Brightening the dim modes of plasmonic nanostructures

Tiny metal structures can act like WiFi antennae for light, connecting single molecules or artificial atoms to light that travels out and can be detected. This project will look at that radiated light, measuring the pattern it makes in space to learn more about the tiny structure that it came from - and then using that pattern to look even more closely at the molecules inside the structure.

Light usually travels as an electromagnetic wave through free space. However, just as radio waves can cause an electric current in the receiving aerial, light can make the electrons in tiny metal structures move back and forth in time with its electric field. If the metal structures are the right shape and size, the electrons resonate with the light, and interact very strongly. Effectively, the light is concentrated into the metal and then scattered back out again. The light that comes back is not uniform - it all comes from the same point, but it radiates in different directions with different strengths. The direction of the light's electric field (the polarisation) also varies with the direction it's going in. That can make it very hard to detect with normal microscopes.

This project will create new microscopes that are able to measure the way the light varies with the direction of travel - its "spatial mode". By detecting different spatial modes, we will learn the modes that are emitted by nanostructures and be able to measure the different ways electrons resonate in the metal structures. If we understand the resonances, it will allow us to design better structures.

Concentrating the light into tiny volumes means it becomes incredibly bright; this allows us to detect very weak scattering signals from the vibrations of individual molecules. This process, known as Raman scattering, is particularly useful because it can identify molecules without needing to create a chemical dye that allows us to detect them. That means we can use it to detect tiny quantities of biomarkers in blood or urine samples for diagnostics, or screen for trace contaminants in foods.

As well as using spatial modes to probe resonances that correspond to visible light, we will create a new microscope that can see in UV and infra-red light as well, using the same spatial modes that couple well to self-assembled nanostructures. By measuring across a wide range of wavelengths, it's possible to detect similar resonances in metals like aluminium and gallium, as well as the silver and gold that are normally used, because they resonate at visible frequencies. Using a wider range of metals could make the structures cheaper, but also lets us separate out what's due to the metal and what's due to the particular shape of the nanostructure. Extending into the infra-red will allow us to see modes that we have theoretically predicted, but rarely been able to measure. Being able to use UV light makes it possible to measure structures with resonances that match those of many biomolecules, which could have important applications in detecting those molecules.

Our work will understand, then control, the patterns of light that go into, and come back out of, tiny metal structures. By acting like WiFi antennae for light, they can connect light to molecules, and let us make new sensors and devices. The work in this project will let us "tune in" more efficiently to a range of different nanostructures, potentially making these miniature sensors more sensitive and more efficient.

This research will lead to better optimised, better understood plasmonic structures for enhanced Raman scattering, directed single photon emission, and data storage. This has immediate benefits for industrial R&D as well as academic research, and longer-term impacts in sensors and components that will be embedded in consumer products enabling health monitoring and quality control. There are also applications in data storage that will have an impact on the way data is stored in server farms of the future, and enhanced quantum emitters to create more efficient on-chip single-photon sources for next-generation secure communications.

We will measure and optimise the way localised plasmons couple with propagating light. This will strengthen the scientific understanding behind the nanostructured substrates often used to enhance Raman scattering. By improving the underpinning science, our work will have a long-term impact on the engineering of these substrates and surrounding optics, leading to more efficient integrated sensors.

As part of our experimental programme, we will develop lab-scale prototypes of more efficient ways to couple light (the usual method of driving and reading out plasmonic sensors) to plasmonic nanostructures. Our novel measurement techniques will be of immediate interest to other research groups in the field, as well as instrumentation companies who supply them. We will build on existing relationships (e.g. with Renishaw and Meadowlark Optics) to explore commercialisation options for our instruments, primarily as a way to achieve impact in industrial and academic R&D labs.

Once we have determined the spatial modes that are important our measurement techniques can also be scaled down into inexpensive integrated optical components. This will form the basis of a future project, and it is these components that will find their way into consumer products. Efficient, plasmonically enhanced Raman scattering has the potential to transform continuous healthcare monitoring, and greatly enhance quality control of food and drugs. For example, it could monitor trace biomarkers in urine, blood or saliva to pick up medical conditions before the patient has noticeable symptoms through integration with devices that are used daily.

Through improved scientific understanding, and better measurement techniques, we will advance the current state of the art in self-assembled nanostructures. Our work will have both an immediate impact in the research community, and a much wider long-term impact through better sensing, storage, and communications technologies. We will particularly pursue applications in sensing, creating efficient, low-cost ways to address self-assembled nanostructures for more sensitive molecular detection.