Tip-enhanced near-field optical microscopy with a parabolic mirror microscope: Enhancing luminescence emission and Raman scattering by inelastic tunneling

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Based on the successful implementation of the research plans of the previous two funding periods, we aim in the next funding period at combining parabolic mirror assisted near-field optical microscopy with scanning tunneling microscopy. The main goal is to investigate the structure, the optical and the electronic properties of (single) molecule(s) in a tunneling junction as a function of bias voltage by tip-enhanced luminescence- and Raman-spectroscopy. More specifically we will investigate the underlying fundamental physics mechanisms responsible for the novel and important observations from our preliminary work. Such as 1) the modification of Raman spectra of molecules chemisorbed to the gold-substrate as a function of the bias voltage; 2) the enhanced luminescence emission and Raman scattering from tunneling junctions as a function of the bias voltage; 3) the influence of the relative position energy levels with respect to the bias-voltage. For instance: i) the relative energetic arrangement of the highest occupied molecular orbital (HOMO) and the d-band of the Au substrate; and ii) the energetic levels of the virtual state (Raman) or the lowest unoccupied molecular orbital (LUMO) and the Fermi-level of the metal. Furthermore, we will explore under what conditions feedback from the plasmon oscillations between a tip and a substrate can lead to stimulated Raman-scattering and luminescence emissinon. In general, we target at a deeper understanding of the fundamental processes governing the interaction of molecules with a metal surface such as binding, polarization, charge transfer, electronic excitation of the molecule and emission of photons from the gap on one hand and the interaction between nearby molecules on the other hand. The observations and the mechanisms explored in this project will find a variety of applications in the following fields: optical microscopy and spectroscopy with single molecule spatial resolution and chemical contrast; efficient single-molecule optical switches and efficient and stable single-molecule point-like light sources.