Ultrafast Moiré Exciton Dynamics Revealed by Two-Dimensional Coherent Spectroscopy

Abstract

Moiré superlattices of van der Waals heterostructures confine excitons into discrete energy levels in finite-sized samples, yet their internal relaxation pathways remain obscured by inhomogeneous broadening. Here, using two-dimensional coherent spectroscopy (2DCS), we directly resolve seven discrete energy levels of the moiré-confined intralayer $\mathrm{WS}_{2}$ A exciton in an hBN-encapsulated $\mathrm{WS}_{2}/\mathrm{WSe}_{2}$ heterostructure, corresponding to a confinement depth of 57.02 meV. By tracking the population of these levels over picosecond delays, we uncover a two-stage thermalization process: an effective exciton temperature drops from 171.8 to 99.3 K within 1.2 ps through rapid intra-manifold population redistribution, followed by a slower recovery toward quasi-equilibrium mediated by exciton--environment interactions. Kullback--Leibler divergence analysis confirms the progressive transition from a non-equilibrium to a Boltzmann-distributed population. Our results establish state-resolved access to moiré exciton dynamics and provide a foundation for exploring nonequilibrium many-body phenomena in moiré quantum materials.