Plasmonics in graphene and related materials
12 May 2016
UAntwerp, Campus Groenenborger, U0.25 - Groenenborgerlaan 171 - 2020 Antwerpen
Organization / co-organization:
Department of Physics
Ben Van Duppen
François Peeters & Marco Polini
PhD defence Ben Van Duppen - Department of Physics
This thesis presents an inquiry into the plasmonic properties of graphene and related two-dimensional materials. Plasmons are high frequency collective density oscillations of the electron liquid. For five distinct platforms the plasmonic response is scrutinised: strongly doped graphene, a graphene-hexagonal boron nitride heterostructure with valley imbalance, silicene in the presence of external fields, graphene with a strong electric current, and an array of graphene nanostripes.
For the strongly doped graphene sheet, it is found that even though the single-particle energy dispersion is trigonally warped, the plasmon dispersion remains isotropic. If graphene is deposited on hexagonal boron nitride, it is shown that under specific circumstances, valley polarisation induces two new kinds of plasmon modes. One of them is an acoustic mode that is charge neutral in the long-wavelength limit and is slightly damped. The other one is an undamped mode that exists for short wavelengths and is partly charge neutral. Silicene also hosts plasmons in its electron liquid. We show that that external electric and exchange fields can be used to change the plasmon damping by switching single-particle excitations of a specific spin and valley type on and off.
When graphene is subjected to a strong electric current, its optical properties are affected. The optical absorption acquires a birefringent character with respect to the direction of the current flow and the plasmon modes are shown to become collimated in the downstream direction. It is demonstrated that the plasmon modes acquire a non-reciprocal dispersion as well. The excitation of plasmons in graphene nanostripes is finally considered in a semiclassical framework. The plasmons change the local density and velocity of the electron liquid. As a result, it is shown that the absorption of the nanoribbons oscillate with twice the frequency of the excited plasmons. This is used as a proof of concept for a high-frequency plasmon-assisted optical modulator.
This thesis provides new insights into the field of plasmonics of graphene and related materials. The results could help to achieve the main goal of plasmonics in 2D materials, i.e. the use of their plasmons in photonic applications.