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New approach strengthens negative refraction

Photonics Spectra
Oct 2012
CAMBRIDGE, Mass. – An “extraordinarily strong” negative refractive index as large as -700 – more than 100 times larger than most previously reported – has been achieved in metamaterials using a technique developed by scientists at Harvard School of Engineering and Applied Sciences (SEAS) in collaboration with Weizmann Institute of Science in Israel.

In a vacuum, light travels so fast that it can circle the Earth more than seven times in the blink of an eye. When light propagates through matter, however, it slows by a factor of typically less than 5. This factor, called the refractive index, is positive in naturally occurring materials and causes light to bend in a particular direction when it shines on water or glass, for example.

Over the past 20 years, scientists have created artificial materials whose refractive indices are negative, defying normal experience by bending light in the “wrong” direction. These metamaterials have been celebrated by both scientists and engineers for their unusual ability to manipulate electromagnetic waves and for their potential to be harnessed for technologies such as 3-D cloaking.


Scientists have demonstrated a new way to achieve negative refraction in metamaterials by applying kinetic inductance. Here, the chamber of the probe station where Donhee Ham’s research group tests the new metamaterials.


Now, SEAS and Weizmann Institute scientists have demonstrated a drastically new way of achieving negative refraction in metamaterials by applying kinetic inductance, the manifestation of the acceleration of electrons subjected to electric fields, according to Newton’s second law of motion.

“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,” said principal investigator Donhee Ham, Gordon McKay Professor of Electrical Engineering and Applied Physics at SEAS.

The change in strategy from using magnetic inductance to kinetic inductance is based on a simple shift in ideas.


The experimental setup in Ham’s lab, tests new metamaterials, which are fabricated on tiny chips. The metamaterials themselves are inside the probing chamber at the bottom right. Imaged through the black microscope, they appear on the screen at the top of this image.


“Magnetic inductance represents the tendency of the electromagnetic world to resist change according to Faraday’s law,” Ham said. “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,” said 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.”

Ham and Yoon used a 2-D electron gas (2DEG) to obtain the large kinetic inductance. The very “clean” 2DEG sample, fabricated by Vladimir Umansky of Weizmann Institute, forms at the interface of two semiconductors: GaAs and AlGaAs.

The findings, supported by the US Air Force Office of Scientific Research, were reported in Nature (doi: 10.1038/nature11297).


GLOSSARY
photonics
The technology of generating and harnessing light and other forms of radiant energy whose quantum unit is the photon. The science includes light emission, transmission, deflection, amplification and detection by optical components and instruments, lasers and other light sources, fiber optics, electro-optical instrumentation, related hardware and electronics, and sophisticated systems. The range of applications of photonics extends from energy generation to detection to communications and...
refraction
The bending of oblique incident rays as they pass from a medium having one refractive index into a medium with a different refractive index.
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