New insight into graphene plasmons

Plasmons are collective excitations of charges which are coupled to electromagnetic radiation in a material. They are responsible for the colour of stained glass, found for example in the windows of churches, or in the famous Lycurgus Cup, a 4th century Roman glass cup which displays a different colour depending whether it is illuminated from the front or the back. Plasmons could be very important in technology in the near future, especially in the field of medicine where devices based on plasmon resonances capable of discriminating between different substances have been tested.

In the examples above, plasmons modulate the absorption of a material, allowing only certain colour to pass and blocking others. This is related to the material which is actually sustaining the plasmons. In stained glass this material is very small metal nanoparticles. These are typically gold or silver spheres which are only 10-50 nm in diameter (this is approximately a ten-thousandth of the diameter of a human air). The electrons contained in such metallic particles react to the electromagnetic field of the incident light and “resonate” at specific frequencies, which depend on the diameter of the particles.

In graphene, a single layer of carbon atoms arranged in an hexagonal structure, such oscillations have peculiar characteristics. Graphene plasmons have a wavelength which is up to 100 times smaller than the wavelength of  the electromagnetic radiation. Intense research in the past six years led to the discovery that graphene

s-SNOM experiment on graphene plasmons.
s-SNOM experiment on graphene and corresponding image of surface plasmons. Reproduced with permission from Nano Lett. 2017, 17 (10), pp 5908–5913.

plasmons are highly tunable, therefore open the door to a plethora of applications, spanning from telecommunication devices, to sensors and light modulators.

One of the main problem with graphene plasmons is that the electrons which oscillate in the electromagnetic field can couple to anther type of oscillation in the material, the phonons. These are mechanical oscillations and they are related to the constant vibration of atoms. When this coupling happens, plasmons loose energy and the expected effect vanishes.

In our recent study, we measured the strength of such coupling in highly-doped graphene. The work was conducted between the University of Exeter and the Institute of Photonic Sciences (ICFO) in Barcelona (ES), a collaboration between the group of Prof. Saverio Russo and the group of Prof. Frank Koppens. In this work we used a technique pioneered in ICFO to “take a picture” of the plasmons in graphene, called scattering Scanning Near-Field Optical Microscopy (s-SNOM). This “picture” looks like a series of bright and dark “waves” on the surface of the material, where the distance between to crests is double the wavelength of the plasmons.

Studying the plasmon-phonon interaction in graphene at very high levels of doping is very important because this is the regime at which graphene plasmons can be used in future telecommunications technologies. in order to reach the required levels of doping, we employed Iron Chloride Intercalated graphene (FeCL3-FLG). Our findings show how the presence of a particular phonon mode, which belongs to bi-layer graphene, can interact with the plasmons damping these oscillations. These findings pave the way toward the realisation of active plasmonic platforms based on graphene.

You can read the paper at the following link (Open Access): De Sanctis A. et al.”Intrinsic Plasmon–Phonon Interactions in Highly Doped Graphene: A Near-Field Imaging StudyNano Lett. 17 (10), 5908–5913 (2017)

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