INITIALLY developed for electron
microscopy, holograms have found a
range of applications from gift shop
CHIJositics to identification labels on
credit cards, Recently, a new application
Was added to this list - holographic
data stora-e. R H Berg and his collaborators at the Riso National Laboratory
in Deninark have investigated a new
molecule which could potentially
rcVolutionise data storage (Nature,
vol 383, No 6600).
The main advantage of holographic
storage is its tremendous data density -
of the order of hundreds of gigabits per
cubic centimetre. The essential idea is to
replace the information content of an
image by the data one wants to store and
retrieve it by standard methods. There
have, however, been several difficulties
in Putting this idea into practice. The
material on w1iich data is to be stored
has to be thermally stable, reusable
(so as to be able to have read ad write
memory) and also efficient enough to
allow tile data to be read quickly. Several
materials (such as photorefractive
polymers) have been unsuccessfully
explored in tile last few years.
Berg and his team have designed
a new class of organic materials -
peptide oligomers (a polymer
whose molecules consist of
fewer units), containing azobenzene chromophores (parts
of molecule responsible for a
compound's colour). The
oligomer, called DNOS (diamino
acid-N-alpha substituted oligopeptide) has azobenzene chains
linked to a peptide-like back-
bone. In the DNO molecule, the
backbone is made up of
ornithine and the steps consist
of azobenzenes which are cigar-
The structure is similar to
that Of DNA in which the back-
bone is made up of deoxyribose
linked by phosphate molecules
while the steps are made up of'
pairs of bases. The rod-like
structure of azo dyes has been used previously for a range of applications from
holographic storage to liquid crystal
films. This unique structure of the molecule is what makes it suitable for use as a
holographic data storage medium.
The basic technique in making a
holograrn is to expose a light- sensitive
material to a laser beam that has been
split into two parts by a beam splitter.
One of the split beams is directed on the
object and reflected. When this reflected
beam interferes with the other half of
the split beam (called the reference
beam), we get an interference pattern.
The hologram is made by recording this
pattern which gives us a three-dimensional picture when the reference laser
beam shines on it.
The laser acts on the azobenzenes
but not on the ornithine. The interaction of the azobenzene and the laser is
such that the azobenzene molecules
become perpendicular to the polarisation of the incident laser beam. And
since the azobenzene is closely packed
and held in place with the backbone into
a helix, the whole molecule responds ill
a similar fashion rather than in a
random way. This important property
makes DNO an ideal candidate for holographic storage.
Berg and his colleagues made films
of good optical quality with a thickness
of about five micrometres. This was
done by using hexafluoroisopropanolic
solvents since the DNO is insoluble in
most solvents. The DNO was then filtered
through a fine-grade glass filter and cast
onto a glass substrate and vacuum
dried. The holographic experiments
were carried out using an argon ion
laser with polarisation beam splitters.
The researchers have created holographic gratings (a surface ruled with
equally-spaced parallel lines used to
produce spectra by diffraction) with a
resolution of about 500 lines per mm.
The practical implications of the
DNO in holographic memories is difficult to predict since there are several
hurdles to be crossed. Firstly, the write
time is still quite long - of the order of
a few seconds. Some changes in the
structure of the molecule seems to speed
up the time, but not very appreciably.
Secondly, it is not clear whether the
same mechanism which works for thin
films will work for thick films which are
needed for high data densities.
But the new material has the advantage of being extremely stable since the
holograms have survived without any
loss for over a year at room temperature. In any case, the new molecule has
immense potential to revolutionise
optical data storage.