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An applied electric voltage can prompt a centimeter-square
slice of graphene to change and control the transmission of electromagnetic
radiation with wavelengths from the terahertz to the midinfrared.
The experiment at Rice
University advances the
science of manipulating particular wavelengths of light in ways that could be
useful in advanced electronics and optoelectronic sensing devices.
In previous work, the Rice laboratory of physicist Junichiro
Kono found a way to use arrays of carbon nanotubes as a near-perfect terahertz
polarizer. This time, the team led by Kono is working on an even more basic
level; the researchers are wiring a sheet of graphene to apply an electric
voltage and thus manipulate what’s known as Fermi energy. That, in turn, lets
the graphene serve as a sieve or a shutter for light.
The discovery by Kono and his colleagues at Rice and the Institute of Laser
Engineering at Osaka
University was reported
online in Nano Letters.
In graphene, “electrons move like photons, or light. It’s
the fastest material for moving electrons at room temperature,” said Kono, a
professor of electrical and computer engineering and of physics and astronomy.
He noted many groups have investigated the exotic electrical properties of
graphene at zero- or low frequencies.
“There have been theoretical predictions about the unusual
terahertz and midinfrared properties of electrons in graphene in the
literature, but almost nothing had been done in this range experimentally,”
Kono said.
Key to the new work, he said, are the words “large area” and “gated.”
“Large because infrared and terahertz have long wavelengths
and are difficult to focus on a small area,” Kono said. “Gated simply means we
attached electrodes, and by applying a voltage between the electrodes and
(silicon) substrate, we can tune the Fermi energy.”
Fermi energy is the energy of the highest occupied quantum
state of electrons within a material. In other words, it defines a line that
separates quantum states that are occupied by electrons from the empty states. “Depending on the value of the Fermi energy, graphene can be either p-type
(positive) or n-type (negative),” he said.
Making fine measurements required what is considered in the
nano world to be a very large sheet of graphene, even though it was a little
smaller than a postage stamp. The square centimeter of atom-thick carbon was
grown in the lab of Rice chemist James Tour, a co-author of the paper, and gold
electrodes were attached to the corners.
Raising or lowering the applied voltage tuned the Fermi
energy in the graphene sheet, which in turn changed the density of free
carriers that are good absorbers of terahertz and infrared waves. This gave the
graphene sheet the ability to either absorb some or all of the terahertz or
infrared waves or let them pass. With a spectrometer, the team found that
terahertz transmission peaked at near-zero Fermi energy, around plus-30 V; with
more or less voltage, the graphene became more opaque. For infrared, the effect
was the opposite, he said, as absorption was large when the Fermi energy was
near zero.
“This experiment is interesting because it lets us study the
basic terahertz properties of free carriers with electrons (supplied by the
gate voltage) or without,” Kono said. The research extended to analysis of the
two methods by which graphene absorbs light: through interband (for infrared)
and intraband (for terahertz) absorption. Kono and his team found that varying
the wavelength of light containing both terahertz and infrared frequencies
enabled a transition from the absorption of one to the other. “When we vary the
photon energy, we can smoothly transition from the intraband terahertz regime
into the interband-dominated infrared. This helps us understand the physics
underlying the process,” he said.
They also found that thermal annealing—heating—of the
graphene cleans it of impurities and alters its Fermi energy, he said.
Kono said his laboratory will begin building devices while
investigating new ways to manipulate light, perhaps by combining graphene with plasmonic
elements that would allow a finer degree of control.
Source: Rice University
