A new light-responsive material could lead to discs the size of today's DVDs that store four orders of magnitude more data. Traditional DVDs and CDs store data on their surface in two dimensions, and holographic discs can store it in three. Now researchers have for the first time demonstrated what they call a five-dimensional optical material. It can record data in three spatial dimensions and in response to different wavelengths and polarizations of laser light.

The material is being developed by researchers led by Min Gu, director of the Centre for Micro-Photonics at the Swinburne University of Technology in Victoria, Australia. The material is made up of layers of gold nanorods suspended in clear plastic spun flat on a glass substrate. Multiple data patterns can be written and read within the same area in the material without interfering with each other. Using three wavelengths and two polarizations of light, the Australian researchers have written six different patterns within the same area. They've further increased the storage density to 1.1 terabytes per cubic centimeter by writing data to stacks of as many as 10 nanorod layers. In a paper published online today in the journal Nature, Gu's group reports recording speeds of about a gigabit per second.

"You can record each bit by one laser pulse," says Gu. The writing laser melts and reshapes the gold particles, which are less than 100 nanometers long. The changes affect how the nanorods interact with light from a laser-imaging system, allowing the data to be read.

The Australian researchers tailored the gold nanoparticles to respond to different wavelengths of light by controlling their dimensions. When pulsed with a focused beam of green light, for example, some of the nanorods will change shape, while others very close by but of a different size will not be affected. The response of the nanorods, which are scattered throughout the plastic randomly, also depends on the angle of propagation of the incoming light. When the polarization of the light is aligned with the rods' long axis, the rods absorb it more strongly than they do light coming from other angles. The patterns can't be erased and rewritten, but they should be stable over time.

Previous work on this kind of multiplexed optical storage relied on light-responsive polymers. "The absoption spectrum of those materials is very broad," says Gu, which makes it difficult to record at high density using multiple colors of light. The advantage of the gold nanorods and of quantum dots, another nanomaterial Gu is exploring for rewritable five-dimensional storage, is that they respond to much narrower bands of light.