Y. P. Lee

Flexible and elastic metamaterial absorber for low frequency, based on small-size unit cell

Using a planar and flexible metamaterial (MM), we obtained the low-frequency perfect absorption 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 periodicity/thickness (the figure of merit for perfect absorption) is achieved to be 10 and 2 for single-snake-bar and 5-snake-bar structures, respectively. The ratio between periodicity and resonance wavelength (in mm) is close to 1/12 and 1/30 at 2 GHz and 400 MHz, respectively. The absorbers are specially designed for absorption peaks around 2 GHz and 400 MHz, which can be used for depressing the electromagnetic noise from everyday electronic devices and mobile phones.

Polarization-insensitive and polarization-controlled dual-band absorption in metamaterials

We numerically and experimentally investigated dual-band absorption of sandwich-structure metamaterials which include periodic metal coupled rings at the front separated from the metal plane at the back by a dielectric layer. The properties are demonstrated in both GHz and mid-IR regimes of electromagnetic (EM) wave. The dual-band perfect absorber with polarization independence is observed under normal incidence. In order to understand the EM properties of dual-band perfect absorber, the plasmonic excitation was clarified for both peaks. Finally, by connecting the rings, the perfect-absorption peaks can be controlled with the polarization angle of incident EM wave.

Critical behavior in Ti-doped manganites LaMn1−xTixO3 (0.05⩽x⩽0.2)

The critical properties of the ferromagnetic insulating system LaMn1−xTixO3 (0.05⩽x⩽0.2) are investigated based on the static magnetization measurements around Curie temperature TC. The values of critical exponents, derived from the magnetic data using the Kouvel-Fisher method, yield 0.359⩽β⩽0.378, 1.24⩽γ⩽1.29, and 4.11⩽δ⩽4.21 with a TC of 95–173K. The exponent values are close to those expected for three-dimensional Heisenberg ferromagnets with short-range interactions, and the magnetic transition is understood in the context of Griffiths phase, arising from the random-temperature effects.

Metamaterial-based perfect absorbers

Metamaterials are artificially-engineered materials, possessing properties which are not readily observable in materials existing in nature. Since they show very novel properties such as left-handed behavior, negative refractive index, classical analog of electromagnetically-induced transparency, extraordinary transmission, negative Doppler effect, and so on, they can be used for perfect lens, invisibility cloaking, perfect absorption and transmission, etc. Metamaterial-based perfect absorbers (MMPAs) are promising candidates for the practical application of perfect absorbers. MMPA is usually composed of three layers. The first layer is periodically-arranged metallic patterns, whose structure and geometrical parameters should be carefully adjusted to fulfill the impedance-matching condition with the ambient, allowing no reflection of incident electromagnetic (EM) waves. The second layer is a dielectric layer, which allows a space for the EM waves to be dissipated, and sometimes plays a role of resonance cavity to prolong the time taken by the EM waves inside the second layer. Finally, the third layer is a continuous metallic plate, blocking remnant transmission. For practical usage, several aspects of MMPAs are to be considered seriously. Some of them are broadband operation, polarization-independent response, omni-directional response, and tunability. These aspects are basically determined by the structures of MMPA. Another important aspect is flexibility, which is determined by the material used in the fabrication. In this review, the basic operating principles of MMPAs and brief introduction of recent progresses in the field of MMPAs operating in different frequency ranges (GHz, THz and infrared/visible) are presented. Perspectives and future works for the investigation and the real application of MMPAs are also presented.

Enhanced ferromagnetic properties in Ho and Ni co-doped BiFeO3 ceramics

The magnetic properties of polycrystalline Bi1-xHoxFe1-yNiyO3 (x = 0, 0.1; y = 0, 0.03), which were prepared by the solid-state method, have been investigated. The powder X-ray diffraction reveals that all the samples are polycrystalline and show rhombohedral perovskite structure. The micro-Raman scattering studies confirm that Bi0.9Ho0.1Fe0.97Ni0.03O3 has a compressive lattice distortion induced by the simultaneous substitution of Ho and Ni ions at A and B-sites, respectively. From the magnetization dependences at room temperature, Bi0.9Ho0.1Fe0.97Ni0.03O3 has enhanced magnetization (0.2280 emu/g) and low coercive field (280 Oe). It was revealed that the Ni dopant plays an important role for the improved ferromagnetic properties and the Ho dopant favors the magnetic exchange interactions in the co-doped ceramic.

