Sensing molecules by light using a substrate of nanoscale gold vocals

Cambridge’s new NanoPhotonics Centre is creating novel properties of light and matter at the nanoscale.

The trick in nanophotonics is to internally "chop up" materials on the size scale of the wavelength of light, which is several hundred nanometres for visible light.

Most components in our bodies are fantastically sophisticated assemblies of molecules working on size scales from atoms to cells to organs. As optical materials, however, they are inert transparent jelly. But can this assembly be adapted to build the optical devices of the future? Our knowledge of how such structures can self-assemble on the nanometre scale – on the order of a billionth of a metre in size – is expanding at the same time that we are discovering the extraordinarily rich possibilities of making new nanomaterials that have unusual optical properties.

The intricate arrangement of metals, glasses and active light emitters in three-dimensional architectures controls how photons of light interact with them; nanophotonics is the study of how photons behave with materials at the nanometre scale. The recent explosion of interest in this discipline heralds entirely new ways of manipulating light for applications ranging from healthcare to energy production. Co-opting the processes honed in the natural world for making tuneable optical materials is one of the aims of Cambridge’s NanoPhotonics Centre, funded by the Engineering and Physical Sciences Research Council (EPSRC) and opened in April 2008.

Photonic crystals and artificial opals

The trick in nanophotonics is to internally ‘chop up’ materials on the size scale of the wavelength of light, which is several hundred nanometres for visible light. Light can become schizophrenic in such environments, flipping from one constituent into the other depending on its exact colour. So-called ‘photonic crystals’ have allowed researchers at the Centre to make super-prisms that separate the colours of light thousands of times more widely than glass, enabling new generations of optical chips for biosensing.

Recently, new ways have been found to squeeze together regular plastic nanoparticles in ways that assemble them into regular photonic crystal stacks on the nanoscale – the resulting sheets are artificial opals with intense iridescent ‘structural colour’ coming purely from their internal structure. Such materials might be used to replace toxic dyes and often exhibit unusual properties: for instance, it is possible to extrude photonic threads that change colour dramatically when stretched.

Towards meta-materials

Coinage metals such as gold or silver are also unusual optical environments as they can trap light, which surfs along their surface. Light in this form is compressed so that the critical dimensions for nanostructuring drops to tens of nanometres. Such structures can act as aerials for light, greatly enhancing absorption and emission by single molecules or nanoparticles. A whole new class of ‘plasmonic’- or ‘meta’-materials is emerging that combines nanostructured metals with active and passive materials to produce novel effects, such as: sub-wavelength imaging for viewing working nanomachinery inside living cells; ultra-high-sensitivity spectroscopy to watch surface catalysis in real time; ultra-small lasers that require minimal energy to turn on; and electromagnetic cloaking to hide objects from view.

The NanoPhotonics Centre

Turning this combined vision of advanced physics and novel nanomaterials into practice at the purpose-built state-of-the-art laboratories of the NanoPhotonics Centre requires diverse interdisciplinary partnerships. Interactions between electrochemists, polymer and materials scientists, device engineers, biochemists and healthcare professionals around Cambridge are becoming fruitful and widespread, and are yielding unexpected and exciting research directions. Just as important are the Centre’s interactions with industrial partners (including Kodak, Renishaw and Merck) who are interested in this rapidly expanding research area. Not only does this encourage research to focus on the practicality of manufacturing nanomaterials on the large scale, but it also effectively brings NanoPhotonics out of the lab and into our hands.

For more information, please contact the author Professor Jeremy J Baumberg (jjb12@cam.ac.uk),

Director of the NanoPhotonics Centre (www.np.phy.cam.ac.uk), at the Department of Physics.


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