Scientific Method / Science & Exploration

Storing light

Metamaterials may offer a path towards storing light. Could this be the memory …

Light-based computing has intrigued scientists for a long time. There is an implicit promise of speed because light travels fast. Since optical operations don't rely on injecting electrical currents into a very small volume, problems such as heat dissipation and junction transit time disappear. Similarly, capacitive coupling between signals is much smaller. For the quantum computation fanatics among us, entangled photons are the most robust form of qubit available, making light a leading contender in that particular race (D-Waves' unsubstantiated statements aside). All in all, it seems like a good prospect. However, one big thorn in the side of the optical computing community is the lack of an optical memory. You just can't stop light very easily.

Currently, storing light is very difficult. Usually light is stored by not storing the light itself; instead it is encoded into some substance through a process that involves creating coherence between the light to be stored and a gas, solid, or single atom. Unfortunately, solids vibrate, meaning that the coherence only lasts for a short time. Gases would be great to store light if it weren't for the fact that the individual atoms move. By the time the light memory is retrieved, the atoms that stored it are in different locations. Storing light in a single atom is probably the most effective method demonstrated so far. Unfortunately, this involves an enormous vacuum system and a very carefully constructed optical system using very highly reflective mirrors with nearly perfect surfaces—not something you can etch out with a fab plant at $30 per 2GB of storage. Clearly, a new approach to storing light is necessary if photonic computation is to be scaled beyond the few bit register level.

Another promising approach is the use of photonic crystals. A photonic crystal consists of a regularly varying structure—picture drilling tiny holes in a block of glass—where the variation takes place at about the same distance scale as the wavelength of the light we are interested in. The regularly spaced interfaces between the materials that make up the structure reflect light. These reflections will add up in such a way that light can be reflected, transmitted, or even guided. The problem is that the structure that would store light efficiently is the same as the one that reflects it. You could store the light, if you could get it in there.

Some theoretical physicists in England have come up with a unique proposal that combines some of the properties of photonic crystals with optical waveguides and metamaterials. Metamaterials are similar to photonic crystals in that they also have a medium that is structured at the same length scale as the wavelength of light. In this case, however, a transparent material is modified by the inclusion of metallic blocks, rings, or spheres. The spacing between metal elements and their optical properties gives the metamaterial a negative refractive index. This has many consequences, such as light bending the opposite direction than expected.

The researchers have studied how light propagates down a waveguide (think optical fiber), where the core—this is the part of the waveguide where the light is confined—is made of a metamaterial. The material surrounding the core are normal but with a refractive index that is smaller than the absolute value of the core, this is necessary to confine the light to the core of the waveguide. As the light hits the interface between the core and the outer layers, the light's phase is modified, which is the equivalent of shifting the light along the waveguide. The key insight of the researchers was to recognize that the, for some geometries, a negative phase shift occurs and for others a positive phase shift occurs. This can be broken into three cases: if the core is too small (but still large enough for the light to "fit" into the waveguide), the light is reflected from the waveguide (like the photonic crystal) because the phase shift is negative. If the core is too large, the light propagates down the waveguide with a positive phase shift making it travel a bit quicker. If the waveguide is tapered, the light will enter the waveguide and propagate to the point where the back-shift in the narrow section is exactly counterbalanced by the positive-shift in the wide section, effectively storing the light in that section of the waveguide. Furthermore, the exact diameter range for which this occurs depends on the color of the light, so different colors of light are stored in different sections of the waveguide.

Now for the caveat section of the write up. As with most metamaterials articles, they haven't actually built anything. They are theoretical physicists and they will only enter the lab to break important bits of equipment. More importantly, metals are not perfect conductors and, for the light to be stored without loss, which is something the researchers claim in the article, the metal would have to be perfect. This means that, like all other light storage solutions, this one will have a limited storage time as well—think of this as RAM rather than flash memory. Finally, this doesn't get around the fact that optical component are big. Really, really big. The semiconductor industry already has a roadmap that has feature sizes in the 30nm range already scheduled, while optical components still require sizes on the order of the wavelength of light (400nm, minimum) so we are talking an order of magnitude in component size. An order of magnitude more performance per component will be required before the jump to optical computing can be justified.

Nature, 2007, DOI: 10.1038/nature06285

Expand full story

You must to comment.


You May Also Like