Graphene plasmons: So NIR

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  • Published: Dec 1, 2016
  • Author: David Bradley
  • Channels: Infrared Spectroscopy
thumbnail image: Graphene plasmons: So NIR

Efficient enhancement

Probing graphene plasmon in nanodisks by FTIR. Credit: Xiaolong Zhu from DTU Nanotech

Efficient absorption enhancement at a wavelength of 2 micrometres by graphene has been demonstrated with the plasmons of nanoscale graphene discs by researchers in Denmark.

A lot of hyperbole surrounded the discovery of yet another allotrope of carbon, the monolayer form of graphite, known as graphene. The material has many unique optical, electronic, and physical properties. However, faced with experimental reality, new wonder materials sometimes do not quite live up to headline and grant proposal expectations. One property that has been something of a hindrance for researchers hoping to exploit graphene in telecommunications where the material might switch between the electronics and optical worlds is the lack of direct, efficient interaction with light, particularly in the near-infrared (NIR) region of the spectrum, the well-trodden domain of telecommunications applications.

Amplified plasmons

Graphene has highly mobile electrons, which makes it an excellent conductor. Moreover, just as a pebble dropped into a pool of water will create ripples, so electronic oscillations can arise in freely moving conduction electrons by absorbing light energy. This leads to collective, coherent motions of the electrons known as plasmons, which can amplify the strength of the absorbed light’s electric field at close proximity. Plasmons are being used increasingly in various optoelectronic applications where highly conductive metals can be easily integrated. But, technologists would like to use graphene instead, because it is less costly than precious metals and brings with it a whole range of additional benefits for fabrication and connection. To do so, they need to find the right pebble to throw in the graphene pool so that the ripples can surmount the NIR barrier. Writing in the journal Optics Letters, scientists at the Technical University of Denmark describe one such answer to this problem.

"The motivation of our work is to push graphene plasmons to shorter wavelengths in order to integrate graphene plasmon concepts with existing mature technologies," explains team leader Sanshui Xiao.

To do so, Xiao, Zhongli Wang, Tao Li, Kristoffer Almdal, Asger Mortensen, and Sokol Ndoni took inspiration from recent developments at the university’s Center of Nanostructured Graphene (CNG), where they demonstrated a self-assembly method resulting in large arrays of graphene nanostructures. The team exploited geometry to boost the graphene plasmon effect at shorter wavelengths essentially by decreasing the size of the graphene structures.

Fundamental degrees

The team prepared lithographic masks using a block copolymer based self-assembly method, which could then be used to create arrays of graphene nano discs. They controlled the finished size of these discs by exposing the array to an oxygen plasma for ten seconds which etched the discs, until the average diameter was about 18 nanometres. These discs exhibited an obvious resonance at a 2 micrometre wavelength of light, the shortest wavelength resonance ever observed in graphene plasmons. The team used Fourier transform infrared (FTIR) spectroscopy to probe the plasmons.

The team suggests that extending the etching time might result in smaller discs that could resonate at even shorter wavelengths. However, anything smaller than 18 nanometres would require the team to take into account quantum effects that would ultimately interfere with the plasmon formation process and take such entities into an entirely different realm of physics than the one in which the researchers are working. As an alternative, the team hopes to find a way to tune the graphene plasmon resonances at smaller scales by using electrical gating methods. In such as setup the local concentration of electrons and electric field profile would alter resonant wavelength. To this end, instead of using solid graphene discs, the team intends to experiment with graphene antidots, a complete graphene sheet in which regular holes have been punched. Such a structure would be more amenable to the back-gating technique than graphene nano discs, Xiao explains.

Of course, there are other fundamental considerations that must be taken into account as these put limits on the physics. At ever shorter wavelengths, the interband transition will increasingly play a role in broadening the resonance. The experiments will thus have to balance the weak coupling of light with graphene plasmons and this broadening effect. The back-gating method and use of graphene antidots should help the team overcome such fundamentals to a degree.

Related Links

Opt Lett 2016, 41, 5345-5348: "Experimental demonstration of graphene plasmons working close to the near-infrared window"

Article by David Bradley

The views represented in this article are solely those of the author and do not necessarily represent those of John Wiley and Sons, Ltd.

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