NY Times, June 12, 2007
Light Fantastic: Flirting
With Invisibility
Increasingly, physicists are
constructing materials that bend light the ÒwrongÓ way, an optical trick that
could lead to sharper-than-ever lenses or maybe even make objects disappear.
Last October, scientists at Duke
demonstrated a working cloaking device, hiding whatever was placed inside,
although it worked only for microwaves.
In the experiment, a beam of
microwave light split in two as it flowed around a specially designed cylinder
and then almost seamlessly merged back together on the other side. That meant
that an object placed inside the cylinder was effectively invisible. No light
waves bounced off the object, and someone looking at it would have seen only
what was behind it.
The cloak was not perfect. An
alien with microwave vision would not have seen the object, but might have
noticed something odd. ÒYouÕd see a darkened spot,Ó said David R. Smith, a
professor of electrical and computer engineering at Duke. ÒYouÕd see some
distortion, and youÕd see some shadowing, and you would see some reflection.Ó
A much greater limitation was
that this particular cloak worked for just one particular Òcolor,Ó or
wavelength, of microwave light, limiting its usefulness as a hiding place.
Making a cloak that works at the much shorter wavelengths of visible light or
one that works over a wide range of colors is an even harder, perhaps
impossible, task.
Nonetheless, the demonstration
showed the newfound ability of scientists to manipulate light through
structures they call Òmetamaterials.Ó
Obviously the military would be
interested in any material that could be used to hide vehicles or other
equipment. But such materials could also be useful in new types of microscopes
and antennae. So far, scientists have written down the underlying equations,
performed computer simulations and conducted some proof-of-principle
experiments like the one at Duke. They still need to determine the practical
limitations of how far they can bend light to their will.
The method is not magic, nor are
the materials novel. Physicists are taking ordinary substances like fiberglass
and copper to build metamaterials that look like mosaics of repeating tiles.
The metamaterials interact with the electric and magnetic fields in light
waves, manipulating a quantity known as the index of refraction to bend the
light in a way that no natural material does.
ÒThere are some things that
chemistry canÕt do on its own,Ó said John B. Pendry, a physicist at Imperial
College London. ÒThe additional design flexibility with introducing structure
as well as chemistry into the equation enables you to reach properties that just
havenÕt been accessible before.Ó
When a ray of light crosses a
boundary from air to water, glass or other transparent material, it bends, and
the degree of bending is determined by the index of refraction.
Air has an index of 1. WaterÕs
index of refraction is about 1.3. That is why rippling water waves distort the
view of a pond bottom, for instance. It is refraction that makes a straw in a
glass of water look as if it is bending toward the surface, and fish swimming
in a pond look closer to the surface than they really are.
Diamonds have a refractive index
of 2.4, giving them their sparkling beauty.
For visible light, transparent
materials like glass, water and diamonds all have an index of 1 or higher,
meaning that when the light enters, its path bends inward, closer to the
perpendicular. Because the index is uniform throughout a material, the bending
occurs only as the light crosses a boundary.
But with metamaterials,
scientists can now also create indexes of refraction from 0 to 1. In the Duke
cloaking device, the index actually varies smoothly from 0, at the inside
surface of the cylinder, to 1, at the outside surface. That causes the path of
light to curve not just at the boundaries, but also as it passes through the
metamaterial.
Metamaterials first took center
stage in a scientific spat a few years ago over a startling claim that the
index of refraction could be not just less than 1, but also negative, less than
0. Light entering such a material would take a sharp turn, almost as if it had
bounced off an invisible mirror as it crossed the boundary.
The refractive index depends on
the response of a material to electric and magnetic fields. Typically within a
material, electrons flow in a way to minimize the effects of an external
electric field, producing an internal electrical field in the opposite
direction. But that is not universally true. For some metals like silver, an
oscillating electric field induces a field in the same, not opposite,
direction.
Victor G. Veselago, a Russian
physicist, realized in the 1960s that if it were possible to find a material
that responded in a contrarian way not just to electric fields and but also
magnetic fields, a result would be a negative index of refraction.
Dr. Pendry was among the first
to start making metamaterials in the late Õ90s, building a structure of thin
wires that responded to electrical fields in a way opposite most materials. He
also designed one that reacted similarly to magnetic fields.
Dr. Smith, then at the University
of California, San Diego, attended a talk by Dr. Pendry at a conference in
1999. He and his colleagues built the first metamaterial to combine electric
and magnetic behavior.