Tunable dual-band perfect absorbers based on extraordinary optical transmission and Fabry-Perot cavity resonance.

Magnetic resonance is considered to be a necessary condition for metamaterial perfect absorbers, and dual-band absorbers can be composed of a pair of metallic layers with anti-parallel surface currents. We designed and fabricated a tunable dual-band perfect absorber based on extraordinary-optical-transmission (EOT) effect and Fabry-Perot cavity resonance. The idea and the mechanism are completely different from the absorber based on the near-field interaction. The important advantage of our structure is that we can switch a single-band absorber to a dual-band absorber by changing the distance between two metallic layers and/or incident angle. The peak originating from the EOT effect becomes significantly narrower, resulting in an increase of the Q-factor from 16.88 to 49. The dual-band absorber can be optimized to be insensitive to the polarization of the incident electromagnetic wave by slightly modifying the absorber structure.

Triple-band perfect metamaterial absorption, based on single cut-wire bar

In general metamaterial perfect absorber, the single meta-pattern makes the single absorption band. Therefore, the multi-absorption bands need the corresponding kinds of meta-patterns. Here, we introduce the triple-band metamaterial perfect absorber utilizing only single kind of pattern. We also demonstrate the absorption mechanism of the triple perfect absorption. The first and the second absorption bands were induced by the fundamental magnetic resonance of the major and the minor axes, respectively, of cut-wire bar. The third peak was caused by the third-harmonic magnetic resonance of the major axis. Additionally, the unexpected third band was formed, which was the overlapping of the third absorption peak with another absorption peak, which was made by pairs of antiparallel currents parallel or antiparallel to the incident electromagnetic wave.

One-dimensional photonic crystal with a complex defect containing an ultrathin superconducting sublayer

The influence of the variation in the incidence angle on the photonic band gap spectra of a one-dimensional dielectric photonic crystal with a complex defect layer, consisting of ultrathin superconducting and dielectric sublayers, was theoretically investigated. The behavior of the defect modes with different polarizations as a function of the incidence angle variation is studied numerically for different thicknesses of the superconducting sublayer. The pronounced contrast in behavior of TE- and TM-polarized modes was demonstrated. The intensity of the TE-polarized defect mode decreases with increasing incidence angle, whereas the intensity of the TM-polarized defect mode increases. The increase in the superconducting defect sublayer thickness leads to a shift in the defect mode of TM-polarization to higher frequencies.

Dependence of the distance between cut-wire-pair layers on resonance frequencies

We studied both experimentally and theoretically the influence of the distance between adjacent cut-wire-pair layers on the magnetic and the electric resonances in the microwave-frequency regime. Besides the dependence on the separation between cut-wire pairs, along the electric-field direction, the electric resonance strongly depends on the distance between cut-wire-pair layers, while the magnetic resonance is almost unchanged. This contrast can be understood by the difference in the distribution of induced-charge density and in the direction of the induced current between the electric and magnetic resonances. A simple model is proposed to simulate our experimental results and the simulation results are in good agreement with the experiment. This result provides important information in obtaining left-handed behavior when the cut-wire pairs are combined with the continuous wire.

Left-handed behavior of combined and fishnet structures

The left-handed (LH) behavior of a combined structure of cut wire pair and continuous wire was experimentally studied. It was found that the LH behavior of a combined structure can still be observed as the width of cut wire pair increases until the cut wire joins the continuous wire, the so-called fishnet structure. These structures were designed, fabricated, and measured in the microwave frequency regime. In addition, we report the experimental results of the parametric study on the fishnet structure, which are also compared with the previous theoretical studies.