The metamaterial crystal
Indian researchers have joined the global race for metamaterials by making the country's first metamaterial crystal1. The crystal was on display at a national meet on metamaterials at the Bhaba Atomic Research Centre (BARC) on August 17.

A joint research team from the Institute of Radio Physics and Electronics (IRPE) of Calcutta University (CU) and the Society for Applied Microwave Electronics Engineering and Research (SAMEER), Kolkata has designed and fabricated the crystal.

Metamaterials reverse several known physical phenomena and bend light in the wrong directions. They can make things invisible and may even catch such minute details of objects that no other sophisticated optical microscope can. Metamaterials have helped science leap from the pages of science fiction to the fringes of reality. Researchers across the globe are yielding astonishing results with metamaterials, also known as Left Handed Materials or LHM.

"Our metamaterial crystal contains a uni-dimensional periodic structure of labyrinth resonators (LRs) — concentric couple of double-slit rings — and thin wires (TWs) of copper," said team leader Subal Kar of CU, explaining the electrodynamics and application of the crystal at the meet organised jointly by BARC, the Board of Research in Nuclear Sciences (BRNS) and India's Department of Atomic Energy .

The crystal has been designed to respond to microwave frequency, he said.

British physicist John Pendry, while working on a radiation-absorbing carbon material, discovered in the mid-1990s that the ability to absorb radiation came not from the carbon but from the shape of its long thin fiber.

The researchers (left to right): Subal Kar, Arijit Majumder, Shantanu Das & Tapashree Roy.
When an electromagnetic wave (such as a ray of light) travels through a material, the electrons within the material's atoms or molecules feel a force and move in response. Electromagnetic waves contain both an electric field and a magnetic field. Each component induces a characteristic motion of the electrons in a material—back and forth in response to the electric field (electrical permittivity) and around in circles in response to the magnetic field (magnetic permeability). Both permittivity and permeability are related to refractive index.

Magnetism involves charge going around in a circle as electrons do in atoms. Taking cues from this, Pendry first hypothesised and then carved metallic thin wires with negative permittivity and split-ring structures (mimics circular path of electrons in atoms) with negative permeability using copper. Both negative permittivity and permeability gave rise to metamaterials with negative refractive index 2.

A member of the Indian team Tapashree Roy said the TW and LR sheets were imprinted on a special type of dielectric material (Rogers RT-Duroid). Ten sheets each of TWs and LRs were placed alternately one behind the other in polypropylene frames just like the pages of a closed book, Roy said.

Aided by computer-based models, the researchers found that upon exposure to microwave radiation, the metamaterial crystal displays its characteristic properties. The magnetic field of the microwave radiation affects the ring structures and the electric field affects the thin wire structures on the crystal.

"The magnetic field generates circulating current through the ring eventually giving rise to magnetism and negative permeability," Roy explained. The electric field affects the thin wire structures giving rise to negative permittivity. Both negative permeability and permittivity culminate in negative refractive index.

The hallmark of metamaterial crystal is that it does not absorb or scatter any part of the microwave radiation. Light could also be made to flow smoothly around the metamaterial like water flowing past a smooth pebble. Such outcome has far-reaching implications.

BARC director Srikumar Banerjee speaking at the meet.
When light hits a small object, it generates waves that vanish without a trace. Such waves carry minute details of a small object. Metamaterials could be designed to capture such evanescent waves giving rise to supermicroscope that can image even an individual strand of DNA. Besides conjuring up the possibility of invisibility cloak, metamaterials are on the brink of revolutionizing the interaction of electromagnetic waves with objects, from modems and MRI to radios and radar.

"LHM is a new field", said BARC director Srikumar Banerjee. "It is close to matter, but not yet matter," he said.

LHM research in India was first proposed in a Vision 2020 document in 2004 at BARC. Given its vast potential, a proposal was jointly made by the Reactor Control Division of BARC, Calcutta University and SAMEER, Kolkata to set up an LHM Centre. The seed funding for LHM research worth over 10 million rupees is awaiting BRNS approval. "Once the fund is through, we will set up the LHM Centre," says Shantanu Das of BARC.