is possible with this system.

Organic materials with second-order nonlinear optical (NLO) activity have interesting potential for many applications in optics and optoelectronics, such as light-frequency conversion, terahertz-wave generation, electric-field detection, and electro-optic (EO) modulation. These materials are based on NLO molecules (chromophores) with a high molecular nonlinearity, which are most often dipolar. In order to achieve a macroscopic second-order NLO response, such molecules need to be arranged in a noncentrosymmetric way in a material. This can be done by incorporating the chromophores in a polymer matrix and subsequent electric-field poling, molecular self-assembly into amorphous acentric structures, or self-assembly into single-crystalline acentric structures. In a single-crystalline form, these materials feature a high thermal and photochemical stability, which is important for the majority of device implementations using organic materials. This chapter discusses state-of-the-art, second-order NLO, single-crystalline organic materials, including molecular and crystal engineering approaches, as well as processing in bulk and thin-film single-crystalline forms. We also present the most promising photonic applications of single-crystalline organic NLO materials, including integrated EO devices, terahertz-wave generation, and terahertz-wave detection.

We report on multi-functional supramolecular building blocks for obtaining a non-centrosymmetric crystal structure, which is the essential requirement for achieving a macroscopic second-order nonlinear optical activity. The supramolecular building blocks PM (piperidin-4-ylmethanol) and PO (piperidin-4-ol) consist of a piperidino group acting as a strong electron-donating group and a hydroxyl group acting as a strong hydrogen-bonding group, which also may act as a ‘site-isolation’ group. The quinolinium crystals PMQ-T and POQ-T that introduce PM and PO blocks, respectively, exhibit a non-centrosymmetric crystal structure with nonlinear optical activity, while analogous quinolinium DAQ-T crystal with the widely used N,N-dimethylamino (DA) group exhibits a centrosymmetric crystal structure without nonlinear optical activity. Compared to the DAQ-T crystal, the PMQ-T and POQ-T crystals exhibit a lower tendency to form a hydrate phase in the crystalline state. The PMQ-T crystal exhibits a higher molecular optical nonlinearity in the crystalline state than the DAQ-T crystal, which is attributed to the ‘site-isolation’ effect resulting from the bulky PM block. In addition, the PMQ-T crystal exhibits large macroscopic optical nonlinearities with more than 3 times higher off-diagonal effective hyperpolarizability than that of benchmark stilbazolium crystals. Therefore, multi-functional supramolecular building blocks PM and PO can give new opportunities for developing non-centrosymmetric ionic crystals for nonlinear optical applications.

The ideal laser source for nonlinear terahertz spectroscopy offers large versatility delivering both ultra-intense broadband single-cycle pulses and user-selectable multi-cycle pulses at narrow linewidths. Here we show a highly versatile terahertz laser platform providing single-cycle transients with tens of MV/cm peak field as well as spectrally narrow pulses, tunable in bandwidth and central frequency across 5 octaves at several MV/em field strengths. The compact scheme is based on optical rectification in organic crystals of a temporally modulated laser beam. It allows up to 50 cycles and central frequency tunable from 0.5 to 7 terahertz, with a minimum width of 30 GHz, corresponding to the photon-energy width of Delta E=0.13 meV and the spectroscopic-wavenumber width of Delta(lambda(-1))=1.1 cm(-1). The experimental results are excellently predicted by theoretical modelling. Our table-top source shows similar performances to that of large-scale terahertz facilities but offering in addition more versatility, multi-colour femtosecond pump-probe opportunities and ultralow timing jitter. (C) 2020 Optical Society of America under the terms of the OSA Open Access Publishing Agreement

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.

High-peak-electric-field THz pulses (>200 kV/cm) can be efficiently generated in a recently developed organic crystal pumped by a femtosecond ytterbium laser in a collinear configuration.

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.

