Ultrafast Floquet engineering of Fermi-polaron resonances in charge-tunable monolayer WSe devices.

Fermi polarons are emerging quasiparticles when a bosonic impurity immersed in a fermionic bath. Depending on the boson-fermion interaction strength, the Fermi-polaron resonances exhibit either attractive or repulsive interactions, which impose further experimental challenges on understanding the subtle light-driven dynamics. Here, we report the light-driven dynamics of attractive and repulsive Fermi polarons in monolayer WSe devices. Time-resolved polaron resonances are probed using femtosecond below-gap Floquet engineering with tunable exciton-Fermi sea interactions. While conventional optical Stark shifts are observed in the weak interaction regime, the resonance shift of attractive polarons increases, but that of repulsive polarons decreases with increasing the Fermi-sea density. A model Hamiltonian using Chevy ansatz suggests the off-resonant pump excitation influences the free carriers that interact with excitons in an opposite valley, thereby reducing the binding energy of attractive polarons. Our findings may enable coherent Floquet engineering of Bose-Fermi mixtures in ultrafast time scales.