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

光学显微镜中的透镜是由玻璃制成的--它必须是透明的。你用一块玻璃做一面镜子，在上面涂上金属--它能反射光。但是如果你在金属薄膜上钻一系列的小孔，比光的波长小得多，那么，神奇的是，光就能穿过金属。这种非凡的行为是由等离子体激元驱动的-金属中电子气的振荡。等离子体激元可以通过吸收光而产生，也可以通过发射光而衰减：换句话说，它们提供了一种传输光学信息的新方法。存在不止一种等离子体激元-小颗粒（簇）具有局部等离子体激元模式，但在扩展的金属表面上，您可以获得传播模式，其显示色散。这意味着表面等离子体激元的频率取决于波长。我们可以想象，如果我们把一个金属簇放在金属表面上，两种不同的等离子体模式将耦合在一起，并且在特定的波长下，这种耦合可能非常强。像这样的效应可以用于新型光学器件、光子电路或生物传感器。该项目的目的是使用一种称为电子能量损失光谱的技术来测量表面上装饰有选定大小和形状的原子团簇的等离子体分散，并探索这些系统中等离子体的局部化和耦合现象。我们需要设计这样的系统，其中团簇模式的频率与表面模式的频率相差不远。金表面上的银团簇是一个很好的选择，特别是如果银团簇的形状从球形变形为椭圆形，这会降低其等离子体频率。然后我们可以看到两个模式在特定波长处的共振耦合。我们还可以设想将其他种类的局部激发模式耦合到表面等离子体，例如半导体团簇或分子中的所谓电子-空穴对激发。实验的原理是一样的。但我们也想尝试一种完全不同类型的测量，我们在纳米尺度上测量真实的空间中的等离子体激元场，在单个集群周围和之间。这是探索团簇和表面等离子体激元之间耦合效应的补充方式，并利用了我们发明的一种新技术，称为扫描探针能量损失光谱（SPELS）。就像等离子体激元一样，这是魔法！

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!