Solid‐State Molecular Motions in Organic THz Generators

Kang

Puc

et al. 2021

Self Cite

ordering in the crystalline state should be simultaneously considered when designing organic π-conjugated crystals to achieve the desired physical properties. However, the prediction of the molecular ordering of organic π-conjugated chromophores in the crystalline state is still significantly difficult. This is because most organic π-conjugated chromophores exhibit several complex intermolecular and intramolecular interactions (and interionic interactions) with multiple possible molecular conformations during the self-assembling process in the crystalline state. [17,18] For each specific target application, a different targeted control of the molecular ordering of chromophores in the crystalline state is required, which is still an important issue in the molecular (and crystal) engineering of various organic π-conjugated crystalline materials.This issue is more critical for organic nonlinear optical (NLO) crystals and will be the focus of this review. Organic NLO crystals can be used in diverse nonlinear photonic applications, such as frequency down-and up-conversion, terahertz (THz) wave generation and detection, phase and amplitude electro-optic modulation, and high-speed telecommunication integrated optics. [19][20][21][22][23] Organic crystals must possess a noncentrosymmetric molecular ordering of chromophores to achieve second-order optical nonlinearity. However, in addition to the aforementioned complicated molecular interactions, the introduction of strong polar substituents (electron-donating groups (EDGs) and electron-withdrawing groups (EWGs)) on widely used push-pull π-conjugated chromophores to increase their molecular nonlinearity also increases their dipole moment. In many cases, a high dipole moment leads to a centrosymmetric ordering of chromophores with antiparallel dipole-dipole aggregation in the crystalline state (i.e., there is no macroscopic second-order optical nonlinearity). This tendency further complicates the development of new organic NLO crystals with large second-order optical nonlinearity. This is one of the reasons for only a few classes of organic crystals with state-of-theart optical nonlinearity being reported in the last few decades. Moreover, in addition to large macroscopic second-order optical nonlinearity, different NLO applications require different targeted physical properties (e.g., refractive index, dispersion of the refractive index, dielectric constant, and absorption coefficient) and crystal characteristics (e.g., direction of the polar axis, morphology, aperture size, thickness, and facets). Therefore, it 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 d...

Section: Self-absorption By Phonon Modessupporting

confidence: 72%

Section: Self-absorption By Phonon Modesmentioning

confidence: 88%

Highly Nonlinear Optical Organic Crystals for Efficient Terahertz Wave Generation, Detection, and Applications

Kang

Puc

et al. 2021

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“…For example, in benchmark organic THz crystals in the THz frequency range, different www.advopticalmat.de crystal axes exhibit largely different absorption characteristics and refractive indexes. [49,50] In many organic benchmark nonlinear optical crystals, the coherence length is often in the range of 0.1-1.0 mm at different optical pump wavelengths. Therefore, using an optimal crystal thickness of PM7ClQ-T crystals is also expected to further improve the optical-to-THz conversion efficiency.…”

Section: High-performance Broadband Thz Wave Generatorsmentioning

confidence: 99%

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

Seok

et al. 2021

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include the submillimeter wavelengths of terahertz (THz) frequencies. Examples of such applications are THz wave generation and detection, cross-phase modulation, and THz nonlinear optics. [1][2][3][4][5][6][7] Compared to inorganic crystals, the organic alternatives exhibit superior macroscopic second-order optical nonlinearity, which often shows one order of magnitude larger nonlinear optical and electro-optic coefficients. [8] There is still a strong demand for improving the macroscopic optical nonlinearity of organic crystals, which is presently below the level achieved in the organic poled polymers. For example, the electro-optic coefficient of benchmark organic crystals is at the level of ≈50 pm V −1 at 1.3 µm. [9][10][11] On the other hand, the poled polymer systems may exhibit much larger electro-optic coefficients, up to ≈500 pm V −1 at 1.3 µm. [12][13][14] However, progress on designing a strategy to exceed the limits of the state-of-the-art optical nonlinearity of current benchmark organic crystals seems to be slow. Moreover, besides the large macroscopic optical nonlinearity desired, many other crystal characteristics that make possible to grow high-quality bulk crystals with plate-like crystal morphology and high thermal stability are required for real-world photonic applications.Developing new organic crystals with large macroscopic second-order optical nonlinearity is a challenging issue 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 finetuning 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.

“…[ 21 ] The vibrational motions, including the phonon modes in organic electro‐optic crystals (i.e., THz generator), lead to self‐absorption of the generated THz waves. [ 10 ] For inorganic electro‐optic crystals such as ZnTe, GaP, and LiNbO 3 , the phonon‐induced self‐absorption is relatively large, which typically limits the generation and detection bandwidth of these materials up to few THz only. Note that the damage threshold of organic electro‐optic crystals may be lower than for inorganic crystals at the same experimental conditions.…”

Section: Resultsmentioning

confidence: 99%

“…However, the vibrational modes still limit the achievable dynamic range with organic crystals at certain THz frequencies. [ 10,29 ]…”

Section: Resultsmentioning

confidence: 99%

High‐Density Organic Electro‐Optic Crystals for Ultra‐Broadband THz Spectroscopy

Seok

Puc

et al. 2021

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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.