"[The optical trap] turned out to be a pretty important discovery. It led to Steve's [Chu] Nobel Prize..."
Arthur Ashkin
Radiation pressure is the force exerted by a beam of light when it is reflected from or absorbed by a body. Normal light beams are wide and brutal, but a focused laser beam can apply extremely delicate forces.
If a small object has a mass less than about 1 gram and is dielectric, it can diffract a laser beam in such a way that the difference between the momentum of the radiation entering the object and that leaving it can be made to produce a force that traps the object in a specific position. A single laser beam can thus act like a pair of tweezers and individual atoms, molecules, and biological cells can be micro-manipulated using the beam. In 1970, Arthur Ashkin (b. 1922) of Bell Laboratories detected optical scattering and showed that particles can be trapped in this way. It took another fifteen years before he created a workable process, derived from the laser-cooling techniques achieved by Steven Chu of Bell Laboratories. Ashkin was trapping very small biological bodies by 1987.
When investigating strands of DNA, bacteria, or viruses, these have to be biochemically attached to a dielectric sphere of glass or polystyrene that is being positioned by the "tweezers." Care must be taken, in choosing the wavelength and power of the laser light, so that the specimen does not heat up or become damaged by the radiation. The positioning of the trapped particle is usually controlled using acousto/ electro-optical systems that are computer-controlled.
Minute details and movements on the nanometer scale can be studied. The whole system is, however, extremely complicated and expensive and needs skillful operation.
