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Chameleon Metals: from nanostructures for plasmon engineering to molecular detection

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The interaction of light with metallic surfaces is strongly enhanced when the surface is patterned on the wavelength scale. Arrays of sub-wavelength holes exhibit many interesting properties mediated by the plasmonic surfaces. Here we demonstrate spectral and angular dispersion measurements of both surface and localised plasmons on nano-structured surfaces composed of spherical voids. We reveal a new strong coupling between the plasmonic-crystal modes and the localised plasmons, which allows plasmonic atoms to communicate. We show that such nanostructured plasmonic substrates have widespread application in molecular sensing. Nano-structured surfaces are formed by electrochemical deposition through an ordered template of self-assembled latex spheres.[1,2] The technique of casting intricate 3D objects was invented five thousand years ago in Mesopotamia. We have found a way to re-invigorate this technique on scales of billionths of a metre, to help us make nanostructures with extremely unusual optical properties.[3] Control over the charge while plating allows precise control of the thickness of the metallic mesh. This process is used to grade the thickness of the sample from shallow dishes to almost entirely encapsulated spherical voids. Surface plasmons are efficiently excited due to the regular array of close-packed dishes; localised plasmons on the other hand reside in the deep cavities at larger sample thicknesses. We show there exists a critical geometry which induces full localisation, corresponding to a Mott transition for surface plasmons. By measuring the spectral response of the samples at different thickness and incident angles the full dispersion is revealed. Of particular interest is the strong mixing and anticrossing of surface and localised plasmons at intermediate thicknesses. These new mixed states correspond to the crossover to hopping plasmonic transport.[4] Understanding the mixing of plasmon states leads to an in-depth knowledge of the electric-field distribution on the nano-structured surfaces allowing the optimisation of efficient plasmonic devices, such as SERS sensors [5,6]. [1] S. Coyle et al., Phys. Rev. Lett. 87, 176801 (2001) [2] P.N. Bartlett et al., Faraday Discussions 125, 117 (2003) [3] J.J. Baumberg, commentary, Nature Materials 5, 2 (2006) [4] T.A. Kelf et al., Phys. Rev. Lett. 95 116802 (2005) [5] J.J. Baumberg et al., Nano. Lett. 5, 2262 (2005) [6] [7] S. Cintra et al., Faraday Discussions, 136, 16 (2005)

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