Materials Mimic Mechanics

Astronomical phenomena such as black holes could be studied in a tabletop laboratory setting if the nascent field of artificial optical materials is combined with celestial mechanics, researchers say.

Albert Einstein’s theory of general relativity describes how the gravity of a massive object, such as a star, can curve space and time. This theory has been used to successfully predict astronomical observations such as the bending of starlight by the sun, small shifts in the orbit of the planet Mercury and the phenomenon known as gravitational lensing. But now it may soon be possible to study the effects of general relativity in benchtop laboratory experiments.

University of California, Berkeley, professor Xiang Zhang, a faculty scientist with Lawrence Berkeley National Laboratory, led research determining that light-matter interactions with space-time 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 tabletop laboratory setting,” Zhang said. “We have introduced a new class of specially designed optical media that can mimic the periodic, quasiperiodic and chaotic motions observed in celestial objects that have been subjected to complex gravitational fields.”

Zhang, a principal investigator with Berkeley Lab’s Materials Sciences Div. and director of UC Berkeley’s Nano-scale Science and Engineering Center, has been a pioneer 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 bend light backward, a property known as a negative refraction that is unprecedented in nature. (See Closing in on Invisibility Cloak)

More recently, his team fashioned a “carpet cloak” from nanostructured silicon that concealed the presence of objects placed under it from optical detection. (See Swept Under the Carpet Cloak) These efforts not only suggest that true invisibility materials are within reach, Zhang said, but also represented a major step toward transformation optics that would “open the door to manipulating light at will.”

Through the optical-mechanical analogy, metamaterials and other advanced optical materials can be used to study such celestial phenomena as black holes, strange attractors and gravitational lenses. Here an air-GaInAsP metamaterial mimics a photon-sphere, one of the key black hole phenomena in its interactions with light. (Image: Xiang Zhang)

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 space-time and propagation of the light in optical metamaterials engineered in a laboratory is referred to as the “optical-mechanical analogy.”

Zhang said 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,” he said. “This is very similar to what happens to starlight when it moves through a gravitational potential and experiences curved space-time. 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.

A paper describing the work, “Mimicking Celestial Mechanics in Metamaterials,” is now online in the journal Nature Physics. Co-authors were Zhang’s postdoctoral students Dentcho Genov and Shuang Zhang.

In the paper, the researchers 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 benchtop laboratory setting on demand.

“Unlike astronomers, we will not have to wait 100 years to get experimental results,” Zhang said.

The research was supported by the US Army Research Office and by the National Science Foundation, which funds the UC Berkeley Nano-scale Science and Engineering Center.

For more information, visit: www.lbl.gov