Hybrid Plasmons for Manipulating Molecular and Excitonic Energy Redistribution Pathways

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The availability of multiple molecular energy redistribution pathways following photoexcitation plays a fundamental role in limiting the yield of a targeted photochemical or photophysical outcome such as photoluminescence or photoinduced charge separation. Optical methods such as ultrafast coherent control have thus been used to try to alter and direct energy flow within molecules. We report here a new approach to this problem by harnessing strong coupling of molecular electronic states with tunable surface plasmon resonances to control electronic energy redistribution pathways in molecules adsorbed on a silver film [1]. In our experiments. ultrafast excitation of porphyrinic molecular J-aggregates into the S2 state is followed by a second pulse of varying incident wavevector to produce a tunable plasmon in the metal film. When the plasmon overlaps the S1 state, energy flows from S2 to S1 at high efficiency through a surface plasmon gated internal conversion. The conversion yield is controlled by the intensity of the gated signal (i.e. the surface plasmon). If the plasmon hybridizes with the S2 state (strong coupling regime), the excitation remains in the S2 vibrational manifold during quenching to the ground state. The redistribution process is analogous to an ultrafast molecular optical transistor where the SPP is the gating signal, the S1 state corresponds to the drain and the S2 state to the source (see Fig. 1)