Saturday, January 25, 2014

MetaTHz-Terahertz metamaterials

Metamaterials are man-made artifical structures that present electromagnetic properties not occurring in natural materials. Typically, their design is based on periodic patterns of metallic elements of sub-wavelength scale. The collective response of these structures in the presence of electromagnetic fields leads to exciting properties, such as a negative effective index of refraction. By tailoring the design, shape and geometry of the metamaterial constitutive elements, these properties can span over a huge spectrum, ranging from microwaves to optical frequencies. Terahertz technology can also benefit from metamaterials, as these can provide the building blocks for THz devices, and offer advanced transmission, absorption and phase control. Various designs have been demonstrated thus far as candidates for THz metamaterials, e.g. split-ring resonators [1], cut-slabs and fishnet structures [2,3]. 


The layout of such a fishnet THz metamaterial structure is shown in Figure 1(a), where a unit cell is replicated in order to compose a metamaterial slab. The dimensions of the unit cell are 150x150 μm, which is deeply subwavelength compared to the target frequency of 1 THz. The transmittance, reflectance, and absorbance of this structure are shown in Figure 1(b). Absorption is maximized near the metamaterial resonance and stems from losses in the metallic films and the dielectric spacer, which is considered to be benzo-cyclo-butene. By assuming an effective thickness of the metamaterial, the effective parameters of the fishnet metamaterial can be calculated for the case of normal incidence [4,5], as shown in Figure 1(c). A figure of merit (FOM) typically used in the assessment of metamaterial performance is the ratio -Re{n}/Im{n}, where is the metamaterial effective index.
[1] W. J. Padilla, A. J. Taylor, C. Highstrete, M. Lee, and R. D. Averitt, "Dynamic electric and magnetic metamaterial response at terahertz frequencies," Phys. Rev. Lett., art. no. 107401, 2006.
[2] M. Kafesaki, I. Tsiapa, N. Katsarakis, Th. Koschny, C. M. Soukoulis, and E. N. Economou, "Dynamic electric and magnetic metamaterial response at terahertz frequencies," Phys. Rev. B, vol. 75, art. no. 235114, 2007.
[3] P. Ding, E. J. Liang, W. Q. Hu, L. Zhang, Q. Zhou, and Q. Z. Xue, "Numerical simulations of terahertz double-negative metamaterial with isotropic-like fishnet structure," Photon. Nanostruct. - Fund. Appl., vol. 7, pp. 92-100, 2009.
[4] D. R. Smith, S. Schultz, P. Markoš, and C. M. Soukoulis, "Determination of effective permittivity and permeability of metamaterials from reflection and transmission coefficients," Phys. Rev. B, vol. 65, art. no. 195104, 2002.
[5] D. R. Smith, D. C. Vier, Th. Koschny, and C. M. Soukoulis, "Electromagnetic parameter retrieval from inhomogeneous metamaterials," Phys. Rev. E, vol. 71, art. no. 036617, 2005.

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