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Data storage enters the 'fifth dimension'

22 May 2009

Optical data storage over one terabit per cubic centimetre possible, say researchers.

The first DVD-sized discs with storage capacities well over one terabyte could be available in as little as five years, according to researchers in Australia who have invented a new storage technique. The concept, which the researchers have already demonstrated on test media, uses layers of gold nanorods to achieve "five-dimensional recording".

Optical discs, such as CDs and DVDs, store data as a spiral track of microscopic pits etched onto their surface. To read the data, light from a laser diode is reflected from the surface and the reflected light drops in intensity every time the beam hits a pit.

With just one layer of pits, the storage is two dimensional, and with multiple layers – a method employed in the highest capacity DVDs, providing capacities up to about 17 Gb (17 × 109 bytes) – the storage is three-dimensional.

More dimensions needed

To reach higher capacities, particularly above 1 Tb (1012 bytes) per disc, scientists believe that they will need to be able to record in even more "dimensions". In recent years there has been success in adding one extra dimension in the form of sensitivity to either the polarization or colour of the laser light, a technique called multiplexing.

Now, however, James Chon and colleagues from Swinburne University of Technology in Melbourne have combined both types of multiplexing for five-dimensional recording.

"Previously there has never been an effort to record in all five dimensions," Chon told physicsworld.com. "This is due to a lack of material that can respond in all five-dimensional recording conditions – colour, polarization and spatial."

Nanorods to the rescue

For its recording media the Swinburne group uses gold nanorods, which respond to different colours and polarizations depending on their apparent size and orientation. When a collection of these nanorods are irradiated with laser light, only those that are aligned to the light's polarization and have an absorption cross-section matching the light's wavelength will absorb it, melt and change in shape. Because there are nanorods left unaffected after one recording, more recording cycles can still take place.

To read the data a laser again illuminates the nanoparticles, which begin to resonate with quasi-particles known as plasmons. The plasmon resonance is very sensitive to the incident light's polarization and colour, and requires a laser that is only a hundredth as powerful, so no more melting takes place.

Compatible with existing technology

In tests using media with three layers of gold nanorods, Chon and colleagues achieved a data storage density of 1.1 Tbit per cubic centimetre, which would equate to 1.6 Tb for a DVD-sized disc. The researchers think that by using thinner spacers between layers the capacity could be increased to 7.2 Tb. Moreover, they say that recording speeds could be as fast as 1 Gbit/s, and the discs would be compatible with existing technology.

Chon says the group is collaborating with the electronics manufacturer Samsung to commercialize the concept, and hopes to see the first devices on sale in five years. "We have only conducted proof-of-principle experiments," he adds. "It is our future work for this technology to be transferred to industry, where many challenges will have to be overcome."

The work was reported in Nature.

• This article first appeared on our sister website physicsworld.com

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