From radio to television to the Internet, telecommunications transmissions are simply information conveyed by light waves and converted into electrical signals.
Silicon-based optical fibers are currently the best structures for high-speed, long-haul transmissions, but graphene – an all-carbon, ultra-thin and adaptable material – could improve performance even further.
In a study published on April 16 in ACS PhotonicsResearchers at the University of Wisconsin-Madison have fabricated graphene into the smallest ribbon structures to date using a method that simplifies scaling. When testing these tiny ribbons, scientists found that they came close to the properties they needed to make graphene useful in telecommunications equipment.
“Previous research suggested that to be viable for telecommunications technologies, graphene would have to be prohibitively structured over large areas, (which is) a manufacturing nightmare,” says Joel Siegel, graduate student at UW- Madison in the group of physics professor Victor Brar and co-lead author of the study. “In our study, we created a scalable fabrication technique to fabricate the smallest graphene ribbon structures to date and found that with modest additional reductions in ribbon width, we can begin to achieve the reach of telecommunications.
Graphene is considered a miracle material for technologies such as telecommunications or solar cells because it is easy to use, relatively inexpensive, and has unique physical properties such as being both an insulator and a conductor. electricity.
If modified to interact with higher energy light, graphene could be used to modulate telecommunications signals at ultra-fast speeds. For example, it could be used to block unwanted communication frequencies.
One way to improve the performance of graphene is to cut it into microscopic, nanoscale ribbon structures, which act like tiny antennae that interact with light. The smaller the antenna, the higher the light energies with which it interacts. It can also be “tuned” to interact with multiple light energies when an electric field is applied, further extending its performance.
The researchers, including teams led by UW-Madison materials science and engineering professors Michael Arnold and Padma Gopalan, initially wanted to make a device out of graphene ribbons that were narrower than anything yet manufactured. By building ribbon-like polymers out of graphene and then stripping off some of the surrounding material, they ended up with precisely drawn and incredibly thin ribbons of graphene.
“It’s very useful because there aren’t good manufacturing techniques to come up with the size of the item we made, 12 nanometers wide over a large area,” says Siegel. “And there is no difference between creating patterns at the centimeter scale that we are working with here and the giant six inch wafers useful for industrial applications. It is very easy to scale. “
Once the devices were made, the researchers could then test how the ribbons interacted with light and how well they could control that interaction.
Together with the group of Prof. Mikhail Kats, professor of electrical and computer engineering at UW-Madison, they projected different wavelengths of infrared light into structures and identified the wavelength where the ribbons and light most strongly interacted, known as the resonant wavelength.
They found that as the width of the ribbon decreases, the resonant wavelength of light also decreases. Lower wavelengths mean higher energies, and their devices interacted with the highest energies measured to date for structured graphene.
The researchers were also able to tune the ribbons by increasing the intensity of the electric field applied to the structures, thereby further reducing the resonant wavelength of the structures. The researchers determined that a structure has the expected flexibility necessary for the technological applications it aims to achieve.
They then compared their experimental data to the predicted behaviors of structured graphene on three different ribbon widths and three electric field strengths. The wider ribbons created by the researchers closely matched the predicted behaviors.
But for the narrower ribbons, they saw a so-called blueshift, or a switch to higher energies than expected. The blueshift can be explained by the fact that the electrons in the small ribbons would be more likely to interact – and repel – with each other.
“The blueshift we have observed indicates that telecommunications wavelengths can be achieved with much larger structures than previously expected – around 8 to 10 nanometers – which is only slightly smaller than 12 nanometer structures. that we made, ”says Siegel.
With the eight to 10 nanometer target being much closer than expected, researchers are now trying to fine-tune their manufacturing methods to make the ribbons even narrower. These new graphene nanostructures will also make it possible to explore the fundamental physics of light-matter interactions, research that Siegel and his colleagues are currently pursuing.
This work was supported by the Defense Advanced Research Projects Agency (YFA D18AP00043). SNM-IS (1727523), US Army Research Office (W911NF-12-1-0025 and W911NF-18-1-0149), US Department of Energy (DE-SC0016007) and Air Force Office of Research (FA9550-18 -1 -0146).