Incoherent dark excitons extend Floquet-like physics across time and momentum
Floquet phenomena - where a coherent, time-periodic drive hybridizes the conduction band with energy-replicated copies of the valence band - have been demonstrated with intense optical fields and, more recently, other bosonic excitations. However, within mean-field approximations and the current understanding of Floquet phenomena, these effects survive only for the coherence time of the drive, limiting practical use and current demonstrations to the sub-picosecond regime. Here, consistent with recent theoretical predictions beyond the mean-field picture, using time- and angle-resolved photoemission spectroscopy in WSe$_2$/MoS$_2$ heterostructures, we show that incoherent, dense populations of dark interlayer excitons hybridize the conduction band with energy-replicated copies of the valence band, mimicking Floquet-like phenomena - now persisting for nearly a nanosecond. With increasing interlayer exciton density, the valence band and its replica evolve from an inverted-parabolic to a Mexican-hat dispersion, a signature of the predicted non-zero static coupling that hybridizes the bands and scales linearly with the exciton density. By tuning twist angle, we further show that dark excitons with net momentum create replicas that hybridize with bands offset not just in energy but also in momentum, providing another powerful degree of freedom beyond conventional Floquet engineering. These results establish a robust, long-lived route to engineering electronic states, beyond a coherent, zero-momentum drive.