The storage of digital information using light is a key technology for producing long-term archives of large amounts of data. Researchers from Peking University1 have now developed a unique low-power optical storage concept based on the ordered self-assembly of nanoparticles.

Fig. 1: The switchable nanoparticle-based optical data storage system. Luminescent nanoparticles (green spheres) are assembled into ordered patterns and covered with a layer of photoactive molecules. When the photoactive molecules are switched into their inactive form (the purple region) by exposure to ultraviolet light, the green emission of the nanoparticles is suppressed. The process is fully reversible, and read out in the near-infrared band is non-destructive.

The bottom-up fabrication of nanoscale devices offers many advantages, such as easy device manufacturing using low-cost processes. The Chinese research team led by Chun-Hua Yan has proposed an optical storage mechanism based on the two-dimensional assembly of nanoparticles made from an optically active inorganic phosphor (Fig. 1). The phosphor absorbs near-infrared light, and emits a characteristic green luminescence. “The two-dimensional assembly promises higher data density and therefore larger storage capacity,” says Yan.

In their device, an organic photochromic molecule is deposited alongside the nanoparticles to switch the light emission from on to off. The photochromic molecules absorb light at the same wavelength as the nanoparticle emission. Thus, the light emission from the nanoparticles is effectively suppressed by the absorption bands of the photochromic molecules.

Illumination with visible light induces a small structural change in the organic molecules. The altered molecules absorb light at a different wavelength, allowing light emission from the nanoparticles to occur without absorption. The molecules revert back to their original state upon illumination with ultraviolet light for several minutes, which makes this technique fully reversible.

The writing of information using visible light can be applied selectively to particular regions of the device, allowing data to be written at high density. “In principle, information can be written on the level of a single nanoparticle,” says Yan. This would bring the storage density of such films into the region of next-generation devices. However, as with all optical techniques, expanding the storage density by such a large degree requires optical instruments that are able to operate beyond the diffraction limit of classical optics, for example, by bringing the light emitter very close to the film surface.

Nevertheless, the novel device design based on the combination of light-emitting nanoparticles and photo-switchable light-absorbing molecules suggests that further advances can be expected. For example, the use of other photo-switchable molecules may lead to improved device performance and faster switching times. The concept presented by Yan's team therefore opens a novel class of self-assembled optical storage devices.