Our Isotropic Design Featured in OPN [Nov 1, 2008]
Ilustration of our blueprint meandering wire design on a four-layer stacked photonic metamaterial background (adapted from OPN).
The meandering wire design which we have proposed for the fabrication of isotropic metamaterials was featured by Ekmel Ozbay in the Optics and Photonics News magazine. Metamaterials have opened a world of new possibilities that would have seemed like magic or science-fiction a decade ago, including perfect lenses and invisibility cloaking. Ozbay's article [1] explores the progress of the field. The following paragraphs are extracted from his feature article.
"Photonic metamaterials are usually fabricated by well-established 2D fabrication technologies, such as electron-beam lithography, focused ion beam etching, evaporation of metal films, etc. The reported structures have a few layers, with the exception of Zhang's 10-layer fishnet metamaterial structures [2]. However, all of the reported photonic metamaterials exhibit a negative index along a certain propagation direction, essentially making them one-dimensional metamaterials.
On the other hand, isotropic 2D and 3D bulk photonic metamaterial designs with low absorption and high transmission are needed to explore all of the potential applications of photonic metamaterials. Direct laser writing (DLW) is a promising technique for fabricating truly 3D large-scale photonic metamaterials. Researchers at the University of Karlsruhe have recently demonstrated the feasibility of this method at near-infrared frequencies assisted with silver chemical vapor deposition [3].
The samples go through 10 chemical vapor deposition cycles, resulting in an estimated silver thickness of approximately 50 nm. The coating is uniform around the structures, even in 3D; this is in sharp contrast to the usual 2D evaporation process. The films are somewhat granular but they are all connected. This crucial aspect is demonstrated by the fact that the silver films exhibit good DC conductivity, by a reflectance of R > 95% in the 1 to 4 mm wavelength range.
The researchers used retrieval methods to determine the electromagnetic properties of the metamaterials grown by DLW. The magnetic permeability exhibits the anticipated negative values around the 100 THz frequency range. This clearly shows that the fabrication of a magnetic metamaterial using DLW and silver CVD is possible. However, the influence of bi-anisotropy of the fabricated structure results in a positive index of refraction. Therefore, there is a clear need to come up with a better design that is suitable for DLW.
In a new paper, Costas Soukoulis and his colleagues [Guney, Koschny, and Kafesaki] offer an effective solution to this problem [4]. They theoretically analyze and demonstrate the first truly bulk NIM design, which is feasible to fabricate with DLW at around telecom wavelengths. [In the above figure] the blueprint design shows the unit cell for a 2D photonic metamaterial consisting of two pairs of SRRs. Magnetic permeability and electric permittivity are simultaneously negative in the region of negative index of refraction.
The operation frequency of the metamaterial is close to the telecom band (150 THz) with a THz bandwidth of about 20. The designed structure has a 2D isotropy in the sense that the propagation is in two directions with fixed polarization and a negative index of refraction. Combined with such new and suitable theoretical blueprints, the DLW and silver CVD techniques can be used to overcome the major hurdles in the field of photonic metamaterials.
Our paper [4] pointed by Ozbay was submitted to Optics Letters.
References
[1] E. Ozbay, The magical world of photonic metamaterials, Optics Photonics News 19, 22 (2008).
[2] J. Valentine, S. Zhang, T. Zentgraf, E. Ulin-Avila, D. A. Genov, G. Bartal, and X. Zhang, Three-dimensional optical metamaterial with a negative refractive index, Nature 455, 376 (2008).
[3] M. S. Rill, C. Plet, M. Thiel, I. Staude, G. von Freymann, S. Linden, and M. Wegener, Photonic metamaterials by direct laser writing and silver chemical vapour deposition, Nature Mater. 7, 543 (2008).
[4] D. O. Guney, Th. Koschny, M. Kafesaki, and C. M. Soukoulis, Connected bulk negative index photonic metamaterials, submitted to Optics Letters.