Fano-like resonances arising from long-lived molecule-plasmon interactions in colloidal nanoantennas.

We examine ultrafast dynamics in a coupled molecule-plasmon system. Using a new ultrafast Raman technique called surface enhanced-femtosecond stimulated Raman spectroscopy (SE-FSRS), we prove that plasmonic nanoparticles and adsorbed molecules are coupled by the appearance of Fano-like lineshapes, which arise from the interaction of narrowband vibrational coherences and the broadband plasmon resonance. We probe the effect of plasmon energy on the vibrational lineshapes and observe changes in the phase of the line shape dispersion. Finally, we examine the effect of plasmon-molecule coupling on the molecular vibrational coherence lifetime. Surprisingly, coupling of the molecular vibration to the plasmon does not significantly shorten the vibrational coherence dephasing time. Better understanding of the ultrafast dynamics of excited vibrational states and vibrational coherences in coupled molecular-plasmonic systems should assist in developing a mechanism-based view of plasmonically enhanced photovoltaic and photocatalytic systems.