Even Albert Einstein might have been impressed. His theory of general relativity,
which describes how the gravity of a massive object, such as a star, can curve
space and time, has been successfully used to predict such astronomical observations
as the bending of starlight by the sun, small shifts in the orbit of the planet
Mercury and the phenomenon known as gravitational lensing. Now, however, it
may soon be possible to study the effects of general relativity in bench-top
laboratory experiments.
Xiang Zhang has been one of the pioneers in the creation of artificial optical materials and their applications to such phenomena as negative refraction, electromagnetic invisibility devices and microscopy with super-resolution. (Photo by Roy Kaltschmidt, Berkeley Lab Public Affairs)
Xiang Zhang, a faculty scientist with the U.S.
Department of Energy's Lawrence Berkeley National Laboratory (Berkeley
Lab) and professor at the University of California Berkeley, lead a study
in which it was determined that the interactions of light and matter with spacetime,
as predicted by general relativity, can be studied using the new breed of artificial
optical materials that feature extraordinary abilities to bend light and other
forms of electromagnetic radiation.
"We propose a link between the newly emerged field of artificial optical materials
to that of celestial mechanics, thus opening a new possibility to investigate
astronomical phenomena in a table-top laboratory setting," says Zhang. "We have
introduced a new class of specially designed optical media that can mimic the
periodic, quasi-periodic and chaotic motions observed in celestial objects that
have been subjected to complex gravitational fields."
A paper describing this work is now available on-line in the journal Nature
Physics. The paper is titled: "Mimicking Celestial Mechanics in Metamaterials."
Co-authoring it with Zhang were his post-doctoral students Dentcho Genov and
Shuang Zhang.
Zhang, a principal investigator with Berkeley Lab's Materials Sciences
Division and director of UC Berkeley's Nano-scale Science and Engineering
Center, has been one of the pioneers in the creation of artificial optical materials.
Last year, he and his research group made headlines when they fashioned unique
metamaterials - composites of metals and dielectrics - that were able to bend
light backwards, a property known as a negative refraction that is unprecedented
in nature. More recently, he and his group fashioned a "carpet cloak"
from nanostructured silicon that concealed the presence of objects placed under
it from optical detection. These efforts not only suggested that true invisibility
materials are within reach, Zhang said, but also represented a major step towards
transformation optics that would "open the door to manipulating light
at will."
Now he and his research group have demonstrated that a new class of metamaterials
called "continuous-index photon traps" or CIPTs can serve as broadband
and radiation-free "perfect" optical cavities. As such, CIPTs can
control, slow and trap light in a manner similar to such celestial phenomena
as black holes, strange attractors and gravitational lenses. This equivalence
between the motion of the stars in curved spacetime and propagation of the light
in optical metamaterials engineered in a laboratory is referred to as the "optical-mechanical
analogy."
Zhang says that such specially designed metamaterials can be valuable tools
for studying the motion of massive celestial bodies in gravitational potentials
under a controlled laboratory environment. Observations of such celestial phenomena
by astronomers can sometimes take a century of waiting.
"If we twist our optical metamaterial space into new coordinates, the
light that travels in straight lines in real space will be curved in the twisted
space of our transformational optics," says Zhang. "This is very
similar to what happens to starlight when it moves through a gravitational potential
and experiences curved spacetime. This analogue between classic electromagnetism
and general relativity, may enable us to use optical metamaterials to study
relativity phenomena such as gravitational lens."
In their demonstration studies, the team showed a composite of air and the
dielectric Gallium Indium Arsenide Phosphide (GaInAsP). This material provided
operation at the infrared spectral range and featured a high refractive index
with low absorptions.
In their paper, Zhang and his coauthors cite as a particularly intriguing prospect
for applying artificial optical materials to the optical-mechanical analogy
the study of the phenomenon known as chaos. The onset of chaos in dynamic systems
is one of the most fascinating problems in science and is observed in areas
as diverse as molecular motion, population dynamics and optics. In particular,
a planet around a star can undergo chaotic motion if a perturbation, such as
another large planet, is present. However, owing to the large spatial distances
between the celestial bodies, and the long periods involved in the study of
their dynamics, the direct observation of chaotic planetary motion has been
a challenge. The use of the optical-mechanical analogy may enable such studies
to be accomplished in a bench-top laboratory setting on demand.
"Unlike astronomers, we will not have to wait 100 years to get experimental
results," Zhang says.
This research was supported by the U.S. Army Research Office and by the National
Science Foundation which funds the UC Berkeley Nano-scale Science and Engineering
Center.