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 metamaterials1,2,3. The range of various structured artificial ‘atoms’ has promised a vast variety of otherwise unexpected physical phenomena3,4,5,6,7,8,9,10,11,12,13,14,15,16,17, 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 optics9,10,11,12,13,14. 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 frequencies18. Previous approaches using metamaterials were not successful in realizing broadband high refractive indices19,20,21. A broadband high-refractive-index metamaterial structure was theoretically investigated only recently22, 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.
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We thank B. Kang for help in the fabrication of micrometre-gap metamaterials, J. S. Chang for proofreading the manuscript before submission, and D. S. Kim for discussions. This work was supported by National Research Foundation of Korea grants funded by the Korean government (numbers 2009-0069459, 2008-0062235 and 2010-0012058). K.-Y.K. acknowledges support by the IT Research and Development programme of MKE/KEIT (number 2006-S-005-04). Y.-H.L. acknowledges support from the National Research Foundation of Korea (number 2007-0093863) and N.P. acknowledges support from National Research Foundation (numbers GRL K20815000003 and 2010-0001859), funded by the Korean government.
The authors declare no competing financial interests.
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Choi, M., Lee, S., Kim, Y. et al. A terahertz metamaterial with unnaturally high refractive index. Nature 470, 369–373 (2011). https://doi.org/10.1038/nature09776
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