Half-light and half-matter exciton polaritons have demonstrated profound impacts on coherent quantum phenomena. Two-dimensional (2D) organic−inorganic perovskite semiconductors, exhibiting large exciton binding energy and spin-based behavior, are an excellent platform for the study of exciton polaritons at room temperature. However, the implementation of these fascinating coherent phenomena is still constrained by the lack of an understanding of the crucial polariton-polariton interaction effects. Here, an experimental observation of spin-dependent polariton-polariton interactions in a prototype 2D organic−inorganic perovskite microcavity by circularly polarization-resolved transient absorption (TA) measurements is reported. A power-dependent blue shift of the lower polariton bands with the same spin is clearly revealed, whereas the energy band of the polariton bands with the opposite spin is almost not changed. Spectacularly, it is found that the energy band blue shift mainly takes place before the spin depolarization, which provides accurate evidence for the spin-dependent polariton-polariton interactions. Furthermore, the strong coherent polariton-phonon coupling of perovskite microcavity has also been detected firstly in ultrafast TA dynamics. These results demonstrate that microcavity polaritons inheriting the matter properties from their excitonic constituents exhibit strong nonlinear interactions and pave the way for realizing coherent quantum phenomena at room temperature.
Abstract2D lead halide perovskites (LHPs) show strong excitonic and spin‐orbit coupling effects, generating a facile spin injection. Besides, they possess a polaron character due to the soft crystal lattice, which can prolong the spin lifetime, making them favorable materials for spintronic applications. Here, the spin dynamics of 2D PEA2PbI4(MAPbI3)n−l thin films with different layers by temperature‐ and pump fluence–dependent circularly polarization‐resolved transient absorption (TA) measurements is studied. These results indicate that the spin depolarization mechanism is gradually converted from the Maialle–Silva–Sham (MSS) mechanism to the polaronic states protection mechanism with the layer number increasing from <n> = 1 to 3, which is determined by the interplay between the strength of Coulomb exchange interaction and the strength of polaronic effect. While for <n> ≥ 4, the Elliot–Yafet (EY) impurities mechanism is proposed, in which the formed polaronic states with free charge carriers no longer play the protective role.
Van der Waals (vdW) heterostructures (HSs) built on 2D materials provide an ideal platform for research of energy migration at the nanoscale. However, the underlying charge transfer mechanism in type II vdW HSs is still not well understood. Here, ultrafast exciton dynamics are investigated in trilayer-WS 2 -MoS 2 -WSe 2 and trilayer-MoS 2 -WSe 2 -WS 2 HSs by broadband pump-probe spectroscopy. A two-step process of exciton transfer in trilayer-WS 2 -MoS 2 -WSe 2 is directly observed when the band edge exciton of WSe 2 is excited. The electrons in WSe 2 are initially transferred to the high lying electronic state of MoS 2 -WS 2 on a time scale of tens of femtoseconds, and then electrons eventually relax into the conduction band minimum of MoS 2 within 1 ps. Furthermore, the transfer of interlayer excitons is observed for the first time in trilayer-MoS 2 -WSe 2 -WS 2 . Both transfer processes can be better understood by the Dexter charge exchange model. Due to the nature of Dexter type transfer that the exchange rate exponentially depends on the donor−acceptor distance, the interlayer exciton transfer rate is nearly a hundred times slower than that of exciton transition in bilayer HSs. The results deepen the understanding of charge transfer in 2D vdW HSs and also indicate that the exciton effect and orbital hybridization make HS a strong coupling system.
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