Copyright information see: https://www.osapublishing.org/submit/review/copyright_permissions.cfm#oal We present the generation of high-peak-electric-field terahertz pulses via collinear optical rectification in a 2-(4-hydroxy-3-methoxystyryl)-1-methilquinolinium-2,4,6-trimethylbenzenesulfonate (HMQ-TMS) organic crystal. The crystal is pumped by an amplified ytterbium laser system, emitting 170-fs-long pulses centered at 1030 nm. A terahertz peak electric field greater than 200 kV/cm is obtained for 420 µJ of optical pump energy, with an energy conversion efficiency of 0.26% - about two orders of magnitude higher than in common inorganic crystals collinearly pumped by amplified femtosecond lasers. An open-aperture Z-scan measurement performed on an n-doped InGaAs thin film using such terahertz source shows a nonlinear increase in the terahertz transmission of about 2.2 times. Our findings demonstrate the potential of this terahertz generation scheme, based on ytterbium laser technology, as a simple and efficient alternative to the existing intense table-top terahertz sources. In particular, we show that it can be readily used to explore nonlinear effects at terahertz frequencies.

In article number 1805257, Fabian Rotermund, O‐Pil Kwon, and co‐workers report new organic three‐component single crystals exhibiting large macroscopic optical nonlinearity, excellent optical quality and efficient optical‐to‐THz conversion. The three‐component single crystals are developed by using the so‐called ‘pseudo‐isomorphic co‐crystallization’ for nonlinear optical and THz photonic applications. A new organic three‐component single crystals are developed using the so‐called “pseudo‐isomorphic cocrystallization” for nonlinear optical and terahertz (THz) photonic applications. The pseudo‐isomorphic cocrystallization is based on two homocrystals exhibiting similar molecular ordering feature in the crystalline state, but different crystallographic space groups. Such new organic cocrystals consist of three components, highly nonlinear optical 2‐(4‐hydroxystyryl)‐1‐methylquinolinium (OHQ) cation, and two different counter anions. Compared to homocrystals having two components (OHQ cation and a single anion type), OHQ‐based cocrystals by isomorphic cocrystallization from isomorphic homocrystals exhibit an isomorphic crystal structure with very similar physical properties. In contrast, OHQ‐based cocrystals by pseudo‐isomorphic cocrystallization provide a different molecular ordering with a different crystallographic space group, resulting in remarkably distinguishable crystal characteristics and physical properties, while maintaining large macroscopic optical nonlinearity with excellent optical quality and morphology suitable for diverse optical experiments. To show a potential for nonlinear optical applications, THz wave generation is demonstrated by optical rectification pumped at fundamental wavelength of 1300 nm. A 0.92 mm thick OHQ‐based cocrystal by pseudo‐isomorphic cocrystallization delivers efficient optical‐to‐THz conversion with one order of magnitude higher peak‐to‐peak THz electric field than the 1.0 mm thick inorganic standard ZnTe crystal and presents a broad spectral bandwidth of up to 8 THz.

A new organic π-conjugated quinolinium derivative was designed and synthesized in order to investigate the influence of the anion structure on characteristics of the crystal. The quinolinium derivative consists of a 2-(4-hydroxy-3-methoxystyryl)-1-methylquinolinium (HMQ) cation and a 4-vinylbenzenesulfonate (VBS) counter anion, which contains an electron-rich vinyl group. Single HMQ-VBS crystals, grown by a solution growth method in methanol, exhibited a monoclinic P21/c space group symmetry. Compared to the previously reported HMQ analogous crystal (HMQ-T), which bears a 4-methylbenzenesulfonate counter anion, the HMQ-VBS crystals generally have similar features of molecular alignment, but show different characteristics of hydrogen bonds related to the anion–anion and anion–cation interactions, as well as π–π stacking associated with the cation–cation interactions. Consequently, HMQ-VBS exhibits significantly different physical properties, including linear absorption and fluorescence in the crystalline state, even though these properties are identical to HMQ-T's in the solution state.

Highly efficient nonlinear optical quinolinium single crystals are newly designed and synthesized. The quinolinium single crystals consist of a 2-(4-hydroxystyryl)-1-methylquinolinium cation possessing large molecular optical nonlinearity and a 2,4,6-trimethylbenzenesulfonate counter anion. The quinolinium crystals exhibit monoclinic acentric space group P2 1 symmetry, resulting in large macroscopic nonlinear optical response, which is comparable with those of highly nonlinear optical configurationally locked polyene crystals. Plate-shaped bulk quinolinium crystals with an area of up to 56 mm 2 are successfully grown by the solution growth method. Single quinolinium crystals exhibit a large transparency range (550–1600 nm) in the optical region and a relatively low absorption coefficient in the THz region.

