Organic Broadband THz Generators Optimized for Efficient Near‐Infrared Optical Pumping

Abstract

New organic THz generators are designed herein by molecular engineering of the refractive index, phonon mode, and spatial asymmetry. These benzothiazolium crystals simultaneously satisfy the crucial requirements for efficient THz wave generation, including having nonlinear optical chromophores with parallel alignment that provide large optical nonlinearity; good phase matching for enhancing the THz generation efficiency in the near-infrared region; strong intermolecular interactions that provide restraining THz self-absorption; high solubility that promotes good crystal growth ability; and a plate-like crystal morphology with excellent optical quality. Consequently, the as-grown benzothiazolium crystals exhibit excellent characteristics for THz wave generation, particularly at near-infrared pump wavelengths around 1100 nm, which is very promising given the availability of femtosecond laser sources at this wavelength, where current conventional THz generators deliver relatively low optical-to-THz conversion efficiencies. Compared to a 1.0-mm-thick ZnTe crystal as an inorganic benchmark, the 0.28-mm-thick benzothiazolium crystal yields a 19 times higher peak-to-peak THz electric field with a broader spectral bandwidth (>6.5 THz) when pumped at 1140 nm. The present work provides a valuable approach toward realizing organic crystals that can be pumped by near-infrared sources for efficient THz wave generation.

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

“…This behavior is often observed in phenolic organic push–pull chromophores with a strong EWG. [ 41 ]…”

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

“…A classic approach to detect coherent THz waves generated using newly developed organic NLO crystals is still to utilize standard EOS schemes based on ZnTe or GaP crystals, as discussed in Section 4.1, which currently present the main bandwidth limitation in high-power THz systems using organic crystalline THz generators. [142,143]…”

“…[47] HMQ-TMS crystals are still being investigated in various optical configurations and conditions for efficient THz wave generation (e.g., megahertz (MHz) repetition rate, emerging femtosecond laser sources). [111,172] After the success of the acentric cationic core structure of the phenolic HMQ cation for nonlinear optics and THz wave generation, [45,[173][174][175] [33,143,[176][177][178][179][180][181] Several crystals based on the above-mentioned phenolic acentric cationic cores exhibit state-of-the-art macroscopic optical nonlinearity and the diagonal component of the effective hyperpolarizability tensor β iii eff greater than 100 × 10 -30 esu with parallel-type phenolic cationanion assemblies (Figure 13b), such as the OHQ-based crystals illustrated in Figure 15c. This results in efficient THz wave [45] Copyright 2013, Springer Nature.…”

High‐Density Organic Electro‐Optic Crystals for Ultra‐Broadband THz Spectroscopy

“…In previous studies, yellowcolor cations with an absorption maximum wavelength λ max near 400 nm exhib ited a high probability of achieving good phasematching at the optical pump wavelengths of 1140-1300 nm and above. [22,23] When a strong electrondonating dialkylamino group is intro duced with strong electronwithdrawing pyridinium or benzo thiazolium groups in chromophores, the absorption maximum wavelength λ max shifts to near 500 nm. [26][27][28] Therefore, in this work, we selected the phenolic electron donor group as it has a relatively weak electron donating strength.…”

“…To achieve ultrabroad band THz wave generation with simple collinear setups, the nonlinear optical crystals should possess the following properties: i) large macroscopic secondorder optical nonlin earity, ii) good phase matching (velocity matching) between the optical and THz waves, and iii) small optical and THz wave absorption. [21][22][23][24][25][26] We consider these three requirements in the design of the new nonlinear optical crystals in this work. To obtain a large macroscopic nonlinear optical coef ficient in the crystal (effective hyperpolarizability tensor components eff β ijk of ≥100 × 10 -30 esu), strong electronwith drawing pyridinium and benzothiazolium are introduced to the OHP (4(4hydroxystyryl)1methylpyridinium1ium) and OHB (2(4hydroxystyryl)3methylbenzothiazol3ium) cat ions, respectively.…”