Exciton dynamics in marginally twisted WS e 2 homobilayer: Role of interlayer coupling, phonons, and intervalley scattering

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In moiré materials, excitons serve as optical probes because of the sensitivity to both valley-dependent electronic structure and many-body excitonic interactions. For angle-tuned twisted bilayers, lattice reconstruction becomes significant at marginal twist angles, raising interesting questions about the optical characteristics of excitons both in the steady-state and nonequilibrium regimes. In this work, we investigate both the steady-state and the ultrafast transient exciton behaviors in a twisted WSe2 homobilayer (t-WSe2) using reflection contrast, polarization-resolved photoluminescence, and ultrafast pump–probe spectroscopy. We report the emergence of two distinct intralayer excitons in the lattice reconstructed t-WSe2, which are used to probe local electronic asymmetries and interlayer coupling in moiré domains. These excitons possess distinct temperature- and doping-dependent valley coherence and population dynamics, arising from the asymmetric interlayer coupling. Theoretical modeling via the Lindblad master equation highlights that the pure dephasing rate increases with hole doping, attributed to the enhanced electron-hole interactions. Ultrafast degenerate pump-probe spectroscopy reveals distinct fast decaying dynamics (<1 ps) for the two intralayer excitonic absorption species, 𝑋1 and 𝑋2, where the asymmetric interlayer coupling contributes more to the faster 𝑋1 decay than 𝑋2. Optical-pump and white-light probe spectroscopy further unveils a biexponential decay, where the fast component (𝜏fast≈0.7–0.9ps) signifies rapid radiative recombination with repopulation effects in the optical light cone. The slow component (𝜏slow≈100–300ps) is linked to exciton-phonon scattering and population relaxation via interlayer breathing phonons. We also present that intervalley scattering pathways, i.e., mediated by the 𝐾⁢𝑄 transitions, are distinct from the monolayer counterpart. This work provides detailed insights into the exciton population dynamics in twisted homobilayers, highlighting the role of intervalley and exciton-phonon interactions in the transient multiple exciton complex behaviors.

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