The U.S. Navy is behind a push to exploit one of the “hottest” areas of the electromagnetic spectrum, the terahertz band. The Office of Naval Research contributed to a breakthrough project at Lawrence Berkeley National Laboratory last fall with the help of graphene nanoribbons, and just last month a team of ONR-funded researchers at the University of Notre Dame announced another new milestone.
The attraction of the terahertz band
Terahertz waves are situated between the microwave and optical light frequencies, at the “farthest end of the far infrared.” In communications, they could transmit far greater amounts of information than either radio waves or microwaves.
In imaging, terahertz frequencies could lead to the development of diagnostic equipment that avoids the health risks of x-rays.
However, expanding the real-world applications of this part of the spectrum has been stuck for want of a material that can be used to manipulate terahertz waves with precision.
Graphene and terahertz waves
The terahertz worm began to turn in 2004, when a team of researchers in the U.K. literally used sticky tape to lift a one-atom thin sheet of carbon from a chunk of graphite.
Called graphene, the new material possesses outsized strength and unique electrical properties, which have made it the focus for bringing about the next generation of super fast, super small, flexible and even transparent electronic devices.
As Notre Dame researcher Berardi Sensale- Rodriguez explained in a prepared statement:
“A major bottleneck in the promise of THz technology has been the lack of efficient materials and devices that manipulate these energy waves. Having a naturally two-dimensional material with strong and tunable response to THz waves, for example, graphene, gives us the opportunity to design THz devices achieving unprecedented performance.”
Graphene nanoribbons to the rescue
Last fall’s breakthrough at Lawrence Berkeley involved the fabrication of graphene nanoribbons, made by etching patterns into a sheet of carbon atoms laid over a silicon oxide substrate. An overlay of ion gel was used to complete the gated structure of a semiconductor system.
The team was able to “tune” or manipulate the ribbons in to control the movement of electrons. This collective movement, or oscillation, of electrons is referred to as a plasmon.
According to Berkeley research Feng Wang, plasmons can be observed by eye, in the unique glow from medieval-era stained glass which is caused by electrons oscillating on the surface of metal nanoparticles including gold and copper.
A similar effect occurs in graphene but at lower frequencies, which are not visible to the naked eye.
The Berkeley team discovered that altering the width of the graphene nanoribbons will cause the electron waves to “slosh” back and forth at different frequencies, which makes the ribbons absorb different frequencies of light.
The demonstration marked a step along the way to practical, real-world applications partly because the team was able to measure the difference in absorption rates at room temperature, in contrast to other research tracks that require temperatures in the absolute zero range.
The findings of the Notre Dame team, published in mid-April, also involved the development of a practical, room-temperature operation. The team was able to demonstrate proof of concept for a graphene based modulator, building on previous research into the use of an electron gas to manipulate terahertz waves.
The idea of using an electron gas dates all the way back to 2006, so given the pace of research in both the Berkeley and Notre Dame cases a practical graphene/terahertz device is far from bouncing out of the laboratory door and onto retail shelves.
Aside from challenges within the research itself, the commercialization of graphene devices depends on the development of cost effective methods for fabricating mass quantities of graphene, and sticky tape will only get you so far. At this point there have been some promising developments, but the goal has proved elusive.
Not to worry, though – the Navy is all over that one, too. Through a separate ONR-funded program, researchers at Rice University are developing a simple, one-step process for creating nanoscale graphene discs.
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