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Lasers beams build and hold nanoscale structures
16:45 23 January 2006.
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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.
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 told New Scientist.
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. "But
we really don''t understand the basic physics well enough."
Evanescent wave
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.
Light work
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 reseae world are also exploring optical binding.
Kishan
Dholakia, from one such 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," he told New Scientist. "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.
Journal reference: ChemPhysChem (DOI: 10.1002/cphc.200500348)