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Kim, Sung Kyun,Bhatia, Ravi,Kim, Tae-Ho,Seol, Daehee,Kim, Jung Ho,Kim, Hyun,Seung, Wanchul,Kim, Yunseok,Lee, Young Hee,Kim, Sang-Woo Elsevier 2016 Nano energy Vol.22 No.-
<P><B>Abstract</B></P> <P>Due to the interesting semiconducting and optical properties of transition metal dichalcogenides, they have received particular attention for novel electronics and optoelectronics. In addition it is expected that piezoelectric properties of two-dimensional (2D) layered materials are very useful to realize next generation mechanically powered transparent flexible charge-generating devices. Here we report directional dependent piezoelectric effects in chemical vapor deposition grown monolayer MoS<SUB>2</SUB> for flexible piezoelectric nanogenerators (NGs). It was found that the output power obtained from the NG with the armchair direction of MoS<SUB>2</SUB> is about two times higher than that from the NG with the zigzag direction of MoS<SUB>2</SUB> under the same strain of 0.48% and the strain velocity of 70mm/s. This study provides a new way to effectively harvest mechanical energy using novel flexible piezoelectric NGs based on 2D semiconducting piezoelectric MoS<SUB>2</SUB> for powering low power-consuming electronics and realizing self-powered sensors.</P> <P><B>Highlights</B></P> <P> <UL> <LI> Directional dependent piezoelectric effects in chemical vapor deposition (CVD) grown monolayer MoS<SUB>2</SUB> for flexible piezoelectric nanogenerators (NGs). </LI> <LI> Lateral piezoresponse force microscopy measurements revealed that the magnitude of the piezoelectric polarization in monolayer MoS<SUB>2</SUB> significantly depends on the atomic orientation axis of MoS<SUB>2</SUB>. </LI> <LI> Flexible piezoelectric NGs were successfully fabricated using the CVD-grown single-crystalline monolayer MoS<SUB>2</SUB> flakes. </LI> <LI> The output power obtained from the NG with the armchair direction of MoS<SUB>2</SUB> is about two times higher than the zigzag direction of MoS<SUB>2</SUB>. </LI> <LI> New effectively harvest mechanical energy using novel flexible piezoelectric NGs based on 2D semiconducting piezoelectric MoS<SUB>2</SUB> for powering low power-consuming devices and realizing self-powered electronics. </LI> </UL> </P> <P><B>Graphical abstract</B></P> <P>We report directional dependent piezoelectric effects in chemical vapor deposition grown monolayer MoS<SUB>2</SUB> for flexible piezoelectric nanogenerators. The output power obtained from the nanogenerator with the armchair direction of MoS<SUB>2</SUB> is about two times higher than that from the nanogenerator with the zigzag direction of MoS<SUB>2</SUB> under the same strain.</P> <P>[DISPLAY OMISSION]</P>
Confidence limits for patient‐specific IMRT dose QA: a multi‐institutional study in Korea
Kim, Jung‐,in,Chung, Jin‐,Beom,Song, Ju‐,Young,Kim, Sung Kyu,Choi, Yunseok,Choi, Chang Heon,Choi, Won Hoon,Cho, Byungchul,Kim, Jin Sung,Kim, Sung Jin,Ye, Sung‐,Joon John Wiley and Sons Inc. 2016 Journal of applied clinical medical physics Vol.17 No.1
<P>This study aims to investigate tolerance levels for patient‐specific IMRT dose QA (DQA) using the confidence limits (CL) determined by a multi‐institutional study. Eleven institutions participated in the multi‐institutional study in Korea. A total of 155 DQA measurements, consisting of point‐dose differences (high‐ and low‐dose regions) and gamma passing rates (composite and per‐field) for IMRT patients with brain, head and neck (H&N), abdomen, and prostate cancers were examined. The Shapiro‐Wilk test was used to evaluate the normality of data grouped by the treatment sites and the DQA methods. The confidence limit coefficients in cases of the normal distribution, and the two‐sided Student's <I>t</I>‐distribution were applied to determine the confidence limits for the grouped data. The Spearman's test was applied to assess the sensitivity of DQA results within the limited groups. The differences in CLs between the two confidence coefficients based on the normal and <I>t</I>‐distributions were negligible for the point‐dose data and the gamma passing rates with 3%/3 criteria. However, with 2%/2 criteria, the difference in CLs were 1.6% and 2.2% for composite and per‐field measurements, respectively. This resulted from the large standard deviation and the more sensitive criteria of 2%/2. There was no noticeable correlation among the different QA methods. Our multi‐institutional study suggested that the CL was not a suitable metric for defining the tolerance level when the statistics of the sample group did not follow the normality and had a large standard deviation.</P><P>PACS number: 87.55.Qr</P>
