A wave penetrates a material: usually this leads to wave interference, to
darker and brighter areas.
Materials that locally amplify or absorb light allow
surprising new kinds of light waves – this has now been shown by calculations
at TU Wien (Vienna).
(August 10, 2015) When a light wave penetrates a material,
it is usually changed drastically. Scattering and diffraction leads to a
superposition of waves, resulting in a complicated pattern of darker and
brighter light spots inside the material. In specially tailored high-tech
materials, which can locally amplify or absorb light, such effects can be
completely suppressed. Calculations at TU Wien (Vienna University of
Technology) have now shown that these materials allow new kinds of light waves,
which have the same intensity everywhere inside the material, as if there was
no wave interference at all. Due to their unusual properties, these new
solutions of the wave equation could be useful for technological applications.
Obstacles Change the
Wave
When a light wave travels through free space, its intensity
can be the same everywhere. But as soon as it hits an obstacle, the wave is
diffracted. At some points in space, the wave becomes brighter, in other places
it becomes darker than it would have been without hitting the object. This is
the reason we can see objects that do not emit light by themselves.
In recent years, however, experiments have been carried out
with new materials which have the ability to modify light in a special way:
they can locally amplify light, similar to a laser, or absorb light, like
sunglasses do. “When such processes are possible, we have to employ a
mathematical description of the light wave which is quite different from the
one we use for normal, transparent materials,” says Professor Stefan Rotter (TU
Wien). “In this case we speak of non-hermitian media.”
Specially designed non-hermitian materials remain completely unperturbed.
New Solutions for the Wave Equation
Konstantinos Makris and Stefan Rotter from TU Wien, together
with Ziad Musslimani and Demetrios Christodoulides from Florida (USA),
discovered that this alternative description allows new kinds of solutions for
the wave equation. “The result is a light wave with the same brightness at each
point in space, just like a wave in free space, even though it travels through
a complex, highly structured material”,
says Konstantinos Makris. “In some sense, the material is completely invisible
to the wave, even though the light passes through the material and interacts
with it.”
The new concept is reminiscent of so-called
“meta-materials”, which have been created in recent years. These materials have
a special structure, which allows them to diffract light in unusual ways. In
certain cases the structure can bend the light around the object, so that the
object becomes invisible, much like Harry Potter’s invisibility cloak. “The
principle of our non-hermitian materials, however, is quite different”, says
Stefan Rotter. “The light wave is not bent around the object, but fully
penetrates it. The way the material influences the wave is, however, fully
cancelled by a carefully tuned interplay of amplification and absorption.” In
the end, the light wave is exactly as bright as it would have been without the
object – at each and every point in space.