  High harmonic generation (HHG) in solids reflects the underlying
nonperturbative nonlinear dynamics of electrons in a strong light field and is
a powerful tool for ultrafast spectroscopy of electronic structures.
Photo-carrier doping allows us to understand the carrier dynamics and the
correlations between the carriers in the HHG process. Here, we study the effect
of incoherent electron-hole pairs on HHG in an atomically thin semiconductor.
The experimentally observed response to photo-carrier doping is successfully
reproduced in numerical simulations incorporating the photo-excited carrier
distribution, excitonic Coulomb interaction and electron-electron scattering
effects. The simulation results reveal that the presence of photo-carriers
enhances the intraband current that contributes to high harmonics below the
absorption edge. We also clarify that the excitation-induced dephasing process
rather than the phase-space filling effect is the dominant mechanism reducing
the higher order harmonics above the absorption edge. Our work provides a
deeper understanding of high harmonic spectroscopy and the optimum conditions
for generating extreme ultraviolet light from solids.
