Moving objects with light beams

Researchers have found a way to use light to pull objects over tens of centimeters — a precedent in the field of optical trapping. 

Publishing in Nature Photonics this week¹, the team, including researchers from Australia, USA and Qatar, show that it is possible to make such a ‘tractor’ laser beam that traps and pulls objects towards the source of illumination.

They also show that the direction of motion of the optically trapped object can be switched from backward to forward by simply changing the polarization of the laser beam. 

To do that experimentally, the team placed gold-coated hollow glass spheres on the axis of a laser beam. When the thickness of the gold coat is less than 25 nm, the spheres are semitransparent, allowing electromagnetic energy from the laser to be absorbed more in the rear side of the sphere, resulting in a photophoretic force that moves the sphere towards the light source.

Within this range, when the laser beam is polarized along the circumference, the spheres are pulled to the laser source. When the beam is polarized along the radius, the spheres are pushed away. It is thus possible to toggle between opposite directions of motion by manipulating the polarization state. 

This ease of controllability, stability and long-range action would allow applications in various fields of science and technology such as laboratory-based studies of aerosols and remote sampling, possibly for the first time.

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