Transient absorption (TA) spectroscopy is a common pump-probe technique that measures the excited state dynamics of molecular systems. A “pump” pulse photogenerates an excited species which evolves through different electronic states as a function of time. Populations of and transfer rates between different excited and ground states can be resolved by measuring changes in the absorption of a “probe” pulse as a function of time delay between the two pulses. Typical TA instruments employ beams loosely focused to tens of microns. The signal is merely an ensemble average of the excited state dynamics of a range of nanoscale structures within the beam profiles, missing the unique dynamics for each structure. We developed a high repetition rate broadband TA microscope that elucidates highly localized transient processes by tightly focusing both beams using a high magnification objective, enabling sub-diffraction resolution of transient signals. Signal is measured as the probe beam is scanned across the pump beam profile using galvo mirrors. These rasterized transient images are collected over a range of pump-probe time delays. This enables measurements of spatially dependent processes that are precluded from typical TA including exciton and charge diffusion rates and excited state processes of sub-micron domains and interfaces such as charge transfer, singlet fission, and triplet fusion. We present transient spectra of silicon nanowires and perylene diimide thin films to demonstrate the capabilities and benchmarks of the instrument. This TA microscope is one of the first to be integrated into a shared user facility, the Center for Dynamics and Control of Materials at the University of Texas at Austin. We present the design features that enable this instrument to be fully operable by external users. This instrument will support future studies of nanostructured molecular systems and, as a user-based instrument, will become a valuable asset to the scientific community.

Design and implementation of a shared-user super-resolution transient absorption microscope

ACS Spring 2022

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