A terahertz metamaterial with unnaturally high refractive index

Abstract

Controlling the electromagnetic properties of materials, going beyond the limit that is attainable with naturally existing substances, has become a reality with the advent of metamaterials. The range of various structured artificial 'atoms' has promised a vast variety of otherwise unexpected physical phenomena, among which the experimental realization of a negative refractive index has been one of the main foci thus far. Expanding the refractive index into a high positive regime will complete the spectrum of achievable refractive index and provide more design flexibility for transformation optics. Naturally existing transparent materials possess small positive indices of refraction, except for a few semiconductors and insulators, such as lead sulphide or strontium titanate, that exhibit a rather high peak refractive index at mid- and far-infrared frequencies. Previous approaches using metamaterials were not successful in realizing broadband high refractive indices. A broadband high-refractive-index metamaterial structure was theoretically investigated only recently, but the proposed structure does not lend itself to easy implementation. Here we demonstrate that a broadband, extremely high index of refraction can be realized from large-area, free-standing, flexible terahertz metamaterials composed of strongly coupled unit cells. By drastically increasing the effective permittivity through strong capacitive coupling and decreasing the diamagnetic response with a thin metallic structure in the unit cell, a peak refractive index of 38.6 along with a low-frequency quasi-static value of over 20 were experimentally realized for a single-layer terahertz metamaterial, while maintaining low losses. As a natural extension of these single-layer metamaterials, we fabricated quasi-three-dimensional high-refractive-index metamaterials, and obtained a maximum bulk refractive index of 33.2 along with a value of around 8 at the quasi-static limit.

Single-layer terahertz metamaterials with bulk optical constants

“…Type 15 metamaterials achieve a more complicated and undesirable response compared to Type 50 (50µm thick) metamaterials. Although this may often be an unplanned interaction, this effect has been utilized in some cases to achieve unit cells with both electric and magnetic response 24,[34][35][36][37] . Other examples include electromagnetic inducted transparency 38 and energy level hybridization [39][40][41] .…”

Electromagnetic Waves in Variable Media

“…The vector field v, the carrier, shall be chosen such that the remaining two equations (30) and (31), too, can be solved. Such a choice will be possible if the response functions depend only on one spatial variable, say ζ , as declared in (15), and thus…”

Electrically Controllable Asymmetric Split-Loop Terahertz Resonator with Outer Square Loop

“…이러한 메타물질은 자연계에는 존재하지 않는 특이한 성질인 음의 굴절률이나 투명한 형상 등을 탄생시킬 수 있어 다양한 분야에서 연구가 이루어지고 있다 [4,5] . 메타물질을 구현하기 위해서는 그 구 조가 파장 대비 매우 작은 크기로 형성되어야 하므로, 광학 주파수 대역보다는 일반적인 광리소그래피 기술로 메타물질 구현이 가능한 테라헤르츠 대역에서 그 활용도가 크므로 테 라헤르츠 대역에서 메타물질 기반 필터나 센서 등의 소자로 응용하는 연구가 활발히 진행되고 있다 [6,7] . 테라헤르츠 필 터의 컷오프 및 센서의 민감도 향상을 위해서는 메타물질의 품질 인자를 높이는 것이 중요하다.…”