X‐Shaped Alignment of Chromophores: Potential Alternative for Efficient Organic Terahertz Generators

Advanced Optical Materials

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.

Section: High-performance Broadband Thz Wave Generatorsmentioning

confidence: 99%

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

Seok

Advanced Optical Materials

et al. 2021

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“…A simple solution for organic NLO crystals with a nonoptimal as-grown morphology and/or higher off-diagonal NLO tensor coefficients is to rotate the crystal with respect to the propagation direction of the pump/THz wave to increase the generation efficiency, which maintains a relatively simple collinear geometry, as shown in Figure 9. [125]…”

Section: Crystal Orientationmentioning

confidence: 99%

“…In 2020, experimental demonstrations and theoretical studies of efficient THz wave generation using crystals with a dominant off-diagonal macroscopic optical nonlinearity were successfully performed. [125] The acentric core 5-fluoro-2-(4-(4-(hydroxymethyl)piperidin-1-yl)styryl)-3-methylbenzothiazol-3-ium (PFB) cation exhibits a significantly large maximal first hyperpolarizability β max of ≈274 × 10 −30 esu and shows an X-type alignment in the crystalline state, with θ p close to the optimal value for maximizing the off-diagonal optical nonlinearity (Figure 17). Consequently, PFB-based crystals exhibit a large off-diagonal optical nonlinearity with β ijj eff (i ≠ j) > 100 × 10 −30 esu.…”

Section: X-type Crystals With Large Off-diagonal Optical Nonlinearitymentioning

confidence: 99%

“…In THz wave generation using off-diagonal macroscopic optical nonlinearity, PFB-based crystals generate a THz amplitude that is one order of magnitude higher than that of ZnTe crystals (Figure 9). [125] This work presents new possibilities for the design of new organic NLO crystals by enhancing the diagonal as well as off-diagonal optical nonlinearity for THz wave generation, with a larger variation in phase-matching possibilities for various available pump laser sources.…”

Section: X-type Crystals With Large Off-diagonal Optical Nonlinearitymentioning

confidence: 99%

mentioning

confidence: 99%

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

Advanced Optical Materials

Kang

Puc

et al. 2021

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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...

A New Class of Organic Crystals with Extremely Large Hyperpolarizability: Efficient THz Wave Generation with Wide Flat‐Spectral‐Band

et al. 2022

Adv Funct Materials

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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 β iii eff of 335 × 10 −30 esu. This is in sharp contrast to zero β iii 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.