A cloaking device made from sheets of transparent carbon nanotubes takes advantage of the mirage effect — an optical phenomenon in which light rays are bent to produce a displaced image of distant objects or the sky — to make objects disappear. The design, which features an "on and off" switch and is best suited for under-water use, consists of one-molecule-thick sheets of carbon wrapped up into cylindrical tubes. Carbon nanotubes have been studied extensively for a variety of applications because they offer unique properties, including the density of air and the strength of steel. But their exceptional ability to conduct heat and transfer it to surrounding areas is what makes them ideal for exploiting the mirage effect. Underwater mirage effect created by a 5 x 12 cm2 carbon nanotube sheet. The text "invisibility cloaks" placed behind the carbon nanotube sheet disappears when the sheet is resistively heated. Images courtesy of Ali Aliev, University of Texas at Dallas. Through electrical stimulation, the transparent sheets of highly aligned nanotubes can easily be heated to high temperatures. They then have the ability to transfer that heat to surrounding areas, causing a steep temperature gradient. Similar to a mirage, this gradient causes the light rays to bend away from the object concealed behind the device, making it appear invisible. "The most exciting (expected and only partially realized) result is the observed ability of the freestanding multiwalled carbon nanotube sheet to rapidly — in a fraction of milliseconds — reach the temperature of 2500 K, enabling high-frequency modulation of sheet temperature over an enormous temperature range, thereby providing a sharp, rapidly changing gradient of refractive index in surrounding liquid or gas," explained Dr. Ali Aliev, a research scientist at the University of Texas. "This ability, including high thermal conductivity of carbon nanotubes and extremely low heat capacity of the aerogel density sheet, provides very effective photothermal deflection, also known as a mirage effect." With this method, Aliev and colleagues observed that it is more practical to demonstrate cloaking underwater, because all of the apparatus can be contained in a petri dish. The ease with which the carbon nanotubes are heated is what gives the device its unique on-and-off switch feature. Their findings, which were published Oct. 3 in Nanotechnology (doi: 10.1088/ 0957-4484/22/43/435704), could provide insight into the optimization of nanotube sheets as thermoacoustic projectors for sonar and loudspeaker applications, where sound is produced by heating via an alternating electrical current, Aliev said.