Jongtaek Kim

High-power Broadband Organic THz Generator

The high-power broadband terahertz (THz) generator is an essential tool for a wide range of THz applications. Here, we present a novel highly efficient electro-optic quinolinium single crystal for THz wave generation. For obtaining intense and broadband THz waves by optical-to-THz frequency conversion, a quinolinium crystal was developed to fulfill all the requirements, which are in general extremely difficult to maintain simultaneously in a single medium, such as a large macroscopic electro-optic response and excellent crystal characteristics including a large crystal size with desired facets, good environmental stability, high optical quality, wide transparency range, and controllable crystal thickness. Compared to the benchmark inorganic and organic crystals, the new quinolinium crystal possesses excellent crystal properties and THz generation characteristics with broader THz spectral coverage and higher THz conversion efficiency at the technologically important pump wavelength of 800 nm. Therefore, the quinolinium crystal offers great potential for efficient and gap-free broadband THz wave generation.

Acentric nonlinear optical N-benzyl stilbazolium crystals with high environmental stability and enhanced molecular nonlinearity in solid state

We have developed a new cation core structure, N-benzyl stilbazolium nonlinear optical chromophore with a non-polar benzyl group to achieve acentric molecular ordering in the crystalline state. New N-benzyl stilbazolium crystal, BP3 (N,N-dimethylamino-N′-2,5-dimethylbenzyl-stilbazolium p-toluenesulfonate), exhibits an acentric crystal structure with the monoclinic P21 phase with a large macroscopic optical nonlinearity of 540 times the powder second harmonic generation efficiency of urea at the non-resonant wavelength of 1.9 µm. Compared to conventional rod-shaped N-alkyl stilbazolium crystals, an enhanced hyperpolarizability of the chromophoresβ0 in the solid state can be utilized in bent-shaped N-benzyl stilbazolium crystals. This is attributed to the decreased inter-chromophore interactions due to the larger chromophore–chromophore separation induced by the bulky and bent-shaped N-benzyl group, so-called site-isolation effect. Moreover, by introducing the non-polar dimethylbenzyl group, BP3 crystals show a high environmental stability: they exhibit almost one order of magnitude smaller solubility in water than conventional stilbazolium crystals and also do not form a hydrated centrosymmetric phase even if crystallized in water-containing solvents. We have grown good optical quality crystals large enough for optical characterization. With as-grown BP3 crystals without additional polishing and cutting procedures we have demonstrated THz generation by optical rectification using 180 fs pulses at the pump wavelength of 836 nm.

New acentric quinolinium crystal with high order parameter for nonlinear optical and electro-optic applications

We have designed and synthesized a new, highly efficient electro-optic vanillin-based quinolinium crystal. The HMQ-NS (2-(4-hydroxy-3-methoxystyryl)-1-methylquinolinium naphthalene-2-sulfonate) crystals exhibit an acentric monoclinic space group symmetry Pn with a large effective molecular hyperpolarizability in the crystal with the diagonal component βeff333 = 149 × 10−30 esu, which is comparable to that of benchmark ionic stilbazolium crystals, and a good thermal stability. Therefore, the newly developed HMQ-NS crystal is an attractive candidate for nonlinear optical and electro-optic applications.

Electronic and chelation effects on the unusual C2-methylation of N-(para-substituted)phenylaziridines with lithium organocuprates

Density functional theory calculations with the B3LYP functional were performed for the title ring‐opening reaction to understand the intrinsic activating and directing effects of the N‐substituents, as well as the electron donating effect of the para‐substituted (Y = Cl, H, Me) phenyl group at the more hindered benzylic C2 atom. The N‐tosyl group (i.e., N‐Tos) or the N‐(2‐pyridyl)sulfonyl group (i.e., N‐Py) was introduced to activate the ring nitrogen atom (N1) and the para‐substituted (Y = Cl, H, Me) phenyl group for the activation of the C2 atom. Conformational searches and geometry optimizations were performed for the N‐(para‐substituted)phenylaziridines (1∼6). Calculations indicate that the aziridine 6 (i.e., Py/Me) has the most elongated C2N1 bond intrinsically due to the electronic activating effects, implying the aziridine 6 to be the most potent candidate for the more‐hindered C2 opening. Transition states (TSs) were investigated for the prospective ring‐opening paths (I∼IV), considering the types of intermolecular push–pull interactions between the N‐activated phenylaziridines and the cuprate. The N‐Py group provides an unique C2‐favored TS along the path IV, which the N‐Tos group cannot afford, due to the less charge transfer from the nucleophilic CH  3δ− of the cuprate into the electrophilic C2 atom. Furthermore, the e‐donating effect of the para‐substituents (Y = Cl, H, Me) enhances the C2 opening for the path IV. This study enables us to understand the unusual ring‐opening phenomena in terms of electronic and directing effects and hence may serve as a tool to design substrates for highly regioselective ring openings.