Unleashing Plasmonics

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.

Photonics is a major growth area and is a major enabler for an increasing array of technologies that span communications, data storage, health care and energy provision. Nanophotonics involves the merger of nanotechnology and photonics to manipulate light in ways that allow greater levels of miniaturisation, functionality and integration. World-wide there is the equivalent of an arms race to drive innovation in nanophotonics as a means to commercial success. Such innovation requires new approaches, of which plasmonics is seen as a key component. Plasmonics is however at a cross-roads, one that needs the development of new basic science tif its potential is to be realised.

The proposed project is just such a basic science enquiry, it is therefore appropriate and important that activities to promote impact are directed where they will have most effect - this is primarily a variety of academic communities. The plasmonics community will benefit from a better understanding of the way gain can modify plasmonics responses and in particular how gain can help mitigate the losses associated with absorption in the metallic nanostructures - it is a much sought after development. Through presentation of project results at conferences we will engage the plasmonics community from the outset of the project. The second community to be engaged will be the multiple scattering community. Access to this community will come primarily through the collaborators in the Netherlands (COPS) who are world-leaders in this area.

At present the plasmonics and light-scattering communities are rather separate. Since this project builds on both activities it provides an ideal opportunity to bring these communities together. We will organise a special workshop at Exeter to foster this joining of communities, and to promote the creation of new directions through the exchange of ideas. Dr Sapienza has agreed to co-organise this workshop with us in 2014, the workshop is planned as an integral part of the project. The project will thus act as a catalyst to promote these interactions, with a special emphasis being placed on a genuine exchange of ideas to allow collaborations to grow. Further impact is likely to develop through the collaboration with the project partner involved in the nanoparticle synthesis (Mulvaney) - this will give us access to a different, more chemistry-focused community.
Other communities that will benefit from the research include metamaterials where gain is seen as a key ingredient in taking successful advantage of the metamaterials concept. The PI is a co-investigator on the QUEST programme grant (EP/I034548/1), a multi-institution project to develop metamaterials for Transformation Optics. This project provides an excellent opportunity to engage the metamaterials community with this work, especially through contacts developed with the Industrial Advisory Board, including representatives of BAE systems, dstl, Flann Microwave and NPL).

More specific potential developments, polymer lasers and lab-on-a-chip lasers, were discussed above - they offer potential routes for impact derived from the research proposed here.

Should intellectual property emerge from the project the applicant pursue patent application. The applicant has had success with 5 previous such applications, and institutional help is available from the Research and Knowledge Transfer office at the University of Exeter.

Finally, a valuable impact from basic science projects comes from the training of skilled researchers. The two people on this project, the PDRA (funding requested) and PhD student (funding already secured). They will contribute to the growing cadre of young photonics-related researchers now being trained in the UK.