Exploring Ultrafast Dynamics and Vibronic Effects in Plexcitonic Nanostructures

This thesis explores the ultrafast dynamics of plasmonic and plexcitonic nanostructures by combining pump-probe, two-dimensional electronic spectroscopy, and microscopy techniques. It begins with investigation of gold nanorods with low aspect ratio, where femtosecond pump--probe and two-dimensional electronic spectroscopy (2DES) provide complementary insights into coherent plasmon dephasing and incoherent hot-electron relaxation. These studies reveal distinct fluence-dependent behaviors of longitudinal and transverse plasmon resonances and enable direct estimation of plasmon dephasing times in colloidal ensembles, establishing a crucial reference for more complex hybrid systems. Building on this foundation, we characterize plexcitonic nanohybrids formed by coupling gold nanorods with cyanine dye aggregates. Linear optical analysis, supported by simplified modeling, confirms the occurrence of strong coupling and provides the parameters required to interpret their nonlinear response. Under these conditions, 2DES reveals the emergence of vibronic coherences within disordered plexcitonic manifolds, where molecular vibrations mediate resonant energy exchange between bright and gray states and sustain coherent dynamics beyond Rabi oscillations. Finally, angle-resolved reflectivity, fluorescence imaging, and stroboSCAT microscopy were used to characterize the transport properties of plexcitonic nanostructures prepared with metallic thin films and the same molecular aggregates. These measurements demonstrate that vibronic modes assist long-range plexciton transport, extending exciton propagation lengths from hundreds of nanometers to several microns. The propagation efficiency is maximized when the lower plexciton is resonant with a vibrational quantum below the exciton reservoir, establishing a direct link between vibronic interactions and nanoscale transport. This coupling sustains the highly propagating bright states and enhances the overall transport mechanism. Altogether, these results show that molecular vibrations do not merely limit coherence in hybrid systems but can actively sustain energy redistribution and long-range propagation, highlighting vibronically assisted plexciton transport as a promising route for nanoscale energy flow and photonic technologies.