The chamber of the probe station where Donhee Ham’s research group tests the new metamaterials. Photo by Eliza Grinnell, SEAS Communications. |
In
a vacuum, light travels so fast that it would circle the Earth more
than seven times within the blink of an eye. When light propagates
through matter, however, it slows by a factor typically less than 5.
This factor, called the refractive index, is positive in naturally
occurring materials, and it 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.
Researchers
at the Harvard School of Engineering and Applied Sciences (SEAS),
collaborating with the Weizmann Institute of Science in Israel, have now
demonstrated a drastically new way of achieving negative refraction in a
metamaterial.
The advance, reported in the August 2 issue of Nature,
results in an “extraordinarily strong” negative refractive index as
large as -700, more than a hundred times larger than most 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.
The
underlying physics of previous work in this field has often involved an
entity called magnetic inductance. Ham’s research group instead
explored kinetic
inductance, which is the manifestation of the acceleration of electrons
subjected to electric fields, according to Newton’s second law of
motion.
At
its heart, the researchers’ change in strategy from using magnetic
inductance to kinetic inductance stems from a simple shift in ideas.
“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 experimental setup in Donhee Ham’s lab shown here is used to test the 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. Photo by Eliza Grinnell, SEAS Communications. |
To
obtain the large kinetic inductance, Ham and Yoon’s work employs a
two-dimensional electron gas (2DEG), which forms at the interface of two
semiconductors, gallium arsenide and aluminum gallium arsenide. The
very “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.
The
primary advantages of the new technology are its ability to localize
electromagnetic waves into ultra-subwavelength scales and its
dramatically 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.
For
now, 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.
Kitty Y. M. Yeung, a graduate student in applied physics at SEAS, also contributed to the work as coauthor.
This research was supported by the Air Force Office of Scientific Research.
Source: Harvard University