Electron recoil effect in electrically tunable MoSe2 monolayers

Radiative recombination of excitons dressed by the interactions with free charge carriers often occurs under simultaneous excitation of either electrons or holes to unbound states. This phenomenon, known as the electron recoil effect, manifests itself in pronounced, asymmetric spectral line shapes of the resulting emission. We study the electron recoil effect experimentally in electrically tunable monolayer semiconductors and derive it theoretically using both trion and Fermi polaron pictures. Time-resolved analysis of the recoil line shapes is employed to access transient, nonequilibrium states of the exciton-carrier complexes. We demonstrate cooling of the initially overheated populations on picosecond timescales and reveal the impact of lattice temperature and free carrier density. Both thermally activated phonons and the presence of free charges are shown to accelerate equilibration. Finally, we find similar values of relaxation times from recoil analysis and luminescence rise times, providing additional insight into trion dynamics.