Recent progress in highly nonlinear organic crystals for THz wave generation

Fabian Rotermund

Department of Physics, Korea Advanced Institute of Science and Technology (KAIST), Daejeon 34141, Korea

Over the years, considerable advancements have been made in the realm of nonlinear organic crystals, demonstrating their effectiveness in generating broadband terahertz (THz) waves. This success is mainly attributed to their unique characteristics, such as large second-order optical nonlinearities and excellent phase matching capabilities. Among the array of existing organic crystals, organic π-conjugated crystalline materials possessing high optical nonlinearity are widely used in various fields of optics and photonics applications, including nonlinear optics, electro-optics, and photodetection [1]. The physical and optical characteristics of these organic π-conjugated crystalline materials are closely related to the molecular ordering of the constituent π-conjugated molecules and their chemical compositions. Designing organic π-conjugated crystals with desired properties necessitates simultaneous consideration of the chemical structures of chromophores and their molecular ordering in the crystalline state. However, predicting such molecular ordering of organic π-conjugated chromophores in the crystalline state remains a formidable challenge. This complexity arises from the manifold complex intermolecular and intramolecular interactions exhibited by most organic π-conjugated chromophores, as well as interionic interactions with multiple possible molecular conformations during self-assembling process in the crystalline state. Tailoring the molecular ordering of chromophores in the crystalline state to meet specific application requirements remains a pivotal concern in the molecular (and crystal) engineering of various organic π-conjugated crystalline materials, particularly for designing highly nonlinear organic crystals. Such crystals must possess a non-centrosymmetric molecular ordering of chromophores to achieve second-order optical nonlinearity. However, in addition to complex molecular interactions, the introduction of strong polar substituents, such as electron-donating groups and electron-withdrawing groups, on widely used push–pull π-conjugated chromophores to increase their molecular nonlinearity also increases their dipole moment.
In this talk, recent advancements and developments of organic nonlinear optical crystals applicable for efficient broadband THz wave generation are presented. Exemplary applications of newly developed organic crystals in achieving efficient THz waves, extending up to 10 THz, will be highlighted [2,3]. Additionally, different approaches aimed at suppressing phonon-mode absorptions, which often induce pronounced modulations in the generated THz spectra in most organic crystals, will be discussed [4-6].

References
[1] M. Tonouchi, Nat. Photon. 1, 97 (2007)
[2] S.-J. Kim et al., Adv. Opt. Mater. 9, 2101019 (2021)
[3] B. J. Kang et al., Opt. Express 24, 11054 (2016)
[4] B. J. Kang et al., Adv. Funct. Mater. 28, 1707195 (2018)
[5] B.-R.-Shin et al., APL Mater. 11, 011101 (2023)
[6] S.-J. Kim et al., Adv. Funct. Mater. 33, 2209915 (2023)