Abstract
Plasmonics takes advantage of the properties of surface plasmon polaritons, which are localized or propagating quasiparticles in which photons are coupled to the quasi-free electrons in metals. In particular, plasmonic devices can confine light in regions with dimensions that are smaller than the wavelength of the photons in free space, and this makes it possible to match the different length scales associated with photonics and electronics in a single nanoscale device1. Broad applications of plasmonics that have been demonstrated to date include biological sensing2, sub-diffraction-limit imaging, focusing and lithography3,4,5 and nano-optical circuitry6,7,8,9,10. Plasmonics-based optical elements such as waveguides, lenses, beamsplitters and reflectors have been implemented by structuring metal surfaces7,8,11,12 or placing dielectric structures on metals6,13,14,15 to manipulate the two-dimensional surface plasmon waves. However, the abrupt discontinuities in the material properties or geometries of these elements lead to increased scattering of surface plasmon polaritons, which significantly reduces the efficiency of these components. Transformation optics provides an alternative approach to controlling the propagation of light by spatially varying the optical properties of a material16,17. Here, motivated by this approach, we use grey-scale lithography to adiabatically tailor the topology of a dielectric layer adjacent to a metal surface to demonstrate a plasmonic Luneburg lens that can focus surface plasmon polaritons. We also make a plasmonic Eaton lens that can bend surface plasmon polaritons. Because the optical properties are changed gradually rather than abruptly in these lenses, losses due to scattering can be significantly reduced in comparison with previously reported plasmonic elements.
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Acknowledgements
The authors acknowledge funding support from the US Army Research Office (MURI programme W911NF-09-1-0539) and the US National Science Foundation (NSF Nanoscale Science and Engineering Center CMMI-0751621).
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T.Z., Y.L. and J.V. conceived and designed the experiments. T.Z. and M.H.M. performed the experiments and analysed the data. Y.L. designed the structures and performed the numerical simulations. J.V. and M.H.M. fabricated the samples. X.Z. guided the theoretical and experimental work. All authors discussed the results and co-wrote the manuscript.
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Zentgraf, T., Liu, Y., Mikkelsen, M. et al. Plasmonic Luneburg and Eaton lenses. Nature Nanotech 6, 151–155 (2011). https://doi.org/10.1038/nnano.2010.282
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DOI: https://doi.org/10.1038/nnano.2010.282
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