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Graphene Controls Colour of Plasmonic Nanoantennas J. Mertens1, A. L. Eiden2, C. Tserkezis3, J. Aizpurua3, A. C. Ferrari2, J. J. Baumberg1 1 NanoPhotonics Centre, Cavendish Laboratory, Department of Physics, University of Cambridge, UK 2 Cambridge Graphene Centre, University of Cambridge, UK 3 Materials Physics Centre, Donostia-San Sebastián, Spain Why nanoantennas? • APPLICATIONS: Nanoantennas concentrate light in small gaps. Trapped light in the antenna provides a nanometre-scale sensor that changes colour depending on the environment • FUNDAMENTAL: We can observe quantum processes for gaps in the subnanometre regime using light • light causes the electrons in metallic nanoparticles (NPs) to oscillate back and forth Nanoparticle on mirror light field What is plasmonics? time oscillating electrons • dimer geometry is useful for surface enhanced Raman scattering to study substances in gap • problem: Dimers are hard to fabricate / handle • solution: Equivalent geometry – NP on a mirror the particle shape and size define the frequency of oscillation, i.e. the colour light is strongly enhanced in a gap when NPs are close together (dimer geometry) NP and its reflection one atomic thin gap • graphene: thinnest possible spacer robust, stable, and controlled subnanometre gap Graphene sets the colour We observe coupling between a NP and its reflection using a microscope: bulk graphite white light dark field microscopy few layer graphene 1 Scattering [normalised] dark field scattering spectra of single NPs on different graphene layers 0 1 monolayer graphene 𝑺𝑷 𝑸 5 layer graphene 𝑷 𝒄𝒐𝒖𝒑𝒍𝒆𝒅 𝑺𝑷 0 500 600 700 Wavelength [nm] 800 The coupled resonance shifts with the number of graphene layers: we observe different plasmon modes SP: single particle resonance – visible without a mirror coupled: plasmon-image coupling – depends on junction between NP and mirror • splitting of coupled resonance only for single graphene layer (1) Interpretation: Plasmonic dipole coupling The spectroscopy indicates a mixing of plasmonic dipoles: global dipole: electron tunnelling through graphene layer (charge transfer) gap dipole: highly localised charges at gap • parallel and antiparallel alignment leads to different energies • quantum mechanical effect sets splitting due to gap of 0.34 nm P -resonance Q -resonance low energy high energy What can it be used for? • tuning plasmon resonances: optically with semiconductor spacers (2) or electronically via gating • non invasive optical probing of electronic and optical properties of the spacer material (1) Mertens et al., Nano Lett., 2013, 13 (11) (2) Sigle & Mertens et al., to be published, 2014 www.np.phy.cam.ac.uk