IEEE Spectrum February, 2014 - 37

Photos, these Pages: left, karlsruhe institute of teChnology/ oPtiCal materials exPress;
right, karlsruhe institute of teChnology/oPtiCs exPress

radiation does, albeit with some lag and at a lower speed. As with wavelength radiation, in the microwave part of the spectrum. To
the incoming field, both the energy and phase vectors are posi- make metamaterials that can act on visible light, which has a wave-
tive. But in a material with a negative index of refraction, energy length range of roughly 400 to 750 nanometers, the "atoms" must
and phase move in opposite directions. Although the energy of
be on the order of 100 to 200 nanometers or smaller.
the light-and the light itself-still moves in the same direction as
How do you make such small structures? You might think that
when it entered the material, the individual peaks and troughs
a natural place to start is with the technology that's already been
actually move backward.
developed by the semiconductor industry. There are, after all,
To picture how this would look in the real world, imagine a straw already powerful patterning tools, such as photolithography and
resting inside a half-filled glass. If the straw is standing in water, electron-beam lithography, that are routinely used to fabricate
it will appear much as it does in air, with only a slight kink at the
submicrometer and nanoscale structures.
interface between the two materials. But if the glass contained a
Indeed, groups such as those of Harald Giessen at the University
liquid with a negative refractive index, the straw would look bent, of Stuttgart, in Germany; Vladimir Shalaev at Purdue University, in
as if it were leaning in the opposite direction.
Indiana; Costas M. Soukoulis at Iowa State University; Xiang Zhang
A material with a negative index of refraction could potentially be
at the University of California, Berkeley; Nikolay Zheludev at the
used to make superlenses capable of imag-
University of Southampton, in England; and
ours at the Karlsruhe Institute of Technol-
ing objects at resolutions far below the wave-
length of light that's shone on them. But the
ogy (KIT), in Germany, have successfully
ability to control both components of light
used these techniques to create arrays of
also offers other possibilities. Meta-atoms
simple metamaterial structures, including
designed to act on both the electric and
split-ring resonators, that are small enough
magnetic components of radiation equally,
to act on visible or infrared light.
for example, could be used to create an
But these lithographic approaches begin
"invisibility cloak" that works on the naked
to break down when you try to fabricate
eye, steering light so that an object can be
even seemingly simple 3-D objects, which is
hidden from view.
what you need for a metamaterial that can
We could also potentially make a meta-
act on light regardless of direction. Lithog-
material with electric and magnetic responses
raphy is designed to pattern 2-D features,
a negative bent: artificial materials can be
that destructively interfere, so that no
so to construct an array of tiny helical coils,
made to have a negative index of refraction,
radiation is reflected. If we could make that a light-bending property not seen in nature.
for example, you'd have to construct them
the interface of air and water-two substances
happen in a system that also absorbs light, we
layer by layer in hundreds of steps. It's a
with positive indices-creates a small kink in
could create a perfectly black material that the appearance of a straw [left]. if the liquid had
time-consuming process that requires care-
a negative index of refraction, the straw would
neither reflects nor transmits light, raising
ful alignment. Even research groups that are
appear to bend in the opposite direction [right].
the possibility of more-sensitive detectors
adept at the process might need an entire
and more-efficient solar cells.
day to make a single layer.
One other possible application is to filter out light with either
Fortunately, there is a better way to make 3-D structures. The
a right- or left-handed circular polarization. The basic mecha- key is to use laser light, and some tricks developed over the last
few years, to write in three-dimensional space. You could consider
nism is similar to the way that sugar water, DNA, and other chiral
materials-molecules with a physically distinct mirror image- this type of optical lithography-direct laser writing-as a micro-
rotate the polarization of a light wave. In those materials, the
scopic version of 3-D printing. Just as with rapid prototyping, or
interaction is fairly weak, requiring light to travel for centimeters
stereolithography, which was patented by Charles W. Hull in 1986,
through the substance before it registers a strong change in
light is used to sketch out a shape. In this case, however, shapes
polarization. A metamaterial, structured as an array of tiny
are not made layer by layer. Instead they're all made in one go, out
helices, can accomplish its filtration task over much shorter dis- of a single volume of material. Solvent washes away what isn't ex-
tances. That could allow us to build compact devices that distin- posed to light, much as Michelangelo might say an artist creates a
guish, for example, between pharmaceuticals and their mirror sculpture: by chipping away excess stone.
images, which may be identical in composition but have very
different biological effects.
as in other lithographic techniques, direct laser writing uses a com-
pound called resist-in this case, a photosensitive mixture called
that's only a sMall saMpling of what might be done with metamateri- photoresist. To make metamaterials, we start with a thick layer
of the stuff, which coats a glass slide or some other substrate and
als. To get from blueprint to reality, we have to find a good way to
is mounted on a microscope. When we shine a laser through the
make the underlying structures-the meta-atoms.
When the metamaterials field got its start a bit more than a de- microscope optics and onto the photoresist, the light breaks mole-
cules apart and causes the exposed material to polymerize and hard-
cade ago, meta-atoms tended to be macroscopic: centimeter-size
metallic split rings and wires printed on standard circuit boards. en. When we're done, we can wash away the unexposed material
But these structures were so large they could act only on longer- with solvent, and only the exposed material remains.
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Table of Contents for the Digital Edition of IEEE Spectrum February, 2014