Terahertz absorption of organic materials is closely linked to molecular arrangements and their intermolecular interactions and is important for material identification as well as THz generation. Theoretical calculations of solid-state vibrations known as phonons help to understand intermolecular interactions responsible for THz absorption but frequently are of limited use without considering dispersion interaction. In this study, we have calculated the THz phonon modes of an organic model crystal 2,2′-bithiophene, considering dispersion intermolecular interactions assuming the fixed cell dimensions. Both energies and intensities of phonon modes at low frequencies were interpreted concentrating on the intermolecular level in conjunction with hydrogen bonds and showed an excellent agreement with the experimental results. This approach to identify the phonon modes responsible for strong THz absorptions and to interpret those modes in terms of intermolecular vibrations is also expected to be applicable to the field of THz generation using nonlinear optical organic crystals.

We investigate theoretically and experimentally the phonon modes of organic electro-optic crystals for THz-wave photonics. THz phonon modes significantly influence the characteristics of THz-wave generation and detection through the THz absorption and refractive-index dispersion. For a model as THz generation material, electro-optic PNP (2-(N-prolinol)-5-nitropyridine) molecular crystal containing a relatively small numbers of atoms in the unit cell is chosen for phonon-mode calculation. THz phonon modes with their frequencies and intensities are calculated by periodic density functional theory, and the resulting spectrum exhibits a good agreement with the experimental absorption spectrum measured by THz time-domain spectroscopy. THz-wave generation experiment is performed with PNP as THz generator and the organic electro-optic OH1 (2-(3-(4-hydroxystyryl)-5,5-dimethylcyclohex-2-enylidene)malononitrile) crystal as THz detector. Considering the phase matching condition and THz absorption, which are both influenced by the THz phonon modes of THz generation and detection materials, the calculated THz generation spectrum also matches well with the experimental THz spectrum obtained by optical rectification at the pump wavelength of 1300 nm. Therefore, the phonon mode identification is very important for optimizing and designing new THz source and detection materials as well as for THz spectroscopy.

Organic electrooptic crystals based on a new acentric core structure 2‐(4‐hydro­xystyryl)‐1‐methylquinolinium provide optimal molecular packing for maximizing the electrooptic response. They allow intense broadband terahertz generation with over 30 times higher terahertz generation efficiency compared to the widely used inorganic ZnTe.

Recently, nonlinear organic crystals have been proposed as promising materials for efficient generation and detection of broadband terahertz (THz) waves delivering high electric fields [1], because they exhibit much larger optical susceptibility and excellent optical-to-THz energy conversion efficiency at room temperature than nonlinear inorganic crystals and controllability of phase matching condition covering broad spectral bandwidth [2]. For growth of organic crystals, simple techniques based on solution and surface roughness of grown crystals below few nanometer scale without polishing are also beneficial for optical and THz photonic applications [3]. However, it is difficult to synthesize a single organic crystal possessing all the requirements for efficient THz wave generation. Especially, a major bottleneck as THz generator is strong re-absorption of generated THz waves caused by phonon modes resonance which is mainly attributed to the intrinsic constituents consisting of the crystal structure. Such self-absorption of THz waves in the crystal leads to drastic decrease in THz electric fields and undesirable modulation of the spectral shape with many dimples. When the generated THz waves exhibit strong absorption gaps with the distorted time trace, the applicability of THz waves is limited by additional parasitic effects and low signal-to-noise ratio at frequencies where the absorption dimples are located. Until now, it has been rarely reported how to effectively suppress the influence of phonon modes without changing intrinsic material properties of nonlinear organic crystals. One possible strategy was previously reported only by the change of chemical structures [4]. Since there is trade-off between suppression of phonon mode intensity and enhancement of macroscopic nonlinearity, this method is also limited for generation of efficient gap-free THz spectrum.