STROBE Seminar: “Ultrafast nano-imaging resolving carrier and lattice dynamics on the nanoscale”

Abstract: Ultrafast infrared spectroscopy in its extension to nano-imaging provides access to vibrational and low energy carrier dynamics in molecular, semiconductor, quantum, or polaritonic materials. In addition, to simultaneously probe both ground and excited state dynamics we have developed ultrafast heterodyne pump-probe nano-imaging with far-from-equilibrium excitation. In ultrafast movies with simultaneous spatial, spectral, and temporal resolution we can image heterogeneities in electron-phonon, cation-lattice, and coupled polaron dynamics on their elementary time and length scales. As exemplary application we use ultrafast pump-probe nanoimaging to provide a real-space and real-time view of the coupled electron-lattice dynamics underlying the photo-physical response of hybrid organic-inorganic perovskites resolving the heterogeneous evolution of polaron lifetimes. From subsequent excited state spectroscopic nanoimaging of the formamidinium (FA) cation vibration as local probe of the molecular environment, we link a transient vibrational blueshift to local variations in lattice polarizability and polaron stability. The high degree of local variation in polaron-cation coupling dynamics points towards the missing link between the optoelectronic heterogeneity and associated carrier dynamics. The results suggest that there is a lot of room for improved synthesis and device engineering and that perovskite photophysical performance is far from its fundamental limits.

Speaker Bio: Markus Raschke is professor at the Department of Physics at the University of Colorado at Boulder. His research is on the development and application of nano-scale nonlinear and ultrafast spectroscopy to control the light-matter interaction on the nanoscale. These techniques allow for imaging structure and dynamics of molecular and quantum matter with nanometer spatial resolution. He received his PhD in 2000 from the Max-Planck Institute of Quantum Optics and the Technical University in Munich, Germany. Following research appointments at the University of California at Berkeley, and the Max-Born-Institute in Berlin, he became faculty member at the University of Washington in 2006, before moving to Boulder in 2010. He is fellow of the Optical Society of America, the American Physical Society, the American Association for the Advancement of Science, and the Explorers Club.