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Terahertz conductivity of high-quality indium films deposited using a substrate cooling method
Choi Geunchang,Choi Dong-Hwan,Ryeom Junho,Park Dae Young,Kim Wu-Sin,Kim Ju-Jin,Bahk Young-Mi,Jeong Mun Seok 한국물리학회 2023 Current Applied Physics Vol.52 No.-
High-quality indium thin films were fabricated by substrate cooling method during the metal deposition. The cooling effect reduced the threshold thickness for insulator-to-metal transition, which originates from the interconnections between the indium nano-islands. The complex conductivities of the films were measured by terahertz time-domain spectroscopy. We showed that the conductivity of the indium films prepared by substrate cooling were ~10 times higher than that deposited at room temperature, even though thickness of the former was lower than that of the latter film. Our results suggest that substrate cooling is promising for the applications of metal thin films and contacts.
Evaluation of moisturizing cream using terahertz time-domain spectroscopy
Choi Geunchang,Lee Do-Hee,Park Ikpyeong,Kang Daecheol,Lee Hyun Kyung,Rhie Jiyeah,Bahk Young-Mi 한국물리학회 2022 Current Applied Physics Vol.39 No.-
Terahertz time-domain spectroscopy is a useful tool for material characterization because it provides both temporal and frequency domain information. In this work, we perform terahertz time-domain spectroscopy for evaluation of commercial moisturizing cream cosmetics. By measurement of terahertz transmission, the change of complex refractive index by exposure of the cosmetic samples in ambient condition is obtained. The tendency of terahertz transmission change is different from that of mass change, which implies that terahertz response becomes a new characterization tool for moisturizing cosmetics. Our results show that terahertz time-domain spectroscopy has potential application on cosmetic testing.
Terahertz Nanoprobing of Semiconductor Surface Dynamics
Choi, Geunchang,Bahk, Young-Mi,Kang, Taehee,Lee, Yoojin,Son, Byung Hee,Ahn, Yeong Hwan,Seo, Minah,Kim, Dai-Sik American Chemical Society 2017 NANO LETTERS Vol.17 No.10
<P>Most semiconductors have surface dynamics radically different from its bulk counterpart due to surface defect, doping level, and symmetry breaking. Because of the technical challenge of direct observation of the surface carrier dynamics, however, experimental studies have been allowed in severely shrunk structures including nanowires, thin films, or quantum wells where the surface-to-volume ratio is very high. Here, we develop a new type of terahertz (THz) nanoprobing system to investigate the surface dynamics of bulk semiconductors, using metallic nanogap accompanying strong THz field confinement. We observed that carrier lifetimes of InP and GaAs dramatically decrease close to the limit of THz time resolution (similar to 1 ps) as the gap size decreases down to nanoscale and that they return to their original values once the nanogap patterns are removed. Our THz narioprobing system will open up pathways toward direct and nondestructive measurements of surface dynamics of bulk semiconductors.</P>
Plasmon Enhanced Terahertz Emission from Single Layer Graphene
Bahk, Young-Mi,Ramakrishnan, Gopakumar,Choi, Jongho,Song, Hyelynn,Choi, Geunchang,Kim, Yong Hyup,Ahn, Kwang Jun,Kim, Dai-Sik,Planken, Paul C. M. American Chemical Society 2014 ACS NANO Vol.8 No.9
<P>We show that surface plasmons, excited with femtosecond laser pulses on continuous or discontinuous gold substrates, strongly enhance the generation and emission of ultrashort, broadband terahertz pulses from single layer graphene. Without surface plasmon excitation, for graphene on glass, ‘<I>nonresonant laser-pulse-induced photon drag currents</I>’ appear to be responsible for the relatively weak emission of both <I>s</I>- and <I>p</I>-polarized terahertz pulses. For graphene on a discontinuous layer of gold, only the emission of the <I>p</I>-polarized terahertz electric field is enhanced, whereas the <I>s</I>-polarized component remains largely unaffected, suggesting the presence of an additional terahertz generation mechanism. We argue that in the latter case, ‘<I>surface-plasmon-enhanced optical rectification</I>’, made possible by the lack of inversion symmetry at the graphene on gold surface, is responsible for the strongly enhanced emission. The enhancement occurs because the electric field of surface plasmons is localized and enhanced where the graphene is located: at the surface of the metal. We believe that our results point the way to small, thin, and more efficient terahertz photonic devices.</P><P><B>Graphic Abstract</B> <IMG SRC='http://pubs.acs.org/appl/literatum/publisher/achs/journals/content/ancac3/2014/ancac3.2014.8.issue-9/nn5025237/production/images/medium/nn-2014-025237_0007.gif'></P><P><A href='http://pubs.acs.org/doi/suppl/10.1021/nn5025237'>ACS Electronic Supporting Info</A></P>
Park Hajung,Kim Nahun,Yu Jaeyoung,Rhie Jiyeah,Choi Geunchang,Bahk Young-Mi 한국물리학회 2022 THE JOURNAL OF THE KOREAN PHYSICAL SOCIETY Vol.81 No.12
Drugs or foods in various forms, such as hydrate and anhydrous forms, can afect the diferent therapeutic efect, which is crucial to identify pseudo-polymorphic state. Cafeine, taken in various routes such as cofee, energy drinks, and drugs, has diferent characteristics in the form of cafeine hydrate or anhydrous cafeine. Here, we investigate a dehydration process of cafeine hydrate by monitoring temporal evolution of terahertz absorption spectra under various exposure times in ambient. It was observed that the main absorption peak of cafeine hydrate at 2.3 THz moves to 1.3 THz indicating anhydrous cafeine during the dehydration process. Moreover, we represent the dehydration process with a simple parameter by extracting the ratio between the absorption values at 2.3 THz and 1.3 THz corresponding to the main absorption resonances of cafeine hydrate and anhydrous cafeine, respectively. Our results demonstrate the feasibility of terahertz spectroscopy technique to monitor dehydration in real time during the manufacturing process in pharmaceutical or biological industries.
Control of optical nanometer gap shapes made via standard lithography using atomic layer deposition
Rhie, Jiyeah,Lee, Dukhyung,Bahk, Young-Mi,Jeong, Jeeyoon,Choi, Geunchang,Lee, Youjin,Kim, Sunghwan,Hong, Seunghun,Kim, Dai-Sik SPIE - International Society for Optical Engineeri 2018 Journal of Micro/nanolithography, MEMS, and MOEMS Vol.17 No.2