Researchers at Northwestern University have created a cloaking material that can render objects invisible in the terahertz range. Although the design can’t translate into an invisibility cloak for the visible spectrum, it could have applications in diagnostics, security and communication. The cloak uses microfabricated gradient-index materials to manipulate the reflection and refraction of terahertz wavelengths. The material was designed by Cheng Sun, an assistant professor of mechanical engineering, To render an object invisible, light must be manipulated so that it will neither scatter at an object’s surface nor be absorbed or reflected by it. To manipulate light in the terahertz frequency, Sun and his group developed a metamaterial comprising tiny, prism-shaped structures less than 10 mm long, using a technique called electronic transfer microstereolithography. The researchers used a data projector to project an image onto a liquid polymer and then light to transform the liquid layer into a thin solid layer. Each of the prism’s 220 layers has tiny holes that are much smaller than terahertz wavelengths, which means that they can vary the refraction index of the light and render invisible anything located beneath a bump on the prism’s bottom surface; the light then appears to be reflected by a flat surface. Sun said that the purpose of the cloak is not to hide items but to get a better understanding of how to design materials that can manipulate light propagation. “This demonstrates that we have the freedom to design materials that can change the refraction index,” he said. “By doing this, we can manipulate light propagation much more effectively.” The terahertz range has been historically ignored because the frequency is too high for electronics. But many organic compounds have a resonant frequency at the terahertz level, which means they could potentially be identified using a terahertz scanner. Sun’s research into terahertz optics could lead to safer detection of certain kinds of cancers and to better ways of using terahertz scanners at airports. Next, Sun hopes to use what he has learned through the cloak to create its opposite: a terahertz lens. He has no immediate plans to extend his invisibility cloak to visible frequencies. “That is still far away,” he said. “We’re focusing on one frequency range, and such a cloak would have to work across the entire spectrum.” For more information, visit: www.northwestern.edu