When two pieces of fine mesh are placed one on top of the other and then rotated, new, more complicated patterns appear. As you keep on twisting, the patterns change like in a kaleidoscope and so-called moiré patterns form. Such patterns have been recently observed in scanning tunnelling microscope (STM) images of stacked layers of graphene (a sheet of carbon just one atom thick) with the twists causing dramatic changes in the material’s electronic properties.
Graphene also has extraordinary optical properties thanks to the fact that it supports strong surface plasmon polaritons. A polariton is a particle-like entity (or quasiparticle) that can be used to describe how light interacts with semiconductors and other materials that have been made to resonate at certain frequencies. It has two different components: an electron-hole pair (or exciton) and a photon, which is emitted when the electron and hole recombine. When a photon is emitted, it is immediately reabsorbed to reform an exciton, so the cycle is repeated. This continuous exchange, or coupling, of energy between photons and excitons can be described in terms of polariton states.

Tunable plasmonic resonance bands

Polaritons will play an important role in future photonics devices that exploit light instead of electricity to process information. Such devices will be much faster and use less energy than their electronic counterparts and the strong coupling of polaritons will be crucial for the success of this new photonics.
“Graphene metasurfaces show plasmonic resonance bands that can be tuned from the mid-infrared to the terahertz regimes,” explains Maruthi Nagavalli Yogeesh, who is a member of Deji Akinwande’s team in Texas. “These plasmonic bands could be exploited for biosensing, spectroscopy, light modulation and communications applications.”

Multiband surfaces are better

The problem is that current graphene plasmonic metasurfaces are usually single band and it would better to be able to make multiband surfaces for more advanced applications such as single-molecule detection, surveillance and communication.
Now, the Austin researchers have succeeded in making such a surface using a cheap and simple technique to pattern large area graphene into moiré metasurfaces having tunable multiband resonance peaks.

Obtaining various moiré patterns

“In this work, we patterned graphene grown by chemical vapour deposition into moiré metasurfaces by combining moiré nanosphere lithography (MNSL) and oxygen reactive ion etching (RIE),” explains Zilong Wu, a member of Yuebing Zheng’s team in Texas. “In brief, we self-assemble colloidal polystyrene (PS) nanospheres into a monolayer on substrates with the graphene. We then deposit a second monolayer of PS nanospheres on top of the first one using a similar process. We can control the relative rotation angle between the first and second layers to obtain various moiré patterns.”
An additional RIE step creates voids between closely packed nanospheres and etches away graphene that has been exposed to the plasma. “After removing the residual nanospheres, graphene sheets with moiré patterns are then left on the substrates,” adds Wu.

Towards protein biosensors

“By varying the relative rotational angle between the top and bottom monolayers of PS nanospheres during MNSL, we are able to significantly change the size and shape of the graphene nanostructures in the metasurfaces. This means that we can tune the multiband resonance peaks in the material from the infrared to the terahertz.” team member Wei Li tellsnanotechweb.org.
The team, reporting its work in Advanced Optical MaterialsDOI: 10.1002/adom.201600242, says that it is now working on making protein biosensors from the graphene metasurface. “We also hope to integrate it with THz photodetectors,” says Zheng.