Tractor Beams: A possibility?

Researchers at the University of New York have conducted the first experimental demonstration of a tractor beam – a travelling wave that comes from one direction and pulls its target towards the beam’s origin.

Up until now, beams have been used to move particles by working as optical tweezers and optical conveyor belts. Optical tweezers work over small distances, trapping particles for manipulation; whilst optical conveyor belts work over larger distances and use two opposing beams to set up a “conveyor belt” of light to move trapped particles.

The tractor beam is different from these other methods because it has been used over much larger distances – micrometers instead of picometers – and comes from only one direction. A tractor beam works by redirecting more photons downstream from the target particle than upstream once they’ve hit the target – the force of being hit by photons is weaker than the force imparted by the redirected photons moving downstream, and so the particle is forced upstream towards the beam’s point of origin. How did the researchers do this? They used a Bessel beam.

A Bessel beam is a beam that doesn’t refract at all and “self-heals” – after hitting a target, the beam reforms on itself downstream. The tractor beam just demonstrated works by firing a Bessel beam in the shape of a circular ring, with another ring-shaped Bessel beam around the first.

The second beam interferes with the first, causing it to rapidly change in intensity and snag the target particle, moving it upstream. The researchers were even able to use two tractor beams to simultaneously move one particle upstream and another downstream.

Whilst these beams are working over much smaller distances than in Sci-Fi, they’re still a revolutionary method for working on microscopic objects.

DOI: 10.1103/PhysRevLett.109.163903 DOI: 10.1103/PhysRevLett.58.1499