Abstract:
This dissertation investigates the excitation, coupling, and radiation behaviour of hybridised plasmonic modes in particle-on-mirror, tip-mirror and collective plasmonic systems. The goal of this research is to improve the understanding of how the enhancement and scattering directionality of plasmon-enhanced Raman scattering depend on the incident laser polarisation or angle of incidence. To achieve this, novel experimental techniques are developed to observe angle-resolved effects during plasmon-enhanced Raman scattering.
Studies on gold nanoparticle-on-mirror systems demonstrate that hybridised plasmonic modes, particularly with dipolar and quadrupolar character, can be efficiently excited with different laser beam polarisations. Strong variability in the polarisation-dependent Raman response and emission directionality between individual hotspots is observed, which is caused by geometric imperfections of the nanospheres and in turn different plasmonic modes. To address this, super-spherical nanoparticles with substantially improved geometric uniformity are employed and combined with a refined sample fabrication technique. This improves the homogeneity of the optical response and enables a reliable correlation between optical data collected in different spectroscopy and imaging methods. An adapted form of energy-momentum spectroscopy is developed to assess the spatial distribution of Raman scattering and gold photoluminescence signal within a parabolic mirror collection aperture.
The same techniques are applied to explore the angular distribution of tip-enhanced Raman scattering in a gold-tip gold-mirror system, which is the first experimental study of its kind. The radiation directionality in tip-enhanced Raman spectroscopy strongly depends on the tip geometry, plasmonic resonance matching, and vibrational geometry of different Raman bands. The tip-sample distance has lower influence here. This work highlights the necessity to consider the spatial distribution of Raman scattering because of the different detection configurations used for TERS.
To explore the influence of the angle of incidence on surface-enhanced Raman scattering, the angle-dependent coupling of a plasmonic lattice with a slab waveguide is investigated. Illumination with different diffraction conditions and varying electric field orientations reveals a large impact on the Raman scattering response. This highlights the strong role of excitation conditions for plasmon-enhanced Raman spectroscopy.
SERS