Some fifteen years ago a paper reporting a combinatorial approach to materials discoveries revolutionized materials research and other disciplines such as chemistry and pharmacology [1]. Here we report on how a combinatorial approach combined with advanced nanofabrication helps to discover photonic metamaterials optimized for prescribed functionalities

Chen, W.T.

Fedotov, V.A.

Plum, E.

Tanaka, K.

Tsai, D.P.

Zheludev, N.I.

English

Southampton (e-Prints Soton)

A combinatorial approach to metamaterials discoveryE. Plum1, K. Tanaka1, W.T. Chen2, V.A. Fedotov1, D.P. Tsai2 and N.I. Zheludev11 Optoelectronics Research Centre, University of Southampton,Southampton, Hampshire SO17 1BJ, United Kingdom,email: erp@orc.soton.ac.uk2 Department of Physics, National Taiwan University,Taipei, 10617, TaiwanAbstractSome ﬁfteen years ago a paper reporting a combinatorial approach to materials discoveries revo-lutionized materials research and other disciplines such as chemistry and pharmacology [1]. Here wereport on how a combinatorial approach combined with advanced nanofabrication helps to discoverphotonic metamaterials optimized for prescribed functionalities.1. IntroductionIn this paper we report on a new approach to discovery of electromagnetic metamaterials and their op-timization that relies on a parallel synthesis of spatially addressable libraries of materials. A library ofregular arrays of split ring meta-molecules with different combinations of size, asymmetry and line widthwere generated by electron beam lithography on a single glass wafer. The ability to generate and screencombinatorial libraries of metamaterials, when coupled with theory and empirical optical characteriza-tion, may signiﬁcantly increase the rate at which novel optical materials are discovered and optimizedand theoretical predictions tested.In this work we demonstrate the combinatorial approach for photonic metamaterials based on asymmet-rically split rings. Such structures are important for a number of applications such as photonic switches,slow light devices, spectral ﬁlters and the lasing spaser.2. Results and DiscussionAs illustrated by Fig. 1, combinatorial metamaterial discovery consists of two main steps: (i) the creationof a spatially addressable library of metamaterial samples with systematically varying design parametersand (ii) metamaterial characterization with respect to certain desirable properties. Furthermore, simula-tions may be used to identify ranges of promising design parameters, thus speeding up the convergenceon the optimal metamaterial design.Here we have created a spatially addressable library of photonic metamaterials that are based on split ringapertures. Using electron beam lithography such aperture arrays with a size of 30£30 ¹m2 were writtenin a 30 nm thick gold ﬁlm supported by a silica substrate. Key design parameters of such structures arethe split ring asymmetry and the meta-molecule size, see Fig. 2 (a). As illustrated by Fig. 2 (b), ourlibrary contains metamaterials with unit cell sizes s between 400 nm and 500 nm (25 nm steps) and splitring asymmetries t=l between 0.1 and 0.7 (steps of 0.1).It should be noted that for smaller asymmetries no trapped mode resonance can be identiﬁed. For largerasymmetries, simulations indicate even narrower resonances, however, due to the very small splitting ofthe ring such structures are very difﬁcult to manufacture. Remarkably, the dependence of the resonancewidth on the split ring asymmetry is exactly opposite to microwave metamaterials of this type, for whichnarrow trapped mode resonances arise from small asymmetries. This behavior can be explained by thefact that trapped modes at microwave frequencies are governed by scattering losses, while in photonicmetamaterials they are governed by absorption losses.Intriguingly, the ﬁgure of merit also depends on the size of the metamaterial unit cell. While increasinglosses in metals at shorter wavelengths should generally lead to relative resonance broadening for smallerstructures, here we see exactly the opposite behavior. Smaller split rings of the same asymmetry have ahigher ﬁgure of merit. This counter-intuitive behavior may be explained by the plasmonic properties ofgold.    Asymmet r y,   t /l  0.7 0.6 0.5 0.4 0.3 500  475  450  425  400  500  475  450  425  400 Meta-molecule size, s (nm) 1.6 3.2 F OM Simulation  Experiment Narrow resonances Broad resonances Broad resonances Narrow resonances Fig. 3: Combinatorial optimization of photonic metamaterials for switching and sensing applications.The narrowest resonances [high ﬁgure of merit ( FOM)] are achieved for split ring apertures of highasymmetry t=l and small size s.4. ConclusionIn summary we introduce a combinatorial approach to the design of metamaterials. We apply this tech-nique arrays of asymmetrically split ring apertures, which show narrow trapped mode resonances andcould form the basis of a huge range of applications. Here the split ring design showing the best perfor-mance (narrowest resonance) has a large asymmetry of t=l = 0:7 and a unit cell size of s = 400 nm.The resonant properties of split ring nanostructures also depend on the gap position and the width ofthe nano-slits. At the conference we will present a comprehensive combinatorial study taking also thesedesign parameters and the resonance depth into account. Thus we will provide the complete recipe formanufacturing planar photonic metamaterials with high quality factor resonances for applications fromsensing, switches and electromagnetically induced transparency to possibly even a lasing spaser.References[1] X.-D. Xiang, X. Sun, G. Brice˜ no, Y. Lou, K.-A. Wang, H. Chang, W. G. Wallace-Freedman, S.-W. Chen andP. G. Schultz, A Combinatorial Approach to Materials Discovery, Science, vol. 268, p. 1738, 1995.

A combinatorial approach to metamaterials discovery

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