Electrodynamics of quasi-particles (plasmons, excitons) in graphene

Graphene is known to be a support for quasi-particles such as plasmons or excitons. Those modes present many interests to design sensors, electro-optic light modulator, etc.. A part of our research work centres around model building to describe electrodynamics in graphene.

In a recent work, we have reported on the enhancement of surface plasmon resonances in a holey bidimensional grating of subwavelength size, drilled in a gold thin film coated by a graphene sheet (see figure above). The enhancement originates from the coupling between charge carriers in graphene and gold surface plasmons. A lower constraint on the gold-induced doping concentration of graphene has been specified and the interest of this architecture for molecular sensing was also highlighted [1].Scheme

Fig. 1. Sketch of the device.

For instance, above (panel (a)), we measured the relative absorption of the graphene-coated versus uncoated gold for various incidence angles. The black curve represents the relative absorption for a non structured metal film. For the sake of clarity, each curve is shifted vertically by 25 units with respect to the previous one. In panel (b), we show the numerical simulation of the absorption of the graphene-coated device for various angles of incidence. For the sake of clarity, each curve is shifted vertically by 0.5 units with respect to the previous one. We then quite well retrieve the experimental behaviour [1].

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Fig. 2. (a) Relative absorption and (b) numerical simulations of the absorption of the graphene-coated versus uncoated gold for various incidence.

In another context, using a theoretical approach previously considered to describe a Universe made of two braneworlds (in the context of High Energy Physics), we have proposed a new theoretical description of the phenomenology of two twisted graphene sheets. The model considers that some graphene bilayers can be described by a two-sheeted (2+1)-spacetime in the formalism of the noncommutative geometry (see figure above). The model has been justified by means of a tight-binding approach, and the noncommutative geometry emerges from K-K’ couplings between graphene layers. This suggests a new way to describe multilayer graphene, which deserves further studies. We have shown that the transfer of excitons between the two graphene sheets is allowed for some specific electromagnetic conditions. While the excitons are produced by incident light on the first graphene layer, photons could be recorded in front of the second graphene layer where the swapped exciton decays. An experimental device was suggested which could be used as a new kind of electro-optic light modulator. The described effect is a solid-state realization of a two-brane Universe, for which it has been shown that matter swapping between two braneworlds could occur. As a consequence, any experimental evidence of this effect in graphene bilayers would also be relevant in the outlook of braneworld studies [2].

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Fig. 3. Two parallel graphene layers can be considered as two two-dimensional worlds embedded in a three-dimensional space (the bulk). This is reminiscent of the concept of braneworld considered in high energy physics. In such a context, it can be proved that a graphene bilayer can be described as a two-sheeted spacetime in the context of the noncommutative geometry when considering exciton dynamics. The effective distance between both spacetime sheets is related to the real distance between graphene layers in a non-trivial way.

 

 

 

[1] N. Reckinger, A. Vlud, S. Melinte, J.-F. Colomer, M. Sarrazin, 2013, Applied Physics Letters, 102, 211108.

[2] M. Sarrazin, F. Petit, 2014, The European Physical Journal B, 87, 26.