ULTRA THIN DETECTORS SEE BELOW THE SURFACE
New research at the
University of Maryland could lead to a generation of light detectors that can
see below the surface of bodies, walls, and other objects. Using the special
properties of graphene, a two-dimensional form of carbon that is only one atom thick,
a prototype detector is able to see an extraordinarily broad band of
wavelengths. Included in this range is a band of light wavelengths that have
exciting potential applications but are notoriously difficult to detect:
terahertz waves, which are invisible to the human eye
A research paper
about the new detector was published online Sept. 7, 2014 in the journal Nature
Nanotechnology. Lead author Xinghan Cai, a UMD physics graduate student,
said a detector like the researchers' prototype "could find applications
in emerging terahertz fields such as mobile communications, medical imaging,
chemical sensing, night vision and security."
The light we see
illuminating everyday objects is actually only a very narrow band of
wavelengths and frequencies. Terahertz light waves' long wavelengths and low
frequencies fall between microwaves and infrared waves. The light in these
terahertz wavelengths can pass through materials that we normally think of as
opaque, such as skin, plastics, clothing and cardboard. It can also be used to
identify chemical signatures that are emitted only in the terahertz range.
Few technological
applications for terahertz detection are currently realized, however, in part
because it is difficult to detect light waves in this range. In order to
maintain sensitivity, most detectors need to be kept extremely cold, around 4
Kelvin, or -452 degrees Fahrenheit. Existing detectors that work at room
temperature are bulky, slow and prohibitively expensive.
The new room
temperature detector, developed by the UMD team and colleagues at the U.S.
Naval Research Lab and Monash University, Australia, gets around these problems
by using graphene, a single layer of interconnected carbon atoms. By utilizing
the special properties of graphene, the research team has been able to increase
the speed and maintain the sensitivity of room temperature wave detection in
the terahertz range.
Using a new operating
principle called the "hot-electron photothermoelectric effect," the
research team created a device that is "as sensitive as any existing room
temperature detector in the terahertz range and more than a million times
faster," says Michael Fuhrer, professor of physics at UMD and Monash
University.
Graphene, a sheet of
pure carbon only one atom thick, is uniquely suited to use in a terahertz
detector because when light is absorbed by the electrons suspended in the
honeycomb lattice of the graphene, they do not lose their heat to the lattice
but instead retain that energy.
The concept behind the
detector is simple, says UMD Physics Professor Dennis Drew. "Light is
absorbed by the electrons in graphene, which heat up but don't lose their
energy easily. So they remain hot while the carbon atomic lattice remains
cold." These heated electrons escape the graphene through electrical
leads, much like steam escaping a tea kettle. The prototype uses two electrical
leads made of different metals, which conduct electrons at different rates.
Because of this conductivity difference, more electrons will escape through one
than the other, producing an electrical signal.
This electrical signal
detects the presence of terahertz waves beneath the surface of materials that
appear opaque to the human eye--or even X-rays. You cannot see through your
skin, for example, and an X-ray goes right through the skin to the bone,
missing the layers just beneath the skin's surface entirely. Terahertz waves
see the in-between. The speed and sensitivity of the room temperature detector
presented in this research opens the door to future discoveries in this
in-between zone.
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