In this picture light from two lasers (red and green) is traveling from left to right while confined in a gradually tapering optical waveguide. As the waveguide narrows, the effective speed of light decreases until it reaches zero and then the light stops. This picture taken by Vera and Igor Smolyaninov and others at Towson University in Maryland shows the green light penetrating further than the red before stopping. If a blue laser had been used its light would have been trapped slightly further down the guide.
The trapped rainbow experiment is part of a larger program to manipulate light in sophisticated ways using "metamaterials" nanofabricated to produce exotic outcomes such as the ability to render an object invisible like the Romulan "cloaking device" in Star Trek. For more information about the Smolyaninovs' cloaking experiments see Those Damn Romulans and the 2Physics blog.
Metamaterials (MMs) are usually constructed by forming periodic structures whose periodicity is slightly smaller than the wavelength of the wave you are trying to control. Since the wavelength of microwaves is on the order of an inch, one can almost construct microwave metamaterials by assembling Lego blocks. Metamaterials that manipulate visible light, however, would have to be constructed of Lego blocks tens of thousands of times smaller.
The Smolyaninovs and their collaborators have cleverly bypassed the difficulty of fabricating metamaterials for visible light by demonstrating that a tapered optical waveguide already possesses some of the desirable qualities of MMs such as the ability to partially cloak small objects and to produce a "trapped rainbow" by slowing and stopping light. It is a big step from hiding tiny gold wires to cloaking whole star ships but a more practical near-term outcome from experiments such as these could be "super lenses" that produce higher resolution images than are possible with glass or, more likely, devices that coax light to perform unnatural acts no one has yet imagined.