Compiled by Photonics Spectra staff
Optical cloaking is getting a boost from
metamaterials in a new technology called “carpet cloaking,” which can
conceal a much larger area than other cloaking techniques of comparable size.
The new carpet cloak, based on an alternating-layer structure
on a silicon-on-insulator (SOI) platform, introduces a flexible way to address the
size problem. It was developed by an international team of physicists from Technical
University of Denmark (DTU), the University of Birmingham in the UK and Imperial
College London. Their approach appeared in OSA’s open-access journal Optics
Express (doi: 10.1364/OE.19.008625).
The cloak was designed with a grating structure that is simpler
than previous metamaterial structures for cloaks, said Jingjing Zhang, a postdoctoral
researcher at DTU. The grating structure – a series of slits or openings that
redirect a beam of light – channels light of a particular wavelength around
an object.
“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,” Zhang
said.
The measured output image
from a flat surface (left) and a cloaked protruded surface (right) at (a) 1480 nm,
(b) 1550 nm and (c) 1580 nm. Courtesy of Technical University of Denmark and Optics
Express.
“Filling factor” refers to the size of the grating
structure and determines the wavelengths of light that are affected by the cloak.
By precisely restoring the path of the reflecting wave 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 to 1580 nm. This means the
carpet cloaks essentially distinguish an object from light, making it appear like
a flat ground plane.
The parameters of the cloak could be tweaked by tuning the filling
factor and the orientation of the layers, which would allow the layered materials
to bypass the limitation of natural materials at hand and give the team extra freedom
to design the devices as desired, Zhang said.
The cloak absorbs a negligible fraction of energy because it is
made exclusively of dielectric materials that are highly transparent to infrared
light.
The researchers still want to improve the technology: They reported
in their paper that, although the cloaking ensures that the beam shape is unaffected
by the presence of the object, the beam intensity is slightly reduced. Attributing
this to reflection at the cloak’s surface and partly because of imperfections
in the fabrication, they discovered that adding a layer of material around the cloak
to improve the uniformity of the grating would help to eliminate the reflection
and scattering issues.
With more precise fabrication, the team hopes, the cloak will
work in the microwave and visible parts of the spectrum, opening doors for futuristic
defense applications.