Plasmonics

Key personnel: Prof Bill Barnes, Prof Roy Sambles

Controlling the interaction between light and matter is fundamental to science and to technology – from probing entanglement in quantum physics to harnessing the spectacular information carrying capacity of optical fibres. Nanoscale fabrication (such as focussed ion beam lithography techniques) allow one to make new materials with increasing sophistication and freedom of design, but controlling light at the nanoscale remains a challenge. Traditionally light can only be controlled on length scales down to a little below the wavelength of light, a few hundred nanometres, hence the usual resolution limit of optical microscopes and telescopes. Without a means to control light on length scales down to a few nm, both nanoscience and nanotechnology will be much the poorer. However, a new paradigm called plasmonics is emerging, an approach based on using the localised surface plasmon resonances of metal particles to control light below the wavelength limit, down to nanometre length scales.

Localised surface plasmon resonances (LSPR) comprise electromagnetic fields that are bound to a metallic object, for example a nanoparticle, through the interaction of the electromagnetic field with the free electrons in the surface of the metal. This interaction is resonant at a frequency (wavelength) that depends strongly on the particle’s size, shape, composition and environment. At resonance the fields associated with these modes are very significantly enhanced, they are also evanescent or near-field in character, falling exponentially in strength with distance away from the particle. Crucially, this means that the light may be localized into a volume of space only ~10 nm in dimension. Through such an approach, and through related approaches demonstrated in the microwave regime, there is now the very real prospect of controlling visible light at the scale – a new optics at the nanoscale can thus be envisaged. Surface plasmons are also encountered in more extended metal structures where the field enhancement can be exploited for monitoring bimolecular interactions, they can also be manipulated through suitable surface structuring. Based as it is on the plasmon modes of metals this field is known as plasmonics.

(Right) Some of the particles produced by Andy - the top row shows scanning electron microscope pictures of four different particles made using electron-beam lithography. The lower row shows the appearance of these individual particles as they appear by eye in a dark-field microscope. Notice how small changes in shape and size of the particles leads to dramatically different colours. These colours arise because of the way the electrons in the particles can be set into resonant motion by incident light. Different shapes/sizes of the particles lead to different resonant frequencies, and hence different colours - demonstrating one aspect of the control available with plasmonics.

The remarkable progress in plasmonics in the past few years, both in developing a new photonics, and in concentrating light into ever smaller volumes, opens up even more opportunities for the future. In particular it opens the way to new approaches in controlling the optical properties of molecules, and in using optics to monitor molecules. Understanding the new physics involved will enable progress in biology, chemistry and materials science, and will enable new devices to be made. At Exeter our work is focused on exploring the extent to which plasmonics can be used to control light, and in particular exploiting the unique attributes of plasmonics to develop new materials – and extending the concepts of plasmonics into other spectral regions, especially THz. For a review of the field see the article "Surface plasmon subwavelength optics".