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Particle manipulation using light made accessible

BioPhotonics
Mar 2012
Ashley N. Paddock, ashley.paddock@photonics.com

URBANA, Ill. — Dexterous optical tweezing and size-sorting of particles can now be done by tuning the properties of laser light illuminating arrays of metal nanoantennas.

In work conducted at the University of Illinois at Urbana-Champaign, assistant professor of mechanical science Kimani Toussaint Jr. and his research team have demonstrated for the first time the use of gold bowtie nanoantenna arrays for multipurpose optical trapping and manipulation of submicrometer- to micrometer-size objects. The findings could prove useful for the growing interest in lab-on-a-chip devices.

The field enhancement and confinement properties of bowtie nanoantenna arrays also make them accessible for formation of optical matter, manipulation of biological matter with reduced specimen photodamage and for basic physics studies of colloidal dynamics.


Concept art depicting the various potential bowtie nanoantenna array trapping states. Courtesy of Kimani C. Toussaint Jr., University of Illinois.


"We believe that our work shows that optical nanoantennas could serve as integral components in potential lab-on-chip devices," Toussaint said. "The basic idea is to use nanoantennas to augment the optical forces on objects (e.g., cells) in aqueous environments. This will considerably relax the requirements on the optics used for such experiments.

Using empirically obtained "optical trapping phase diagrams" to achieve the desired trapping response, the researchers demonstrated several types of particle manipulation, including single-beam optical tweezing of single particles over the entire nanoantenna area, single-beam optical tweezing of two-dimensional hexagonal-packed particles over the entire nanoantenna area, and optical sorting of particles by size. They also showed stacking of submicron- to micron-size particles in three dimensions.

For a given particle size, wavelength and desired type of manipulation, the trapping phase diagrams provide information on the input power and nanoantenna array spacing required to achieve the desired task. They "are empirically derived and become an easy way to harness the forces in the nanoantenna platform that are otherwise complex to fully model," Toussaint said.

This is the first demonstration of a range of particle manipulation behavior for a given nanoantenna array, according to Toussaint. "Perhaps the most immediate impact is that we have helped to make general particle manipulation (using light) accessible," he said. "Single- and multiple-particle trapping, as well as sorting, is now very doable using a fixed nanoantenna platform and without the use of high-power lasers, extremely tight focusing or multiple laser beams."

In fact, his team conducted most of its experiments using an average power at least 1000 times less than that of a standard laser pointer. A laser pointer was used for some proof-of-concept experiments to show that ubiquitous off-the-shelf technology could be used easily with the nanoantenna system because of the structure's ability to enhance optical fields, he said.

"The beam quality of a laser pointer is often not ideal for conventional optical trapping experiments without a fair amount of spatial filtering, but for our system it does not matter," he said. "We could get away with using relatively 'cheap and dirty' optics.

The research appeared online Dec. 30 in Nano Letters (doi: 10.1021/nl203811q).


GLOSSARY
laser pointer
Handheld optical laser device containing a semiconductor or DPSS source. The output is corrected via internal collimating optics. For typical use the output is reduced to eyesafe operation power. (5 mW)
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