So it turns out that Sony did something right and it, along with a large consortium of supporting folks, won the format war. Blu-ray is now the HD disc format pf choice, but even so, the data storage capability won't keep users happy indefinitely. Some recent research from Northwestern University shows that much higher storage densities are possible using technology based on existing optical media.

The limiting factor is the area on which light can be focused, set by the diffraction limit. Using conventional optics, light can only be focused to a spot that is about the same as the wavelength—for Blu-ray, the wavelength is about 405nm, and the spot size is about 580nm. This means that the pits and landings that represent the data need to be a significant fraction of this size (~100nm) and be separated by a significant fraction of the spot-size as well (100-300nm).

This sets limits on the data density and, currently, the only way to overcome these limits is to layer the disc. That can only go so far, as each layer has to reflect a significant amount of light so that the data can be read. For instance, if each layer of a dual layer disc is 50 percent reflective, then the signal level can be expected to drop by a factor of four between two adjacent layers.

Recently, it was discovered that the interaction between light and a simple glass sphere has some really odd properties. In particular, on the side opposite to the illuminating light, a very narrow and very bright stream of light, termed a photonic jet, exits the sphere. Within a very short distance—on the order of a micrometer for a blue-light laser—the light stream begins to expand rapidly. But very close to the sphere it has dimensions that are much smaller than the wavelength of light. The researchers realized that if they placed the pits and landings within this narrow beam of light, they should be able to store data at much higher densities.

The researchers also realized that this would be difficult to test this using blue-light lasers because it would be very difficult to manufacture and align a suitable demonstration surface at the micron scale. Instead, they did a proof-of-principle using microwave radiation, which has a wavelength of 10 millimeters. The microwaves illuminated an acrylic sphere to create a photonic jet. An optical disc was approximated via a polished aluminum plate that had pits and landings machined into the surface; the plate was coated with the equivalent of the protection layer that comes standard on all optical media, making the whole device as close to a scaled-up optical drive as possible.

Their experimental results and modeling indicated that the light reflected from a pit was about 700 times weaker than from a landing, making the difference very easy to detect. Furthermore, modeling results showed that the reflected signal varied strongly with pit depth, opening up the possibility of encoding additional data by pit depth.

Two big questions remain: how much more data can be stored, and can the reader be be scaled down? If scaled directly, the pits would be approximately 50-80nm long, making the per-layer capacity a factor of two larger than current blu-ray discs. However, the depth encoding could easily provide another two to four times the capacity. Furthermore, this is the first experiment, so we can probably expect another factor of two to four from lessons learned during the development process—the optimist in me says that this could give us 30 times more data than a single layer disc.

The question of scaling is more tricky. The sphere would be on the order of two micrometers in diameter, which is pretty easy to make. However, the distance between the surface of the disc and the sphere would be only around 200nm, which would be very difficult to maintain while the disc was spinning. If that problem can be addressed, then this technology would probably be a winner, particularly since it would be compatible with existing discs.

Applied Physics Letters, 2008, DOI: 10.1063/1.2936993