During recent years, the picture of dynamic charge carrier screening in lead halide perovskites (LHPs) evolved to explain long carrier lifetimes and diffusion lengths, leading to the nowadays famous optoelectronic properties of this material class. However, the most important prerequisite for such anticipated structural screening, the direct observation of related lattice- or organic cation motion within the estimated polaron formation time (<1ps), remains elusive. By developing two-dimensional optical Kerr effect (2D-OKE) spectroscopy, we unveil that the ultrafast polarization response to near-bandgap excitation pulses is overwhelmed by the instantaneous electronic polarizability [1]. In this case, the nonlinear mixing of highly anisotropic and dispersive light propagations is responsible for oscillatory Kerr-type modulations at terahertz (THz) frequencies [1, 2]. Therefrom-arising 2D-OKE fingerprints can be harnessed to trace lattice anisotropy across non-trivial phase transitions or in multi-cation alloyed LHPs [3]. Despite this useful characterization of static lattice distortions, the dynamic lattice response seems elusive for the OKE with excitation pulses in the visible spectral range.

Ultimately, to study the ultrafast lattice response to a transient electric field, we move the excitation to the THz spectral region, resonant to the optical modes of the lead halide lattice, by employing intense phase-stable THz fields. Conceptually, this close to single-cycle electric field with a rise-time below 1 ps, can be seen as proxy for ultrafast electron-hole separation during optical excitation and consecutive thermalization. By probing the THz-induced Kerr effect (TKE), we observe a strong THz polarizability in both inorganic CsPbBr₃ and in the hybrid MAPbBr₃ materials. Also here, we show that it is crucial to account for dispersion and optical anisotropy in interpreting the transient responses via rigorous four-wave mixing simulations. Nevertheless, we finally observe a contribution from the lattice polarizability by witnessing the nonlinear excitation of coherent phonons, in full agreement with static Raman spectra. Moreover, we pinpoint a single phonon mode which dominates the lattice polarizability, providing another stepping stone towards a better understanding of how lattice dynamics enable charge carrier protection in this class of defect tolerant semiconductors.

References:

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

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

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