SCOTS scientists have found a way to slow down the speed of light in a move that could boost science and medical diagnosis.
Professor Kishan Dholakia and Dr Yoshihiko Arita of the School of Physics and Astronomy at the University of St Andrews teamed up with Dr Mark Scullion and Professor Thomas Krauss of the University of York to create a specially fabricated nanostructure and used it to drive particles at high speed along a track of light.
Dholakia said: “The team has performed a very exciting study that shows a new way to speed up the microscopic world, making it easier to transport fragile biological material that in turn could have major applications in fundamental science and even medical diagnosis.”
The research could open up more rapid methods of understanding disease or the way in which we look at the biological world in general.
As light bends through a transparent object such as a marble, it exerts a minuscule but important force. The marble would not move as the force is too weak, but the force is sufficient to move and propel particles the size of blood cells or smaller.
Dholakia explained: “We are very excited about this. It turns out that light can exert a small force, in other words it can exert motion on an object.
"So if I shine light on a mirror, for example, it bounces back. A physicist might say ‘why doesn’t the mirror move?’. But that light force is very small. In science fiction we have the idea of a tractor beam where light can somehow grab and move objects without damaging them.
"It turns out science fiction is actually science fact. We can’t move objects the size of a wine glass or a ball but if we go down to the size of a cell in our body or smaller, that force of light that’s shining off the mirror, though incredibly weak, is strong enough to move a cell, or squash a cell.
"It can look at DNA and allows us to understand the microscopic world. People can understand the basics of disease and separate diseased cells from non-diseased cells and so on, which is great.”
Light moves at 186,000 miles per second but can be slowed down in glass by a third. The researchers designed special nanostructures made from silicon that affect the motion of photons, called photonic crystals, to reduce the speed further.
The effect is like placing speed bumps into the light’s path, and the researchers managed to slow the light down by 95 per cent, corresponding to a reduction by a factor of 20. The team then placed particles on top of the photonic crystal and used the slow light effect to actually increase the propulsion of the particles, as the slower light exerts a stronger force on the particles than would otherwise be the case.
Optical forces can allow scientists to move, collect and sort minuscule particles in the biological world, including separating abnormal from healthy cells or viruses.
Dholakia added: “Light force is very weak so if we could get that same amount of light but somehow make it work harder for us that would be really very helpful.
"Thus instead of having light moving at the speed of light in a vacuum, we can create little micro-structured elements, made from silicon, and what happens is the light in these structures, instead of just going at that free space speed, it sloshes around in there for a while.
"It is like taking three steps forward and two steps back.”
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