Scientists Trap a Rainbow

An anonymous reader writes to tell us that Physicists from both the University of Surrey and Salford University have devised a method to trap a multi-colored rainbow of light inside a prism. "Previous attempts to slow and capture light have involved extremely low or cryogenic temperatures, have been extremely costly, and have only worked with one specific frequency of light at a time. The technique proposed by Professor Hess and Mr Kosmas Tsakmakidis involves the use of negative refractive index metamaterials along with the exploitation of the Goos Hänchen effect, which shows that when light hits an object or an interface between two media it does not immediately bounce back but seems to travel very slightly along that object, or in the case of metamaterials, travels very slightly backwards along the object."

Comments

The Source

[Anonymous Coward]

It would be nice if the "journalists" bothered to mention there's an article in Nature.

Re:Scientists Trap a Rainbow

[kebes]

First off, for those interested (and with subscriptions) let me provide a reference to the actual paper (from last week's Nature):
Kosmas L. Tsakmakidis, Allan D. Boardman & Ortwin Hess 'Trapped rainbow' storage of light in metamaterials Nature 450, 397-401 (15 November 2007) | doi: 10.1038/nature06285. (See also summary comment box, doi 10.1038/450330a.)

They propose a method that might. The meta-materials needed to do this with visible light don't exist yet. Your caution is quite correct. The paper is theoretical. An actual device has not yet been built. However this result is still significant because what they are showing is that the various results on "slow light" and "trapped light" can be realized in optical metamaterials. This is significant because metamaterials are in principle more amenable to technological deployment than the more exotic techniques of slowing light (ultra-cold condensates, etc.).

It's also worth noting that metamaterials at various wavelengths (e.g. microwave band and IR) have already been made. We are getting very close to optical metamaterials. For instance, see this review of the field:
Vladimir M. Shalaev Optical negative-index metamaterials Nature Photonics 1, 41 - 48 (2006) doi: 10.1038/nphoton.2006.49.

We already have prototype metamaterials at wavelengths of 780 nm, which is on the edge of the visible spectrum. Significantly, we already have metamaterials that operate in the IR band, which is what is used for modern fiber-optics, telecommunications, etc. The materials to date are not optimized, so it will of course be awhile before all these great applications of metamaterials are implemented in real telecom devices. But, still, we are getting quite close to these applications. In particular, I expect we'll see a commercial 'rainbow trapping' device for communications before we see a commercial 'invisibility cloak'!

Re:Light Labyrinth?

[kebes]

Has anyone worked on making devices or materials that channel light along a very long internal optical path folded up inside a small volume? It's a neat idea, and in real-world optics such tricks are sometimes used. For instance you can set up two mirrors, and have the beam bounce back-and-forth between them, in order to introduce a known delay into a particular beam path (you can increase the traveled path by a rather large amount). Another simple trick is just to launch a pulse into a big roll of fiber-optic.

The main problem with such techniques is losses. Even if your mirror is 99.9% reflective (and mirrors that good are expensive, by the way), you quickly lose all your signal intensity if you are reflecting thousands or millions of times. Your idea of using a photonic crystal is neat, but you would be hard-pressed to make a very long path length without making the crystal large, too. And if you cap the end with a mirror (to trap the light for longer), you run into losses from that.

That's one of the reasons the research mentioned in TFA is significant: in principle it allows a pulse to be trapped for an arbitrary amount of time with no losses (and for a broad range of wavelengths).

Re:Light Labyrinth?

[kebes]

The inefficiency in reflections is light converting to heat when interacting with the medium. What exactly is that mechanism called? There are a variety of effects that lead to losses. There is simple absorption, where light is converted to heat. Any real material will have a non-zero absorbance. Also, to achieve high reflectivity you want a high refractive index contrast. Vacuum has a nice low refractive index, but of course there is no material with an infinite refractive index, so you will always get some transmission into the material. Unless the surface is truly perfect you will also get some amount of scattering that sends off light in other directions.

There are similar problems with refraction: the refractive index contrast is not infinite, so some amount of light is always transmitted. At glancing angles (below the critical angle), you theoretically get perfect 100% internal reflection. This is how fiber-optics work: by having a glancing-angle internal reflection, the losses at the boundary are quite low. However the beam is then propagating inside a material, and there is absorption from the material itself. (Even if the absorption was somehow zero, the refraction at the boundary would never be perfect: imperfections and evanescent waves would cause some amount of light to escape.)

So, while theoretically one could build a light-trap using reflection or refraction, using any known material would involve some imperfections or losses preventing long-term trapping.