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Mapping surface plasmons

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It is well known that a silver fork reflecting light appears grey, or that a gold ring is golden. Stained-glass church windows exhibit various colours, although they are also made up of gold or silver. The colour difference stems from the different morphologies of the metals: continuous films in the first case, nanoparticles in the second. This effect is known since ancient times, and has been theoretically described since a century: when metal particles have dimensions lower than the wavelength of the light illuminating them (typically, a few hundreds of nanometers), the colour of these particles depends on their size and their shape. To illustrate this effect, we can use the analogy of a vibrating piano string. For a given tension on the string, the note produced by the vibrating string depends on its length: the shorter the string is, the more quickly it vibrates and the sharper the sound produced. For nanoparticles under light illumination, a similar effect occurs. If one sends an electromagnetic wave (the light) on a nanoparticle, the latter will be set in "vibration". These vibrations of the electromagnetic field are called "surface plasmons��. Just like in the case of the string, the smaller the nanoparticle, the faster the electromagnetic vibrations will be, and the nanoparticle will appear bluer in colour. We thus understand that the colour of nanoparticule depends on its size. Moreover, nanoparticles can be of different morphologies, bi or tri-dimensional (spheres, cubic, planar triangles...), and thus, can sustain several characteristic vibrations, i.e several colours. In the far field, the colour of the nanoparticle will be a mixture of the various constituent colours. What happens if now one investigate the surface of the nanoparticle : can one assign specific colours to the different parts of the nanoparticle? This question is less simple that it may seem at first sight, since the concept of colour is in general not well defined for sub-wavelength dimensions. To answer this question, we performed Electron Energy Loss Spectroscopy measurements in a Scanning Transmission electron Microscope on individual silver nanoparticles. We have shown that it was possible to measure the colour of these nanoparticules with an unprecedented spatial resolution (at least 100 times higher than the wavelength). Moreover, we showed that the constituent colour displays specific distributions on the surface of the nanoparticles (see figure). This work, resulting from a European collaboration including a Spanish team for sample production, a French team for the experiments and two teams (one Belgian, the other Spanish) for the theory, provides a new and promising tool for the characterization of structures used in the emergent field of "nano-plasmonic".