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PHYSICISTS in the US have used light from a laser beam to bind matter
in a new way. Michael Burns and his team at the Rowland Institute for Science
in Cambridge, Massachusetts formed what they call an ‘optical molecule’
– a pair of tiny plastic spheres held on ripples of lightlike atoms in a
molecule. Now, they are attempting to create more complex structures, which
they have christened ‘optical crystals’ (Physical Review Letters, vol 63,
p 1233).

Å®ÉúСÊÓƵs have known since the 1970s that they can attract and trap
small particles with the aid of an intense laser beam. The initial discovery
was made by Arthur Ashkin at Bell Laboratories in New Jersey, who found
that he was able to use a beam to trap small bacteria. At the Rowland Institute,
Burns and his colleagues decided to study Ashkin’s trapping mechanism in
the laboratory. They realised that a similar ‘optical force’ might arise
between particles if they were exposed to laser light, and that the force
might be capable of holding the particles in a fixed arrangement.

To investigate the optical force, the scientists shone a laser beam
with a wavelength of 0.387 micrometres on two plastic spheres, each only
1.43 micrometres in diameter. They found that when the spheres were separated
by a distance equal to an integer multiple of the laser’s wavelength, the
spheres formed stable ‘molecules’. The spheres were more tightly bound when
they were closer together and exposed to a stronger laser beam. As soon
as the researchers turned off the laser beam, the forces between the spheres
disappeared, and random thermal motion took over.

According to Burns, the spheres form stable ‘molecules’ because each
sphere scatters a small fraction of the laser light in the form of spherical
waves. These create a pattern of ripples in space, rather like those produced
by a stone dropped into water. The ‘light ripples’ from the two spheres
combine with the incident laser light to produce what physicists refer to
as an interference pattern; that is, in some parts of space, the waves reinforce
each other, while in others they cancel each other out. It is this interference
pattern, or ‘field of force’ that is responsible for the optical forces
that hold the spheres together.

The pattern of ripples changes with the distance between the two spheres.
When the spheres are separated by integer multiples of the beam’s wavelength,
the ripples reinforce each other. Each sphere then finds itself in the vicinity
of a series of peaks and troughs which become deeper towards the centre
of the pattern. The troughs, which physicists call ‘potential wells’, are
positions of low energy in which the spheres can sit – provided, that is,
that the well is sufficiently deep to prevent their thermal energy from
jiggling them out of it.

When both spheres are in nearby troughs in the interference pattern,
the ‘molecule’ is stable. As the distance between the spheres changes, the
fields destroy each other. The spheres then encounter forces that move them
together until they reach the next stable configuration.

According to Burns, optical molecules are only the first stage in his
team’s investigation. He and his colleagues hopeto create one- and even
two-dimensional ‘crystals’ using the laser beam. Because the optical forces
have a long range, it may be possible for spheres to arrange themselves
in a regular pattern so that all the scattered fields reinforce one another.
In such a situation, the potential wells will get deeper and the structure
more stable as each new sphere is added to the pattern. This is precisely
the criterion for growing a crystal structure.

The scientists say that it is still too early to think of definite applications
of their work. ‘At the moment,’ says Burns, ‘we are just having a lot of
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