Unleashing Plasmonics

利用等离子体激元在纳米尺度上控制光是一个令人兴奋的前景，但受到一个严重的瓶颈，即由于吸收的损失。等离子体涉及使用纳米结构的金属材料来操纵光深入到亚波长范围。允许这种前所未有的控制水平的相同金属也吸收一些光，而正是这种吸收是问题的根源。增益材料的添加被广泛认为是克服这些损耗的少数方法之一。然而，进展缓慢，其背后的物理机制仍远未明朗。在这个项目中，我们将进行一系列新颖的实验，以建立正确理解等离子体激元光放大材料之间相互作用的基础。我们的长期目标是提供克服吸收瓶颈所需的理解，从而释放等离子体的全部能量。就像钟声可以发出特定的铃声一样，光照射在金属纳米颗粒上可以使金属环中的电子。这种振铃模式被称为等离子体激元模式，发生在光学频率处，并且是等离子体激元的核心。就像铃声有一定的音符一样，振铃电子与某种颜色的光强烈相互作用，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，将光限制和增强在远远超过衍射极限的纳米级区域中，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，通过使用等离子体（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颗粒被染料分子包裹，当被激发时，可以放大光。等离子体激元和分子之间的强相互作用意味着当一个激发的分子释放其储存的能量时，而不是作为光子出现，能量反而以与金属纳米颗粒相关的等离子体激元模式的形式出现。这种等离子体激元可能会触发其他被激发的分子以等离子体激元的形式释放能量，导致等离子体激元雪崩。尽管最近的演示令人兴奋，但人们对潜在的物理学知之甚少。另一种激光范例-随机激光-为探索这一新领域提供了一种新的方法。在传统的激光器中，放大是通过使用腔来实现的;相比之下，在随机激光器中，来自嵌入增益材料中的纳米颗粒的随机排列的多重散射用于控制放大。随机激光提供了一种直接的方法来探讨一些关键问题，等离子体模式和增益材料如何相互作用。在这个项目中，我们将合成一系列介电和金属纳米颗粒，包括一些掺杂有能够放大光的发光分子。然后，我们将从这些颗粒中制造胶体，并将研究它们表现出的随机激光行为如何取决于与金属纳米颗粒相关的等离子体共振，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了解增益材料如何修改等离子体模式。我们的研究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.