¿ìè¶ÌÊÓƵ

LITTLE glass cubes the size of sugar lumps may one day replace the vast
arrays of disc drives we need to store enormous databases. The secret is a new
holographic material developed by researchers in Canada and Spain. They hope to
record text and pictures at different levels in the holographic cube.

Pavel Cheben of the National Research Council in Ottawa and Maria Calvo of
Complutense University in Madrid suspended photosensitive chemicals in porous
silica glass to form a 1-centimetre cube which can store a hundred times more
data than a DVD.

In holographic recording, two lasers create an interference pattern which
represents pages of information. An organic material called a photopolymer
captures the interference pattern like a photograph. One layer can store many
thousands of holograms if the angle at which beams hit the storage medium is
slightly altered. The readout beam moves through matching angles to read out
different pages.

Until now, the technique has only worked if the polymer supporting the
photopolymer layer is optically very pure, so that the beam angles remain true.
And the polymer must not change its shape with age or temperature. This means
the photopolymer can be no more than 0.1 millimetres thick, limiting storage
volume.

Cheben and Calvo believed a better way of storing more data in thicker media
might be to use a material that is not completely organic. By impregnating the
photopolymer into a rigid but porous silica glass matrix, they’ve made a hybrid
organic-inorganic material that’s more stable. The material is far less likely
to cause data distortion with heating.

In tests, the pair have now successfully stored and retrieved data. In
theory, they say, a 1-centimetre cube of the material should store 6 million
megabits (6 terabits) of data. And because data pages are read all in one go,
rather than as a sequential stream of bits, they can be read a hundred times
faster than from a DVD.