Abstract-Terahertz wave interaction with metallic nanostructures

Ji-Hun Kang, Dai-Sik Kim,  Minah Seo,

https://www.degruyter.com/downloadpdf/j/nanoph.2018.7.issue-5/nanoph-2017-0093/nanoph-2017-0093.pdf

Understanding light interaction with metallic structures provides opportunities of manipulation of light, and is at the core of various research areas including terahertz (THz) optics from which diverse applications are now emerging. For instance, THz waves take full advantage of the interaction to have strong field enhancement that compensates their relatively low photon energy. As the THz field enhancement have boosted THz nonlinear studies and relevant applications, further understanding of light interaction with metallic structures is essential for advanced manipulation of light that will bring about subsequent development of THz optics. In this review, we discuss THz wave interaction with deep sub-wavelength nano structures. With focusing on the THz field enhancement by nano structures, we review fundamentals of giant field enhancement that emerges from non-resonant and resonant interactions of THz waves with nano structures in both sub- and super- skin-depth thicknesses. From that, we introduce surprisingly simple description of the field enhancement valid over many orders of magnitudes of conductivity of metal as well as many orders of magnitudes of the metal thickness. We also discuss THz interaction with structures in angstrom scale, by reviewing plasmonic quantum effect and electron tunneling with consequent nonlinear behaviors. Finally, as applications of THz interaction with nano structures, we introduce new types of THz molecule sensors, exhibiting ultrasensitive and highly selective functionalities

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-Ultrasensitive terahertz sensing of gold nanoparticles inside nano slot antennas

Yong-Sang Ryu, Dong-Kyu Lee, Ji-Hun Kang, Sang-Hun Lee, Eui-Sang Yu, and Minah Seo

https://www.osapublishing.org/oe/abstract.cfm?uri=oe-25-24-30591

We introduce a robust control method of terahertz (THz) transmission by tuning filling factors of Au nanoparticles (AuNPs) inside nano slot antennas. AuNPs in sub-100 nm diameters were spread over the nano slot antennas, followed by sweeping them into the slots. AuNPs can be efficiently localized and inserted into nano slots where the THz fields are greatly enhanced, by a “squeegee” made of the polydimethylsiloxane (PDMS). The sweeping of the AuNPs results in further dramatic reduction of THz transmission by suppressing the fundamental resonance mode of the nano slot, as compared to a typical random dropping case. It definitely works for an accurate THz transmission control, as well as the removal of unwanted ions that occasionally confuse signal accuracy from the target signals. Our approach provides a complete reinterpretation of sample deposition for further steady demands in developing ultrasensitive terahertz (THz) molecule sensors.

© 2017 Optical Society of America under the terms of the OSA Open Access Publishing Agreement

Abstract-Highly sensitive and selective sugar detection by terahertz nano-antennas

• Dong-Kyu Lee
• , Ji-Hun Kang
• , Jun-Seok Lee
• , Hyo-Seok Kim
• , Chulki Kim
• , Jae Hun Kim
• , Taikjin Lee
• ,Joo-Hiuk Son
• , Q-Han Park
•  & Minah Seo

http://www.nature.com/articles/srep15459

Molecular recognition and discrimination of carbohydrates are important because carbohydrates perform essential roles in most living organisms for energy metabolism and cell-to-cell communication. Nevertheless, it is difficult to identify or distinguish various carbohydrate molecules owing to the lack of a significant distinction in the physical or chemical characteristics. Although there has been considerable effort to develop a sensing platform for individual carbohydrates selectively using chemical receptors or an ensemble array, their detection and discrimination limits have been as high in the millimolar concentration range. Here we show a highly sensitive and selective detection method for the discrimination of carbohydrate molecules using nano-slot-antenna array-based sensing chips which operate in the terahertz (THz) frequency range (0.5–2.5 THz). This THz metamaterial sensing tool recognizes various types of carbohydrate molecules over a wide range of molecular concentrations. Strongly localized and enhanced terahertz transmission by nano-antennas can effectively increase the molecular absorption cross sections, thereby enabling the detection of these molecules even at low concentrations. We verified the performance of nano-antenna sensing chip by both THz spectra and images of transmittance. Screening and identification of various carbohydrates can be applied to test even real market beverages with a high sensitivity and selectivity.

Abstract-Observation of terahertz-radiation-induced ionization in a single nano island

Minah Seo,

Ji-Hun Kang,

Hyo-Suk Kim,

Joon Hyong Cho,

Jaebin Choi,

Young Min Jhon,

Seok Lee,

Jae Hun Kim,

Taikjin Lee,

Q-Han Park

& Chulki Kim

http://www.nature.com/srep/2015/150522/srep10280/full/srep10280.html

Terahertz (THz) electromagnetic wave has been widely used as a spectroscopic probe to detect the collective vibrational mode in vast molecular systems and investigate dielectric properties of various materials. Recent technological advances in generating intense THz radiation and the emergence of THz plasmonics operating with nanoscale structures have opened up new pathways toward THz applications. Here, we present a new opportunity in engineering the state of matter at the atomic scale using THz wave and a metallic nanostructure. We show that a medium strength THz radiation of 22 kV/cm can induce ionization of ambient carbon atoms through interaction with a metallic nanostructure. The prepared structure, made of a nano slot antenna and a nano island located at the center, acts as a nanogap capacitor and enhances the local electric field by two orders of magnitudes thereby causing the ionization of ambient carbon atoms. Ionization and accumulation of carbon atoms are also observed through the change of the resonant condition of the nano slot antenna and the shift of the characteristic mode in the spectrum of the transmitted THz waves