Nonlinear spectroscopy of a single nanoobject via a plasmonic waveguide

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How does Nature optimize the absorption of light and the transport of energy for photosynthesis? Does quantum mechanical coherence play a role in the warm and wet environment of biology? Are all protein complexes identical or do different solutions exist for the same function? The answer to these questions requires ultrafast spectroscopy of a single protein complex. Single molecule (or protein) fluorescence imaging and spectroscopy is a well-established technique. It circumvents ensemble averaging of spectroscopic properties and gives access to their temporal evolution without the need to synchronize the ensemble. However, the accessible timescales are limited by the fluorescence lifetime and faster processes such as decay of coherence and transport of excitation are hidden from the observer. Nonlinear optical spectroscopy allows us to access these ultrashort timescales. A single emitter has thus to interact with two photons within a short delay. Under these conditions, photobleaching of the fluorophore limits the total number of detectable photons to about one million. Only very few and rather limited nonlinear experiments on single molecules have been successful so far. Here we propose to combine two recent innovations to overcome this limitation. We will use plasmonic waveguides to increase the photostability of fluorophores by the Purcell effect. This will boost the total number of detected photons and thus the signal-to-noise ratio, making tiny nonlinear effects visible. We will combine this sample design with phase-modulated fluorescence-detected two-dimensional spectroscopy. This method of fluorescence excitation and data analysis will allow us to extract the maximum information from the limited number of photons. To validate our new setup, we will investigate the vibrational coherence in a single dye molecule, which is not accessible by linear spectroscopy but is thought to have a decisive influence on light harvesting. We then switch to single photosynthetic protein complexes of purple bacteria. 2d spectra will elucidate the degree of coherence between the absorption bands, its decay, and its variation from complex to complex.