One of the hardest tasks faced by scientists developing optical computers is building a high-speed optical memory. Recently, engineers have been experimenting with 'thick' holograms to meet these specifications, but the resulting equipment is large, delicate and complex to maintain. Now Atusi Kurita and colleagues from Osaka University and NAIST in Japan have discovered a new technique that exploits both the wavelength of the light being used, and the angle at which it enters the storage medium (Phys. Rev. Lett. 83 1582).
The medium is made of nanocrystals of zinc sulphide doped with samarium. Although the nanocrystals are only 3 nanometres long, they clump together into larger particles to form a powder that can be used as the storage medium. In their experiments Kurita and co-workers used a laser to measure the energy excitation spectrum of the crystal. Then they used a much stronger laser to burn a narrow “hole” in this spectrum, probably by ionizing the samarium in the crystal. This hole can be measured by using the first laser again.
The depth of the hole decreased with time, but as much as half of the initial hole was found to remain after 24 hours. Moreover, the position of the hole can change – or even disappear – as the crystal is rotated with respect to the laser beam. This behaviour has not been observed before and could, say the researchers, “bring a significant advantage over conventional two-dimensional optical memories.”
