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Graphene has caused a lot of excitement among scientists since the extremely
strong and thin carbon material was discovered in 2004. Just one atom thick,
the honeycomb-shaped material has several remarkable properties combining
mechanical toughness with superior electrical and thermal conductivity.
Now a group of scientists at Iowa State University, led by physicist Jigang
Wang, has shown that graphene has two other properties that could have
applications in high-speed telecommunications devices and laser technology—population
inversion of electrons and broadband optical gain.
Wang is an assistant professor in the Department of Physics and Astronomy in
the College of Liberal
Arts and Sciences at Iowa State University. He also is an associate
scientist with the Department of Energy’s Ames Laboratory.
Wang’s team flashed extremely short laser pulses on graphene. The
researchers immediately discovered a new photoexcited graphene state
characterized by a broadband population inversion of electrons. Under normal
conditions, most electrons would occupy low-energy states and just a few would
populate higher-energy states. In population-inverted states, this situation is
reversed: more electrons populate higher, rather than lower, energy states.
Such population inversions are very rare in nature and can have highly unusual
properties. In graphene, the new state produces an optical gain from the
infrared to the visible.
Simply stated, optical gain means more visible light comes out than goes in.
This can only happen when the gain medium is externally pumped and then
stimulated with light (stimulated emission). Wang’s discovery could open doors
for efficient amplifiers in the telecommunication industry and extremely fast
optoelectronics devices.
Graphene as a gain medium for light
amplification
“It’s very exciting,” Wang said. “It opens the possibility of
using graphene as a gain medium for light amplification. It could be used in
making broadband optical amplifiers or high-speed modulators for
telecommunications. It even provides implications for development of
graphene-based lasers.”
Wang’s team unveiled its findings in Physical
Review Letters. In addition to Wang, the paper’s other authors are Tianq
Li, Liang Luo, and Junhua Zhang, Iowa State physics graduate students; Miron
Hupalo, Ames Laboratory scientist; and Michael Tringides and Jörg Schmalian,
Iowa State physics professors and Ames Laboratory scientists.
Wang is a member of the Condensed Matter Physics program at Iowa State
and the Ames Laboratory. He and his team conduct optical experiments using
laser spectroscopy techniques, from the visible to the mid-infrared and
far-infrared spectrum. They use ultrashort laser pulses down to 10
quadrillionths of a second to study the world of nanoscience and correlated
electron materials.
In 2004 United Kingdom
researchers Andre Geim and Konstantin Novoselov discovered graphene, which led
to their winning the 2010 Nobel Prize in Physics. Graphene is a 2D (height and
width) material with a growing list of known unique properties. It is a single
layer of carbon only one atom thick. The carbon atoms are connected in a
hexagonal lattice that looks like a honeycomb. Despite a lack of bulk, graphene
is stronger than steel, it conducts electricity as well as copper and conducts
heat even better. It is also flexible and nearly transparent.
An understanding gap existed, Wang explained, between the two scientific
communities that studied the electronic and photonic properties of graphene. He
believed his group could help bridge the gap by elaborating the non-linear
optical properties of graphene and understanding the non-equilibrium electronic
state. Wang explained that linear optical properties only transmit light—one
light signal comes into a material and one comes out. “The non-linear
property can change and modulate the signal, not just transmit it, producing
functionality for novel device applications.”
Graphene in a highly non-linear state
Wang said other scientists have studied graphene’s optical
properties, but primarily in the linear regime. His team hypothesized they
could generate a new “very unconventional state” of graphene
resulting in population inversion and optical gain.
“We were the first group to break new ground, to start looking at it in
a highly excited state consisting of extremely dense electrons—a highly
non-linear state. In such a state, graphene has unique properties.”
Wang’s group started with high-quality graphene monolayers grown by Hupalo
and Tringides in the Ames Laboratory. The researchers used an ultrafast laser
to “excite” the material’s electrons with short pulses of light just
35 femtoseconds long. Through measurements of the photo-induced electronic
states, Wang’s team found that optical conductivity (or absorption) of the
graphene layers changed from positive to negative—resulting in the optical gain—when
the pump pulse energy was increased above a threshold.
The results indicated that the population inverted state in photoexcited
graphene emitted more light than it absorbed. “The absorption was
negative. It meant that population inversion is indeed established in the
excited graphene and more light came out of the inverted medium than what
entered, which is optical gain,” Wang said. “The light emitted shows
gain of about one percent for a layer a mere one atom thick, a figure on the
same order to what’s seen in conventional semiconductor optical amplifiers
hundreds of times thicker.”
The key to the experiments, of course, was creating the highly non-linear
state, something “that does not normally exist in thermal
equilibrium,” Wang said. “You cannot simply put graphene under the
light and study it. You have to really excite the electrons with the ultrafast
laser pulse and have the knowledge on the threshold behaviors to arrive at such
a state.”
Wang said a great deal more engineering and materials perfection lies ahead
before graphene’s full potential for lasers and optical telecommunications is
ever realized. “The research clearly shows, though, that lighting up
graphenes may produce brighter emissions as well as a bright future,” he
said.
