Researchers from Towson University have come close to making science-fiction an everyday reality by “trapping” a rainbow. Most people are familiar with the idea of a cloaking device or invisibility cloaks thanks to Star Trek and Harry Potter, but a real-world experiment designed to understand how to use and manipulate light involved using an array of 25,000 “invisibility cloaks,” each smaller than a hair’s breadth, to snare a rainbow, slowing light to a near-stop.
Led by Vera Smolyaninova, the Towson University team used a commercially available “micro-lens array” – a grid of tiny lenses just 50 millionths of a meter across – that they then coated with a thin film of gold and placed above a flat, gold-coated sheet of glass. When light was shone in from the side, it was guided around each tiny lens, creating a small cloaked region in the center of each one. This has an overall effect of ‘slowing down’ the light. It also effects the color. As in a prism, the effect was slightly different for light of different hues and spread out into its constituent colors across the surface.
This “trapped rainbow” effect was first predicted in 2007 in a paper published in Nature by Ortwin Hess and colleagues. Speaking to the BBC, Professor Hess, who now teaches at Imperial College London, said the experiment was “encouraging and exciting. When coming up with that general concept of the trapped rainbow, it seemed to be a very fundamental effect and have wide application,” he told BBC News. “So taking this forward to the experimental stage is a very nice thing to see.”
Traditionally, the idea of a cloaking device sees light bent or guided around a specific object, so that it appears invisible to anyone watching. Most products have had limited success due to their size or the range of colors or angles of light that they work with. However, the Towson team’s experiment has revealed that by engineering light in this manner, there are a great many applications besides invisibility that it could benefit, including communications.