Scientists from the UK and Spain describe in this week's Science Express (8 July) how artificial materials with tiny grooves and holes drilled into their surfaces could channel and focus light beams on a chip.
When light hits the surface of a metal such as silver, as well as a reflection, another form of light is excited at the surface. This light, bound to the surface as a small mixture of light and electrons, is called a surface plasmon, its behaviour likened to waves on the surface of a 'sea' of electrons. For many years a curiosity, the properties of plasmons have only recently been fully explored.
In their paper this week, the theorists show that holes perforating a surface can spoof the creation of these plasmons, and they suggest that the effect could be harnessed to channel light at tiny scales, overcoming one of the constraints facing designers of the first optical computer.
"They aren't really plasmons but they behave like them," says Professor Sir John Pendry of Imperial College London and first author of the paper. "They capture light and lock them up in very tiny spaces."
The holes, which may be just a few tens of nanometres wide, can be made using a special 'drill' called an ion-beam. A human hair is 100 times larger in diameter by comparison.
This work suggests that engineered surface plasmons could be as simple as drilling holes in a perfectly conducting material.
"It opens up a new dimension of design for the people looking to use surface plasmons to put light on a chip," says Sir John.
By analogy with an electronic chip full of transistors, the most basic requirement is to join the bits together with wires. But in using light instead of electrons the challenge is how to replace the wires to move light around the chip. Optical fibre is not the answer as it is 50 microns wide and as big as the chip.
"Instead of etching a path on a chip, now we could drill holes to make a path to control light on a chip," says Sir John. "The plasmons contain the same signals as the light exciting them and therefore can be used to transport information across the surface."
"Alternatively we could send the plasmons across the surface in free flight, rather than in channels. We could drill holes to make lenses to focus it."
Another use could be in shaping light. As light goes through holes in surfaces, smaller drilled grooves around the hole act to stop the light spreading out, focusing it instead, and in effect forming one of the tiniest lenses in the world at just a few microns wide.
Research by Thomas Ebbesen and colleagues in 1998 at the University Louis Pasteur, Strasbourg, demonstrated a way of forcing light to go through tiny holes at the surface of a metal. By turning light into a surface plasmon then back again, they demonstrated that the effect worked, but only with the metals silver and gold. The theorists speculated that a material could be engineered that does not naturally have surface plasmons yet still has the same effect.
"It turns out that if you take something completely inert, just by drilling holes you can make it behave as if it's got these surface excitations," says Sir John. "If you've got holes and you try to bounce light off the surface some light stays stuck in the holes, just as if it were stuck to the surface of silver in a surface plasmon."
Surface plasmons were first described by Rufus Ritchie in the 1950s and subsequently applied by Ritchie and others to energy loss by the high voltage electrons in an electron microscope.
This work is part of a European collaboration between researchers at Imperial, University of Exeter, Strasbourg, Madrid and Zaragoza. The European Commission provided financial support for this research under project FP6-NMP4-CT-2003-505699.
For further information, please contact:
Professor Sir John Pendry
Condensed Matter Theory Group
Blackett Laboratory, Department of Physics
Imperial College London Press Office
Notes to Editors:
This research appears online in the journal Science Express on Friday 8 July 2004. www.sciencexpress.org
Title: 'Mimicking Surface Plasmons with Structured Surfaces'
Authors: JB Pendry 1, L Martín-Moreno 2, and FJ Garcia-Vidal 3.
(1) Imperial College London, Dept. of Physics, Blackett Laboratory, London, SW7 2AZ, UK.
(2) Departamento de Fisica de la Materia Condensada, ICMA-CSIC, Universidad de Zaragoza, E-50009 Zaragoza, SPAIN.
(3) Departamento de Fisica Teorica de la Materia Condensada, Universidad Autonoma de Madrid, E-28049 Madrid, SPAIN.
Short biography of Professor Sir John Pendry FRS
Professor Sir John Pendry FRS BA MA PhD FInstP, aged 60, has been Professor of Theoretical Solid State Physics at Imperial College London since 1981. Professor Pendry is a theoretical physicist renowned for his work on the structure of surfaces and their response to electrons and photons. He has published over 200 scientific papers and was elected a Fellow of the Royal Society and the Institute of Physics in 1984. At Imperial, he was Dean of the Royal College of Science from 1993-1996; Head of the Department of Physics from 1998-2001; and first Principal of the Faculty of Physical Sciences from 2001-2002. From 1975-1981 he was Head of the Theory Group at the Daresbury Laboratory. Sir John was knighted in the Queen's Birthday honours list in June 2004 for services to science.
About Imperial College London
Consistently rated in the top three UK university institutions, Imperial College London is a world leading science-based university whose reputation for excellence in teaching and research attracts students (10,000) and staff (5,000) of the highest international quality. Innovative research at the College explores the interface between science, medicine, engineering and management and delivers practical solutions that enhance the quality of life and the environment - underpinned by a dynamic enterprise culture.
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