Abstract-Electrically Controllable Terahertz Second‐Harmonic Generation in GaAs

Joo Kang, Won Tae Kim, Hyeon‐Don Kim, Soojeong Baek, Kwang Jun Ahn, Bumki Min,Fabian Rotermund

https://onlinelibrary.wiley.com/doi/abs/10.1002/adom.202000359?af=R

Terahertz radiation and its nonlinear optical manipulation may possess potential for a variety of applications in next‐generation electronics and optics. Pioneering studies have shown that the nonlinearity of carrier drift in semiconductors and graphene can be utilized for nonlinear optical processes at terahertz frequencies. However, because of the symmetric response of carriers to the terahertz field direction, most experiments have confirmed only the presence of odd‐order nonlinear processes. In this study, electric‐field‐induced terahertz second‐harmonic generation (SHG) in photoexcited gallium arsenide is demonstrated, where an applied bias field breaks the directional symmetry of the drift transport of electrons. The amplitudes of odd‐ and even‐harmonic waves are found to be highly controllable using the bias field. The measured conversion efficiency of SHG reaches beyond 10−5, substantially higher than the value previously reported. This terahertz harmonic generation platform with electrical controllability may be useful for future nonlinear applications at terahertz frequencies.

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-Electrical control of terahertz frequency conversion from time-varying surfaces

Kanghee Lee, Jagang Park, Jaehyeon Son, Bong Joo Kang, Won Tae Kim, Seong Cheol Lee, Bumki Min,  Fabian Rotermund,

Fig. 1 Experimental information. (a) Schematic of experimental setup. (b) Spectrum of single-cycle THz waves generated from LiNbO3 crystal with waveform in time domain (inset). (c) Spectrum of multi-cycle THz waves modulated through stacked band-pass filters with waveform in time domain (inset).

https://www.osapublishing.org/oe/abstract.cfm?uri=oe-27-9-12762

We investigate the electrical control of frequency conversion from a time-varying interdigitated photo-conductive antenna (IPCA) and time-varying metasurface in the terahertz (THz) frequency range. Ultrafast near-infrared (NIR) optical pulses rapidly modify the conductivities of the IPCA and metasurface; however, external voltages can retard this conductivity transition. Thus, external voltages can be used to control the frequency conversion process based on the interaction between the THz waves and the time-varying surfaces. In the IPCA, both frequency up- and down-conversion processes are suppressed by external voltages. However, in the metasurface, the down-conversion is dramatically suppressed by external voltages, whereas the suppression on the up-conversion is less effective.

© 2019 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-Electrically Controlled Second-Order Nonlinear Generation of Terahertz Waves

Kanghee Lee, Jagang Park, Bong Joo Kang, Won Tae Kim, Bumki Min, and Fabian Rotermund
https://www.osapublishing.org/abstract.cfm?uri=NLO-2017-NW3A.2

We report electric-field-induced second-harmonic generation of terahertz waves in photodoped gallium arsenide with direct-current bias fields. A fundamental wave of 0.6 THz was selectively incident and its second-harmonic wave of 1.2 THz was clearly observed.

© 2017 OSA

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

Bong Joo Kang, In Hyung Baek, Seung-Heon Lee, Won Tae Kim, Seung-Jun Lee, Young Uk Jeong, O-Pil Kwon, and Fabian Rotermund
https://www.osapublishing.org/oe/abstract.cfm?uri=oe-24-10-11054

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.

© 2016 Optical Society of America

Full Article  |  PDF Article

Abstract-Electromagnetic Saturation of Angstrom-Sized Quantum Barriers at Terahertz Frequencies

Young-Mi Bahk, Bong Joo Kang, Yong Seung Kim, Joon-Yeon Kim, Won Tae Kim, Tae Yun Kim, Taehee Kang, Jiyeah Rhie, Sanghoon Han, Cheol-Hwan Park, Fabian Rotermund, and Dai-Sik Kim

Phys. Rev. Lett. 115, 125501 – Published 16 September 2015

http://journals.aps.org/prl/abstract/10.1103/PhysRevLett.115.125501

Metal-graphene-metal hybrid structures allow angstrom-scale van der Waals gaps, across which electron tunneling occurs. We squeeze terahertz electromagnetic waves through these λ/10 000 000 gaps, accompanied by giant field enhancements. Unprecedented transmission reduction of 97% is achieved with the transient voltage across the gap saturating at 5 V. Electron tunneling facilitated by the transient electric field strongly modifies the gap index, starting a self-limiting process related to the barrier height. Our work enables greater interplay between classical optics and quantum tunneling, and provides optical indices to the van der Waals gaps.

• 
• 
•