Action-detected 2D spectroscopy: Technical development and practical applications

This doctoral thesis presents a comprehensive investigation into action-detected two-dimensional electronic spectroscopy (A-2DES), from the technical development of a state-of-the-art apparatus to its application in elucidating ultrafast dynamics in advanced materials and biological systems. The work is founded on the design and implementation of an A-2DES setup centered on a single acousto-optic pulse shaper, a configuration that offers inherent phase stability and compactness. A significant portion of this research was dedicated to the rigorous characterization and mitigation of instrumental artifacts, particularly pulse-shaper-induced nonlinearities and distortions arising from data acquisition protocols. The development of robust post-processing and background-subtraction strategies was crucial for ensuring the high fidelity of the spectroscopic data. The validated methodology was subsequently applied to three distinct systems, showcasing the versatility of coherent multidimensional spectroscopy. First, coherent 2DES was used to study a highly polydisperse ensemble of CdSe quantum dots, directly visualizing the effects of quantum confinement on the electronic structure and resolving the size-dependent biexciton binding energy, a key many-body interaction. Second, photocurrent-detected 2DES was employed to investigate operational perovskite solar cells, providing direct insight into how the A-site cation composition (formamidinium
vs. methylammonium) influences sub-picosecond charge carrier cooling and relaxation dynamics. These measurements, performed on fully encapsulated devices, established a direct link between microscopic carrier dynamics and the functional photocurrent output, with a detailed kinetic model quantitatively describing the phonon-assisted intraband relaxation cascade. Finally, a comparative study of the light-harvesting complex II (LHCII) at cryogenic temperatures using both coherent- and action-detected 2DES revealed the complementary nature of these techniques. While coherent 2DES clearly mapped population relaxation and energy transfer pathways between chlorophyll states, the action-detected spectra were dominated by features attributed to incoherent mixing, providing a sensitive probe of efficient exciton-exciton annihilation. Collectively, this research establishes A-2DES not merely as an alternative to conventional coherent techniques but as a distinct and powerful tool that offers unique access to the functional dynamics and many-body interactions governing energy conversion and dissipation. The work presented herein provides a validated framework for A-2DES and demonstrates its broad utility in addressing
fundamental questions in materials science and biophysics.