DSpace Community: KAIST College of Natural SciencesKAIST College of Natural Scienceshttp://hdl.handle.net/10203/112017-03-24T09:33:53Z2017-03-24T09:33:53ZColoring graphs without fan vertex-minors and graphs without cycle pivot-minorsChoi, IlkyooKwon, O-JoungOum, Sang-ilhttp://hdl.handle.net/10203/2204332017-03-22T02:52:34Z2017-03-01T00:00:00ZTitle: Coloring graphs without fan vertex-minors and graphs without cycle pivot-minors
Authors: Choi, Ilkyoo; Kwon, O-Joung; Oum, Sang-il
Abstract: A fan F-k is a graph that consists of an induced path on k vertices and an additional vertex that is adjacent to all vertices of the path. We prove that for all positive integers q and k, every graph with sufficiently large chromatic number contains either a clique of size q or a vertex-minor isomorphic to F-k. We also prove that for all positive integers q and k >= 3, every graph with sufficiently large chromatic number contains either a clique of size q or a pivot-minor isomorphic to a cycle of length k. (C) 2016 Elsevier Inc. All rights reserved.2017-03-01T00:00:00ZPhotoconductive dipole antennas for efficient terahertz receiverKim, Won TaePark, IkmoBark, Hyeon SangRotermund, Fabian MarcelKang, Bong JooJeon, Tae-InNguyen, Truong KhangLee, JaejinKim, Kanghohttp://hdl.handle.net/10203/2145822017-03-16T00:46:59Z2017-01-01T00:00:00ZTitle: Photoconductive dipole antennas for efficient terahertz receiver
Authors: Kim, Won Tae; Park, Ikmo; Bark, Hyeon Sang; Rotermund, Fabian Marcel; Kang, Bong Joo; Jeon, Tae-In; Nguyen, Truong Khang; Lee, Jaejin; Kim, Kangho
Abstract: We designed various photoconductive antennas applicable to efficient terahertz (THz) receivers and experimentally investigated their detection characteristics. Three different antennas based on Grischkowsky (H-), I-, and bowtie shapes were fabricated on a 1.2- mu m-thick low-temperature GaAs layer that was grown on a semi-insulating GaAs substrate and subsequently attached to extended hemispherical silicon lenses. The experimental results showed different characteristics for detection responsivity and agreed well with the theoretical prediction. Measurements of the peak-to-peak amplitudes of the detected THz photocurrent were approximately 67, 42, and 59 nA for the H-shaped, I-shaped, and bowtie-shaped antennas, respectively. The I- and bowtie-shaped antennas provided higher THz detection sensitivities than the H-shaped antenna in the low-frequency region, i.e., below 0.6 THz. At a frequency of 0.2 THz, the I- and bowtie-shaped antennas offered an approximately 3.6-fold and 6-fold enhancement, respectively, in THz detection sensitivity compared to the H-shaped antenna. The bowtie-shaped antenna produced better peak amplitude and a wider spectral bandwidth than the I-shaped antenna. The observed detection peak frequencies of the I-shaped and bowtie-shaped antennas possessing very long dipole arms indicate that the lowest limit of the frequency detected in a typical THz-TDS using a GaAs photoconductive antenna as emitter/detector is around 0.2 THz. (C) 2016 Elsevier B.V All rights reserved2017-01-01T00:00:00ZCollaborative effects of wavefront shaping and optical clearing agent in optical coherence tomographyLee, Peter JaehyunJo, Young JuJeong, YongYu, HyeonseungPark, Yong KeunTuchin, ValeryLee, Kyeo Rehhttp://hdl.handle.net/10203/2147882017-03-16T00:49:21Z2016-12-01T00:00:00ZTitle: Collaborative effects of wavefront shaping and optical clearing agent in optical coherence tomography
Authors: Lee, Peter Jaehyun; Jo, Young Ju; Jeong, Yong; Yu, Hyeonseung; Park, Yong Keun; Tuchin, Valery; Lee, Kyeo Reh
Abstract: We demonstrate that simultaneous application of optical clearing agents (OCAs) and complex wavefront shaping in optical coherence tomography (OCT) can provide significant enhancement of penetration depth and imaging quality. OCA reduces optical inhomogeneity of a highly scattering sample, and the wavefront shaping of illumination light controls multiple scattering, resulting in an enhancement of the penetration depth and signal-to-noise ratio. A tissue phantom study shows that concurrent applications of OCA and wavefront shaping successfully operate in OCT imaging. The penetration depth enhancement is further demonstrated for ex vivo mouse ears, revealing hidden structures inaccessible with conventional OCT imaging.2016-12-01T00:00:00ZConstruction of class fields over cyclotomic fieldsYoon, Dong SungKoo, Ja-Kyunghttp://hdl.handle.net/10203/2145602017-03-17T05:47:02Z2016-12-01T00:00:00ZTitle: Construction of class fields over cyclotomic fields
Authors: Yoon, Dong Sung; Koo, Ja-Kyung
Abstract: Let l and p be odd primes. For a positive integer mu, let k(mu), be the ray class field of k = Q(e(2 pi i/l)) modulo 2p(mu). We present certain class fields K-mu, of k such that k(mu) subset of K mu subset of k(mu+1), and we provide a necessary and sufficient condition for K-mu = k(mu+1). We also construct, in the sense of Hilbert, primitive generators of the field K-mu over k(mu) by using Shimura's reciprocity law and special values of theta constants.2016-12-01T00:00:00Z