Laser patterns particles in 3D

By Kimberly Patch, 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 said.

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," he said.

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, said Dholakia.

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
Funding:   Government
TRN Categories:  Nanotechnology; Optical Computing, Optoelectronics and Photonics
Story Type:   News
Related Elements:  Technical paper, "Creation and Manipulation of Three-dimensional Optically Trapped Structures," Science, May 10, 2002.


May 15/22, 2002

Page One

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Laser patterns particles in 3D


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