Project C1 is a subproject of the DFG Collaborative Research Centre 1375/2 NOA and investigates chemical processes on the nanoscale down to the single-molecule level. The aim is to analyse the dynamics of local chemical reactions triggered by plasmonic hotspots. These hotspots generate spatially confined stimuli such as strong electromagnetic fields, local heating, or hot electrons, which can initiate chemical transformations.

The project systematically combines, for the first time, two enhancement mechanisms of Raman spectroscopy: plasmonic field enhancement in hotspots and coherent nonlinear excitation of Raman vibrations. This combination enables the investigation of reaction dynamics with extremely high sensitivity and temporal resolution.

During the first funding period, the focus was on establishing the methodological and technological foundations for single-molecule measurements. Key achievements include the development and optimisation of broadband and multi-resonant plasmonic nanostructures, high-precision nanofabrication of structures with gap sizes below five nanometres, and the implementation of a time-resolved broadband SECARS measurement system. The latter allows effective suppression of disturbing signals such as non-resonant four-wave mixing and two-photon photoluminescence from gold. At the same time, broadband CARS spectroscopy enables continuous spectroscopic monitoring of chemical reactions.

These efforts removed major experimental obstacles on the path towards single-molecule SECARS and form the basis for the second funding phase.
The second and current project phase focuses on two closely interlinked research topics. The first priority is the further development of time-resolved single-molecule vibrational spectroscopy, with the goal of achieving the first realisation of time-resolved, single-pulse broadband SECARS at the single-molecule level with the highest possible temporal resolution.

The second focus addresses the investigation of the dynamics of hotspot-induced local chemical reactions. Using time-resolved SECARS and B-CARS measurements, reactions triggered by local hotspot stimuli—such as hot electrons, local heating, or strongly localised optical fields—are analysed in detail.

This project is funded by the German Research Foundation (Deutsche Forschungsgemeinschaft, DFG) under project number 398816777.