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Optimizations for ultra-small and wide-incident-angle metamaterial perfect absorbers at low frequency

Optimizations for ultra-small and wide-incident-angle metamaterial perfect absorbers at low frequency
Bui Xuan Khuyen
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In recent years, the advances in material science have contributed to the development of better technologies. In looking for new exciting materials and effects, an artificial material named “metamaterial (MM)” was designed and has changed our understanding of light-matter interactions in nature. MMs are regarded as a class of composite materials, artificially structured to exhibit extraordinary properties that are not readily observed in natural materials. From the first MM, which was theoretically proposed by V. G. Veselago in 1968, the so-called left-handed material (LHM), the unnatural electrodynamic effects were established such as the double-negative refractive index (NRI) material, the opposite direction of electromagnetic (EM)-wave propagation and its power flux, and inverse Cherenkov radiation. In 2000, Smith et al. successfully fabricated the first NRI. Nowadays, the novel properties of MM have been investigated in every frequency range from radio to visible for practical applications such as slow light, super resolutions, superlens, electromagnetically-induced transmission, EM cloaking, MM memory, and wireless power transferring. In this thesis, I studied the exotic phenomenon of perfect absorption in MM by simultaneous optimization of the perfect impedance matching and the fundamental magnetic resonances at very low frequency. These results are promising candidates for applications in radio broadcasting and telecommunications. Firstly, a metamaterial perfect absorber (MPA) is designed by the sandwiched structure (metal-dielectric-metal) at 400 MHz. The ratios of the periodicity of unit cells and the thickness to the absorption wavelength (at 400 MHz) are designed as 1/12 and 1/94, respectively. This MPA maintains the impedance-matching condition with the free space quite well in a relatively-wide range of incident angles up to 30o for both transverse-electric and transverse-magnetic polarization. A self-asymmetric structure was also investigated to obtain the dual-band perfect absorption in the same range. Secondary, I apply a model of four connected split-square resonators for a planar MPA structure, which has a thickness 240-times smaller than the absorption wavelength at 250 MHz. The performance of this MPA is experimentally tested for a wide range of incident angles up to 45o. Then, by adjusting the lumped capacitors and the vertical interconnects, this new kind of MPA is miniaturized to be only λ/816 and λ/84 for its thickness and periodicity with respect to the operating wavelength (at 102 MHz), respectively. Moreover, I improved this structure to realize a nearly-perfect dual-band absorber in the same range. The results were obtained by both simulation and experiment at oblique incidence angles up to 50o. Finally, by adapting only the lumped capacitors on a simple meta-surface, dual-band metamaterial perfect absorbers (DMPAs) with thickness of 1/378 and 1/320 with respect to the operating wavelength at 305 and 360.5 MHz, respectively, are demonstrated. The operation of this DMPA is also explored over a wider range of incident angles (up to 55o). In order to further study this integrated method, a triple-band MPA is considered by using an add-on capacitor. All of the absorption spectra are examined by the measurement in the radio-frequency range and compared with the simulation. My approach contributes significantly to the development of multi-band MPAs at MHz frequencies. This will be the basis for ultra-broadband MPAs operating at very low frequency in the future. In addition, these results are promising for potential practical applications in the radio band, such as radio-frequency shielding devices, single/dual-frequency filters, and single/multi-mode switching devices.
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GRADUATE SCHOOL[S](대학원) > PHYSICS(물리학과) > Theses (Ph.D.)
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