patterns particles in 3D
Technology Research News
For a couple of decades now, nanotechnology
researchers have been able to use beams of light to move microscopic particles.
But the optical tweezers method has been limited to moving individual
particles or several particles as a group.
Researchers from Scotland and Mexico have improved the tool, making it
possible to use photons to arrange microscopic particles into three-dimensional
structures, and to rotate these nanostructures. The improved method also
opens the door for bioengineering applications that involve observing
and affecting the way molecules move in three dimensions.
Optical tweezers are lasers whose photons move tiny objects much like
wind energy moves windmills. A glass particle, for example "bends, [or]
refracts the light and can act like a lens," said Kishan Dholakia, a lecturer
at the University of St. Andrews in Scotland. Changing the direction of
the light passing through a particle "causes forces to be exerted on this
particle," said Dholakia.
When the particle in question is very small, the force of an intense beam
of photons changing direction is enough to pull the particle toward the
brightest region of the beam. This bright region of light can be used
like tweezers to move small objects around.
The St. Andrews researchers noticed that when a group of particles were
drawn toward the light beam at once, they stacked up. "With lots of particles,
they all got drawn into the bright region near the focus of the beam and
aligned themselves as a long tower in the direction [of] the light beam,"
said Dholakia. "We realized that if we had many bright sites we [could]
replicate this in many places, making an extended three-dimensional structure
like [a] cube," he said.
The researchers used a trick of light -- the way two laser beams interfere
with each other -- to make multiple laser tweezers. "The way we made the
many bright spots was to interfere two light beams, analogous to interfering
water waves made by throwing two stones into a still lake," said Dholakia.
In contrast to the flat, pancake-like shape of a water wave, however,
the wave front of light is helical, like a spiral staircase. "If you take
two spirals, each spiraling in opposite directions and add them together
you get a series of [bright] spots," said Dholakia. Even though the spots
are the results of two spirals, the pattern is stationary and each spot
can attract and trap a stack of particles, said Dholakia.
Another key to making the technique work was finding a way to make the
pattern of bright spots rotate in addition to making up-and-down and side-to-side
motions. Achieving this gave the researchers full three-dimensional control
over the particles. "Creating the pattern... and also looking at ways
to get the pattern to move and rotate," were necessary to make the plan
work, said Dholakia.
To do this the researchers used the angular Doppler effect, which is a
more complicated version of the familiar linear Doppler effect heard when
a train whistle seemingly changes pitch as it moves away from a listener.
"We can make the pattern of spots spin around its axis using the angular
Doppler effect" to rotate the three-dimensional nanostructures, Dholakia
To use the effect this way, the researchers started with light beams that
were circularly polarized, meaning the plane of the electric field surrounding
the light rotates. By sending one of the tweezer beams through a waveplate
device the researchers were able to speed or slow, and thus control this
rotation. When the researchers added the controlled tweezer beam to the
second beam "we got our pattern of [spots] to go round," said Dholakia.
"This is a neat way to get interference patterns, in general, to move
and has wide applicability," he added.
The researchers demonstrated their method by making three-dimensional
stacks of silica particles, and transporting and rotating the structures.
The method gives nanotechnology researchers a way to assemble and rotate
microscopic, three-dimensional structures, said Dholakia.
The method could be used in bioengineering, Dholakia said. "Optical tweezers
are very good at grabbing biological materials [like] cells and chromosomes.
This can be used to create ordered arrays and help study things such as
organ and tissue growth," he said.
Observing the way particles that make up substances like milk, ink, paint
or blood collect or aggregate around a uniform structure is a good way
to learn about those particles, said Dholakia. "The dynamics of how these...
complex systems behave and how the particles within them might organize
themselves under a variety of conditions is the subject of intense worldwide
investigation and of central importance in industry and basic science,"
In order to do this, however, it is necessary to be able to create a uniform
microscopic template. "Our cubic and other structures could allow [for]
three-dimensional investigations of these effects," Dholakia said. The
three-dimensional optical tweezers make it possible to observe microscopic
objects in detail, said Dholakia. "We can, with simple video technology,
follow the way a particle moves and behaves over time."
Studying how substances behave around ordered arrays may be useful not
only for biology and chemistry, but may also give scientists insights
into how atoms, electrons and photons interact with materials to create
effects like superconductivity, Dholakia said.
The researchers' next steps are to create bigger three-dimensional structures,
and to develop a way for inspecting the structures. "We're aiming for
a method that will allow us to create extended 3D arrays of particles
in a pretty determined order and even look at defects," said Dholakia.
Ultimately the researchers are aiming to use the method "to understand
fundamental physics as well as looking at bio-problems such as tissue
growth and organization," he said.
Researchers should be able to use the method to make three-dimensional
arrays of particles for use in this type of research within five years,
Dholakia's research colleagues were Michael P. MacDonald, Lynn Paterson
and Wilson Sibbett of the University of St. Andrews, Karen Volke-Sepulveda
of the Mexican National Institute of Optical and Electronic Astrophysics
(INAOE), and Jochen Arlt of the University of Edinburgh in Scotland.
They published the research in the May 10, 2002 issue of Science. The
research was funded by the UK Engineering and Physical Sciences Research
Council, the UK Medical Research Council, the Royal Society London and
the Mexican National Council of Science and Technology (CONACYT).
Timeline: < five years
TRN Categories: Nanotechnology; Optical Computing, Optoelectronics
Story Type: News
Related Elements: Technical paper, "Creation and Manipulation
of Three-dimensional Optically Trapped Structures," Science, May 10, 2002.
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