The use of graphene in the growing field known as plasmonics—in which the waves of electrons known as surface plasmons that are generated when photons strike a metallic structure—has been transforming the world of photonics and optoelectronics, enabling the possibility of much smaller devices operated by photons rather than electrons.
The Graphene Council has covered the work being performed at one of the leading research institutes in the world in this field of plasmonics, the Institute of Photonic Sciences (ICFO) in Barcelona.
We had the opportunity to visit ICFO last week and speak to a number of their researchers, which we will be sharing in the coming weeks. In particular, we spoke to F. Javier García de Abajo from the Nanophotonics Theory research group at ICFO, who has proposed a revolutionary approach of exploiting graphene for plasmonics.
It’s worth providing a bit of background on the field of plasmonics before jumping to this latest research. The use of photons instead of electrons for something like an integrated circuit has the clear benefit that photons travel much faster than electrons, promising much faster devices. However, the use of light in these applications is limited by the relatively large size of wavelengths of light. Light is fast, but their wavelengths are much larger than nanometer-scale dimensions of most integrated circuits.
Plasmonics provides a way to convert that light—photons—into waves of electrons that can be tuned to have much smaller dimensions than those of light. The dimensions of these plasmon waves can be a hundred times smaller than the smallest wavelengths of light. This means that light can serve as the basis of photonic integrated circuits, but many more devices than that.
The field of plasmonics has really taken in off in the last half-decade, and ICFO has been at the forefront of a lot of that work, especially in using graphene to enable the effect. However, what Garcia de Abajo has proposed is a new theoretical approach to generate visible plasmons in graphene not from light but from tunneling electrons.
In research published in the journal ACS Photonics, Garcia de Abajo and his colleague Sandra de Vega have suggested that there are more efficient ways of generating surface plasmons on graphene than using an external light source and have instead shown through models that graphene plasmons can be efficiently excited via electron tunneling in a sandwich structure formed by two graphene monolayers separated by a few atomic layers of hexagonal boron nitride.
As mentioned, it’s possible to tune the size of the plasmon waves, especially graphene plasmons, which can be changed in size according to the amount of doping level (an addition of other materials). While high doping levels can push the wavelength of the graphene plasmons towards the visible range, these grpahene plasmons primarily reside in the mid-infrared region, which translates into a weak coupling between far-field light and graphene.
What de Vega and García de Abajo have proposed is a methodology for visible-plasmon generation in graphene that requires no light at all. Instead, plasmons are generated from tunneling electrons, which are electrons that are able to pass through a material on the quantum level that they could not otherwise pass through.
To achieve this photon-less plasmonics, the researchers propose a graphene–hexagonal boron nitride (hBN)–graphene sandwich structure. In their model, the hBN layer is 1-nm thick that is sandwiched between two graphene monolayers.
When the right amount of voltage (bias) is applied between the two graphene sheets, it produces tunneling electrons through the gap. The researchers discovered a particular voltage window in which the tunneling electrons lose energy through the excitation of a propagating optical plasmon rather than dissipate through coupling with the vibrations of the crystal lattice of hBN that carry heat, which are known as phonons, (low bias) or electron–electron interactions (high bias).
One of the side benefits of plasmonic devices that operate in this way—without the need for photons—can also be used in reverse as sensors. In this way when a change occurs in the graphene plasmon properties, that change could lead to a voltage readout.