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IR Technique Sees Through Graphene Stacks

Photonics.com
Nov 2013
BUFFALO, N.Y., Nov. 22, 2013 — An IR technique identifies and characterizes individual graphene layers in a large stack, providing information on the electronic properties of each layer. Such information opens the possibility of graphene-based optical devices for communications, imaging and signal processing.

A University at Buffalo-led team developed the technique, which involves shooting a beam of IR light at a stack of graphene sheets and measuring how the light wave’s polarization changes as it bounces off the layers within.


The direction that a light wave is oscillating changes as the wave is reflected by a sheet of graphene. This changing direction of oscillation — also known as polarization — enabled researchers to identify the electronic properties of multiple sheets of graphene stacked atop one another, even when they were covering each other up. Courtesy of Chul Soo Kim, US Naval Research Laboratory.


Graphene, a nanomaterial that consists of a single layer of carbon atoms, has generated huge interest due to its remarkable fundamental properties and technological applications. It is lightweight but also one of the world’s strongest materials.

When a magnetic field is applied and increased, different types of graphene alter polarization in different ways. A graphene layer stacked neatly on top of another will have a different effect on polarization than one that is messily stacked.

“By measuring the polarization of reflected light from graphene in a magnetic field and using new analysis techniques, we have developed an ultrasensitive fingerprinting tool that is capable of identifying and characterizing different graphene multilayers,” said John Cerne, Ph.D., a UB associate professor of physics who led the project.

Cerne’s new research looks at graphene’s electronic properties, which change as sheets of the material are stacked on top of one another, and seeks to answer the question of why all graphene layers don’t affect polarization the same way.

The answer, he said, lies in the fact that different layers absorb and emit light in different ways.

His study showed that absorption and emission patterns change when a magnetic field is applied, so the polarization can be turned on and off either by applying a magnetic field to graphene layers or, more quickly, by applying a voltage that sends electrons flowing through the graphene.

“Applying a voltage would allow for fast modulation, which opens up the possibility for new optical devices using graphene for communications, imaging and signal processing,” said Chase T. Ellis, a former graduate research assistant at UB and current postdoctoral fellow at the Naval Research Laboratory.

Cerne’s collaborators included colleagues from UB and the US Naval Research Laboratory. The findings appeared Nov. 5 in Scientific Reports, (doi: 10.1038/srep03143). Ellis was first author on the paper.

For more information, visit: www.buffalo.edu


GLOSSARY
modulation
In general, changes in one oscillation signal caused by another, such as amplitude or frequency modulation in radio which can be done mechanically or intrinsically with another signal. In optics the term generally is used as a synonym for contrast, particularly when applied to a series of parallel lines and spaces imaged by a lens, and is quantified by the equation: Modulation = (Imax – Imin)/ (Imax + Imin) where Imax and Imin are the maximum and minimum intensity levels of the image.
optical
Pertaining to optics and the phenomena of light.
polarization
With respect to light radiation, the restriction of the vibrations of the magnetic or electric field vector to a single plane. In a beam of electromagnetic radiation, the polarization direction is the direction of the electric field vector (with no distinction between positive and negative as the field oscillates back and forth). The polarization vector is always in the plane at right angles to the beam direction. Near some given stationary point in space the polarization direction in the beam...
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