Negative Refraction In Metamaterials
August 2, 2012

Extraordinarily Strong Negative Refraction In Metamaterials

April Flowers for - Your Universe Online

In a vacuum, light moves extraordinarily fast. Fast enough to circle the Earth seven times before you can literally blink your eye.

When light travels through matter, however, it slows down by just less than a factor of five. This refractive index factor is positive in naturally occurring materials, causing light to bend in a particular direction when it passes through matter like water or a glass, for example.

In the last 20 years, scientists have been able to create artificial materials whose refractive indices are negative. These metamaterials, or artificially created materials with properties that may not be found in nature, defy normal experience by bending light the "wrong" direction. Scientists and engineers alike celebrate these negative index metamaterials for their ability to manipulate electromagnetic waves and their potential for technological application.

Researchers at the Harvard School of Engineering and Science (SEAS) collaborated with the Weizmann Institute of Science in Israel in demonstrating a drastically new way of creating negative refraction in metamaterials. They report an "extraordinarily strong" negative refraction index of -700, which is 100 times more than any other previously reported.

"This work may bring the science and technology of negative refraction into an astoundingly miniaturized scale, confining the negatively refracting light into an area that is 10,000 times smaller than many previous negative-index metamaterials," says principal investigator Donhee Ham, Gordon McKay Professor of Electrical Engineering and Applied Physics at SEAS.

According to the article published in the August 2 issue of Nature, the previous research has relied on magnetic inductance. Ham and his group instead explored the concept of kinetic inductance, or the manifestation of the acceleration of electrons subjected to electric fields, according to Newton's second law of motion. Newton's second law states that the acceleration of a body is parallel and directly proportional to the net force acting on the body, is in the direction of the net force, and is inversely proportional to the mass of the body.

A simple shift of ideas accounts for what might be a major breakthrough in metamaterials.

"Magnetic inductance represents the tendency of the electromagnetic world to resist change according to Faraday's law," explains Ham. "Kinetic inductance, on the other hand, represents the reluctance to change in the mechanical world, according to Newton's law."

"When electrons are confined perfectly into two dimensions, kinetic inductance becomes much larger than magnetic inductance, and it is this very large two-dimensional kinetic inductance that is responsible for the very strong negative refraction we achieve," explains lead author Hosang Yoon, a graduate student at SEAS. "The dimensionality profoundly affects the condensed-matter electron behaviors, and one of those is the kinetic inductance."

The team, whose research was supported by the Air Force Office of Scientific Research, employed a two-dimensional electron gas (2DEG) which forms at the interface of two semiconductors, gallium arsenide and aluminum gallium arsenide, to create the large kinetic inductance. The 2DEG used in this work was fabricated by Vladimir Umansky of the Weisman Institute.

The research team sliced a sheet of the 2DEG into an array of strips and used gigahertz-frequency electromagnetic waves (microwaves) to accelerate electrons in the leftmost few strips.  The resulting movements passed through to the rightmost strips, where electrons are consequently accelerated. The proof-of-concept device creates an effective wave to the right, in a direction perpendicular to the strips, each of which acts as a kinetic inductor due to the electron's acceleration.  This wave displayed a "staggering" degree of negative refraction.

This new technology has many advantages, but primary among them are its ability to localize electromagnetic waves into ultra-subwavelength scales and its dramatically reduced size.  Though the concept was demonstrated with microwaves, if extended to other regions of the electromagnetic spectrum, it may prove important for operating terahertz and photonic circuits far below their usual diffraction limit, and at near field. This could lead to extremely powerful microscopes and optical tweezers, used to capture and study miniscule particles.

Though the current device operates at below 20 degrees Kelvin, Ham and his team are already investigating room temperature operations using terahertz waves.  They are also testing out carbon structure graphene as an alternative to the 2DEG conductor.

"While electrons in graphene behave like massless particles, they still possess kinetic energy and can exhibit very large kinetic inductance in a non-Newtonian way," says Ham.