Abstract-Wide‐Bandgap Organic Crystals: Enhanced Optical‐to‐Terahertz Nonlinear Frequency Conversion at Near‐Infrared Pumping

Deokjoong Kim, Won Tae Kim, Jae‐Hyun Han, Ji‐Ah Lee, Seung‐Heon Lee, Bong Joo Kang, Mojca Jazbinsek, Woojin Yoon, Hoseop Yun, Dongwook Kim, Stein van Bezouw, Jochen Campo, Wim Wenseleers, Fabian Rotermund,   O‐Pil Kwon,

https://onlinelibrary.wiley.com/doi/abs/10.1002/adom.201902099

Enhanced terahertz (THz) wave generation is demonstrated in nonlinear organic crystals through refractive index engineering, which improves phase matching characteristics substantially. Unlike conventional low‐bandgap nonlinear organic crystals, the newly designed benzimidazolium‐based HMI (2‐(4‐hydroxy‐3‐methoxystyryl)‐1,3‐dimethyl‐1H‐benzoimidazol‐3‐ium) chromophore possesses a relatively wide bandgap. This reduces the optical group index in the near‐infrared, allowing better phase matching with the generated THz waves, and leads to high optical‐to‐THz conversion. A unique feature of the HMI‐based crystals, compared to conventional wide‐bandgap aniline‐based crystals, is their remarkably larger macroscopic optical nonlinearity, a one order of magnitude higher diagonal component in macroscopic nonlinear susceptibility than NPP ((1‐(4‐nitrophenyl)pyrrolidin‐2‐yl)methanol) crystals. The HMI‐based crystals also exhibit much higher thermal stability, with a melting temperature Tm above 250 °C, versus aniline‐based crystals (116 °C for NPP). With pumping at the technologically important wavelength of 800 nm, the proposed HMI‐based crystals boost high optical‐to‐THz conversion efficiency, comparable to benchmark low‐bandgap quinolinium crystals with state‐of‐the‐art macroscopic nonlinearity. This performance is due to the excellent phase matching enabled by decreasing optical group indices in the near‐infrared through wide‐bandgap chromophores. The proposed wide‐bandgap design is a promising way to control the refractive index of various nonlinear organic materials for enhanced frequency conversion processes.

Abstract-Generation of high-field terahertz pulses in an HMQ-TMS organic crystal pumped by an ytterbium laser at 1030 nm

Andrea Rovere, Young-Gyun Jeong, Riccardo Piccoli, Seung-Heon Lee, Seung-Chul Lee, O-Pil Kwon, Mojca Jazbinsek, Roberto Morandotti, and Luca Razzari

https://www.osapublishing.org/oe/abstract.cfm?uri=oe-26-3-2509

We present the generation of high-peak-electric-field terahertz pulses via collinear optical rectification in a 2-(4-hydroxy-3-methoxystyryl)-1-methilquinolinium-2,4,6-trimethylbenzenesulfonate (HMQ-TMS) organic crystal. The crystal is pumped by an amplified ytterbium laser system, emitting 170-fs-long pulses centered at 1030 nm. A terahertz peak electric field greater than 200 kV/cm is obtained for 420 µJ of optical pump energy, with an energy conversion efficiency of 0.26% - about two orders of magnitude higher than in common inorganic crystals collinearly pumped by amplified femtosecond lasers. An open-aperture Z-scan measurement performed on an n-doped InGaAs thin film using such terahertz source shows a nonlinear increase in the terahertz transmission of about 2.2 times. Our findings demonstrate the potential of this terahertz generation scheme, based on ytterbium laser technology, as a simple and efficient alternative to the existing intense table-top terahertz sources. In particular, we show that it can be readily used to explore nonlinear effects at terahertz frequencies.

© 2018 Optical Society of America under the terms of the OSA Open Access Publishing Agreement

Abstract-Complementary tandem configuration of nonlinear organic crystals for efficient terahertz spectral filling

Bong Joo Kang, Seung-Heon Lee,   Won Tae Kim,  Seung-Chul Lee,   Kang Hee Lee, Mojca Jazbinsek,   O-Pil Kwon,  Fabian Rotermund

