Interactions of Plasmonic and Dielectric Structures with 2D Materials

Abstract

Three separate systems of nanoparticles coupled to two-dimensional materials are studied. Each system is investigated both experimentally and numerically with FDTD simulations.

The first system involves a nanoparticle-on-mirror approach, with a 150 nm Au sphere on a 100 nm Au film with an intermediate monolayer of graphene in between. The gap size between the Au nanosphere and film, as well as the doping level of the graphene is altered by immersing the system in nitric acid. The scattering spectra for individual nanospheres revealed three multipolar modes, which red-shifted with a reduced gap size and blue-shifted with increased chemical potential of the graphene. Smaller gap sizes increased the electric field strength between the Au sphere and film, increasing the interaction with the graphene. Additionally, it was found that the dipolar mode splits into two new modes due to the coupling of the gap plasmon and the charge transfer plasmon. This work demonstrates the merging of two other theories previously shown to strongly influence the multipolar plasmonic modes in the nanoparticle on mirror system.

The second system consists of single Au nano-bipyramids drop-cast on a monolayer of MoS2. Rabi splitting is achieved between the longitudinal plasmons of the Au bipyramids and the excitons of the MoS2 layer. This strong coupling effect is enhanced by increasing the size of the bipyramids. This is due to the higher electric field strength concentrated at the tips of the larger bipyramids, and also due to the lower aspect ratio of the bipyramid required to keep the plasmon energy overlapped with the energy of the exciton. A lower aspect ratio increases the natural tilt a bipyramid has towards the substrate, tilting the plasmon to overlap more closely with the exciton, and hence increases the strong coupling effect. A bipyramid's sharp tips are also important to increase the Rabi splitting strength, by both increasing the electric field strength and confinement and by tilting the plasmon resonance more directly towards the MoS2 layer. This work is the first demonstration of the Rabi splitting strength increasing with an increased metal volume of nanoparticle, without increasing the number of excitons coupled into the system.

The final system is a monolayer of MoS2 placed on top of a Si disk array on a SiO2/Si substrate. The Si disks are shown to cause a photoluminescence enhancement in the MoS2 for two different pump wavelengths, caused primarily due to the increased absorption at both wavelengths. The system was further optimised with simulations, by replacing the Si in the substrate with Au to act as a back-reflector. This increases the absorption in a monolayer of MoS2 to up to 53%.