A long-distance optical tractor beam that can attract and repel particles 100 times further than previously possible has been developed by scientists from the Australian National University (ANU). The beam is created using a new technique, which in the future, could be scaled up and used for applications such as controlling atmospheric pollution.
The tractor beam is a doughnut shape, which is bright around the edges and dark in the centre, and is capable of moving particles one fifth of a millimetre in diameter and a distance of up to 20 centimetres.
Unlike previous techniques, which used photon momentum to impart motion, the tractor beam relies on the energy of the laser heating up the particles and the air around them. The new technique requires only a single laser beam, making it suitable for uses such as controlling atmospheric pollution or for the retrieval of tiny, delicate or dangerous particles for sampling.
The ANU team demonstrated the effect on gold-coated hollow glass particles. The particles are trapped inside the dark centre of the beam, and energy from the laser hits these particles and travels across the surface, where the energy is absorbed; this creates hotspots on the surface. Air particles colliding with these hotspots heat up and shoot away from the surface, which causes the particles to recoil, in the opposite direction.
To manipulate the particles, 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 Professor Wieslaw Krolikowski, from the Research School of Physics and Engineering at ANU. ‘We can move smoothly from one polarisation to another and thereby stop the particle or reverse its direction at will.’
The researchers also anticipate that in the future the experiment could be scaled up. ‘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.