Publication date: 16th July 2025
We investigate coherent exciton dynamics in CsPbBr₃ nanocrystal (NC) samples with sizes ranging from 8 to 28 nm, employing transient four-wave mixing (FWM) and time-resolved photoluminescence (TRPL) spectroscopy at 2 K. Due to inhomogeneous broadening of optical transitions in the NC ensemble, photon echoes (PE) are generated in the FWM signal. We perform two- and three-pulse PE measurements under resonant excitation to evaluate the exciton coherence time T₂ and population relaxation time T₁, respectively. We find that T₂ is limited by T₁, following the relation T₂ = 2T₁. This indicates that exciton coherence is governed by escape (energy relaxation) dynamics or exciton recombination, while elastic scattering is negligible at low temperature.
The coherent dynamics addressed in the low-energy tail of the absorption spectrum are attributed to zero-phonon excitons, which show the longest relaxation time T₁ compared to excitation at higher photon energies. For different samples with increasing NC size, this time decreases from 100 to 20 ps and is consistent with exciton lifetimes evaluated from the photoluminescence decay in TRPL experiments. The strong size dependence of the lifetime is attributed to an increase in exciton oscillator strength, implying radiative recombination with time constant τrad.
At higher photon energies within the same ensemble, exciton decay processes are accelerated due to energy relaxation via emission of optical phonons and exciton-polaron formation. The exciton-polaron formation is quantified via the decay rate γε = 1/T₁ − 1/τrad, which increases with photon energy. We show that T₂ and T₁ exhibit a consistent dependence on NC size (comparing different ensembles) and on excitation energy (within each ensemble), with the longest coherence times observed near the low-energy tail of the PE absorption spectrum.
Our results provide detailed insight into the size and energy dependence of exciton relaxation processes in perovskite NCs. In particular, we show that the homogeneous linewidth of low-energy zero-phonon excitons is determined primarily by the radiative lifetime (ℏ/τrad), which may enable future applications in quantum communication.
This research was financially supported by the Deutsche Forschungsgemeinschaft (DFG) under project coherent exciton dynamics in lead-free double perovskites. We gratefully acknowledge their support.
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