Impact of incident photon energy and pulse intensity on circularly polarized high harmonic generation in monolayer graphene

Circularly polarized high harmonics provide important light sources for ultrafast detection of chiral molecules and solid materials. Although the circularly polarized high harmonic generation (HHG) has been reported in many solid materials, the comprehensive study about its dependence on the driving pulse parameters is still rare. Here, by using time-dependent density functional theory calculations, we have systematically simulated the generation of high harmonics from monolayer graphene driven by circularly polarized femtosecond pulses with different photon energies and peak intensities. Our calculation results show that when the driving pulses with high photon energy or low peak intensity are adopted, circularly polarized high harmonics can be generated, in which the harmonic radiation is dominated by intraband process and the harmonic order/polarization fulfill the selection rule. However, under driving pulses with low photon energy and high peak intensity, some of low-order harmonics exhibit noise spectra and their corresponding polarizations deviate significantly from the selection rule, which is because under this condition the number of incident photons is increased and the interband transitions between the empty and occupied bands with large density of states are enhanced. Our study clarifies the conditions for the generation of circularly polarized high harmonics in graphene and provides an important reference for selecting driving pulse parameters with the knowledge of electronic band structure. We also discover the correspondence between the HHG noise spectrum and the polarization, which can serve as a simple experimental criterion on whether the interband transitions become prominent in solid materials.