This is an electron micrograph image of an invisibility cloak structure. The polymer-air metamaterial (“logs”) is colored blue, the gold-coated areas are colored yellow. |
“Seeing
something invisible with your own eyes is an exciting experience,” say
Joachim Fischer and Tolga Ergin. For about one year, both physicists and
members of the team of Professor Martin Wegener at KIT’s Center for
Functional Nanostructures (CFN) have worked on refining the structure of
the Karlsruhe invisibility cloak to such an extent that it is also
effective in the visible spectral range.
In
invisibility cloaks, light waves are guided by the material such that
they leave the invisibility cloak again as if they had never been in
contact with the object to be disguised. Consequently, the object is
invisible to the observer. The exotic optical properties of the
camouflaging material are calculated using complex mathematical tools
similar to Einstein’s theory of relativity.
These
properties result from a special structuring of the material. It has
to be smaller than the wavelength of the light that is to be deflected.
For example, the relatively large radio or radar waves require a
material “that can be produced using nail scissors,” says Wegener. At
wavelengths visible to the human eye, materials have to be structured in
the nanometer range.
The
minute invisibility cloak produced by Fischer and Ergin is smaller than
the diameter of a human hair. It makes the curvature of a metal mirror
appear flat, as a result of which an object hidden underneath becomes
invisible. The metamaterial placed on top of this curvature looks like a
stack of wood, but consists of plastic and air. These “logs” have
precisely defined thicknesses in the range of 100 nm. Light waves that
are normally deflected by the curvature are influenced and guided by
these logs such that the reflected light corresponds to that of a flat
mirror.
“If
we would succeed again in halving the log distance of the invisibility
cloak, we would obtain cloaking for the complete visible light
spectrum,” says Fischer.
Last year, the Wegener team presented the first 3D invisibility cloak in the journal Science.
Until that time, the only invisibility cloaks existed in waveguides and
were of practically two-dimensional character. When looking onto the
structure from the third dimension, however, the effect disappeared. By
means of an accordingly filigree structuring, the Karlsruhe invisibility
cloak could be produced for wavelengths from 1500 to 2600 nm. This
wavelength range is not visible to the human eye, but plays an important
role in telecommunications. The breakthrough was based on the use of
the direct laser writing method (DLS) developed by CFN. With the help of
this method, it is possible to produce minute 3D structures with
optical properties that do not exist in nature, so-called metamaterials.
In
the past year, the KIT scientists continued to improve the already
extremely fine direct laser writing method. For this purpose, they used
methods that have significantly increased the resolution in microscopy.
With this tool, they then succeeded in refining the metamaterial by a
factor of two and in producing the first 3D invisibility cloak for
non-polarized visible light in the range of 700 nm. This corresponds to
the red color.
“The
invisibility cloak now developed is an attractive object demonstrating
the fantastic possibilities of the rather new field of transformation
optics and metamaterials. The design options that opened up during the
last years had not been deemed possible before,” emphasizes Ergin. “We
expect dramatic improvements of light-based technologies, such as
lenses, solar cells, microscopes, objectives, chip production, and data
communication.”
The way toward the Karlsruhe Invisibility Cloak
The
“small improvement” of the Karlsruhe metamaterial with a high effect
results from a series of development steps that appeared impossible a
few years ago. Until the early 21st century, it was deemed infeasible to
develop a material, by means of which light can be manipulated such
that the material acts like an invisibility cloak. In 2006, the
fundamentals of an invisibility cloak were described for the first time
by the theory of transformation optics.
Based
on theoretical calculations, first attempts were started to produce
such a material artificially. Sir John B. Pendry (Imperial College,
London, U.K.) and David R. Smith (Duke University, Durham, NC, USA and
Imperial College, London, U.K.) published their results obtained for an
invisibility cloak for radar waves in 2006. In 2008, Jensen Li (City
University of Hong Kong, China) and Sir John B. Pendry presented the
theoretical idea of a carpet invisibility cloak. In 2010, Wegener and
his team from KIT, Karlsruhe, presented their first 3D invisibility
cloak. In 2011, the effects of the Karlsruhe invisibility cloak are also
visible to the bare eye.
DFG Center for Functional Nanostructures
Literature
J.
Fischer, T. Ergin, and M. Wegener, “Three-dimensional
polarization-independent visible-frequency carpet invisibility cloak”,
Optics Letters, in press
