Imagine storing two copies of War and Peace on an area the size of a
pin head. This may soon be possible with a technique which squeezes a beam
of blue-green light to a tenth of its wavelength and could mean computer
discs holding 100 times more information.
Eric Betzig and his team at AT&T Bell Laboratories in New Jersey
squeezed the laser beam by directing it down an aluminium-coated optical
fibre which had been tapered to a fine point. At the tip, the spot of light
emitted is 50 nanometres across, a tenth of the blue-green light’s wavelength.
Laser light is used to read, as well as write, from optical storage
discs. Data is stored as dots the size of which is determined by the width
of the laser’s beam. At present, lenses and mirrors cannot focus light into
a beam much smaller than the light’s wavelength – 500 nanometres for blue-green
lasers. But by squeezing the light into a smaller beam, more dots, or bits
of information, can be stored.
Betzig’s group has made 400 dots in a grid 2.3 micrometres across, corresponding
to 7 billion bits per square centimetre. The dots were about 60 nanometres
across. The team recorded the dots on magneto-optic film. Discs made of
this material can store a lot more data than conventional magnetic discs
and are more rugged.
Data is recorded on the discs while they are in a magnetic field by
focusing a laser beam on to the film. This heats the film and makes a dot
with changed magnetisation. Data is read by passing a weaker laser beam
through the magnetised dot, which changes the polarisation of light. A change
in the polarisation signals the presence of the dot.
Betzig was the developer of a technique called near-field scanning optical
micro-scopy which can achieve a much greater magnification than conventional
optical microscopes. His idea was to create an optical source smaller than
a wavelength of light, and place it close to the target.
Betzig and his team have now adapted this approach. They placed the
optical fibre tip within 10 nanometres of the surface to produce the high-resolution
dots. The result, AT&T claims, is a storage density nearly 100 times
that of the best commercial magneto-optic discs and 300 times that of magnetic
discs. Ultimately, Betzig hopes to reach densities of up to 75 billion bits
per square centimetre.
AT&T is optimistic about practical applications, but many obstacles
remain. Rather than use conventional magneto-optical materials, the experimenters
used platinum-cobalt films developed at Carnegie Mellon University in Pittsburgh.
The fibre tip must be within 10 nanometres of the surface to produce the
smallest dots. This requires the surface to be very clean and the tracking
of the tip extremely accurate.