a1718_1.pdf QWH1.pdf
Experimental Comparison of Circular, Elliptical and Rectangular (Fishnet) Negative Index Metamaterials Zahyun Ku and S. R. J. Brueck Center for High Technology Materials and Electrical and Computer Engineering Department, University of New Mexico, Albuquerque, NM 87106
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Negative index materials consisting of Au-Al2O3-Au films with a 2D array of apertures have been fabricated. Circular, elliptical and rectangular apertures are compared. Comparable figures of merit [-Re(n)/Im(n)] are observed for all three geometries. © Optical Society of America OSIS Codes: (260.0260) physical optics, (160.0160) materials
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Introduction There has been intense interest in negative index materials (NIM) as a result of the unique physical properties of this new class of materials. Over the past two years several experiments have reported negative index in the NIR [1 -3]. The reported structures are all multilayered films (metal-dielectric-metal) with a two-dimensional transverse pattern of either apertures or isolated structures. In [1], results with an array of circular apertures were reported; the overall transmission in the negative index region was low, only about 5%, and a figure of merit [FOM defined as -Re(n)/Im(n)] had a value of about 0.5. Improved results [4] were obtained with an elliptical array of holes, reducing the metal linewidth providing the negative permittivity and providing a better impedance match between the NIM and the substrate and superstrate (air). Transmission was increased to about 20% and the FOM improved to ~0.8. A rectangular array or fishnet structure was introduced in [3] with a different material system (Ag in place of Au), a somewhat different wavelength (1.5 μm compared to 2.0 μm) and with significantly better performance, a transmission of about 60% and a FOM ~ 3. An important issue is the relative contributions of the 1.0 Measurement material systems (lower loss metal), of the structure, Simulation 0.8 and of the fabrication processes (e.g. nonuniformity, roughness, etc.) to this improved result. In [3] a 0.6 suggestion was made that the improved performance 0.4 was at least partially attributable to the rectangular 0.2 aperture geometry in which the width of the aperture does not vary across the aperture, in contrast to the 0.0 1.0 1.5 2.0 2. 1.0 varying chord length along the field direction for an Measurement ellipse. Simulation 0.8 We report an experimental and modeling study 0.6 of this issue. Samples with circular, elliptical and rectangular aperture arrays, all fabricated in the same 0.4 TE material system (Au-Al2O3-Au), with similar TM 0.2 resonant wavelengths are compared. Interferometric 2 lithography was used to fabricate large area (~ 6 cm ) 0.0 1.5 2.0 2. 1.01.0 samples. Characterization was with FTIR, an RCWA Measurement Simulation simulation was used to extract the FOM from the 0.8 experimental transmission results. In previous work 0.6 [1,4] we have shown that this simulation is in excellent agreement with the complete experimental 0.4 TE characterization (amplitude and phase of reflection TM 0.2 and transmission).
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Results Figure 1 shows the structures (left), the experimentally measured transmission (right), and the RCWA modeling fits (insets) for the circular
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Fig. 1: (left) Circular (top), elliptical (center) and rectangular NIM. (right) measured transmission and detailed RCWA fits in the region of negative index.
a1718_1.pdf QWH1.pdf
(top), elliptical (middle) and rectangular (bottom) designs. The transmission results are in good qualitative agreement with previous results with the transmission increasing from circular to elliptical to rectangular apertures. In each case the negative index region is in the vicinity of the “knee” at about 2.0 μm. For the asymmetric elliptical and rectangular structures, experimental results are shown for the two polarizations, with the incident electric field parallel to the narrower linewidth between apertures (the design polarization), or the orthogonal direction parallel to the wider linewidth. As noted above, the impedance match is better for the first polarization as is evident from these results. The present version of simulation software is restricted by the available computational power, to a periodic structure with rectangular holes. Therefore, the dimensions of the circular and elliptical holes in the fits were adjusted keeping both the area of the hole and major/minor axis ratio. Some increase in the scattering frequency is expected for these very thin films (30 nm). Any structural inhomogeneity, which broadens the resonance in the averaging over the ~ 1 cm2 measurement area, will be indistinguishable in the modeling from an effective increase in the scattering frequency. Figure 2 shows the FOM obtained for these three structures. The progression in the FOM (improving from circular to elliptical to rectangular) is similar to that for the transmission. These reported FOMs are about 40% higher than the previous results [1,4] for the circular and elliptical structures in this same material system, suggesting that these samples have an improved 2 homogeneity. The best fit values of the Au scattering NIM with frequency are about 2X the bulk value, while previously we had reported a value of ~3X. CH
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Wavelength( μm) Fig. 2: FOM for the three structures. Essentially identical results are obtained for the elliptical and rectangular structures.
Discussion Experimental and modeling results have been presented for three related NIM geometries. The results are in good agreement with the progression of improved impedance matching in moving from circular to elliptical to rectangular apertures. The FOM are very similar for all three structures, suggesting 1) that the improved impedance match, rather than material loss, is the dominant reason for the improved transmission, and 2) that the improved FOM reported for the Ag system was primarily a result of the different materials used rather than the structure modification from elliptical to rectangular apertures.
1. S. Zhang, W. Fan, N. C. Panoiu, K. J. Malloy, R. M. Osgood and S. R. J. Brueck, “Demonstration of Near-Infrared Negative-Index Metamaterials,” Phys. Rev. Lett. 95, 137404 (2005). 2. V. M. Shalaev, W. Cai, U. K. Chettiar, H-K. Yuan, A. K. Sarychev, V. P. Drachev and A. V. Kildishev, “Negative Index of Refraction in Optical Metamaterials,” Opt. Lett. 30, 3356-3358 (2005). 3. G. Dolling, C. Enkrich, M. Wegener, C. M. Soukoulis, and S. Linden, “low-loss Negative-Index Metamaterial at Telecommunication Wavelengths,” Opt. Lett. 31, 1800 (2006). 4. S. Zhang, W. Fan, K. J. Malloy, S. R. J. Brueck, N.-C. Paniou and R. M. Osgood, “Demonstration of metal-dielectric negative-index metamaterials with improved performance at optical frequencies,” J. Opt. Soc. Am. B23, 434-438 (2006).
