Researchers have managed to trap a rainbow, slowing light to a near-stop, in an impressive set of 25,000 "invisibility cloaks," each smaller than the breadth of a hair. A report published in the 14th edition of the New Journal of Physics, May, 2012, shows how the quest for an invisibility cloak is paving the way to smarter methods of using and manipulating light, operating in the visible frequency range.
Several research efforts in recent years have demonstrated a wide range of cloaking techniques, with various potential applications. Light can be guided around or canceled by a material in the visible frequency range, making an object invisible to an observer. Such cloaks have generally been tiny or had a limited range of colors and angles of light to work with. This type of using and manipulating light has, however, many potential applications besides invisibility.
Moreover, researchers writing in Physical Review B have recently indicated that arranging a number of tiny cloaks in a two-dimensional grid could have applications in biomedicine and sensing, or serve as traditional camouflage.
Demonstration
In a new study, Vera Smolyaninova and her colleagues at Towson University in Maryland, U.S., have taken those ideas and put them into practice, starting with a "micro-lens array." This array basically consists of a grid of tiny lenses just 50 millionths of a meter across, and is commercially available.
The researchers coated the array with a thin film of gold, then placed it above a flat sheet of glass, also coated in gold. As light was shining in from the side, between the micro-lens array and the sheet of glass, it was guided around each tiny lens, thus creating a small cloaked region in the center of each one. The array of lenses had the effect of slowing down the light to a near stop. Similar to a prism, the lenses had a slightly different effect for light of different colors, so that light shining in at one side would be spread out across the surface into its constituent colors.
As the BBC notes, the "trapped rainbow" effect was first predicted back in 2007, in a paper Ortwin Hess and colleagues published in Nature. There are numerous techniques that use many colors of light, from blood analysis to explosive detection, however the sensitivity of many techniques depends on how much light interacts with the material under investigation. Slow light can interact much more intensively with the matter being investigated, and for this reason the trapped rainbow effect has always been considered as a useful way to improve such methods.
"When coming up with that general concept of the trapped rainbow, it seemed to be a very fundamental effect and have wide application," Prof. Hess, now at Imperial College London, told BBC News. "So taking this forward to the experimental stage is a very nice thing to see."
