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-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-Giant phase retardation of terahertz waves by resonant hyperbolic metasurface

Seojoo Lee, WonTae Kim, Ji-Hun Kang, BongJoo Kang, Fabian Rotermund, Q-Han Park,

https://www.spiedigitallibrary.org/conference-proceedings-of-spie/10541/1054122/Giant-phase-retardation-of-terahertz-waves-by-resonant-hyperbolic-metasurface/10.1117/12.2288060.short?SSO=1

Due to the relatively weak birefringence of natural materials in terahertz regime, metasurfaces have been proposed for compact terahertz phase modulators since they show effectively strong birefringence only with ultrathin structures. However, previous designs of metasurface show limited phase modulation reaching only up to the quarter-wavelength phase, and there has been no single metasurface design that works for a terahertz half-waveplate. Here, we present a metasurface that modulates the phase variably up to 180 degrees. The phase modulation is achieved by a hyperbolic metasurface composed of periodically arrayed rectangular metal rings with different periods for horizontal and vertical axis. By controlling each period, we show that our hyperbolic metasurface can possess large positive and negative permittivity values for horizontal and vertical axis and the phase shift can reach up to the 180 degrees. To check the validity of our design, we fabricate reconfigurable metasurface films and demonstrate the phase modulation 90 to 180 degrees. All results show good agreement with numerical simulation results.

© (2018) COPYRIGHT Society of Photo-Optical Instrumentation Engineers (SPIE). Downloading of the abstract is permitted for personal use only.

Abstract-Linear frequency conversion via sudden merging of resonances in time-variant terahertz metasurfaces

Kanghee Lee,   Jaehyeon Son,   Byungsoo Kang,   Jagang Park,  Fabian Rotermund,  Bumki Min

http://ieeexplore.ieee.org/document/8067111/


We experimentally demonstrate linear frequency conversion on time-variant terahertz (THz) metasurface. The proposed metasurface is originally designed to have two resonances, and with optical pumping, the resonances are merged into one. When a THz pulse passes through the metasurface during this merging process, a frequency component corresponding to the merged mode is generated.

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-Terahertz optical bistability of graphene in thin layers of dielectrics

Kwang Jun Ahn and Fabian Rotermund

https://www.osapublishing.org/oe/abstract.cfm?uri=oe-25-8-8484

We theoretically studied in terahertz frequency regime optical bistability of graphene placed at the interface between thin dielectric layers. We solved self-consistently the nonlinear wave equations containing the third-order optical conductivity of graphene in four-layer structures and obtained hysteresis response of the transmitted power as a function of the incident power. We numerically observed that the critical powers for the up and down transitions and the Fermi-energy of graphene required for terahertz optical bistability can be reduced by carefully choosing material properties and the thicknesses of dielectric layers. Furthermore, these values can be substantially decreased when graphene as a randomly stacked multilayer structure is asymmetrically located in thin dielectric layers.

© 2017 Optical Society of America

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-Control of terahertz nonlinear transmission with electrically gated graphene metadevices

• Hyun Joo Choi
• , In Hyung Baek
• , Bong Joo Kang
• , Hyeon-Don Kim
• , Sang Soon Oh
• , Joachim M. Hamm
• , Andreas Pusch
• , Jagang Park
• , Kanghee Lee
• , Jaehyeon Son
• , Young U. k. Jeong
• , Ortwin Hess
• , Fabian Rotermund
•  & Bumki Min

http://www.nature.com/articles/srep42833?WT.feed_name=subjects_materials-science

Graphene, which is a two-dimensional crystal of carbon atoms arranged in a hexagonal lattice, has attracted a great amount of attention due to its outstanding mechanical, thermal and electronic properties. Moreover, graphene shows an exceptionally strong tunable light-matter interaction that depends on the Fermi level - a function of chemical doping and external gate voltage - and the electromagnetic resonance provided by intentionally engineered structures. In the optical regime, the nonlinearities of graphene originated from the Pauli blocking have already been exploited for mode-locking device applications in ultrafast laser technology, whereas nonlinearities in the terahertz regime, which arise from a reduction in conductivity due to carrier heating, have only recently been confirmed experimentally. Here, we investigated two key factors for controlling nonlinear interactions of graphene with an intense terahertz field. The induced transparencies of graphene can be controlled effectively by engineering meta-atoms and/or changing the number of charge carriers through electrical gating. Additionally, nonlinear phase changes of the transmitted terahertz field can be observed by introducing the resonances of the meta-atoms.

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

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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.

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