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Single nanoparticle all-optical memory on the horizon

07 Sep 2007

Latest result brings optically switched gallium nanoparticle system a step closer to market.

Gallium nanoparticles stimulated with nanosecond optical pulses could be used as the basis of next-generation all-optical memory and data-storage technologies, say physicists in the UK. These memories could have reduced cell sizes and higher storage densities than today's devices.

Cell sizes in so-called phase-change recording media, such as those employed in current digital versatile disk (DVD) technology, need to be made smaller in the race for ever-greater storage densities and lower power consumption. Data recording in such materials is achieved by switching between structural forms, which correspond to different logic states (digital 1's and 0's) with distinct optical or electronic properties, using laser or current pulses.

Recently, researchers showed that individual gallium nanoparticles can behave as rewritable memory elements that can be switched from lower to higher energy states using microsecond optical pulses. These optically addressed memory elements could offer storage densities comparable to those expected in next-generation hard drives that exploit perpendicular recording technology. Moreover, they have switching energies that are an order of magnitude lower than those employed in current DVDs and hard disks.

However, the problem is that the material has to be cooled by around 90 K to switch the gallium particle back to a lower energy state. This means that, although single microsecond optical pulses can readily "write" information to the nanoparticle memory, and this information can be "read" optically by probing the particle's reflective properties, the only way to "erase" the information is to substantially cool the particle.

Now, Nikolay Zheludev and colleagues at the University of Southampton have shown that this cooling requirement can be reduced to about 5 K if the particle is excited with higher intensity, lower energy laser pulses that last just a few nanoseconds. The result suggests that it may be possible to achieve both the writing and erasing functions optically at a fixed temperature by using different laser pulses for the two directions of switching.

The Southampton team achieved its result by stimulating and observing the structural transformation in gallium nanoparticles grown on the 100 nm aperture at the tip of a tapered, gold-coated silica optical fibre using a reflective "pump-probe" arrangement. A continuous 1310 nm diode laser, providing around 20 nW at the nanoaperture, was used to probe a particle's reflectivity. A 1550 nm laser producing 3 ns pulses at 30 kHz with a peak power of about 0.1 mW at the nanoaperture was used to excite transitions in the particle.

"Today’s DVD technology is based on thin films of chalcogenide phase-change materials that are switched between amorphous and crystalline states by laser pulses," explained team member Kevin MacDonald. "Particulate phase-change memory materials such as gallium could offer reduced memory cell size and thereby higher storage densities and switching energies. Gallium in particular may also find novel applications because the material can exist in more than two distinct phases, thus offering a logic-base higher than a two-phase binary system," he added.

The team is now working towards demonstrating fixed-temperature bi-directional optically induced switching.

The work was published in Appl. Phys. Lett.

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