Coulomb enhancement of high harmonic generation in monolayer transition-metal dichalcogenides

High harmonic generation (HHG) in monolayer MoS$_2$ is studied using fully microscopic many-body models based on the semiconductor Dirac-Bloch equations and density functional theory. It is shown that Coulomb correlations lead to a dramatic enhancement of HHG. Especially near the bandgap enhancements of two orders of magnitude or more are observed for a wide range of excitation wavelengths and intensities. For excitation near or above the excitonic resonances the Coulomb interaction leads to spectrally broad sub-floors of the harmonics. The widths of these contributions depends strongly on the dephasing time for polarizations. For times on the order of ten femtoseconds the broadenings are similar to the Rabi energies and reach one eV at fields of about 50 MV/cm. The intensities of these contributions are about four to six orders below the peaks of the harmonics.