SAN DIEGO, and BARCELONA, Spain, June 21, 2012 — Graphene has been found to be an excellent host for guiding, confining and electrically manipulating light, two independent studies report. These properties are desirable for plasmonic devices, which offer the possibility to control and guide light at subwavelength scales and could lead to the development of superfast optical computer chips.
Scientists from the University of California, San Diego, and ICFO in Barcelona have created and controlled “plasmons” — electromagnetic waves that spread across the surface of a metal — on the surface of graphene. They observed some of the shortest plasmon wavelengths measured in any material, yet the waves propagated as far as they do in metals such as gold. Unlike plasmons on metals, however, graphene plasmons can be tuned.
Plasmons are expected to have similar effects for grapheme, but have so far been difficult to instigate and detect. In the two studies, plasmons were directly launched and propagated in graphene using near-field optical microscopy. Both groups showed that properties of these plasmons can be controlled using a gate voltage.
In the UCSD experiment, led by Dmitry Basov, scientists made the devices by peeling grapheme from graphite and rubbing it onto silicon dioxide chips. They launched plasmons by shining an infrared laser beam onto the surface of graphene and measured the waves using the ultrasensitive arm of an atomic force microscope. The outgoing waves are impossible to measure, but as they reach the edge of the graphene, they reflect like water waves from the wake of a boat bouncing off a pier.
An infrared laser beam focused on the arm of an atomic force microscope launches plasmons — waves through electrons — on the surface of graphene, a single honeycomb layer of linked carbon atoms. (Image: Basov Lab/UCSD)
Oscillations that returned from the edge added to, or canceled out, subsequent waves, creating a characteristic interference pattern that reveals their wavelength and amplitude. The researchers discovered that this pattern can be altered by controlling an electrical circuit formed with electrodes attached to the graphene surface and a layer of pure silicon beneath the chips.
Similar to how light carries complex signals through fiber optics, plasmons could be used to transmit information, however, within far tighter spaces.
“It’s impossible to confine light at nanometer scales because light wavelengths are many hundreds of nanometers,” said Zhe Fei, a graduate student in Basov’s lab. “We used light to excite surface plasmons with a length scale of 100 nm or less that can travel at very high speed from one side of the chip to the other.”
The Spanish research collaboration, from IQFR-CSIC, ICFO and nanoGUNE, used a near-field microscope that uses a sharp tip to convert the illumination light into a nanoscale light spot that provided the extra push needed for the plasmons to be created. Their observations proved what theoretical physicists have long predicted: It is possible to trap and manipulate light in a highly efficient way, using graphene as a novel platform for optical information processing and sensing.
Their experiment used graphene film deposited by a gas rather than peeled from graphite.
“Our near-field optical images definitely prove the existence of propagating and localized graphene plasmons and allow for a direct measurement of their dramatically reduced wavelengths,” said Rainer Hillenbrand, leader of the nanoGUNE group.
The teams demonstrated that graphene plasmons can be used to electrically control light in a way similar to electrons in a transistor. These capabilities enable new highly efficient nanoscale optical switches which can perform calculations using light instead of electricity.
This image shows optical nanoimaging of graphene plasmons. The upper panel shows a sketch of the imaging method. A laser illuminated scanning tip launches plasmons on graphene. Detection is by recording the light backscattered from the tip. The lower panel shows an optical image of graphene, where the fringes visualize the interference of the graphene plasmons. (Image: nanoGUNE, IQFR-CSIC, ICFO)
“With our work, we show that graphene is an excellent choice for solving the long-standing and technologically important problem of modulating light at the speeds of today’s microchips,” said Javier Garcia de Abajo, leader of the IQFR-CSIC group.
Frank Koppens, leader of the ICFO group, said, “Graphene truly bridges the fields of nanoelectronics and nano-optics.”
In addition, graphene’s ability to trap light in very small volumes could give rise to a new generation of nanosensors with applications in diverse areas such as biodetection, solar cells, light detectors, medicine and quantum information processing. The results open the door to a new field of research, providing the first viable path toward ultrafast tuning of light, which was not possible until now.
“Graphene optoelectronics and information processing are very promising," Basov said. “There also is entirely new, fundamental science coming out of this. By monitoring plasmons, we learn what electrons do in this new form of carbon, how fundamental interactions govern their properties. This is a path of inquiry.”
The two research experiments were published in separate reports in this week’s issue of Nature.
For more information, visit: www.ucsd.edu