The groundbreaking tractor beam, made of a hollow laser beam (a laser bright around the edges and hollow in the centre), was able to move particles 0.2 mm in diameter distances up to 20 cm (7.87 in) -- around 100 times farther than has been achieved in previous experiments.

"Demonstration of a large scale laser beam like this is a kind of holy grail for laser physicists," said Professor Wieslaw Krolikowski from the Research School of Physics and Engineering at The Australian National University.

Previous experiments, such as those conducted by the University of St Andrews, relied upon the motion of photons to propel particles on a microscopic level. The ANU's experiment, however, used the laser differently: rather than photon momentum, the team used heat.
tractor2.jpg Krolikowski et al, Nature Photonics

The team trapped microscopic gold-coated hollow glass particles in the dark centre of the laser beam. Energy from the laser travels across the surface of the particle, where it is absorbed. This create hotspots; when air particles collide with these hotspots, they heat up and shoot away from the particle; in turn, the particle then recoils in the opposite direction.
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In order to then steer the particle, the team carefully controls the polarisation of the laser beam to heat up the desired portion of the particle's surface.

"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 the ANU's Dr Cyril Hnatovsky. "We can move smoothly from one polarisation to another and thereby stop the particle or reverse its direction at will."

This technique has a vast amount of potential: it's versatile, because it uses only a single laser beam. Practical applications could include controlling atmospheric pollution, or retrieving tiny, delicate or dangerous particles for sampling. It could also be scaled up for larger uses.

"Because lasers retain their beam quality for such long distances, this could work over metres. Our lab just was not big enough to show it," said co-author Dr Vladlen Shvedov.