Negative Index Metamaterials Bend Light in the Wrong Direction 100 Times More Powerfully
Researchers at the Harvard School of Engineering and Applied Sciences (SEAS), in collaboration with Weizmann Institute of Science in Israel, have demonstrated a new method achieving negative refraction in a metamaterial, using kinetic inductance, that shows the potential for miniaturization of metamaterials.
Reported in the August 2 issue of Nature, the technique results in an “extraordinarily strong” negative refractive index as large as -700, more than a hundred times larger than most previously reported. In the photo is the lab used to test the new metamaterials, fabricated on tiny chips. The metamaterials themselves are in the probing chamber at the bottom right. Imaged through the black microscope, they appear on the screen at the top of the photo.
The Refractive Index and Metamaterials
In a vacuum, light travels quick; so fast that it could circle the Earth more than seven times in the blink of an eye. When light propagates through matter though, it slows by a factor 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, for example, water or glass.
Over the past two decades, scientists have managed to create artificial materials whose refractive indices are negative; these negative-index metamaterials defy normal experience by bending light in the “wrong” direction. Due to their unusual ability to manipulate electromagnetic waves and their potential to be harnessed for technology (that might, for example, cloak objects from view), negative-index metamaterials have been celebrated by scientists and engineers alike.
“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, Professor of Electrical Engineering and Applied Physics at SEAS.
Magnetic Inductance vs. Kinetic Inductance
The basic physics of previous work in this field has frequently involved a concept called magnetic inductance. Ham’s researchers, in its place, explored kinetic inductance, the expression of the acceleration of electrons subjected to electric fields, according to Newton’s second law of motion.
The change in strategy from using magnetic inductance to kinetic inductance stems from a straightforward shift in ideas.
“Magnetic inductance represents the tendency of the electromagnetic world to resist change according to Faraday’s law,” Ham explained. “Kinetic inductance, on the other hand, represents the reluctance to change in the mechanicalworld, 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. “The dimensionality profoundly affects the condensed-matter electron behaviors, and one of those is the kinetic inductance.”
Two Dimensional Electron Gas
To get the hefty kinetic inductance, Ham and Yoon’s work uses a two-dimensional electron gas (2DEG), which forms at the interface of two semiconductors, gallium arsenide and aluminum gallium arsenide. The highly “clean” 2DEG sample used in this work was fabricated by coauthor Vladimir Umansky, of the Weizmann Institute.
Ham’s team effectively sliced a sheet of 2DEG into an array of strips and used gigahertz-frequency electromagnetic waves (microwaves) to accelerate electrons in the leftmost few strips. The resulting movements of electrons in these strips were “felt” by the neighboring strips to the right, where electrons are consequently accelerated.
In this way, the proof-of-concept device propagates an effective wave to the right, in a direction perpendicular to the strips, each of which acts as a kinetic inductor due to the electrons’ acceleration therein. This effective wave proved to exhibit what the researchers call a “staggering” degree of negative refraction.
Room Temperature Operation Under Investigation
The chief advantages of the new technology are its capability to localize electromagnetic waves into ultra-subwavelength scales and its radically reduced size. This concept demonstrated with microwaves, if extended to other regions of the electromagnetic spectrum, may prove important for operating terahertz and photonic circuits far below their usual diffraction limit, and at near field. It may also one day lead to extremely powerful microscopes and optical tweezers, which are used to trap and study minuscule particles like viruses and individual molecules.
Currently, the device operates at temperatures below 20 degrees Kelvin. The researchers note, however, that a similar result can be achieved at room temperature using terahertz waves, which Ham’s team is already investigating, with the carbon structure graphene as an alternative two-dimensional 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.