Er-doped fiber frequency comb with mHz relative linewidth
Kim, Yunseok,Kim, Seungman,Kim, Young-Jin,Hussein, Hatem,Kim, Seung-Woo The Optical Society 2009 Optics express Vol.17 No.14
<P>A low-noise fiber frequency comb is demonstrated to improve the frequency accuracy and linewidth by suppressing the phase noise caused by the nonlinear self-phase modulation as well as the amplified spontaneous emission within the Er-doped fiber amplifier. The linewidth of the carrier-envelop- offset signal measures less than 1.9 mHz and the frequency stability well follows the reference Rb clock. This achievement will facilitate the use of the fiber frequency comb for industrial applications to precision near-infrared spectroscopy, frequency calibration, optical clocks and length metrology.</P>
Er-doped fiber comb with enhanced f_ceo S/N ratio using Tm:Ho-doped fiber
Kim, Yunseok,Kim, Young-Jin,Kim, Seungman,Kim, Seung-Woo The Optical Society 2009 Optics express Vol.17 No.21
<P>We report that the Tm:Ho-doped fiber can be utilized to improve the frequency stabilization of the Er-doped fiber comb. This rare-earth doped fiber provides photon absorption at 1.2 microm and 1.7 microm wavelengths together with emission at wavelengths between 1.8 microm to 2.1 microm. This unique combination of the absorption and emission regions constructively redistributes the spectral power of the supercontinuum generated by a highly nonlinear fiber to detect the carrier-envelope-offset frequency (f(ceo)) via a self-referencing f-2f interferometer. As a result, the signal to noise (S/N) ratio of the detected f(ceo) signal increases by 10 dB, thereby increasing the potential of enhancing the long-term frequency stability of the fiber frequency comb.</P>
High-harmonic generation by resonant plasmon field enhancement
Kim, Seungchul,Jin, Jonghan,Kim, Young-Jin,Park, In-Yong,Kim, Yunseok,Kim, Seung-Woo Nature Publishing Group 2008 Nature Vol.453 No.7196
High-harmonic generation by focusing a femtosecond laser onto a gas is a well-known method of producing coherent extreme-ultraviolet (EUV) light. This nonlinear conversion process requires high pulse intensities, greater than 10<SUP>13</SUP> W cm<SUP>-2</SUP>, which are not directly attainable using only the output power of a femtosecond oscillator. Chirped-pulse amplification enables the pulse intensity to exceed this threshold by incorporating several regenerative and/or multi-pass amplifier cavities in tandem. Intracavity pulse amplification (designed not to reduce the pulse repetition rate) also requires a long cavity. Here we demonstrate a method of high-harmonic generation that requires no extra cavities. This is achieved by exploiting the local field enhancement induced by resonant plasmons within a metallic nanostructure consisting of bow-tie-shaped gold elements on a sapphire substrate. In our experiment, the output beam emitted from a modest femtosecond oscillator (100-kW peak power, 1.3-nJ pulse energy and 10-fs pulse duration) is directly focused onto the nanostructure with a pulse intensity of only 10<SUP>11</SUP> W cm<SUP>-2</SUP>. The enhancement factor exceeds 20 dB, which is sufficient to produce EUV wavelengths down to 47 nm by injection with an argon gas jet. The method could form the basis for constructing laptop-sized EUV light sources for advanced lithography and high-resolution imaging applications.
Hybrid mode-locked Er-doped fiber femtosecond oscillator with 156 mW output power.
Kim, Seungman,Kim, Yunseok,Park, Jiyong,Han, Seunghwoi,Park, Sanguk,Kim, Young-Jin,Kim, Seung-Woo Optical Society of America 2012 Optics express Vol.20 No.14
<P>We report on an Er-doped fiber oscillator that produces 146 fs pulses with 156 mW average power at a repetition rate of 49.9 MHz. The pulse energy reaches 3.13 nJ, surpassing the conventional power limit in the dispersion-managed soliton regime. Such high pulse power is obtained by devising a hybrid mode-locking scheme that combines saturable absorption with nonlinear polarization evolution. The oscillator also offers excellent temporal purity in the generated pulses with high power, providing a robust fiber-based frequency comb well suited for industrial uses.</P>