Electromagnetic properties of a monolayer of polarisable particles deposited on graphene

Abstract

We theoretically predict several plasmonic effects that arise when a monolayer of small polarisable particles is deposited on graphene, related to surface plasmon-polaritons (SPPs) supported by the latter in the terahertz (THz) spectral range. The electromagnetic coupling between graphene SPPs and dipole moments of the nanoparticles (NPs) gives rise to optical properties that are not present in the individual components of this system. The polarisation characteristics of the particles are modified, namely: (i) individual NP’s polarisability is renormalized and (ii) their dipole-dipole interaction includes, beyond the usual ∝ ρ−3 term (ρ is the particleparticle distance), an indirect particle-particle coupling arises via polarisation charges induced on the graphenecovered interface by each particle – this indirect coupling oscillates with the interparticle distance. We derived coupled-dipole equations taking into account all these effects, allowing us to calculate an effective optical conductivity of the particles’ monolayer and study the optical properties of the composite (G+NPs) system. One of the G+NPs system’s unique features is a collective resonant mode, causing a considerable enhancement of the THz radiation absorption in graphene, while the reflection drops to nearly zero for a broad range of angles of incidence. The frequency of this resonant mode can be adjusted by changing the Fermi energy in graphene via electrostatic gating and therefore it can be used for electrically controlled reflection and transmission of THz radiation. Another remarkable finding is that the composite layer supports s-polarised surface polaritons linked to Drude plasmons in graphene.