http://ieeexplore.ieee.org/document/8086452/

Recently, nonlinear organic crystals have been proposed as promising materials for efficient generation and detection of broadband terahertz (THz) waves delivering high electric fields [1], because they exhibit much larger optical susceptibility and excellent optical-to-THz energy conversion efficiency at room temperature than nonlinear inorganic crystals and controllability of phase matching condition covering broad spectral bandwidth [2]. For growth of organic crystals, simple techniques based on solution and surface roughness of grown crystals below few nanometer scale without polishing are also beneficial for optical and THz photonic applications [3]. However, it is difficult to synthesize a single organic crystal possessing all the requirements for efficient THz wave generation. Especially, a major bottleneck as THz generator is strong re-absorption of generated THz waves caused by phonon modes resonance which is mainly attributed to the intrinsic constituents consisting of the crystal structure. Such self-absorption of THz waves in the crystal leads to drastic decrease in THz electric fields and undesirable modulation of the spectral shape with many dimples. When the generated THz waves exhibit strong absorption gaps with the distorted time trace, the applicability of THz waves is limited by additional parasitic effects and low signal-to-noise ratio at frequencies where the absorption dimples are located. Until now, it has been rarely reported how to effectively suppress the influence of phonon modes without changing intrinsic material properties of nonlinear organic crystals. One possible strategy was previously reported only by the change of chemical structures [4]. Since there is trade-off between suppression of phonon mode intensity and enhancement of macroscopic nonlinearity, this method is also limited for generation of efficient gap-free THz spectrum.

Abstract-Terahertz Phonon Mode Engineering of Highly Efficient Organic Terahertz Generators

Seung-Heon Lee, Bong Joo Kang, Ba-Wool Yoo, Seung-Chul Lee, Seung-Jun Lee, Mojca Jazbinsek, Hoseop Yun, Fabian Rotermund,

http://onlinelibrary.wiley.com/doi/10.1002/adfm.201605583/full

For terahertz (THz) wave generators based on organic electrooptic crystals, their intrinsic phonon modes are playing an essential role in THz generation characteristics. Here, this study proposes an effective design strategy for THz phonon mode engineering of organic electrooptic salt crystals for efficient optical-to-THz frequency conversion. To reduce phonon-mode intensity, strongly electronegative trifluoromethyl group acting as strong hydrogen-bond acceptor is incorporated into molecular anions. New 2-(4-hydroxy-3-methoxystyryl)-1-methylquinolinium 4-(trifluoromethyl)benzenesulfonate (HMQ-4TFS) crystals exhibit a relatively small absorption coefficient in the THz spectral range between 0.5 and 4 THz, which is attributed to suppressed molecular vibrations due to strong hydrogen bonds involving the 4TFS anion. In addition, HMQ-4TFS crystals possess a very large macroscopic optical nonlinearity, comparable (or even higher) to benchmark stilbazolium crystals. Based on the low-intensity THz phonon modes and the large optical nonlinearity, a 0.37 mm thick HMQ-4TFS crystal pumped with 150 fs infrared laser pulses facilitates very efficient THz wave generation by optical rectification, delivering 23 times higher peak-to-peak THz electric field than the widely used standard inorganic ZnTe crystal (1.0 mm thick) and a broader spectral bandwidth. Therefore, strongly electronegative groups introduced into molecular salt electrooptic crystals provide a very promising design strategy of THz phonon mode engineering for developing intense broadband THz sources.

Abstract-Terahertz Phonon Modes of Highly Efficient Electro-Optic Phenyltriene OH1 Crystals

Jongtaek Kim, Seung-Heon Lee, Seung-Chul Lee, Mojca Jazbinsek, Katsuhiko Miyamoto, Takashige Omatsu,Yoon Sup Lee, and O-Pil Kwon

J. Phys. Chem. C, Just Accepted Manuscript

DOI: 10.1021/acs.jpcc.6b07979

Publication Date (Web): October 4, 2016

Copyright © 2016 American Chemical Society

http://pubs.acs.org/doi/abs/10.1021/acs.jpcc.6b07979

Understanding the origin of phonon modes of highly efficient electro-optic crystals is very important for designing materials and for optimizing their photonic applications. Here we investigate the origin of phonon modes in the 0.1–15 THz range of the benchmark electro-optic OH1 (2-(3-(4-hydroxystyryl)-5,5-dimethylcyclohex-2-enylidene)malononitrile) crystal, interesting due to its large electro-optic coefficient and high THz-wave generation efficiency. The phonon modes (and vibrational absorption properties) of OH1 crystals are evaluated theoretically by periodic density functional theory and also experimentally by THz absorption spectroscopy. The theoretical calculations are well-matched with experimental results. The THz absorption properties are highly anisotropic; the amplitude of vibrational absorption is largest along the polar c-axis compared to the other two crystallographic axes. For comparison, vibrational absorption modes of the OH1 molecule in gas phase are also calculated. The calculated vibrational absorption spectrum of OH1 crystalline powder appears similar to that of the OH1 molecule in gas phase. However, the molecular vibrational motions in crystalline state are coupled motions of vibrational motions in gas phase. Interestingly, the vibrational mode of the torsion of the O-H bond with the largest absorption strength in gas phase is in crystal inhibited due to the crystal field effect. The origin of intense phonon modes of OH1 crystals is mainly related to relatively strong distortions of the push-pull -conjugated system including electron donor and acceptor groups.