Unleashing Plasmonics

利用等离子体控制纳米级的光是一个令人兴奋的前景，但却面临着一个严重的瓶颈，即吸收造成的损失。等离子体学涉及使用纳米结构的金属材料来操纵深入亚波长范围的光。同样的金属，允许这种前所未有的控制水平，也吸收了一些光，这是吸收问题的根源。添加增益材料被广泛认为是克服这些损耗的少数几种方法之一。然而，进展缓慢，其潜在的物理原理仍远不清楚。在这个项目中，我们将进行一系列新颖的实验，为正确理解等离子体光放大材料之间的相互作用奠定基础。我们的长期目标是提供所需的理解，如果要克服吸收瓶颈，从而允许释放等离子体的全部功率。就像敲钟可以产生一定的铃声一样，光撞击金属纳米粒子也可以使金属中的电子产生铃声。这种环形模式，被称为等离子体模式，发生在光学频率上，是等离子体的核心。就像一个振铃有一个特定的音符一样，振铃电子与特定颜色的光强烈地相互作用，具体的颜色取决于粒子的大小、形状和周围的光学环境。至关重要的是，电子的运动将光紧紧地束缚在粒子表面，将光限制并增强在纳米级区域，远远超过衍射极限，在那里它可能与分子、量子点等发生非常强烈的相互作用。在过去的几年里，利用等离子体（金属）纳米腔的激光演示产生了许多令人兴奋的结果。支持等离子体模式的金属纳米粒子被染料分子包裹，当染料分子被激发时，可以放大光。等离子体激元和分子之间的强烈相互作用意味着，当其中一个被激发的分子释放其储存的能量时，能量不是以光子的形式出现，而是以与金属纳米粒子相关的等离子体激元模式的形式出现。然后，这个等离子体激元可能触发其他被激发的分子以等离子体激元的形式释放能量，导致等离子体激元的雪崩。尽管最近的演示引起了人们的兴奋，但人们对其潜在的物理原理知之甚少。另一种激光模式-随机激光-为探索这一新领域提供了一种新的方法。在传统的激光放大是通过使用一个腔实现的；相比之下，在随机激光器中，由嵌入在增益材料中的纳米颗粒的随机排列产生的多次散射被用来控制放大。随机激光提供了一种直接的方法来探索等离子体模式和增益材料如何相互作用的一些关键问题。在这个项目中，我们将合成一系列介电和金属纳米颗粒，包括一些掺杂了能够放大光的发光分子的纳米颗粒。然后，我们将从这些粒子中制造胶体，并将研究它们所表现出的随机激光行为是如何依赖于与金属纳米粒子相关的等离子体共振的，并可能受到其控制。将我们的结果与适当的理论模型进行比较，将使我们能够探索潜在的物理学。我们研究的重点将是更好地理解增益材料如何改变等离子体模式。我们的结果将引起广泛的科学和技术界的兴趣，包括；纳米光子学，超材料，光散射，光通信，成像和生物光子学。

The exploitation of plasmonics to control light at the nanoscale is an exciting prospect, but suffers from a serious bottle-neck, that of losses due to absorption. Plasmonics involves using nanostructured metallic materials to manipulate light deep into the sub-wavelength regime. The same metals that allow this unprecedented level of control also absorb some of the light, and it is this absorption that is at the root of the problem. The addition of gain materials is widely seen as one of the few ways to overcome these losses. However, progress is slow, the underlying physics is still far from clear. In this project we will conduct a series of novel experiments to establish the foundations of a proper understanding of the interaction between plasmonics light amplifying materials. Our longer term aim is to provide the understanding that will be needed if the absorption bottleneck is to be overcome, thereby allowing the full power of plasmonics to be unleashed.Just as a bell can be struck to produce a certain ringing note, so light impinging on a metallic nanoparticle can make the electrons in the metal ring. This ringing mode, known as a plasmon mode, occurs at optical frequencies and is at the heart of plasmonics. Just as a ringing bell has a certain note, the ringing electrons interact strongly with light of a certain colour, the specific colour depending on the size, shape and the optical environment around the particle. Crucially, the motion of the electrons binds the light tightly to the surface of the particle, confining and enhancing the light in nanoscale regions well beyond the diffraction limit, where it may interact very strongly with molecules, quantum dots etc.. Much excitement has been generated in the past couple of years by demonstrations of lasing using plasmonic (metallic) nano-cavities. Metallic nanoparticles that support plasmon modes were coated with dye molecules that, when excited, can amplify light. The strong interaction between plasmons and molecules means that when one of the excited molecules releases its stored energy, rather than emerging as a photon, the energy instead appears in the form of a plasmon mode associated with the metal nanoparticle. This plasmon may then trigger other excited molecules to release their energy as plasmons, leading to an avalanche of plasmons.Despite the excitement generated by this recent demonstration, the underlying physics is poorly understood. An alternative lasing paradigm - random lasing - offers a fresh approach to exploring this new field. In a traditional laser amplification is achieved through the use of a cavity; by contrast, in a random laser, multiple scattering from a random arrangement of nanoparticles embedded in the gain material is used to control the amplification. Random lasing offers a straightforward way to probe some of the key questions about how plasmon modes and gain materials interact. In this project we will synthesize a range of dielectric and metallic nanoparticles, including some doped with light-emitting molecules capable of amplifying light. We will then make colloids from these particles, and will investigate how the random lasing behaviour they exhibit depends on, and may be controlled by, the plasmon resonances associated with the metallic nanoparticles. Comparison of our results with appropriate theoretical models will allow us to explore the underlying physics. The focus of our investigation will be to better understand how gain materials modify plasmon modes. Our results will be of interest to a wide range of scientific and technological communities including; nanophotonics, metamaterials, light scattering, optical communications, imaging and bio-photonics.