Young Ju Kim

Metamaterial Absorber for Electromagnetic Waves in Periodic Water Droplets.

Perfect metamaterial absorber (PMA) can intercept electromagnetic wave harmful for body in Wi-Fi, cell phones and home appliances that we are daily using and provide stealth function that military fighter, tank and warship can avoid radar detection. We reported new concept of water droplet-based PMA absorbing perfectly electromagnetic wave with water, an eco-friendly material which is very plentiful on the earth. If arranging water droplets with particular height and diameter on material surface through the wettability of material surface, meta-properties absorbing electromagnetic wave perfectly in GHz wide-band were shown. It was possible to control absorption ratio and absorption wavelength band of electromagnetic wave according to the shape of water droplet–height and diameter– and apply to various flexible and/or transparent substrates such as plastic, glass and paper. In addition, this research examined how electromagnetic wave can be well absorbed in water droplets with low electrical conductivity unlike metal-based metamaterials inquiring highly electrical conductivity. Those results are judged to lead broad applications to variously civilian and military products in the future by providing perfect absorber of broadband in all products including transparent and bendable materials.

Dual broadband metamaterial absorber

We propose polarization-independent and dual-broadband metamaterial absorbers at microwave frequencies. This is a periodic meta-atom array consisting of metal-dielectric-multilayer truncated cones. We demonstrate not only one broadband absorption from the fundamental magnetic resonances but additional broadband absorption in high-frequency range using the third-harmonic resonance, by both simulation and experiment. In simulation, the absorption was over 90% in 3.93-6.05 GHz, and 11.64-14.55 GHz. The corresponding experimental absorption bands over 90% were 3.88-6.08 GHz, 9.95-10.46 GHz and 11.86-13.84 GHz, respectively. The origin of absorption bands was elucidated. Furthermore, it is independent of polarization angle owing to the multilayered circular structures. The design is scalable to smaller size for the infrared and the visible ranges.

Size-efficient metamaterial absorber at low frequencies: Design, fabrication, and characterization

We numerically and experimentally demonstrated a metamaterial perfect absorber (MPA) in MHz region based on a planar sandwiched metal-dielectric-metal structure. First, the single-peak perfect absorption was obtained at 400u2009MHz. The ratios of the periodicity of unit cells and the thickness to the absorption wavelength are 1/12 and 1/94, respectively. The advantage of structural design and the mechanism for the low-frequency MPA are described in detail by the comparison between calculation, simulation, and experiment. Influence of the incident angle of electromagnetic (EM) wave for both transverse-electric (TE) and transverse-magnetic (TM) polarization on absorption was also investigated, and the absorption was maintained to be above 95% at incident angles up to 30°. Finally, we propose a self-asymmetric structure, which induces the dual-band perfect absorption in the same range of frequency. The EM behavior of the excitation modes and the mechanism of the dual-band MPA are clearly explained. Especially, w...

Miniaturization for ultrathin metamaterial perfect absorber in the VHF band

An efficient resolution for ultrathin metamaterial perfect absorber (MPA) is proposed and demonstrated in the VHF radio band (30–300u2009MHz). By adjusting the lumped capacitors and the through vertical interconnects, the absorber is miniaturized to be only λ/816 and λ/84 for its thickness and periodicity with respect to the operating wavelength (at 102u2009MHz), respectively. The detailed simulation and calculation show that the MPA can maintain an absorption rate over 90% in a certain range of incident angle and with a wide variation of capacitance. Additionally, we utilized the advantages of the initial single-band structure to realize a nearly perfect dual-band absorber in the same range. The results were confirmed by both simulation and experiment at oblique incidence angles up to 50°. Our work is expected to contribute to the actualization of future metamaterial-based devices working at radio frequency.

Experimental Realization of Tunable Metamaterial Hyper-transmitter

We realized the tunable metamaterial hyper-transmitter in the microwave range utilizing simple planar meta-structure. The single-layer metamaterial hyper-transmitter shows that the transmission peak occurs at 14u2009GHz. In case of the dual-layer one, it is possible to control the transmission peak from 5 to 10u2009GHz. Moreover, all the transmission peaks reveal transmission over 100%. We experimentally and theoretically investigated these phenomena through 3-dimensional simulation and measurement. The reason for being over 100% is also elucidated. The suggested hyper-transmitter can be used, for example, in enhancing the operating distance of the electromagnetic wave in Wi-Fi, military radar, wireless power transfer and self-driving car.

