Enhancement of high-order harmonic generation in two-dimensional materials by plasmonic fields

High-order harmonic generation (HHG) is responsible for high-energy photons and isolated attosecond pulses. The plasmonic fields excited by nanostructures exhibit both field enhancement and spatial dependence. And two-dimensional (2D) materials exhibit attractive features for coherent HHG. Here, by using ab initio simulations, we explore the possibility of combining 2D materials and plasmonic fields generated by nanoscale dimers to produce efficient HHG. In our scheme, we find the spatial inhomogeneity of plasmonic fields plays a key role for HHG enhancement in a wide range of the high-energy region, which enables the production of isolated attosecond pulses. By the classical trajectory simulation based on the single-electron approximation, we further analyze and confirm the role of the spatial inhomogeneity. We also investigate the HHG enhancement in bilayer 2D materials and analyze the influences of the stacking forms between two layers. According to our simulation results, different stacking forms will induce distinctive interlayer plasmons which cause significant differences in low-energy harmonics. Our exploration supplies a scheme to generate high-energy photons and ultrashort isolated pulses.