Halide perovskite semiconductors represent an attractive system for optoelectronic and photonic applications. While traditional strategies focus mainly on photovoltaics and light emitting diodes, the potential of perovskite nanocrystals in quantum light sources has attracted significant attention. In this respect the energy structure of exciton complexes, as well as their interactions and ultrafast coherent dynamics after excitation with femtosecond pulses represent particular interest. Conventional time-integrated reflectivity or photoluminescence techniques alone often fail to provide clear conclusions about the energy structure due to the inhomogeneous broadening of optical transitions and the complex dynamics of photoexcited carriers. Here, nonlinear optical techniques based on photon echoes or two-dimensional Fourier spectroscopy offer unique insights into the coherent dynamics of excitons in perovskite semiconductors.

We study the coherent dynamics of excitons in halide perovskite semiconductors of different compositions and dimensionality. Due to the inhomogeneous broadening of optical transitions, coherent optical response is represented by photon echoes even in bulk perovskite crystals. In mixed (FA,Cs)Pb(Br,I)₃ crystals, the magnitude of fluctuations of the energy bandgap is in the order of 10-20 meV. Here, we observe exceptionally long exciton coherence times up to 80 ps at a low temperature of 2 K due to the localization of excitons at the scale of tens to hundreds of nanometers [1]. In this case, the homogenous line of 16 µeV is about three orders of magnitude smaller than the inhomogeneous broadening of optical transitions. This allows us to evaluate fine structure splitting between bright and dark excitons of 0.5 meV by analyzing the quantum beats between the exciton spin states in an external magnetic field. Next, the role of exciton-exciton interactions in bulk crystals is evaluated. Here, polarization-resolved transient signals provide rich information about the biexciton binding energy and spin-dependent interactions in dense exciton ensembles [2,3]. Finally, we study coherent optical response from lead-halide nanocrystals where quantum beats in the photon echo signal are observed due to excitons fine structure and interaction with optical phonons. Specifically, we demonstrate a previously unexplored regime of coherent exciton dynamics with coherence times approaching ~300 ps in CsPbI₃ nanocrystal ensembles, revealed through quantum beats between exciton–polaron states [4].