Organic σ‐Hole Containing Crystals with Enhanced Nonlinear Optical Response and Efficient Optical‐to‐THz Frequency Conversion

Abstract

to obtain the desired physical properties correlated to electronic and vibrational states. [1][2][3] In particular, alterations in the intermolecular interactions of organic nonlinear optical materials can result in dramatic changes in the molecular ordering of chromophores, macroscopic nonlinear optical responses, and molecular vibrational and phonon modes, [4][5][6][7] which are important material characteristics in diverse nonlinear optical and THz photonic applications. [8][9][10] The macroscopic nonlinear optical response of organic materials is directly proportional to the first-order hyperpolarizability β of chromophores and their alignments. [8] In many nonlinear optical organic crystals, strong intermolecular interactions to π-conjugated chromophores can adversely affect the resulting macroscopic optical nonlinearity. In well-known and commercially available pyridinium-based crystals, [8,11,12] the first-order hyperpolarizability of chromophores in the crystalline state (β crystal ) is considerably less than in solution (β solution ); e.g., in stilbazolium and merocyanine crystals, β crystal is only about 20% and 5%, respectively, of the corresponding β solution . [6] In a more precise analysis of intermolecular interactions of stilbazoliumbased crystals, reducing intermolecular interactions (edge-toface π-π interactions and hydrogen bonds (H··· − O-S) between 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 β crystal 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 chloroquinolinium 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.

Solid‐State Molecular Motions in Organic THz Generators

“…[6][7][8][9][10][11] Organic THz generators show superior performance to inorganic counter parts such as ZnTe and GaP crystals (e.g., one order of magnitude higher optical-to-THz conversion efficiency and broader spectral bandwidth). [4,5,8,[12][13][14] This is because organic nonlinear optical crystals exhibit larger second-order macroscopic optical nonlinearity, better phase matching between optical pump and the generated THz waves, and lower phononmode amplitudes in wide THz wave range compared to inorganic crystals. [4,5,12] Organic molecular salt crystals exhibit efficient narrowband THz wave generation with one order of magnitude higher optical-to-THz conversion efficiency compared to inorganic ZnTe crystals.…”

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

“…in organic NLO crystals. [164,[191][192][193] In crystals, the introduction of halogen substituents (F, Cl, and Br) has multiple merits. The simultaneous electron-withdrawing and electrondonating effects of halogen substituents (Cl and Br) induce unique and strong multiple interionic interactions with both the existing partially positive σ-holes and the partially negative belts (e.g., Cl substituents in cationic chromophores and anions in Figure 18c [193] ).…”

“…[164,[191][192][193] In crystals, the introduction of halogen substituents (F, Cl, and Br) has multiple merits. The simultaneous electron-withdrawing and electrondonating effects of halogen substituents (Cl and Br) induce unique and strong multiple interionic interactions with both the existing partially positive σ-holes and the partially negative belts (e.g., Cl substituents in cationic chromophores and anions in Figure 18c [193] ). In addition, halogen substituents have considerably similar space-filling characteristics as one of the simplest substituents, the methyl group; thus, the space-filling characteristics are not significantly altered upon introducing halogen substituents instead of the widely used methyl substituent.…”

“…The introduction of halogen substituents in cationic chromophores and/or anions results in the modulation and enhancement of THz wave generation characteristics. [164,[191][192][193] Adv. Optical Mater.…”

“…Copyright 2017, Wiley-VCH. c) Reproduced and adapted with permission [193]. Copyright 2020, Wiley-VCH.…”