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Publications

Conferences

Study on the beam-wave interaction in sine waveguide

2018 IEEE International Vacuum Electronics Conference, IVEC 2018

Lei X., Li Q., Wu G., Ding C., Wei Y., Yin H., Guo G., Wang W., Park G.

Abstract
A theoretical model for analyzing beam-wave interaction in sine waveguide (SWG) is proposed. The 'hot' dispersion equation of the beam with arbitrary thickness is obtained by regarding the electron beam as a moving plasma dielectric. Moreover, the formula of gain growth rate is carried out. © 2018 IEEE.
Hydrothermally prepared reduced graphene oxide free standing film as high curren...

2019 International Vacuum Electronics Conference, IVEC 2019

Hong D., Kim S., Hossain M.M., Min S.-H., Sattorov M., Park G.-S.

Abstract
Uniform, highly conductive, thin film field emitters are in immense need for future development of high power compact terahertz(THz) vacuum electronic devices(VED). Reduced graphene oxide(rGO) based free standing film as high current sheet beam cathode is fabricated by hydrothermal method. This hydrothermal method facilitates highly conductive, thin, compact film. We achieved to get few micrometers to sub-micrometer thick film which is much thinner than conventional vacuum filtered film resulting in higher field enhancement factor, current density, and current. This approach can successfully establish a fabrication method for uniform, thin high current field emitter for high power THz VEDs. © 2019 IEEE.
Enhanced radiation using Cerenkov effect in Fano metamerical

2019 International Vacuum Electronics Conference, IVEC 2019

Kim S., Hong D., Sattorov M., Hossain M.M., Min S.-H., Park G.-S.

Abstract
The Fano metamaterial provides a way of trapping and releasing electromagnetic waves in microwave and terahertz regime. We show that, by utilizing the electromagnetic properties, the efficiency of radiation power from the Cerenkov effect in our recently proposed metallic metamaterial is much larger than the one from the ordinary Cerenkov and Smith-Purcell effects. The optimization of the efficiency was numerically conducted by manipulating the quality(Q) factor of the trapped Cerenkov light. The proposed metamaterial is suitable for developing compact and highly efficient free electron lasers. © 2019 IEEE.
Alignment of Carbon Nanotubes inside the Fibers Through Interfacial Interaction ...

2019 International Vacuum Electronics Conference, IVEC 2019

Hossain M.M., Hong D., Sattorov M., Kim S., Park G.-S.

Abstract
Alignment of carbon nanotubes (CNTs) inside the CNT fiber is very important for the fast electron movement through CNTs. We utilized nanoparticles, such as ZnO for the alignment of CNTs and described how semiconducting ZnO can act as an alignment agent in carbon nanotubes (CNTs) fibers. Due to the alignment of CNTs through the ZnO nanoparticles linking groups, the CNTs inside the fibers were equally distributed by the attraction of bonding forces into sheet-like bunches, such that any applied mechanical breaking load was equally distributed to each CNT inside the fiber, making them mechanically robust against breaking loads. Although semi-conductive ZnO nanoparticles were used here, the electrical conductivity of the aligned CNT fiber was comparable to bare CNT fibers, suggesting that the total electron movement through the CNTs inside the aligned CNT fiber is not disrupted by the insulating behavior of ZnO nanoparticles. Due to the high electrical, mechanical, and thermal properties of CNT fiber, it is a good candidate as field emitter. For using CNT fiber as cathode for sheet beam, CNT fiber further was converted to the sheet structure, which are more important for getting sheet. © 2019 IEEE.
High-power THz Wave Generation through Coherent Cherenkov Radiation based on ...

2019 International Vacuum Electronics Conference, IVEC 2019

Min S.-H., Kwon O., Sattorov M., Kim S., Hong D., Park C., Cho I., Hong B.H., Jung I.S., Hwang W.T., Park G.-S.

Abstract
Generally, as the operating frequency of the electromagnetic wave increases, the maximum output becomes smaller and the wavelength of the wave becomes smaller, so that the size of the circuit cannot but be reduced. Particularly, fabrication of a circuit with a high-power terahertz (THz) wave frequency band of kW or more is limited due to the problem of circuit size on the order of μm to mm. In order to overcome these limitations, this paper proposes a source design technique of 0.1THz-0.3GW levels with a cylindrical shape with a diameter of about 2.4cm. Modeling and computational simulation were performed to optimize the design of high power electromagnetic sources based on the Cherenkov radiation generation technology using the principle of plasma wake-field acceleration with ponderomotive force and artificial dielectrics. An effective design guideline has been proposed to facilitate the fabrication of large-power terahertz wave vacuum devices of large diameter that are less restricted in circuit size through objective verification. © 2019 IEEE.
Collective Dynamics of Nanoconfined Water in Phospholipid Multilamellar Vesicles...

