Trapped by Lasers

The first scientist to use lasers to trap tiny spheres published his groundbreaking study 29 years ago this month. His research led to the development of "optical tweezers," laser-based devices that can manipulate small particles and detect the forces of single molecules, and to Nobel Prize-winning work on using lasers to trap atoms.

Physicist Arthur Ashkin of the Bell Telephone Laboratories in Holmdel, New Jersey, shot a laser beam through a container of water filled with microscopic latex spheres. He found that the spheres were drawn from the edge of the beam toward its center, then were pushed in the direction of the beam's moving photons by so-called radiation pressure. Pointing two laser beams at each other created a stable trap for the spheres, he reported in the 26 January 1970 issue of Physical Review Letters.

"Art had a huge influence on both the field of optical tweezers and atom trapping," says William Phillips, a physicist at the National Institute of Standards and Technology in Gaithersburg, Maryland. Optical tweezers use a tightly focused laser to manipulate microscopic particles. They can gauge the delicate forces of single molecules on a particle based on its slight displacement from the laser's focus. For example, in the past decade researchers used optical tweezers to measure the push and pull between two molecules involved in muscle contraction: myosin and actin. As for atom trapping and cooling, says Phillips, who won a share of the Nobel Prize in physics last year for this kind of work, Ashkin had "a lot of the key ideas."