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New look for optical data storage

16 May 2007

The first optical memory element to be made from a single gallium nanoparticle could compete with DVD and hard disks.

Physicists in the UK have made the first optical memory element with information encoded in the structural phase of a single gallium nanoparticle. The new memory element is about the same size as bit cells in next-generation hard disks, but requires much less energy to switch from one state to another than current DVDs or DVRs.

The secret of the new memory element, made by Bruno Soares and colleagues at the University of Southampton, lies in the fact that four-level optical memory functionality can be achieved using a single nanoparticle. In contrast, today's computer memories use only two states for encoding information. The new device can therefore store twice as much information.

Information-storage technology relies on systems that can be easily "switched" from one configuration to another by applying an external stimulus. Systems that use phase changes to store information are promising candidates for high-density data storage because they use about the same power as electronic memories. In phase-change memories, data recording is usually done by switching the material between different phases, rather like in present-day DVD or DVR technology.

Soares and co-workers have now made a quaternary logical optical memory element with information coded in the structural phase of a single gallium nanoparticle that measures 80 nm across. The researchers did this by growing the nanoparticle on the tip of a tapered gold-coated optical fiber using atomic beam deposition. Information can be stored in the particle by using short optical pulses from a laser to switch between the different Ga crystalline phases, as well as the liquid state.

"Since the different phases have different optical properties, the memory state can be monitored by measuring the particle's optical response," explained Soares. "This principle of operation is similar to the one used in electronic phase-change memories, which are currently thought to be the successors of today’s flash memories."

The Southampton physicists found that the energy required to switch between the different logic states was as low as 1.5 pJ, which is an order of magnitude less than the energy required in today's state-of-the-art hard-disk technology. Moreover, they estimated that the data storage density of an array of Ga nanoparticles could reach 0.2 terabytes per square inch (Tb/in.2). To compare, Blu-ray DVDs have a density of 0.015 Tb/in.2 and hard disks employing "perpendicular recording" technology going beyond the so-called superparamagnetic limit have recently reached record values of 0.23 Tb/in.2.

Soares told nanotechweb.org that the size and power requirements of this new four-level memory element means that this type of nanoscale memory functionality could compete directly with existing technologies. "Moreover, using quaternary memory elements allows for entirely new algorithms in computation where complex number arithmetic is considerably simplified and error accumulation reduced," he added.

The team will now study ways of addressing individual particles within a closely packed array. It also plans to experiment with alternative materials.

The work was reported in Phys. Rev. Lett.

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