A cloak is a device that makes objects invisible to an external observer. The first cloaks that could work with visible light used bulky containers with special optical properties, but, although they concealed the object, the containers could still be visible to an observer. Xiang Zhang and colleagues at the University of California, Berkeley have now reported a proof-of-principle cloaking device made of an 80-nm-thick surface that can be wrapped around objects of any shape, making them invisible to 730-nm light.
This ultrathin cloak reroutes light scattered from an object so that it seems as though the light has been reflected by a mirror. To obtain such an effect, the researchers patterned a gold surface with six differently sized gold nanoantennas with subwavelength dimensions that could compensate for the phase shift acquired by the scattered radiation. Because phase compensation occurs locally and covers the full spectrum (from 0 to 360°), objects with sharp edges can potentially be cloaked. To retain most of the light intensity, the team added a dielectric layer that increases the overall reflectivity of their metasurface. This design, which can in principle be scaled up, works for incoming light within 30° angles with respect to the perpendicular direction.