Copenhagen, Denmark--Until now, invisibility-cloaking techniques have come with a significant limitation--the cloaks need to be orders of magnitude larger than the object being cloaked. This places serious constraints on practical applications, particularly for the optoelectronics industry, where size is a premium.
Physicists from the Technical University of Denmark (DTU), the University of Birmingham (Birmingham, England), and Imperial College London may have overcome this size limitation by creating a carpet cloak with a uniform-period silicon-grating structure that can conceal a much larger area than other cloaks of comparable size.1
Jingjing Zhang, a postdoctoral researcher at DTU, explains that the team's new metamaterial carpet cloak, which is based on an alternating-layer structure on a silicon-on-insulator (SOI) platform, introduces a flexible way to address the size problem with a simpler metamaterial structure.
"The highly anisotropic material comprising the cloak is obtained by adopting semiconductor manufacturing techniques that involve patterning the top silicon layer of an SOI wafer with nanogratings of appropriate filling factor. This leads to a cloak only a few times larger than the cloaked object," says Zhang.
100 nm near-IR range
By restoring the wavefront of a beam of light reflected from the surface, the cloak creates an illusion of a flat plane for a triangular bump on the surface, hiding its presence over wavelengths ranging from 1480 nm to 1580 nm.
"The cloak parameters can be tweaked by tuning the filling factor and the orientation of the layers," says Zhang. "Therefore, layered materials bypass the limitation of natural materials at hand and give us extra freedom to design the devices as desired."
In contrast to previous works based on nanostructures, the cloaking carpet used in this work also shows advantages of easier design and fabrication. The cloak is made exclusively of dielectric materials that are highly transparent to infrared light, so the cloak itself absorbs only a small portion of the beam.
Although the cloaking ensures that the beam shape is unaffected by the presence of the object, the beam intensity is slightly reduced. The researchers attribute this partly to reflection at the cloak's surface and partly to imperfections in fabrication. Adding an antireflection layer and improving uniformity of the grating would help eliminate reflection and scattering issues, say the researchers.
Although our experiment was carried out at near-IR wavelengths, this design strategy is applicable to other spectral ranges such as the visible and microwave regions, notes Zhang.
REFERENCE:
1. Jingjing Zhang et al., Optics Express, Vol. 19, Issue 9, p. 8625, 25 April, 2011.
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