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The ability to observe movement of electrons in complex materials has been challenging due to the lack
of suitable spectroscopic techniques for measuring the motion of electrons with state-specific
resolution. To address this challenge, I designed and constructed a tabletop ultrafast pump-probe X-ray
absorption spectrometer based on high-harmonic generation (HHG), which enables state-specific
probing of electrons with femtosecond time resolutions. In my talk, I will describe this new frontier of
ultrafast science and technology and highlight the recent applications of this method to study electron
dynamics in transition metal oxide semiconductors. Measuring core-to-valence transitions using HHG X-
rays provides femtosecond-resolved chemical fingerprint of materials with atom-specific oxidation state,
spin state, carrier polarity, and coordination geometry. Using this technique, I measured the spectral
signatures of charge transfer excitons, surface electron trapping, exciton dissociation, spin transition,
and interfacial charge transfer dynamics in light harvesting and magnetic materials. However, a detailed
understanding of these ultrafast processes also requires probing of the phonon motions coupled to the
electrons. The second part of my talk will focus on the use of time-domain Raman spectroscopy to probe
molecular like phonon wavepackets, lattice structural dynamics, and exciton-phonon coupling in two-
dimensional perovskites and quantum materials. Therefore, my research on time-resolved X-ray and
Raman spectroscopies provides detailed nanoscale understanding of electron and phonon motions that
govern charge separation, trapping, transport, and recombination processes in materials. These
examples illustrate the abilities and future promises of these ultrafast spectroscopic techniques to
provide rational design principles of functional materials with numerous technologically relevant
applications spanning photocatalysis, photovoltaics, and information storage and processing.
Relevant references:
1) Biswas et al. Accounts of Chemical Research, 2022, 55, 893
2) Biswas et al. Journal of Chemical Physics, 2019, 151, 104701
3) Biswas et al. Nano Letters, 2018, 18, 1228
4) Biswas et al. Physical Chemistry Chemical Physics, 2018, 20, 24545
5) Biswas et al. Physical Review B, 2022, 106, 134303 |