Localisation and Coupling of Plasmon Modes in Size-Selected Cluster Films Probed by EELS

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A lens in an optical microscope is made of glass - it has to be transparent. You make a mirror from a piece of glass by coating it with metal - which reflects light. But if you drill a series of tiny holes, much smaller than the wavelength of light, in a metal film, then, magically, the light goes through the metal. This remarkable behaviour is driven by plasmons - the oscillations of the electron gas in a metal. Plasmons can be created by the absorption of light and can decay by the emission of light: in other words, they provide a new way to transmit optical information. There is more than one kind of plasmon - small particles (clusters) have local plasmon modes but on an extended metal surface you get propagating modes, which show dispersion. This means that the frequency of the surface plasmon depends on the wavelength. We can imagine that if we place a metal cluster on top of a metal surface, the two different kinds of plasmon mode will couple together, and that at a special wavelength this coupling could be very strong. Effects like this could be exploited in novel optical devices, photonic circuits or biosensors. The aim of this project is to use a technique called electron energy loss spectroscopy to measure the plasmon dispersion on surfaces decorated with atomic clusters of selected size and shape, and to explore the phenomena of localisation and coupling of the plasmons in these systems. We need to design systems in which the frequency of the cluster mode is not far away from that of the surface mode. Silver clusters on a gold surface make a good choice, especially if the silver cluster is distorted in shape from spherical to ellipsoidal, which lowers its plasmon frequency. Then we may see a resonant coupling of the two modes at a specific wavelength. We can also envisage coupling other kinds of local excitation mode to the surface plasmons, such as so-called electron-hole pair excitations in semiconductor clusters or molecules. The experiment works the same way. But there is also a completely different type of measurement we want to attempt, where we measure the field of the plasmon in real space, on the nanometre scale, around and between the individual clusters. This is complementary way to explore the effects of the couping between the plasmons of the cluster and the surface, and exploits a new technique we have invented, called Scanning Probe Energy Loss Spectroscopy (SPELS). Like the plasmons, it's magic!