סמינר בחומר מעובה: Driving and Decoding Structural and Polarization Dynamics in the THz Spectral Range

Coherently excited structural dynamics are a unique tool to control material properties, drive phase transitions or even access hidden phases on ultrafast time scales [1]. So far, selective THz excitation of coherent phonons has been restricted to IR-active modes, because purely Raman-active modes are symmetry protected against even intense resonant THz fields. In the first part, I will show, how we overcome this fundamental hurdle with the sum-frequency counterparts of impulsive stimulated Raman scattering [2] and ionic Raman scattering [3], which were conventionally governed by difference-frequency components of the driving field. The resulting map of photonic and ionic difference- and sum-frequency excitation completes the toolkit for THz- or MIR-driven phenomena. In the second part, I will reveal previously assumed structural dynamics in lead halide perovskites (LHPs) as polarization dressed light propagation. In recent studies, a dynamically disordered structure and anharmonic crystal lattice was suggested to be a key component for LHPs’ outstanding optoelectronic properties. Here, we develop two-dimensional optical Kerr-effect (2D-OKE) spectroscopy to spectrally resolve and disentangle contributions to the ultrafast polarization response of MAPbBr3 and its all-inorganic counterpart CsPbBr3 [4]. With this, we finally establish a unified origin of ultrafast Kerr responses in single crystal LHPs near the optical bandgap, dominated by highly dispersive anisotropic light propagation and its nonlinear mixing [4, 5]. Moreover, these 2D-OKE fingerprints can be harnessed to trace lattice anisotropy across non-trivial phase transitions or in multi-cation alloyed LHPs [6].

References:

[1] S. F. Maehrlein et al., Science Advances 4, (2018).

[2] S. F. Maehrlein et. al., Phys. Rev. Lett. 119, 127402 (2017).

[3] D. M. Juraschek, and S.F. Maehrlein, Phys. Rev. B 97, 174302 (2018).

[4] S.F. Maehrlein et al., PNAS, 118,7, e2022268118 (2021).

[5] L. Huber et al., J. Chem. Phys., 154, 9, 094202 (2021).

[6] F. Wang et al., J. Phys. Chem. Lett., 12, 20, 5016–5022 (2021).

Website: https://pc.fhi-berlin.mpg.de/tsd/