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.
|