High-order harmonic generation from two-dimensional materials subjected to intense laser fields

Based on the real-time time-dependent density functional theory, we theoretically investigate the influence of bandgap on the high-order harmonic generation (HHG) from monolayer hexagonal two-dimensional (2D) solids: Gallium Phosphide (GaP), Graphene, Borophene (graphene-like), and Boron nitride (h-BN) under a few-cycle linearly- and/or single circularly-polarized laser pulse. Our results show that interband currents are prominently larger in the zigzag (ZZ) direction in comparison with the armchair (AC) direction, when the laser field is polarized along the ZZ-direction. Accordingly, the high-order harmonics can be produced more efficiently along the ZZ-direction than that of the AC-direction. We exhibit that single-layer 2D materials can generate bulk-like high-order harmonics when they are driven by an in-plane polarized laser field, and atomic-like harmonics when driven by an out-of-plane polarized laser field. Our findings indicate that due to the difference in the effective mass of carriers along AC- and ZZ-directions, the high-order harmonics spectra are different in both directions. In addition, the results illustrate that the dependence of HHG intensity changes according to the polarization of the laser electric field. The bandgap significantly affects the HHG, most importantly through ultrafast modification of the interband polarization of the system. Finally, based on the present study, borophene and GaP have outstanding potential for future utilization in extreme-ultraviolet, efficient table-top HHG sources, and as an ultrafast optical tool to provide possibilities for imaging solid structures.