메타물질의 완전 흡수 특성과 다중 자기 공명에 대한 연구

Abstract

Metamaterials (MMs) are artificial materials fabricated to have electromagnetic (EM) properties, which do not exist in nature. In 1968, materials with negative permittivity and permeability in a certain frequency were first theoretically studied by Veselago. He designated them as left-handed media due to the left-handed triad formed by the electric field, the magnetic field and the electromagnetic wave vector. Because of the inexistence of such materials in nature, the optical and the electromagnetic consequences examined by Veselago could only be observed in nearly 30 years. Based on the results by Veselago, Smith and Schultz experimentally investigated materials with negative refractive index in 2001. Fabricated material consisted of the periodic cells with negative permittivity and permeability in the form of metallic wires and split rings, which was shown to exhibit left-handed properties. MMs, which are effectively homogeneous EM structures consisting of dielectrics and periodically-arranged conductors, have been studied by many researchers recently because of the well-controlled properties for incident EM wave. The major advantage of MMs is that the unit-cell parameters can be adjusted easily to have desired the EM properties of sample. MMs started to get spotlight as negative-index materials, extraordinary optical transmission, classical analogue of electromagnetically-induced transparency and metamaterial absorber. In this thesis, I studied dual-band perfect-absorption (PA) MMs using the bar-type structure, generating the fundamental and the third-harmonic magnetic resonances. The third resonance in MM, which was induced by the magnetic multi-plasmon, was discovered to yield perfect-absorption peak by manipulating the structural parameters. The EM properties were examined in comparison with the conventional at the fundamental resonance. Polarization-independent dual-band PA MMs with the disk- and donut-type structures employing a single pattern were also investigated. I have successfully fabricated two different structures. Two MM structures consist of the arrangement of disk- and donut-type resonators and a metallic background plane, separated by a dielectric substrate FR-4. I theoritically and experimentally realize polarization-independent dual-band PA in the GHz range. The absorption spectra were measured in the microwave-frequency range and compared with the simulated ones. The mechanism of dual-band absorption peaks was also studied. I demonstrate the multiple-plasmonic high-frequency PAs induced by the third harmonic as well as the fundamental magnetic resonances. The donut-type MMs could provide absorption higher than 99% at the third-harmonic magnetic resonance as well as the fundamental one, which are independent of polarization. |메타물질은 일반 자연계에는 존재하지 않으며, 사람들에 의해서 인위적으로 제작된 물질을 의미한다. 최근 십년간 메타물질은 입사하는 전자기파의 성질을 변화시켜 줄 수 있는 다양한 전자기적 특성 때문에 많은 연구자들로부터 큰 관심을 받아왔다. 메타물질은 음의 굴절률, 전자기 유도 투과효과, 특이 광투과 및 전자기파 완전 흡수체 등으로 활용될 수 있는 관점에서 많은 각광을 받아왔다. 메타물질 중에서도 전자기파 완전 흡수체는 태양광 하베스트, 플라즈모닉 센서, 무선 전력전송 등에 활용이 가능하기에 더욱 많은 연구가 진행되고 있는 분야이다. 메타물질 완전 흡수체에서 일어나는 흡수는 전기적 공명과 자기적 공명에 의해서 발생한다고 알려져 있고, 상대적으로 조절이 쉬운 자기적 공명에 초점을 맞춰서 많은 연구가 진행되고 있다. 본 학위논문에서는 완전한 흡수가 가능한 메타물질 흡수체가 가지는 전자기적 특성과 한 종류의 간단한 메타구조를 가지고 기본 자기공명 및 3차 조화 자기공명의 2가지에 의해 2 주파수에서 동시에 완전한 흡수가 나타나는 메타 물질에 대한 연구결과를 다룬다. 최초로 개발된 완전 흡수체는 전기적 공진기와 길이가 긴 금속막대가 조합을 이룬 메타물질로서 하나의 주파수 영역에서 흡수가 가능하였으나, 흡수 대역이 매우 좁고 입사하는 전자기파의 편광방향에 매우 민감하게 반응하였다. 이 후에 많은 연구자들은 흡수 주파수 변경이 가능한 메타물질, 다중 흡수 대역에서 흡수가 가능한 메타물질 및 입사하는 전자기파의 편광방향에 둔감한 메타물질에 대해서 연구가 진행되고 있다. 본 연구에서는 하나의 패턴을 사용하여 두 곳의 주파수 영역에서 완전한 흡수가 발생하는 메타 물질 흡수체를 성공적으로 제작하였다. 일반적으로 메타 물질 흡수체에서는 하나의 패턴을 활용하여 하나의 흡수 밴드만 구현하는 것이 가능하였는데, 그 이유는 기본 자기공명에 의한 흡수만 활용하였기 때문이다. 본 연구에서는 하나의 패턴에서 3차 조화 자기공명이 발생하며 이를 활용하면 또 다른 주파수 영역에서도 흡수 밴드가 나타난다는 것을 이론적 및 실험적으로 증명하였다. 또한 대칭성이 매우 우수한 원판과 도넛 형태의 메타 단위구조를 사용하면 입사파의 편광방향에 영향을 받지 않는다는 것도 증명하였다. 본 연구에서 활용한 방법을 이용한다면, 입사파의 파장과 비슷한 크기의 단위 셀을 이용해도 흡수가 일어날 수 있으며, 메타물질 제작에서 단위 셀의 크기가 갖는 단점을 극복할 수 있다는 것을 제시하였다.; Metamaterials (MMs) are artificial materials fabricated to have electromagnetic (EM) properties, which do not exist in nature. In 1968, materials with negative permittivity and permeability in a certain frequency were first theoretically studied by Veselago. He designated them as left-handed media due to the left-handed triad formed by the electric field, the magnetic field and the electromagnetic wave vector. Because of the inexistence of such materials in nature, the optical and the electromagnetic consequences examined by Veselago could only be observed in nearly 30 years. Based on the results by Veselago, Smith and Schultz experimentally investigated materials with negative refractive index in 2001. Fabricated material consisted of the periodic cells with negative permittivity and permeability in the form of metallic wires and split rings, which was shown to exhibit left-handed properties. MMs, which are effectively homogeneous EM structures consisting of dielectrics and periodically-arranged conductors, have been studied by many researchers recently because of the well-controlled properties for incident EM wave. The major advantage of MMs is that the unit-cell parameters can be adjusted easily to have desired the EM properties of sample. MMs started to get spotlight as negative-index materials, extraordinary optical transmission, classical analogue of electromagnetically-induced transparency and metamaterial absorber. In this thesis, I studied dual-band perfect-absorption (PA) MMs using the bar-type structure, generating the fundamental and the third-harmonic magnetic resonances. The third resonance in MM, which was induced by the magnetic multi-plasmon, was discovered to yield perfect-absorption peak by manipulating the structural parameters. The EM properties were examined in comparison with the conventional at the fundamental resonance. Polarization-independent dual-band PA MMs with the disk- and donut-type structures employing a single pattern were also investigated. I have successfully fabricated two different structures. Two MM structures consist of the arrangement of disk- and donut-type resonators and a metallic background plane, separated by a dielectric substrate FR-4. I theoritically and experimentally realize polarization-independent dual-band PA in the GHz range. The absorption spectra were measured in the microwave-frequency range and compared with the simulated ones. The mechanism of dual-band absorption peaks was also studied. I demonstrate the multiple-plasmonic high-frequency PAs induced by the third harmonic as well as the fundamental magnetic resonances. The donut-type MMs could provide absorption higher than 99% at the third-harmonic magnetic resonance as well as the fundamental one, which are independent of polarization.