'Tractor beam' technology advances

The beam consists of a helix of twisting laser light

By BBC News Online science editor Dr David Whitehouse

A "tractor beam" that can reach out, trap and move objects has been developed by British scientists.

But while fictional tractor beams of the kind depicted in Star Trek can ensnare a giant spaceship, this real version works only at the microscopic level.

The beam consists of a helix of twisting laser light, which is able to seize hold of objects as small as a protein molecule.

Scientists believe it will be an invaluable tool for manipulating parts of living cells or components of micro-machines.

Glass beads

Dr Kishan Dholakia, of St Andrews University, said: "Our technique could be used to drive motors, mixers, centrifuges and other rotating parts in cheap, tiny, automated technologies of the future."

The beam can rotate tiny objects

The researchers speculate that components like these might one day appear on microchip laboratories capable of performing a range of chemical and biological tests.

So far, the scientists have tried out the system with glass beads 100 times thinner than a human hair, and a tiny glass rod which could be used for stirring minute amounts of liquid.

They also used the beam to rotate a hamster chromosome to demonstrate its potential for studying structures inside the cell. Optical tweezers, which allow particles trapped in a tightly focused laser beam to be moved from one spot to another, already exist.

But the devices are severely limited because they cannot change an object's orientation. A proper tractor beam, say scientists, must be able to rotate objects as well as move them.

Just a beginning

The researchers describe in the journal Science how they extended the principles of optical tweezers technology. In both systems, a particle gets trapped in the laser beam because some of the light refracts when it hits the object.

This changes the momentum of the object, which is forced towards the spot in the beam where the light is most intense.

"The beauty of our technique is that we can dictate how far we want the spiral pattern to go around and at what speed," Dr Dholakia said. "That means we can fully control the rotation of that one particle."

He added: "We've only just begun to realise the possibilities for what we might do with this technology."