Attosecond three-stage formation and coherent exciton dynamics in a two-dimensional material under strong field.

Excitons play a crucial role in optical properties of two-dimensional materials. While significant progress has been made in understanding exciton dynamics on femtosecond timescales, the microscopic details of the earliest stages of exciton formation and evolution remain elusive. Here we explore the ultrafast processes of exciton formation, evolution and dissociation in monolayer hexagonal boron nitride using state-of-the-art time-dependent density functional theory simulations incorporating long-ranged interactions. We find that the exciton forms within ~2.5 fs through a three-step process: free carriers are first generated by photoexcitation; electrons and holes subsequently bind to form a metastable "exciton core"; and finally, the exciton core evolves into a fully-formed exciton. The subsequent dynamics are dominated by exciton-exciton interference, which gives rise to oscillatory electron-occupation signals. These signals serve as a predicted phase-sensitive signature, providing a theoretical basis for experimentally probing the exciton envelope phase. This interference can be further modulated and an anisotropic Mott transition is induced upon increasing laser intensity into the strong field regime.