SEM image of a fabricated carpet cloak, the insets show
the oblique view of the carpet cloak (top) and the
cloak/reflector interface (bottom) (Image: Technical University
of Denmark/Optics Express)
Efforts to create a working "invisibility
cloak" have generally involved the use of artificial
materials with a negative refractive index known as
metamaterials. Another
promising technique involves the use of a natural crystal
called calcite that boasts an optical property known as
birefringence, or double-refraction. While both methods have
proven successful in rendering very small objects invisible in
specific wavelengths of light by bending and channeling light
around them, both techniques require the "cloak" to be orders of
magnitude larger than the object being concealed. Researchers
are now reporting progress in overcoming this size limitation
using a technology known as a "carpet cloak."
The carpet cloak technology developed by an international
team of physicists from the Technical University of Denmark
(DTU), the University of Birmingham, UK, and Imperial College
London (which is the same team responsible for the calcite-based
invisibility cloak) doesn't refer to a cloak that can conceal
fabric floor covering, but technology that can conceal a much
larger area than other cloaking techniques of comparable size.
Metamaterials are once again central to the technology, but the
researchers have employed a new grating structure that consists
of a series of slits or openings that redirect a beam of light.
The cloaking material, which is highly anisotropic – meaning
it has different physical properties along different axes – was
created using semiconductor manufacturing techniques that
involve patterning the top silicon layer of a silicon on
insulator (SOI) wafer with nanogratings of an appropriate size
and structure, referred to as the filling factor, which
determines the wavelengths of light that are affected.
"This leads to a cloak only a few times larger than the
cloaked object," says Jingjing Zhang, a postdoctoral researcher
at DTU's Fotonik Department of Photonics Engineering and
Structured Electromagnetic Materials. "The cloak parameters can
be tweaked by tuning the filling factor and the orientation of
the layers. Therefore, layered materials bypass the limitation
of natural materials at hand and give us extra freedom to design
the devices as desired."
By precisely restoring the path of the reflecting wave from
the surface, the researchers have created the illusion of a flat
plane that conceals the presence of a triangular bump on the
surface over wavelengths ranging from 1,480 nanometers to 1,580
nanometers. Additionally, the researchers say the cloaking
carpet is also easier to design and fabricate than other
nanostructure-based cloaking techniques.
Being made exclusively of dielectric materials that are
highly transparent to infrared light, the cloak is also very
efficient and absorbs a negligible fraction of energy. However,
although the cloak ensures that the beam shape is unaffected by
the presence of the object, the beam intensity is slightly
reduced. The researchers attribute this to reflection at the
cloak's surface as well as imperfections of the fabrication, but
they say that adding an additional layer of material around the
cloak and improving the uniformity of the grating would help
eliminate reflection and scattering issues.
"Although our experiment was carried out at near-infrared
frequencies, this design strategy is applicable in other
frequency ranges," notes Zhang. "We anticipate that with more
precise fabrication, our technique should also yield a true
invisibility carpet that works in the microwave and visible
parts of the spectrum and at a larger size – showing promise for
many futuristic defense and other applications."
The researchers describe their carpet cloak technology in a
paper titled "Homogeneous optical cloak constructed with
uniform layered structures" published in the journal
Optics Express.
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