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

Abstract

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...

Design Strategy of Highly Efficient Nonlinear Optical Orange‐Colored Crystals with Two Electron‐Withdrawing Groups

“…From the aforementioned experimental and theoretical results, it is obvious that the diagonal macroscopic optical nonlinearity of OPR crystals is very large and comparable to that of DAST crystals and other various benchmark organic nonlinear optical crystals. [11,34]…”

“…When large macroscopic optical nonlinearity is achieved, possessing the shorter absorption wavelength of OPR crystals is very beneficial for achieving different wavelength dependences of both linear and nonlinear optical characteristics, as well as the photochemical stability. [6,34,39,40] For many nonlinear optical applications, it is essential to avoid the absorption at the pump and/or probe wavelengths used, while some may also prefer to match the wavelengths used with the absorption and rely on resonance effects. OPR and DAST crystals are, therefore, expected to exhibit a remarkably different wavelength dependence of their nonlinear optical response.…”

Phonon‐Suppressing Intermolecular Adhesives: Catechol‐Based Broadband Organic THz Generators

“…For instance, in organic π ‐conjugated materials, the so‐called molecular phonons play an important role for their functional properties, such as the charge mobility, [ 11–13 ] emission characteristics, [ 14,15 ] as well as the absorption and refractive index characteristics at far‐infrared and THz frequencies. [ 16–27 ]…”

“…Such hydrogen bonds present main interionic interactions in several promising recently developed organic salt crystals for THz wave generation. [ 16,17 ] Note that in many organic THz generators, aromatic (i.e., phenolic) and aliphatic OH groups are incorporated. In this work, we introduce two phenolic OH groups for the design of new organic THz device materials.…”

Energy-band echoes: Time-reversed light emission from optically driven quasiparticle wavepackets

“…Coherent optical control of quantum many-body systems has also been recognized as an intriguing problem in modern condensed matter physics, since such a system exhibits a wide variety of phases and thereby provides a rich playground for the control of physical properties. Recent developments in photoinduced phase transitions [25][26][27][28] and Floquet engineering [29][30][31][32][33] have attracted growing interest in studying the ultrafast and nonthermal control of quantum materials, owing to the rapid development of light sources [34][35][36][37] and of timeresolved measurement techniques [25,29,[38][39][40][41][42][43][44][45][46][47][48][49]. However, the coherent control of condensed matter systems still faces challenges, since quantum coherence is quickly destroyed by interactions with environmental degrees of freedom.…”