Dual-absorption metamaterial controlled by electromagnetic polarization

Polarization is one of the fundamental properties of electromagnetic waves, conveying valuable information in signal transmission and sensitive measurements. We investigated the controllability of dual-band absorption metamaterials whose unit cells were designed and fabricated based on the conventional absorber model with proper manipulation of the structural pattern. Dual-band perfect absorption is achieved and gradually switched off according to the polarization angle. Moreover, another mode emerges concomitantly at higher frequencies. The electromagnetic properties were investigated to clarify the mechanisms of dual-band perfect absorption and polarization-sensitive switchable effects. Our work was demonstrated experimentally in the gigahertz range and applied to the terahertz region.

Simple metamaterial structure enabling triple-band perfect absorber

Two resonators in metamaterial usually correspond only to two absorption peaks. In this report, by breaking the symmetry, we could create multi-fundamental resonances at GHz frequencies in both simulation and experiment. First, a dual-band metamaterial absorber (MA) was achieved for 4.6 and 10.6 GHz. Next, by modifying the relative position of inner square, the triple-band MA was obtained with enhanced absorption properties. In addition, dependence on the polarization of the incident electromagnetic (EM) wave was clarified. The mechanism is elucidated to be an alteration of the coupling strength, which is made by changing the geometrical configuration of the inner square and the outer ring. It is shown that our structural configuration can be applied to the fields where the interaction with a wide range of EM waves exists or is needed.

Conformation and hydration of acetylcholine

Abstract Conformational free energy calculations using an empirical potential function and a hydration shell model (program CONBIO ) were carried out on acetylcholine (ACh) in the unhydrated and hydrated states. The results indicate that the electrostatic interaction between O2 of the acetyl ester and N1 of the trimethylammonium group is of significant importance in stabilizing the gauche conformations of the torsion angle Oue5f8CH2ue5f8CH2ue5f8N+ in both states. Hydration is proved to be one of the essential factors in stabilizing the overall conformations of ACh in aqueous solution. The computed conformations are reasonably consistent with the conformation deduced from 1H and 13C NMR as well as from Raman experiments. Several feasible conformations obtained from conformational free energy calculations in the hydrated state indicate that an ensemble of several conformation exists in aqueous solution, rather than a single dominant conformation. This is in good agreement with the recent Raman experiment.

Polarization-independent, wide-incident-angle and dual-band perfect absorption, based on near-field coupling in a symmetric metamaterial

We numerically and experimentally investigated a dual-band metamaterial perfect absorber (MPA), utilizing the near-field coupling of double split-ring resonators (DSRRs). Owing to the near-field coupling between resonators, two arms in each DSRR resonate in different phases, leading to a dual-band perfect absorption. The proposed MPA also exhibits polarization-insensitive behavior and maintains the high absorption above 90% up to a wide range of incident angle more than 45°. Finally, to further consolidate our approach, a multi-band absorption is also studied by exploiting the near-field coupling among a larger number of DSRRs. Our work is expected to be applied to future broadband devices using MPA.

Role of Wood’s anomaly in the performance of metamaterial absorbers with periodicity comparable to wavelength

We examined simple metamaterial absorbers (MMAs) in a frequency regime where the periodicity of the unit cell of MM was comparable to the wavelength of the incident electromagnetic wave, and found that Woods anomaly caused by in-plane propagation played an important role. The investigated structure is composed of a cut-wire array and a continuous metal plate on the front and rear sides of the FR-4 substrate, respectively. We observed two absorption peaks corresponding to the fundamental and its third-harmonic resonances. Besides these two peaks, a minor peak located at a slightly higher frequency than that of the third-harmonic peak was observed. The existence of the minor peak was not manifested in the simulation, in which periodic boundary conditions (PBCs) were routinely used. By using a non-PBC simulation the minor peak was successfully reproduced. The so-called Woods anomaly plays a crucial role not only in the appearance of the minor peak but also the angle-of-incidence dependence of both the major and minor peaks, in which the structural conditions of the metallic patterns on the front side is key. Although our investigations are restricted to MMAs, the results obtained can be easily extended to the performance of other MM-based devices, especially, when operating in frequency ranges that are recommended to be avoided due to diffraction effects.