A research project led by Prof Bill Barnes, University of Exeter (Physics and Astronomy)

This project is funded by the Leverhulme trust

In this project I will bring together what have traditionally been regarded as two different areas of optics, molecules and metals, to create new possibilities. A major problem in exploiting molecules for optics is that of size, molecules are approximately 1000 times smaller than the wavelength of light, as a consequence molecules and light generally interact very weakly. The same electrons that enable gold, silver etc. to conduct electricity so well also give them their alluring lustre. Astonishingly, these same electrons also allow us to manipulate light down to the scale of molecules -- the nanoscale -- way beyond anything that can be accomplished with conventional optics. Controlling light at the nanoscale has great potential, applications as diverse as treatments for cancer, ways to store data, and the development of more efficient means to harvest energy from the sun are being pursued.

The nano-world spans sizes from one nanometer -- for instance, a small molecule -- up to one hundred nanometers, the size of a flu virus, and one ten-thousandth the diameter of the full stop at the end of this sentence. The nano-world is an exciting place: materials behave differently there, radically new properties arise and different scientific disciplines converge. A flu virus cannot be seen by eye, even with the best microscopes, but, amazingly, a metal particle of the same size can be seen with ease, how so? Light impinging on such a particle sets the electrons into a 'ringing' motion: this ringing mode, known as a plasmon mode, is at the heart of a new field of study called plasmonics. Just as a ringing bell has a certain note, the ringing electrons scatter light of a certain colour, the specific colour depending on the size, shape and environment around the particle. Medieval craftsmen exploited this effect unwittingly; the yellow and red colours in some medieval stained glass windows arise from silver and gold nanoparticles formed as impurities when the glass cooled. Importantly, the motion of the electrons binds the light tightly to the surface of the particle, confining the light to a nanoscale volume where it may interact strongly with molecules.

Metals thus overcome the problem of size. But this comes at a heavy price, for though they are good reflectors of light, metals nonetheless absorb some of the light that impinges on them. This absorption, though small, is a critical limitation that hinders the development of the new optics about which I am so curious. Recent research suggests absorption in the metal can be overcome by adding special molecules to the metal structures, molecules that amplify light. Materials that contain such molecules are known as gain media. Combining plasmonics (nanostructured metals) and gain media (molecules that amplify light) offers a radical solution to the absorption problem. The plasmonics and gain combination is almost completely unexplored and is the focus of this project.

Project duration, Summer 2011 - Summer 2014.

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