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Posted: Jan 24, 2006

Laser beams build and hold nanoscale structures

(Nanowerk News) A form of matter held together by nothing more substantial than light has been created by physicists in the UK. The method, known as "optical binding", was used to glue together about 100 polystyrene beads each 400-nanometres in diameter in a flat two-dimensional structure. The findings were reported in the Dec. 27, 2005 issue of ChemPhysChem.
Colin Bain from the University of Durham and Christopher Mellor from the National Institute for Medical Research say the phenomenon might one day provide a simple way to construct, or reconfigure, nanoscale structures. "It is an entirely new way of making regular nanostructures," Bain says. He adds that changing the parameters of the laser used, or the shape of the particles, could generate very different types of structure. "It's a possible way of making much more complicated patterns," he says. " we really don't understand the basic physics well enough."
The researchers used twin laser beams and a prism to make the nano-beads self-organise exploiting the mechanical force exhibited by light on the microscopic scale. The technique is related to that employed by optical tweezers, which use a focused beam of light to trap and manoeuvre an individual particle. But optical binding relies on the way particles scatter light between one another. If they do this in a uniform manner, the beads will automatically align themselves in a checker-board arrangement, with equal spaces between one another, under the influence of a single beam. In the researchers' experiments, two laser beams pass through the prism, on top of which is a solution containing the nanobeads. As the beams hit the top of the prism they are reflected away but a small amount of energy known as an "evanescent wave" escapes from the prism's surface and causes the nanobeads to align. The energy produced by the two lasers secures the beads in one place.
Bain says it is too early to tell how the technique might eventually be employed. But he suggests that it could be used to control the flow of micro-fluids through nanoscopic valves. But the time it takes for the beads to arrange a fraction of a second suggests that the process is too slow to lend itself to computation, he says.
Several other research groups around the world are also exploring optical binding. Kishan Dholakia, from the Optical Trapping Group at the University of St Andrews, UK, says that optical binding could perhaps be used to influence the way crystals grow. "Self-assembly is a big deal. My dream is to shine just a laser pointer and let the particles do all the work for us." Dholakia's group has published several articles on optical binding and he says the group will publish details of more complex experiments later in 2006. But Dholakia also concedes that modelling the way light will scatter between particles, in order to predict optical binding effects, remains a very complex computational problem. "It's one of the biggest challenges," he says.
Source: University of Durham
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