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Junsu Lee,Minwan Jung,Joonhoi Koo,Cheolhwan Chi,Ju Han Lee IEEE 2015 IEEE journal of selected topics in quantum electro Vol.21 No.1
<P>We experimentally demonstrate that a bulk-structured Bi2Te3 topological insulator (TI) film deposited on a side-polished fiber can act as an effective Q-switch for a 1.89-mu m laser. Our bulk-structured Bi2Te3 TI film with a thickness of similar to 31 mu m, was prepared using a mechanical exfoliation method, and the fabricated film was transferred onto a side-polished SNI2000 fiber to form a fiberized saturable absorber based on evanescent field interaction. By incorporating the saturable absorber into a thulium (Tm)-holmium (Ho) co-doped fiber-based ring cavity, it is shown that Q-switched pulses with a minimum temporal width of similar to 1.71 mu s can readily be produced at a vvavelength of 1.89 mu m. The output pulse repetition rate was tunable from similar to 11.54 nJ at a pump power of 250 mW. The output performance of our laser is completed to that of the depending on the pump power. The maximum output pulse energy was,--111.54 nJ at a pump power of 250 The output performance of our laser is compared to that of the 1.98- mu m Q-switched fiber laser based on a nanosheet-based Bi2Te3 TI demonstrated previously by Luo et al.</P>
Factors associated with suicide ideation among subway drivers in Korea
Junsu Byun,Hyoung-Ryoul Kim,Hye-Eun Lee,Se-Eun Kim,Jongin Lee 대한직업환경의학회 2016 대한직업환경의학회지 Vol.28 No.-
Background: There were several suicide events of subway drivers in Korea. The aim of this study is to explore work-related factors associated with suicide ideation among subway drivers. Methods: We analyzed data from 980 male subway drivers. A section of the Korean version of the Composite International Diagnostic Interview (K-CIDI 2.1) was administered by trained interviewers to judge whether a driver has suicide ideation and to diagnose psychiatric disorders. A questionnaire was also administered to collect data on sociodemographic characteristics, work environments, occupational stress, person under train (PUT) experience, and work-related problems. Occupational stress was examined by using the Korean Occupational Stress Scale (KOSS). Logistic regression was applied to evaluate the association between work-related factors and suicide ideation among subway drivers. Results: Regarding work-related problems, conflict with passengers and sudden stops due to the emergency bell were significantly associated with suicide ideation. MDD, PTSD, and panic disorder were strongly associated with suicide ideation. In the analysis of occupational stress, insufficient job control (OR 2.34) and lack of reward (OR 2.52) were associated with suicide ideation even after being adjusted for psychiatric disorders and other work-related factors. Conclusions: Insufficient job control and lack of reward were associated with suicide ideation among subway drivers. Strategies for drivers to have autonomy while working and to achieve effort-reward balance should be implemented. Furthermore, drivers who have experienced negative work-related problems should be managed appropriately.
Electronic properties of a graphene/periodic porous graphene heterostructure
Lee, Junsu,Kim, Gunn Elsevier 2017 Carbon Vol.122 No.-
<P>Recently, two-dimensional van der Waals materials such as graphene and hexagonal boron nitride have attracted interest as research topics. The two-dimensional material of polyphenylene superhoneycomb network (PSN) is similarly interesting because it is a type of periodic porous graphene. In this paper, we report a first-principles study of the geometric and electronic properties of vertical heterostructures comprising graphene and PSN. AA, AA', and AB stacking configurations of a graphene sheet on a PSN sheet produce band gaps of 63, 16, and 3 meV, respectively. We also determine the relationships between the band gap and the interlayer distance between the graphene and PSN sheets. Finally, we present computationally simulated scanning tunneling microscopy images, which indicate the local electronic structures of the surfaces of the graphene and PSN sides. (C) 2017 Elsevier Ltd. All rights reserved.</P>
Oblong-Shaped-Focused Transducers for Intravascular Ultrasound Imaging
Lee, Junsu,Jang, Jihun,Chang, Jin Ho IEEE 2017 IEEE Transactions on Biomedical Engineering Vol.64 No.3
<P>In intravascular ultrasound (IVUS) imaging, a transducer is inserted into a blood vessel and rotated to obtain image data. For this purpose, the transducer aperture is typically less than 0.5 mm in diameter, which causes natural focusing to occur in the imaging depth ranging from 1 to 5 mm. Due to the small aperture, however, it is not viable to conduct geometric focusing in order to enhance the spatial resolution of IVUS images. Furthermore, this hampers narrowing the slice thickness of a cross-sectional scan plane in the imaging depth, which leads to lowering spatial and contrast resolutions of IVUS images. To solve this problem, we propose an oblong-shaped-focused transducer for IVUS imaging. Unlike the conventional IVUS transducers with either a circular or a square flat aperture, the proposed transducer has an oblong aperture of which long side is positioned along a blood vessel. This unique configuration makes it possible to conduct geometric focusing at a desired depth in the elevation direction. In this study, furthermore, it is demonstrated that a spherically shaped aperture in both lateral and elevation directions also improves lateral resolution, compared to the conventional flat aperture. To ascertain this, the conventional and the proposed IVUS transducers were designed and fabricated to evaluate and to compare their imaging performances through wire phantom and tissue-mimicking phantom experiments. For the proposed 50-MHz IVUS transducer, a PZT piece of 0.5 × 1.0 mm<SUP>2</SUP> was spherically shaped for elevation focus at 3 mm by using the conventional press-focusing technique whereas the conventional one has a flat aperture of 0.5 × 0.5 mm<SUP>2</SUP>. The experimental results demonstrated that the proposed IVUS transducer is capable of improving spatial and contrast resolutions of IVUS images.</P>
Lee, Junsu,Koo, Joonhoi,Jhon, Young Min,Lee, Ju Han Optical Society of America 2015 Optics express Vol.23 No.5
<P>We experimentally demonstrate femtosecond harmonic mode-locking of a fiber laser using a bulk-structured Bi(2)Te(3) topological insulator (TI)-deposited on a side-polished fiber as a mode-locker. A bulk-structured Bi(2)Te(3) TI film was prepared at a thickness of ~20 μm using a mechanical exfoliation method. Using the mode-locker in an erbium-doped fiber ring cavity, it was experimentally shown that harmonically mode-locked pulses with temporal widths of 630 ~700 fs could readily be generated upto the 55th harmonics. The pulse repetition rate was shown to be tunable from the fundamental resonance frequency of 14.07 MHz to the harmonic frequency of 773.85 MHz with increasing pump power. The pumping efficiency was measured at ~3.36 MHz/mW. The side mode suppression ratio (SMSR) was observed to be more than 27.3 dB over all harmonic orders, while the corresponding signal-to-noise ratio (SNR) ranged from 46.3 to 63 dB.</P>
Lee, Junsu,Kim, Minjung,Chelikowsky, James R.,Kim, Gunn American Physical Society 2016 Physical Review B Vol.94 No.3
<P>Using ab initio density functional calculations, we predict subatomic-resolution atomic force microscopy (AFM) and scanning tunneling microscopy (STM) images of vertical heterostructures of graphene/hexagonal boron nitride (h-BN) with an intercalated metal atom (Li, K, Cr, Mn, Co, or Cu), and study the effects of the extrinsic metal defect on the interfacial coupling. We find that the structural deformation of the graphene/h-BN layer caused by the metal defect strongly affects the AFM images, whereas orbital hybridization between the metal defect and the graphene/h-BN layer characterizes the STM images.</P>