A team of researchers led by Dr Vladlen Shvedov of the Australian National University’s Laser Physics Center in Canberra, Australia, has built what they say is the first long-distance optical tractor beam.
Australian National University scientists adjusting the hollow laser beam. Image credit: Stuart Hay.
“Demonstration of a large scale laser beam like this is a kind of holy grail for laser physicists,” said Prof Wieslaw Krolikowski of the Laser Physics Center and Texas A&M University at Qatar, Doha, who is the senior author of a paper published in the journal Nature Photonics.
The team’s technique requires only a single laser beam.
The beam can move particles one fifth of a millimeter in diameter a distance of up to 20 cm, around 100 times further than previous experiments.
It could be used in controlling atmospheric pollution or for the retrieval of tiny, delicate or dangerous particles for sampling.
“Because lasers retain their beam quality for such long distances, this could work over meters. Our lab just was not big enough to show it,” Dr Shvedov said.
Unlike previous techniques, which used photon momentum to impart motion, the beam relies on the energy of the laser heating up the particles and the air around them.
Part of the experimental set-up with the beam converter and particle dispenser; the half-wave plates used to change the polarization state of the beam are denoted by λ/2. Image credit: Vladlen Shvedov et al.
Dr Shvedov and his colleagues demonstrated the effect on gold-coated hollow glass particles. The particles are trapped in the dark center of the beam. Energy from the laser hits the particle and travels across its surface, where it is absorbed creating hotspots on the surface. Air particles colliding with the hotspots heat up and shoot away from the surface, which causes the particle to recoil, in the opposite direction.
To manipulate the particle, the scientists move the position of the hotspot by carefully controlling the polarization of the beam.
“We have devised a technique that can create unusual states of polarization in the doughnut shaped laser beam, such as star-shaped (axial) or ring polarized (azimuthal). We can move smoothly from one polarization to another and thereby stop the particle or reverse its direction at will,” said co-author Dr Cyril Hnatovsky, also from the Laser Physics Center.
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Vladlen Shvedov et al. A long-range polarization-controlled optical tractor beam. Nature Photonics, published online October 19, 2014; doi: 10.1038/nphoton.2014.242
