Towards ultrafast microspectroscopy of individual quantum emitters: From solid-state systems to single molecules

The presented thesis contributes two new microscope setups to the field of ultrafast microscopy. The demonstrated techniques are based on action-based ultrafast spectroscopy, which is ideally suited for integration with microscopes due to fully collinear beam geometries and the use of incoherent observables. First, the fundamental concepts underlying the experimental work and the used laser setup were introduced. Then, individual plasmonic nanoslit resonators were investigated using photoemission electron microscopy (PEEM). The local nonlinearity of the photoemission process was determined, and spectral variations in the plasmon resonance of single nanoslits were found in TR-PEEM, which was confirmed in 2D nanoscopy experiments. Higher-order contributions to the 2D spectrum were attributed to a plasmon-polariton quantum wave packet. Further, the new method of ultrafast fluorescence-detected pump–probe microscopy was developed using samples of TBI dye molecules dispersed in a PMMA matrix. After an introduction to the homebuilt microscope, an ensemble sample was discussed as a reference. The technique was then pushed towards single molecules, and the transient nonlinear spectra and ultrafast relaxation dynamics of three single molecules were measured and discussed in relation to the ensemble. Additionally, the information content of measured spectra and observed variations between molecules were addressed. The developed methods offer a significant contribution to the field of ultrafast microscopy as they combine the advantages of action-based ultrafast spectroscopy with the spatial resolution of two different microscopes. This offers the possibility of studying local, ultrafast phenomena on surfaces with spectral resolution while avoiding ensemble averaging. Ultimately, the methods contribute to the long-term goal of establishing an understanding of the fundamental physics in complex nanostructures with embedded quantum emitters.