Ultrafast time-resolved terahertz spectroscopy (TRTS) allows for the direct probing of charge carriers and quasi-particles in semiconductors. The sensitivity of the technique to both the carrier mobility and its density can help elucidate the mechanisms of their temporal evolution. The use of gas photonics provides short, ultra-broadband THz pulses and, thus, offers an improved time-resolution.

Here, we present a study of the early steps of the charge carrier dynamics in lead halide perovskite thin films from the point of view of the THz mobility. Taking advantage of a 200 fs time-resolution, we were able to temporally follow the cooling of hot carriers through the observation of a change in their mobility. This change is understood as resulting from a modification of the carrier effective mass [1].

Our results are compatible with a hot carrier cooling mechanism implying LO phonon emission. This is subjected to a phonon bottleneck [2], and competes with a dynamic screening process, which time-constant was identified as being due to polaron formation [3]. While the screened hot carriers take longer to cool down [4], the dynamic screening process does not produce a change in mobility when only cold carriers are involved.

Measurements applied to perovskite samples of various compositions are compared to elucidate the role of cations and anions in both processes.

[1] C. Beard, G. M. Turner and C. A. Schmuttenmaer, Phys. Rev. B, 2000, 62, 15764-15777.
[2] Fu, Q. Xu, G. Han, B. Wu, C. H. A Huan, et al., Nat. Commun., 2017, 8, 1300.
[3] Miyata, D. Meggiolaro, M. T. Trinh, P. P. Joshi, E. Mosconi, et al., Sci. Adv., 2017, 3, e1701217.
[4] P. Joshi, S. F. Maehrlein and X. Zhu, Adv. Mater., 2019, 1803054.