Schematic illustration of the tractor beam set-up. The particles (red balls) are held between a mirror (lower sheet) and a transparent slide (upper sheet). Laser light (green) is focused at the region between the mirror and slide, causing the particles to arrange themselves neatly at the left. (Courtesy: O Brzobohatý et al.)

Optical tractor beam sorts tiny particles

Jan 24, 2013

The idea that light can grab hold of objects may sound like science fiction, but optical tweezers that hold particles at a laser focus are widely used today. An even more fictional-sounding concept is the tractor beam – a beam of light or sound that can pull an object towards it. Over the past three years or so, physicists have shown that tractor beams are theoretically possible – and there has been a flurry of activity in the lab to try to build one. Now, researchers in the Czech Republic have produced a simple example using two laser beams. And as an unexpected bonus, they have also discovered a potentially powerful technique for sorting microscopic particles.

Optical tweezers can hold a particle still at the focus of a laser; but if you want to move the particle, you have to move the focus by, for example, adjusting a lens. In theory, a tractor beam is a beam of light that can reel in particles towards the source of the radiation without having to fiddle with foci. However, photons carry momentum, which can be transferred to a particle and therefore nudge it away from the source – something that seems to rule out a tractor beam.

However, in 2006 Philip Marston of Washington State University showed that there is a little flexibility. If the particle is irradiated with a specially shaped beam called a Bessel beam, consisting of wavefronts that form concentric circles centred on the object, then the recoil momentum that the particle acquires from scattering photons forward can exceed the momentum that it receives from the incoming photons. According to his calculations, the net force that the particle experiences can be directed back towards the light source.

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