Long before the Klingons came at Captain Kirk with ‘cloaked’ space craft in the 1960s, the imagined possibilities of invisibility have been dreamed of by schoolboys, scientists, the military and almost everyone else. Recent advances have brought these hopes nearer and taken them out from the realms of pure science fiction, yet its application using frequencies in the visible range have proved elusive. Now, a team at Berkeley, California, [Valentine et al., doi:10.1038/NMAT2461] has achieved an experimental demonstration of the effect using a cloak of isotropic dielectric materials.
Microwave frequency cloaking has already been demonstrated and uses transformation optics, but it is effective in only a narrow range due to strong dispersion and makes scaling down to visible-region wavelengths difficult. Xiang Zhang and colleagues have devised a method that overcomes these scaling-down restrictions, etching holes into a layer of silicon in a complex pattern, hiding a small ‘bump’ underneath the cloaking carpet.
In the experiment, a triangular cloak region, with a uniform hole pattern, is used as a background with a constant reflective index. A rectangular cloak region with a 2D variable index profile serves to ‘hide’ the bump. The effect was demonstrated within a silicon slab waveguide, placing the cloak around a bump using quasi-conformal mapping; a 2D lattice of sub-wavelength holes of varying density fabricated to form the index profile. Two gratings were made to couple the light in and out of the waveguide. Electron beam evaporation was used to deposit 100nm gold, directionally onto the waveguide to form the reflective surface.
With the cloaking area placed around the curved surface, or bump, any shape placed behind it will have the reflectance properties of a flat surface. The cloaking carpet was fabricated on a silicon-on-insulator (SOI) wafer, separated from a Si wafer substrate by a layer of silicon oxide (SiO2).
In order to determine the bandwidth of the device, a wide range of wavelength measurements were made. Between 1400 and 1800nm, a single peak expresses the band profile at the output grating, suggesting that the cloak performance is unaffected by the change in wavelength. Below 1400nm, the wavelegth in the slab waveguide becomes comparable to the hole diameter in the cloak and scattering or breakdown occurs. However, performance in this range can be improved with the use of smaller holes, extending the effective medium approximation to a shorter wavelength.
With this experiment, it has been shown that visible-light invisibility can be achieved with low loss and broadband, the mapping design and method of fabrication opening up transformation optics applications in a number of areas. Further improvements in light transmission are expected from using electron beam lithography to drill the holes, in association with reactive ion etching, eliminating some of the losses and providing for much larger cloaking devices.