Using light to move objects

optical tweezer
Optical tweezer diagram. From Wikipedia.

Moving objects using light beams is a very popular science fiction idea that has been around since at least the early 1900s.

In the 1980s, this fiction became reality on the microscopic scale with the invention of optical tweezers. Optical tweezers are focused lasers that can control movement of microscopic objects, from atoms up to bacteria and viruses. This technology has been used for the last 30 years to uncover many biological secrets like how the tiny protein motors we have inside our cells work.

Optical tweezers work by focussing a laser beam through a microscope objective. This creates a narrow part in the beam, called the beam waist. The beam waist has a stronger electric field gradient than other parts of the laser beam. So particles are attracted to the beam waist. A laser can be split into two or more beams to create many ‘tweezer’ arms that can manipulate specimens. Optical tweezers have proven to be a very useful research tool. However, they do have one major side effect: the lasers can cook specimens. This is called opticution.

This side effect has been harnessed by Aussie scientists, Dr Vladlen Shvedov and Dr Cyril Hnatovsky, to move much bigger (0.2mm diameter) glass particles with light. They used a hollow laser beam to heated up areas on tiny gold-covered hollow glass beads. This created hotspots on the surface of the beads. When this happens surrounding air moves away from the hotspot and the particle moves in opposite direction. The scientists can manipulate the laser and control where the hotspots are to control the direction that the bead moves. Laser beams can extend for metres, so it might be possible to scale this up to move bigger particles. One day the Tractor Beam of Sci Fi mythology may become real.

The Scientific Papers:

Shvedov et al. A long-range polarization-controlled optical tractor beam. Nature Photonics. 2014

Zhang et al. High-resolution optical tweezers for single-molecule manipulation. Yale J Bio Med.

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