Efficient Gap‐Free Broadband Terahertz Generators Based on New Organic Quinolinium Single Crystals

Abstract

of the existing organic and inorganic THz wave generators and detectors are hindered by fundamental limitations in spectral generation and detection as well as by bandwidth limits due to intrinsic phonon and molecular vibrational modes. [2][3][4][5] In nonlinear optical frequency conversion processes, such as optical rectification and difference frequency generation, organic electro-optic crystals are more beneficial for broadband THz wave generation and detection as compared to their inorganic counterparts. [6][7][8][9] This is because organic electro-optic crystals exhibit large macroscopic optical nonlinearity with good phase matching ability between optical and THz frequencies and a relatively low absorption coefficient in high-frequency THz spectral regions.When considering THz wave generation and detection, current state-of-the-art organic electro-optic pyridinium-, quinolinium-, and benzothiazolium-based ionic crystals and nonionic phenolic polyene-based crystals exhibit a macroscopic nonlinear optical coefficient one order of magnitude larger than that of inorganic crystals (e.g., ZnTe and GaP) with an effective hyperpolarizability tensor β iii eff > 100 × 10 −30 esu and/or an electro-optic coefficient >50 pm V −1 . [6,[10][11][12][13][14] In addition, benchmark organic electro-optic crystals exhibit much lower absorption coefficients in the high THz frequency region when compared to inorganic crystals. While the upper New organic electro-optic crystals containing orthogonally oriented electronwithdrawing groups are developed for efficient and gap-free THz wave generation with a very flat broadband spectral shape. These crystals consist of 2-(4-hydroxystyryl)-1-methylquinolinium (OHQ) cationic chromophores and nonplanar 4-(trifluoromethoxy)benzenesulfonate (TFO) anions with orthogonally oriented highly electronegative trifluoromethoxy groups capable of strong hydrogen bonds. OHQ-TFO crystals exhibit enhanced macroscopic optical nonlinearity with a second harmonic generation efficiency 2.3 times greater than that of benchmark OHQ-based crystals with conventional planar anions. This enhancement is attributed to reduced edge-to-face π-π interactions between cations and anions due to the orthogonal orientation and electron-withdrawing characteristics of trifluoromethoxy groups. Moreover, OHQ-TFO crystals exhibit excellent THz wave characteristics generated by optical rectification; 0.52 mm thick OHQ-TFO crystal generate a peak-to-peak THz electric field 15 times higher than that of inorganic standard 1.0 mm thick ZnTe crystal and a broader spectrum with an upper cut-off frequency near 8 THz at pump wavelengths of 1140-1500 nm. Unlike previously reported state-of-the-art organic electro-optic salt crystals with strong phonon absorption in the frequency range of 0.8-3 THz, OHQ-TFO crystals facilitate gap-free broadband THz wave generation without strong absorption modulations due to the strong hydrogen bond ability of trifluoromethoxy groups.

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

“…[12][13][14] Various quinolinium electron acceptors instead of pyridinium electron acceptors were introduced into nonlinear optical chromophores to increase their nonlinearity. [8,[22][23][24][25][26][27][28][29][30] The electron withdrawing strength of quinolinium electron acceptors is higher than that of the pyridinium group; e.g., compared to the widely used 1,4-dimethylpyridinium electron acceptor, introducing 1,2-dimethylquinolinium electron acceptor results in enhancing the molecular optical nonlinearity of chromophores by about 50% at 1550 nm in solution. [30] Among quinolinium-based crystals, to our knowledge only phenolic crystals exhibit state-of-the-art macroscopic optical nonlinearity; e.g., eff β iii > 150 × 10 −30 esu.…”

Solid‐State Molecular Motions in Organic THz Generators

“…In previous reports, only few phonon mode engineering approaches have been conceptually suggested for organic nonlinear optical salt crystals. [12,27] For example, introducing substituents with strong multiple hydrogen-bonding ability have resulted in suppressing solidstate vibrations of organic nonlinear optical salt crystals, leading to enhanced optical-to-THz conversion efficiency and reduced the number or the strength of absorption-induced spectral dimples. [12,27] However, the origin of solid-state vibrational motions is presently still not clearly understood.…”

“…[12,27] For example, introducing substituents with strong multiple hydrogen-bonding ability have resulted in suppressing solidstate vibrations of organic nonlinear optical salt crystals, leading to enhanced optical-to-THz conversion efficiency and reduced the number or the strength of absorption-induced spectral dimples. [12,27] However, the origin of solid-state vibrational motions is presently still not clearly understood. This leads to unclear situation for systematic design of optimized nonlinear optical crystals for THz generators and detectors.…”

“…As model crystal, quinolinium-based HMQ-TMS (2-(4-hydroxy-3-methoxystyryl)-1-methylquinolinium 2,4,6-trimethylbenzenesulfonate, Figure 1a) [28] is chosen, because many efficient organic THz generators based on quinolinium-based organic crystals have been recently developed. [4,12,27,28] HMQ-TMS crystals with large diagonal effective hyperpolarizability tensor ( iii β eff = 185 × 10 −30 esu) exhibit very high optical-to-THz conversion efficiency in broadband and narrowband THz generation. [7,15,28] The simulated results of solid-state molecular vibrations along the polar axis of HMQ-TMS crystals show a good agreement with experimental results.…”

Ultra‐Broadband and High‐Dynamic‐Range THz Time‐Domain Spectroscopy System Based on Organic Crystal Emitter and Detector in Transmission and Reflection Geometry

“…For improving the spectrum around and below the strong phonon resonance at 1 THz, which is already considerably improved for DSTMS compared with DAST (see Figure S2b, Supporting Information), as well as reduce the oscillations of the spectrum due to vibrational modes in DAST or DSTMS, other organic crystals being developed [ 42–46 ] may lead to improved performance of such systems in the future. Inserting a conventional 1 mm‐thick (110) ZnTe in its optimal orientation as the THz generator in the same THz setup (with DSTMS detector) results in a THz electric field amplitude of more than two orders of magnitude smaller, which is because of nonoptimal phase‐matching conditions of ZnTe at 1560 nm pump wavelength, as well as a more than one order of magnitude smaller electro‐optical figure of merit of ZnTe compared with DAST or DSTMS.…”