Ultrafast local structural and electronic dynamics in chiral thin films

• Applicants: Professor Dr. Thomas Lenzer; Professorin Dr. Kawon Oum
• Subject Area: Physical Chemistry of Molecules, Liquids and Interfaces, Biophysical Chemistry
• Term: since 2024
• Project identifier: Deutsche Forschungsgemeinschaft (DFG) - Project number 525783257

Ultrafast processes changing the chirality of thin-film materials have important applications in diverse fields, such as films of liquid crystal arrangements and thin layers for spintronics and optical data storage. Such chiral films frequently show substantial inhomogeneities in circular dichroism (CD) on a microscopic length scale and apparent CD signals with hidden contributions from linear dichroism (LD/LD'), circular birefringence (CB) and linear birefringence (LB/LB'). In addition, the locally favored structural order of the films has a strong impact on their chiral response. Understanding the dynamics of such chiral systems requires both ultrafast time resolution and high spatial resolution. In this proposal, we will therefore implement two new methods for chirality-sensitive time-resolved microscopy: ellipsometry-based ultrafast broadband transient circular dichroism microscopy and ultrafast transient circularly polarized luminescence microscopy, both with diffraction-limited spatial resolution. The thin-film characterization using steady-state Müller matrix microscopy will provide complete information regarding polarization changes in terms of the CD, LD/LD', CB and LB/LB' contributions. One of the main investigations in this proposal deals with the time-resolved CD spectroscopy of the structural dynamics in chiral organic thin films. These include the aggregate-induced heterogeneity of the ultrafast local response in chiral copolymer thin films and photoswitch-driven supramolecular rearrangements in pre-aligned cholesteric phases. Another part will address the spatially resolved chiral dynamics of inorganic and hybrid inorganic-organic systems, specifically chiral thin films of perovskite-inspired compounds as well as undoped and doped phase-change materials. Here we aim to understand the dynamics of spin depolarization, i.e. the local "chiral memory" of perovskite materials. The result is relevant to future applications in spintronics. We also investigate the spatial dynamics of low-dimensional perovskites, which are embedded in cholesteric liquid crystals, in order to enhance their chiral optical response. Furthermore, we will use circular polarization as an "added dimension" for ultrafast information storage in phase-change materials. Thin films showing strong "non-CD" contributions will be also characterized using ultrafast Müller matrix spectroscopy.

• DFG Programme: Research Grants