J. S. Hwang

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 14 GHz. In case of the dual-layer one, it is possible to control the transmission peak from 5 to 10 GHz. 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.

Reinforced magnetic properties of Ni-doped BiFeO3 ceramic

Multiferroic materials attract considerable interest because of the wide range of potential applications such as spintronic devices, data storage devices and sensors. As a strong candidate for the applications among the limited list of single-phase multiferroic materials, BiFeO3 (BFO) is a quite attractive material due to its multiferroic properties at room temperature (RT). However, BFO is widely known to have large leakage current and small spontaneous polarization due to the existence of crystalline defects such as oxygen vacancies. Furthermore, the magnetic moment of pure BFO is very weak owing to its antiferromagnetic nature. In this paper, the effects of Ni2+ substitution on the magnetic properties of bulk BFO were investigated. BFO, and BiFe0.99Ni0.01O3, BiFe0.98Ni0.02O3 and BiFe0.97Ni0.03O3 (BFNO: Ni-doped BFO) ceramics were prepared by solid-state reaction and rapid sintering, and analyzed by structural and magnetic-property measurements. The leakage current density was measured at RT by using a standard ferroelectric tester. All the Ni-doped BFO samples exhibited the similar rhombohedral perovskite structure (R3c) to that of BFO. The magnetic properties of Ni-doped BFO were much enhanced with respect to BFO prepared at the same conditions, because the enhanced ferromagnetic interaction is caused by the Fe/Ni coupling.

Flexible perfect metamaterial absorbers for electromagnetic wave

Abstract The flexible metamaterials (MMs) provide a new field in controlling electromagnetic waves. This paper comprises the MMs made on flexible and elastic substrates, together with the relevant techniques and approaches. The perspective is also described, which includes the possible conversion of investigated MMs into practical devices. For an example of the research and development so far in this fascinating area, by using a planar and flexible MM, the low-frequency perfect absorption has been obtained even with very small unit-cell size in snake-shape structure. These shrunken, deep-sub-wavelength and thin MM absorbers were numerically and experimentally investigated by increasing the inductance. The absorbers are specially designed for absorption peaks around 2 GHz and 400 MHz, which can be used for depressing the EM noise from everyday electronic devices and mobile phones. It has been reported new concept of water droplet-based MM perfect absorber absorbing perfectly electromagnetic wave with water, an eco-friendly material which is very ordinary and plentiful on the earth. If arranging water droplets with particular height and diameter on material surface, meta-properties absorbing electromagnetic wave perfectly in GHz broadband were shown. It was possible to control absorption ratio and absorption wavelength band of electromagnetic wave according to the shape of water droplet and apply to various flexible and/or transparent substrates such as plastic, glass, and paper. These results can lead wide applications to variously civilian and military products in the future by providing perfect absorber of broadband in all products including bendable and transparent materials.

Switching and extension of transmission response, based on bending metamaterials

The electromagnetically-induced transparency (EIT)-like effects in planar and non-planar metamaterials (MMs) were investigated for microwave (GHz) frequencies. The specific MMs used in this study consisted of a cut-wire resonator and a ring resonator, where were placed on the top and the bottom layers, respectively. A transmission window was produced, due to the interference between bright- and bright-mode coupling. Using the numerical and the experimental results, we demonstrate that the bending of MM leads to enhanced transmission and bandwidth, as well as an additional EIT-like peak. This provides an effective way of realizing the tunable devices, including the switching sensors.

Ultra-subwavelength thickness for dual/triple-band metamaterial absorber at very low frequency

An integrated model utilizing external parasitic capacitors for a dual-band metamaterial perfect absorber (DMPA) is proposed and demonstrated in the UHF radio band. By adjusting the lumped capacitors on a simple meta-surface, the thickness of absorber is reduced to be only 1/378 and 1/320 with respect to the operating wavelength at 305 and 360.5 MHz, respectively. The simulations and the experiments confirm that the DMPA can maintain an absorption over 91% in a wide range of incident angle (up to 55°) and independent of the polarization of incident radiation. Additionally, we examine the integrated model for smaller dual-band absorber and absorption performance at higher frequencies (LTE band). Finally, we consolidate our approach by fabricating an ultrathin triple-band perfect absorber miniaturized to be only 1/591 of the longest operating wavelength. Our work is expected to contribute to the actualization of metamaterial-based devices working at radio frequency.

Flexible ultrathin metamaterial absorber for wide frequency band, based on conductive fibers

ABSTRACT Flexible and ultrathin wide-band metamaterial absorbers are suggested and demonstrated in the microwave-frequency range. By using resonators of different sizes and conductive fibers on metallic-pattern layer, the total thickness of metamaterial absorber is reduced to be only 1/349 with respect to the operating wavelength at 0.97 GHz. We present the absorption mechanism in terms of the impedance matching with the free space, the distributions of surface current and the three-dimensional distributions for power loss. In simulation, the absorption was over 97% at 0.97–6.12 GHz, and the corresponding experimental absorption band over 97% was 0.87–6.11 GHz. Furthermore, the dielectric substrate of metamaterial absorbers was replaced with flexible substrate in order to have the flexibility and the broadband absorption properties. The absorption band is expanded and the high-absorption performance maintains at the same time. The total thickness of metamaterial absorber comes to be only 1/5194 of the operating wavelength at 0.75 GHz. Our work is expected to contribute to the flexible microwave/electronic devices in the near future. Graphical Abstract

Flexible metamaterials, comprising multiferroic films

The metamaterial (MM) devices on flexible substrates provide a new dimension in manipulating electromagnetic (EM) waves. This work reports MMs realized on flexible and elastomeric substrates, along with the relevant techniques and approaches. Future directions are mentioned with the promise to translate MMs into practical devices. We also present a multiferroic nano-composite film where BiFeO3 (BFO) nanoparticles (NPs) were evenly dispersed into highly-insulating polyvinyl alcohol (PVA) polymer. The multiferroic (MF) properties of the film were revealed, such as the saturated ferroelectric curves due to the cut-off of current leakage. Moreover, the prepared films show high flexibility and their multiferroicities are preserved well even in a high curved condition, reflecting the possibility for fabricating wearable devices based on MF materials.