21st October 2014 Tractor beam can move objects 100 times further Laser physicists at the Australian National University have built a reversible tractor beam, able to move objects 0.2 mm in diameter a distance of up to 20 cm (7.9"). This is 100 times further than was possible in previous experiments.
Tractor beam technology – as depicted in science fiction movies like Star Trek – might become a reality sooner than we think. Following a number of successful experiments in recent years, it is moving further and further into the macro-scale. Laser physicists at the Australian National University (ANU) have now demonstrated the first long-distance optical tractor beam, able to repel and attract objects using a "hollow" beam that is bright around the edges and dark in its centre. It can move particles 0.2 mm in diameter a distance of up to 20 cm (7.9"), about 100 times further than previous attempts. “Demonstration of a large-scale laser beam like this is a kind of holy grail for laser physicists,” said Prof. Wieslaw Krolikowski, from the Research School of Physics and Engineering at ANU. The new technique is versatile because it requires only a single laser beam. It could be used, for example, in controlling atmospheric pollution or for the retrieval of tiny, delicate or dangerous particles for sampling. The researchers can also imagine the effect being scaled up. “Because lasers retain their beam quality for such long distances, this could work over metres,” said co-author Dr Vladlen Shvedov. “Our lab just was not big enough to show it.”
Unlike previous techniques, which used photon momentum to impart motion, the ANU tractor beam relies on the energy of the laser heating up the particles and the air around them. The ANU team demonstrated the effect on gold-coated hollow glass particles. These are trapped in the dark centre of the beam. Energy from the laser hits a particle and travels across its surface, where it is absorbed, creating hotspots on the surface. Air particles colliding with hotspots heat up and shoot away from the surface, which causes the particle to recoil, in the opposite direction. To manipulate the particle, the team move the position of the hotspot by carefully controlling the polarisation of the laser beam. “We have devised a technique that can create unusual states of polarisation in the doughnut-shaped laser beam, such as star-shaped (axial) or ring polarised (azimuthal),” said co-author Dr Cyril Hnatovsky. “We can move smoothly from one polarisation to another and thereby stop the particle or reverse its direction at will.” The work is published this week in Nature Photonics.
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