Electromagnetic metamaterials have brought Harry Potter’s invisibility cloak into practice in the last several years. Unfortunately, only narrow working bandwidth could be achieved owing to the fundamental bandwidth-delay restriction in the cloaks. In an article recently published in Scientific Reports, researchers from the Chinese Academy of Sciences have now demonstrated a broadband two-dimensional metamaterial (metasurface), which could make an object become invisible to detectors operating at broadband frequencies.
Traditionally, there are many methods that do tricks to make objects disappear. One particular example can be seen in the Lockheed F-117 Nighthawk. The nighthawk is designed with multiple planar surfaces, which reflect the radar waves to directions away from the detectors. The drawback of this technology lies in the fact that the optimized shape for invisibility is, however, not optimized for flight. The bad aero- and hydro-dynamics limited its performance.
The new technology presented now is termed as virtual shaping, which means the shape sensed by the electromagnetic detector can be arbitrarily designed, but the physical shape do not alter. This is a property that has been pursued by many scientists and engineers for a long time. However, traditional methods are not suitable to achieve such a goal. In the current work, researchers used the coupling between polarization and phase, i.e. the spin-orbit interaction, in space-variant metasurfaces to change the phase shift of an incident electromagnetic wave. The bandwidth problem is solved by employing a technique called spectral dispersion engineering. The simulation and experimental results show that the novel devices could operate in the entire visible and near-infrared region.
“Strictly speaking, this device is not an ideal invisibility cloak, because the detectors from other directions could still observe the objects. Variation of the phase shift could make a perfect carpet cloak, whereas the practical application is still to be investigated,” said Xiangang Luo, director of the State Key Laboratory of Optical Technologies on Nano-Fabrication and Micro-Engineering (SKLOTNM), Chinese Academy of Sciences. “Nevertheless, we believe the concept of broadband virtual shaping may open many perspectives in this emerging area” he adds.
Article: Pu, M. et al. Spatially and spectrally engineered spin-orbit interaction for achromatic virtual shaping. Scientific Reports 5, 9822 (2015). DOI: 10.1038/srep09822