Ultrafast dynamics of the valley-polarized excitonic correlated states in WS₂/WSe₂ moire superlattice heterostructures

Moiré superlattices in van der Waals heterostructures are emerging as transformative platforms for probing complex quantum phenomena. Upon optical excitation, these structures generate charge-neutral interlayer excitons with out-of-plane electric dipoles, fostering strong onsite dipole-dipole interactions that can form exotic correlated bosonic states, analogous to Mott states in electronic systems. In this study, we employ polarization-dependent, time-resolved optical spectroscopy to explore and manipulate exciton and valley dynamics within hBN-encapsulated WS₂/WSe₂ moiré superlattices. Using a femtosecond optical pump resonant with the A-exciton of WSe₂, we observe a pronounced new resonance approximately 18 meV above the ground state, triggered only when exciton density exceeds one per moiré lattice site. This resonance signals the emergence of a strongly correlated exciton state, where two excitons are confined within a single moiré unit cell, exhibiting a significantly enhanced radiative recombination rate compared to the singly occupied state, and retaining strong valley polarization. Below the one-exciton-per-site threshold, the exciton recombination rates remain largely independent of exciton density, while valley dynamics display a distinct progression: as initial exciton density approaches one per moiré site, the valley recombination rate first increases, then decreases, and finally stabilizes. These findings reveal an unprecedented level of optical control over exciton-exciton correlations and valley dynamics, advancing the tunability of quantum states in engineered moiré superlattices.