The journal Physical Review
Letters rejected his scientific paper describing the experiment, considering it
simplistic and uninteresting. Only then did Dr. Smith come upon Dr. VeselagoÕs
work on negative refraction and the larger implications of the experiment. ÒWe
had it, but we didnÕt realize it,Ó said Dr. Smith, who is now at Duke. ÒThen I
rewrote the abstract, and it was accepted.Ó
That set off a contentious back
and forth that lasted several years between researchers who made and measured
negative-refraction metamaterials and those who said that the experiments
showed nothing of the sort, that negative refraction was at best an illusion
and violated the laws of physics.
Part of the difficulty in
resolving the controversy was that the negative refraction experiments were at
microwave wavelengths. Designing metamaterials for shorter wavelengths and
higher frequencies like visible light is more difficult, because fewer
materials are transparent at the higher frequencies.
ÒJust look around the room,Ó Dr.
Pendry said. ÒHow many things can you see through? Not many. YouÕre running out
of road.Ó
This year, researchers at the
Ames Laboratory in Iowa and Karlsruhe University in Germany reported making a
metamaterial that had a negative index of refraction for a visible wavelength.
Some critics remain unmollified.
Nicol‡s Garc’a of the Spanish National Research Council still calls Dr.
PendryÕs statements on negative refraction Òpropaganda.Ó But today, most
physicists accept the negative refraction interpretation.
The debate did highlight limits
of metamaterials. They are dispersive, meaning the angle of refraction depends
very sensitively on the frequency of light, and they are lossy, meaning that
they absorb energy from the light as it passes through.
Nonetheless, Dr. Pendry has
proposed that negative refraction materials can be used to make a ÒsuperlensÓ
because they sidestep a process called diffraction that blurs images taken via
conventional optics.
Researchers led by Xiang Zhang,
a professor at the University of California, Berkeley, have demonstrated that a
thin, flat piece of silver can indeed produce such images, able to resolve two
thin lines separated by 70 billionths of a meter.
ÒYou put your object on one side
and your image will be projected on the other side,Ó Dr. Zhang said.
The superlens can also preserve
detail lost in conventional optics. Light is usually thought of as having
undulating waves. But much closer up, light is a much more jumbled mess, with
the waves mixed in with more complicated Òevanescent waves.Ó
The evanescent waves quickly
dissipate as they travel, and thus are usually not seen. A negative refraction
lens actually amplified the evanescent waves, Dr. Pendry calculated, and that
effect was demonstrated by Dr. ZhangÕs experiment. A negative refraction could
someday lead to an optical microscope that could make out tiny biological
structures like individual viruses.
The main limit now is that an
object has to be placed very close to the lens, within a fraction of a wavelength
of light.
Another possible use would be
for a DVD-type recorder. The finer focus could allow more data like
high-definition movies to be packed in the same space, perhaps the entire Library
of Congress on a platter the size of todayÕs DVD, Dr. Zhang said.
The metamaterials researchers
also look for new problems to solve. ÒNow itÕs sort of fired up our
imaginations to do this cloaking thing,Ó Dr. Pendry said, Òbecause we realized
we could actually make one using these materials.Ó
In May 2006, Dr. Pendry and Dr.
Smith proposed a design that would cloak a single microwave frequency. By
October, Dr. SmithÕs group at Duke demonstrated a working version, although
simplified and imperfect. Dr. SmithÕs microwave design cannot be adapted to
visible light, because the energy absorption problem becomes too great.
This year, Vladimir M. Shalaev
of Purdue displayed a different design, avoiding the absorption problem. He
said it would cloak visible light, albeit just a single wavelength at a time.
ÒWe can make our cloak for any of these colors but not for all of them
simultaneously,Ó Dr. Shalaev said. ÒAt least, it starts looking like itÕs
doable.Ó
He
said he hoped to build the design, which requires tiny rods arrayed around a
cylinder, in a few years. Metamaterials could also be used for other novel
devices. Dr. Shalaev suggested an ÒanticloakÓ that would trap light of a
certain wavelength. ÒThat could be used as a sensing device,Ó he said.
Whether the cloak could be made
big enough to cover a teenage wizard or an alien spaceship is another question.
ÒIÕm fairly pessimistic knowing what I know now,Ó Dr. Smith said.
Dr. Shalaev said it would be a
challenge. ÒI donÕt know,Ó he
said. ÒWe hope it is possible.Ó
