How to fit 300 DVDs on one disc

The different colours and polarisations of light access different images A new optical recording method could pave the way for data discs with 300 times the storage capacity of standard DVDs, Nature journal reports. The researchers say this could see a whopping 1.6 terabytes of information fit on a DVD-sized disc. They describe their method as "five-dimensional" optical recording and say it could be commercialised. The technique employs nanometre-scale particles of gold as a recording medium. Researchers at Swinburne University of Technology in Australia have exploited the particular properties of these gold "nano-rods" by manipulating the light pointed at them. The team members described what they did as adding three "dimensions" to the two spatial dimensions that DVD and CD discs already have. They say they were able to introduce a spectral - or colour - dimension and a polarisation dimension, as well as recording information in 10 layers of the nano-rod films, adding a third spatial dimension. The scientists used the nanoparticles to record information in a range of different colour wavelengths on the same physical disc location. This is a major improvement over traditional DVDs, which are recorded in a single colour wavelength with a laser. Also, the amount of incoming laser light absorbed by the nanoparticles depends on its polarisation. This allowed the researchers to record different layers of information at different angles. The researchers thus refer to the approach as 5-D recording. Previous research has demonstrated recording techniques based on colour or polarisation, but this is the first work that shows the integration of both. As a result, the scientists say they have achieved unprecedented data density. Their approach used 10-layer stacks composed of thin glass plates as the recording medium. If scaled up to a DVD-sized disk, the team would be able to record 1.6 terabytes - that is, 1,600 gigabytes - or over 300 times the quantity stored on a standard DVD. Significant improvements could be made by thinning the spacer layers and using more than two polarisation angles - pushing the limits to 10 terabytes per disc and beyond, the researchers say. Bit by bit Recent efforts based on holography have shown that up to 500 Gb could potentially be stored on standard DVD-sized disks. Holographic methods take all of the information to be recorded and encode it in the form of a graph showing how often certain frequencies arise in it. That means that the recording process is a complex, all-at-once, all-or-nothing approach that would be difficult to implement on an industrial scale. By contrast, 5-D recording is "bit-by-bit", like current CD and DVD writing processes in that each piece of information is read sequentially. That is likely to mean that recording and read speeds would be comparatively slow, but the approach would be easier to integrate with existing technology. "The optical system to record and read 5-D is very similar to the current DVD system," says James Chon, a co-author on the research. "Therefore, industrial scale production of the compact system is possible." DVD surfaces now are "2-D": just the position on the disc matters Now that the method has been demonstrated in custom-made multi-layer stacks, the team is working in conjunction with Samsung to develop a drive that can record and read onto a DVD-sized disc. Dr Chon says that the material cost of a disc would be less than five cents (£0.03), but there are a number of advantages in moving to silver nano-rods that would bring that cost down by a factor of 100. For optical data storage expert Tom Milster, at the University of Arizona, the beauty of the approach is in its simplicity. "It's not just elegant - there are a lot of experiments that are elegant - it's relatively straightforward," he told BBC News. For the moment, Dr Milster says, the equipment needed to write the data would make a commercial system expensive. However, that has not stopped the development of optical storage solutions in the past. "For example, a Blu-ray player is not a real easy system to realise; they've got some wonderful optics in there," Dr Milster said. "People thought that would be pretty difficult to do but others managed to do it."