Investigating the properties of quasi-particles and their dynamics in a specific environment is an essential part of tailoring functionality in device fabrication. Nonlinear optical spectroscopy, in particular transient absorption, has become a standard tool for probing ultrafast single quasi-particle dynamics by tracking the evolution of the probe pulse spectrum after photoexcitation with a pump pulse. As an extension of this approach, multidimensional electronic spectroscopy enables the simultaneous temporal and spectral resolution of pump and probe interactions so that the spectral overlap of signatures attributed to, e.g., energy transport processes and coupling mechanisms, can be resolved [1].

As most devices operate through solid interfaces, quasi-particle dynamics are usually studied in the solid state. In conventional nonlinear spectroscopy, the signal represents an average over structural inhomogeneities and nanostructured regions of the interface. We have therefore adapted the concept of transient absorption for fluorescence-detected measurements in a light microscope to follow the exciton dynamics of a single molecule [2]. Beyond that, some of us have introduced 2D nanoscopy [3–6], a combination of multidimensional electronic spectroscopy with photoemission electron microscopy (PEEM), to study exciton and surface-plasmon polariton dynamics, coupling and energy transfer processes in extended systems on a spatial scale of a few nanometers.

Because focused beams concentrate light into a small volume, they produce high light intensities that can generate multiple quasi-particle excitations which can interact with each other. Molecular excitons, for instance, can undergo exciton–exciton annihilation, i.e., a process that hinders charge carrier transport in optoelectronic devices. Multiple excitations at high light intensities, which are usually intended to provide a better signal-to-noise ratio, have plagued nonlinear spectroscopy methods for decades. In other cases, many-particle interactions are an essential part of physical systems, e.g., singlet fission, exciton–phonon interactions or Bose–Einstein condensation. Some of us have therefore recently developed the intensity cycling method, which makes it possible to experimentally separate the individual terms of the Taylor expansion in nonlinear light–matter interaction by taking specific linear combinations of measurement data recorded at different excitation intensities [7]. In this way, the signal contribution of N interacting particles can be isolated from the rest of the signal. The method, introduced using transient absorption, is universal and does not depend on the sample system. In addition, the method has just been adapted to multidimensional electronic spectroscopy [8].

Here, I will report on our progress to extend the intensity cycling method to our PEEM setup [9] in order to study quasi-particle interactions with high spatial resolution. In our first steps, we look for exciton–exciton annihilation, a measure for exciton diffusion, in a 5 nm thick film of terrylene bisimide (TBI) molecules on Si(100). As an excitation sequence, we use a 680 nm pump pulse and a 340 nm probe pulse for photoemission.