Detecting single molecules one traces individual molecules both in space and in time, providing high-resolution images of dynamic processes beyond the ensemble. A host of important dynamic processes occurs on fs-ps timescale, such as electronic relaxation/dephasing, energy & charge transfer, vibrational relaxation, photo-dissociation-ionization, photo-isomerization. Yet capturing fast dynamics is fundamentally limited by the lifetime of the detected fluorescence. For the ensemble, fs-ps resolution is obtained by transient absorption and 2D electronic/vibrational spectroscopy.

We challenge ultrafast transient absorption of a single molecule and have set out to probe the non-linear ultrafast response of the single molecule using a broadband laser in an effective 3-pulse scheme with fluorescence detection. 2D electronic spectroscopy of single molecules is getting into reach.

Stimulated emission has been used as alternative detection method and is widely applied to enhance spatial resolution in STED microscopy. We have set out to directly detect the stimulated emission from individual quantum emitters at ambient conditions, to disentangle ultrafast charge dynamics in the excited state, on sub-picosecond timescale.

Finally, local optical near-fields of nano-antennas and plasmonic nanoparticles are known to enhance both sensitivity and resolution in sensing and microscopy. Critical to the optimal coupling and enhancement is the positioning of the molecule at the local hotspot. We apply both deterministic scanning and stochastic mapping of the nanoscale plasmon-molecule interaction, to optimize the coupling strength and ultrafast interaction.