International Conference on Infrared, Millimeter, and Terahertz Waves, IRMMW-THz

Eom K., Jang J., Kim S., Park G.-S.

Abstract
We present the first experimental characterization, using GHz to THz dielectric relaxation spectroscopy, of the collective rearrangement of the hydrogen bond network of water molecules nanoscopically confined in phospholipid multilamellar vesicles. We determined a distinct relaxation peak at 40 GHz and the Kirkwood correlation factor of approximately 1.2, which are the indication of acceleration of the collective reorientation dynamics with nanoconfinement, and strong disruption of the hydrogen bond network compared with that of bulk water. Combined with X-ray observation, it has estimated that 70% of the nanoconfined water contributes to the accelerated relaxation mode. © 2019 IEEE.
Hydration numbers from ab initio water reorientation dynamics

International Conference on Infrared, Millimeter, and Terahertz Waves, IRMMW-THz

Kim S., Jang J., Eom K., Tommaso D.D., Park G.-S.

Abstract
We present a novel approach to determine the hydration number of aqueous electrolytes by means of ab initio molecular dynamics. An hydration status analysis is devised to quantify the cooperative effect of ions on the reorientational dynamics of different water subpopulations in electrolyte solutions. The methodology is applied to predict the hydration numbers, h, as the number moles of water molecules per mole of dissolved salt that no longer participate in bulk-like reorientational dynamics. The obtained hydration number of MgCl2 (h = 15) is consistent with THz-DR experiment, measuring reorientational dynamics of water in solutions. By providing a link with the micro-scale dynamical behaviour of ions and water molecules, this approach represents a generally applicable, welldefined methodology to quantify hydration numbers of ions and molecules in aqueous solutions. © 2019 IEEE.
Controlled Hydration in Epidermal Ridges Probed by THz Time-Domain Spectroscopy

International Conference on Infrared, Millimeter, and Terahertz Waves, IRMMW-THz

Yum S.-M., Baek I.-K., Hong D., Kim S., Eom K., Jang J., Kim S., Sattorov M., Lee M.-G., Kim S., Adams M., Park G.-S.

Abstract
Fingerprints are common to humans, primates, and koalas but how their role in grip activities is poorly understood. Here, we reveal that the fingerprints control the hydration level of the fingertip, as required for precision gripping, by ultimately maximizing the friction. Electromagnetic waves with frequencies in the megahertz, terahertz, infrared, and visible ranges were used to identify the hydrodynamics in fingerprints, which lead to the steady-state hydration condition in 'dry' and 'wet' conditions. The results suggest that the fingerprint structure functions as a moisture channel for facilitating precision grip. © 2019 IEEE.
High-Q THz Fano metamaterial Interacting with Vacuum Electron

International Conference on Infrared, Millimeter, and Terahertz Waves, IRMMW-THz

Hong D., Kim S., Sattorov M., Hossain M.M., Min S.-H., Park G.-S.

Abstract
By manipulating Fano resonance in metallic metamaterial, trapping and releasing of electromagnetic wave can be controlled. Simply, by varying the size of structural asymmetry to the proposed metallic slit structure, Q factor of trapped mode in metallic metamaterial can be tailored. We show that optimizing the electromagnetic property of Fano metamaterial can maximizes the interaction between convection electron and trapped mode, which gives significantly high efficiency of Cerenkov radiation larger than one from the ordinary Cerenkov and Smith-Purcell effects by order of two. We also observed inversion of the spectral asymmetry and the phase shift of high-Q trapped modes in subwavelength slits by manipulating the direction of the structural asymmetry of unit cell paving the way for full control of Fano resonances This finding is thought to be useful for many other potential applications other than Cerenkov lasing such as sensing and optical switching. © 2019 IEEE.
Effect of Confinement on Collective Dynamics of Nanoconfined Water in DMPC Multi...

International Conference on Infrared, Millimeter, and Terahertz Waves, IRMMW-THz

Jang J., Eom K., Kim S., Park G.-S.

Abstract
Confinement effect on collective reorientational dynamics of nanoscopically confined water in DMPC multilamellar vesicle (MLV) was investigated by using GHz dielectric relaxation spectroscopy. The collective dynamics of hydration and beyond the hydration layer was estimated when the interlamellar distance is reduced by increasing PEG concentrations. Beyond hydration layer is accelerated over bulk water and this means nanoconfinement induces longer-range perturbation on water dynamics than generally assumed perturbation range, i.e., hydration layer. As osmotic pressure is increased, both relaxation times begin to change and shows opposite trend. This result demonstrates that confinement induces coupling between hydration and beyond hydration region. © 2019 IEEE.

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