Seung-Jun Lee

Highly nonlinear organic crystal OHQ-T for efficient ultra-broadband terahertz wave generation beyond 10 THz.

We report on efficient generation of ultra-broadband terahertz (THz) waves via optical rectification in a novel nonlinear organic crystal with acentric core structure, i.e. 2-(4-hydroxystyryl)-1-methylquinolinium 4-methylbenzenesulfonate (OHQ-T), which possesses an ideal molecular structure leading to a maximized nonlinear optical response for near-infrared-pumped THz wave generation. By systematic studies on wavelength-dependent phase-matching conditions in OHQ-T crystals of different thicknesses we are able to generate coherent THz waves with a high peak-to-peak electric field amplitude of up to 650 kV/cm and an upper cut-off frequency beyond 10 THz. High optical-to-THz conversion efficiency of 0.31% is achieved by efficient index matching with a selective pumping at 1300 nm.

Benzothiazolium Single Crystals: A New Class of Nonlinear Optical Crystals with Efficient THz Wave Generation

Highly efficient nonlinear optical organic crystals are very attractive for various photonic applications including terahertz (THz) wave generation. Up to now, only two classes of ionic crystals based on either pyridinium or quinolinium with extremely large macroscopic optical nonlinearity have been developed. This study reports on a new class of organic nonlinear optical crystals introducing electron-accepting benzothiazolium, which exhibit higher electron-withdrawing strength than pyridinium and quinolinium in benchmark crystals. The benzothiazolium crystals consisting of new acentric core HMB (2-(4-hydroxy-3-methoxystyryl)-3-methylbenzo[d]thiazol-3-ium) exhibit extremely large macroscopic optical nonlinearity with optimal molecular ordering for maximizing the diagonal second-order nonlinearity. HMB-based single crystals prepared by simple cleaving method satisfy all required crystal characteristics for intense THz wave generation such as large crystal size with parallel surfaces, moderate thickness and high optical quality with large optical transparency range (580-1620 nm). Optical rectification of 35 fs pulses at the technologically very important wavelength of 800 nm in 0.26 mm thick HMB crystal leads to one order of magnitude higher THz wave generation efficiency with remarkably broader bandwidth compared to standard inorganic 0.5 mm thick ZnTe crystal. Therefore, newly developed HMB crystals introducing benzothiazolium with extremely large macroscopic optical nonlinearity are very promising materials for intense broadband THz wave generation and other nonlinear optical applications.

Electro-optic crystals grown in confined geometry with optimal crystal characteristics for THz photonic applications

We report a new crystal engineering technique for the thickness-controlled growth of large-area organic electro-optic crystals in confined geometry optimal for THz wave photonics. We investigate the relationship between the material properties and THz generation characteristics to determine the optimal crystal characteristics for THz applications. Two different state-of-the-art nonlinear optical organic crystals are chosen as model crystals, ionic quinolinium-based and non-ionic polyene-based crystals. Both kinds of electro-optic crystals grown in confined geometry exhibit good optical quality, large area and sub-millimeter-scale thickness, which are optimal crystal characteristics for efficient THz wave generation. In order to investigate the relationship between crystal characteristics and optical-to-THz conversion characteristics, THz experiments with quinolinium crystals are performed and compared with theoretical calculations. The characteristics of the generated THz waves from as-grown quinolinium-based single crystals in confined geometry match well with those from very flat quinolinium crystals prepared from bulk crystals by a cutting method and also with theoretical calculations.

Quinolinium single crystals with a high optical nonlinearity and unusual out-of-plane polar axis

New highly efficient nonlinear optical crystals based on phenolic quinolinium with various benzenesulfonate and thiophenesulfonate anions are reported for photonic applications. All phenolic quinolinium crystals exhibit strong second harmonic generation (SHG) efficiency. Among them, OHQ-MBS crystals (2-(4-hydroxystyryl)-1-methylquinolinium 4-methoxybenzenesulfonate) with acentric monoclinic Pn space group symmetry exhibit very large macroscopic second-order optical nonlinearity with diagonal effective hyperpolarizability of about 120 × 10−30 esu, which is comparable to the state-of-the-art organic nonlinear optical crystals. OHQ-MBS single crystals with a large area of over 5 × 5 mm2 and good optical quality were successfully grown using a solution growth method in mixed solvent systems. Interestingly, as-grown OHQ-MBS single crystals exhibit an out-of-plane polar axis, which is very unusual, because the state-of-the-art organic nonlinear optical as-grown crystals commonly exhibit an in-plane polar axis. The out-of-plane polar axis is very beneficial for applying various optical configurations. In addition, OHQ-MBS single crystals with an in-plane polar axis are also successfully prepared using a simple cleaving method. Therefore, OHQ-MBS crystals with easily accessible both out-of-plane and in-plane polar axes, as well as the state-of-the-art optical nonlinearity may open up a range of new possibilities for photonic applications.

New antibacterial-core structures based on styryl quinolinium

Quaternary quinolinium salts have been widely used as alternative antimicrobial agents. In an effort to improve the current quinolinium compounds and determine the relation between antibacterial activity and substituted functional groups, 10 different styryl quinolinium derivatives with various quaternary ammonium electron acceptors, electron donors, and counter anions were rationally designed. Among the 10 styryl quinoliniums, six compounds exhibited bactericidal effects against Gram-positive bacteria, with minimum inhibitory concentrations (MICs) of 2.4–37.5 μg/mL. In addition, two compounds, namely DA-DMQ1,4-T and DA-DMQ1,4-TMS, showed low MICs of 18.75–75 μg/mL with Gram-negative bacteria. In general, compounds possessing electron acceptor groups with a strong electron-withdrawing ability exhibited high bactericidal activity against diverse bacterial species. Co-administration of quinolinium (1.17–9.36 μg/mL) and broad-spectrum β-lactam antibiotic ampicillin (0.02–2.34 μg/mL) showed synergistic bactericidal effects on both Gram-positive and Gramnegative bacteria. This study provides guidelines for the development of new quinolinium salts with a prominent antimicrobial activity.