Scatterings

'Tractor Beam' Pulls Particles, Instead of Pushing Them

Patricia Daukantas

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For two decades, scientists have used optical tweezers to trap tiny beads, molecules and even atoms in a small space and to push them around with radiation pressure. But researchers can now pull these nanoscale particles with a beam of green light—eerily reminiscent of the Star Trek “tractor beam” (Nature Photon. 7, 123).
 
Researchers at the University of St. Andrews (Scotland) and the Institute of Scientific Instruments (Brno, Czech Republic) demonstrated that a single beam of linearly polarized light, reflected off a mirrored surface, can easily move polystyrene beads in the opposite direction of the photon stream. By rotating the polarization of the incident beam, scientists can change the direction of the moving particles and group them into basic structures.
 
According to Pavel Zemánek, the Czech institute's deputy director, the team developed the method over the past few years, while Tomáš Čižmár, now a research fellow in medical optics at St. Andrews, was working on his doctorate. The group was looking for a new optical manipulation technique that was simple to design and pulled particles farther than about 10 μm.
 
To make the tractor beam, the scientists shone a wide Gaussian beam of 532-nm linearly polarized laser light onto a reflective mirror. The tiny plastic beads floated in water just above the mirror. By changing the beam's orientation from p-polarization to s-polarization, the researchers pushed and pulled the beads around in different directions and clustered them in small groups.
 
The simple setup is easily attached to optical microscopes, Zemánek says, and the scientists will use it to explore the self-organization and self-sorting behavior of nanoparticles as they interact with light.


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