Near-field exploration of light-matter interactions on gold and van der Waals materials

The subwavelength confinement of light is a key requirement for future nanophotonic devices. As such, the subwavelength field confinement of polaritons, such as surface plasmon polaritons (SPPs) or exciton polaritons, makes them attractive for various applications such as nanocircuits, sensing, light generation and solar energy conversion. Due to their subwavelength confinement, these polaritons are however difficult to characterize directly with conventional microscopes. Near-field optical microscopy allows the visualization of strongly confined polaritons. In this PhD work, we first study the near-field amplitude and phase of SPPs on a monocrystalline gold platelet, in the visible spectral range and with a near-field microscope in a reflection configuration. Due to the moderate confinement of these SPPs compared to SPPs in materials such as graphene, the full quantitative characterization of the wavelength and propagation length of these polaritons is challenging. Indeed, multiple excitation pathways of these polaritons lead to complex interference patterns. We measure the near-field amplitude and phase for six different angles between the incident light and the edge of the platelet. We find that the signals from the SPPs excited at an atomic force microscope tip are best isolated from the signals coming from the other excitation pathways at grazing incident angle. Moreover, we introduce a simple model to describe the amplitude and phase profiles. This model explains the π/2 phase shift observed between these profiles. Using this model, the wavelength and propagation length of the tip-launched plasmons are retrieved by isolating and fitting the profiles far from the platelets’ edges. Our experimental results are in excellent agreement with theoretical models using gold refractive index values from the literature. With this acquired knowledge, we study the coupling between the previously characterized SPPs and excitons in a van der Waals material transferred on a gold platelet. The chosen material is tungsten diselenide (WSe2). As WSe2 is a uniaxial anisotropic crystal, we derive 2×2 transmission and propagation matrices to calculate the dispersion relation of polaritons on this multilayered structure. A slightly lower confinement and coupling between the excitons and the SPPs is predicted when the anisotropy of WSe2 is taken into account, compared to the case where WSe2 is considered as isotropic. The theoretical dispersion relation is compared to measurements of the experimental dispersion relation reconstructed from near-field measurements of polaritons at several consecutive excitation energies. A small back-bending of the experimental dispersion relation is observed. The presented methods to fully characterize the SPP complex wavevector could enable the quantitative analysis of moderately confined polaritons occurring in different materials in a reflection configuration and at visible wavelengths.