Australasian Science: Australia's authority on science since 1938

Physicists Build Reversible Laser Tractor Beam

Laser physicists at The Australian National University have built a tractor beam that can repel and attract objects.

The hollow laser beam, reported in Nature Photonics, is bright around the edges and dark in its centre, and can move particles 0.2 mm in diameter a distance of up to 20 cm – around 100 times further than previous experiments.

Prof Wieslaw Krolikowski of the Research School of Physics and Engineering said that the new technique could be used to control atmospheric pollution or remove tiny, delicate or dangerous particles for sampling.

“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.

Unlike previous techniques that 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 team demonstrated the effect on gold-coated hollow glass particles trapped in the dark centre of the beam. Energy from the laser hits the particle and travels across its surface, where it is absorbed and creates 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 team moves 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-worker Dr Cyril Hnatovsky. “We can move smoothly from one polarisation to another and thereby stop the particle or reverse its direction at will.”