High-order harmonic generation employing field enhancement by metallic fractal rough surfaces.

We numerically investigate high-order harmonic generation (HHG) in noble gases in the vicinity of fractal structures of metallic rough surfaces described by the restricted solid-on-solid model. The calculated intensity enhancement factors in the range of 10³ enables HHG up to the 50-th order with low pump intensity down to tens of GW/cm². The increased interaction volume of "hot spots" in the case of grazing incidence of s-polarized pump pulses leads to an efficiency of harmonics in the plateau region of about 10⁻⁷.