Influence of surface roughness on harmonic yield from dielectric targets

High harmonic generation (HHG) from thin films has become a promising tool to reflect the electronic state of bulk materials, to measure laser CEP in bulks, and to serve as a new type of coherent attosecond source for ultrafast studies. However, most of the theoretical models are based on highly idealized assumptions, including atomically flat surfaces, perfectly homogeneous films, and either the absence of a substrate or the presence of an optically inactive one. In this work, we present a comprehensive model for HHG from dielectric thin films that explicitly incorporates surface rough- ness and multilayer geometries. The film is modeled as a two-layer structure—comprising a bulk region and a rough surface layer with distinct optical properties, the latter derived via the Bruggeman effective medium approximation. Assuming a spatially uniform electric field, HHG is computed individually for each sublayer using their respective thickness and complex conductivity. Our results show that roughness can significantly modify the thickness dependent harmonic yield predicted by models using idealized layers. These findings highlight the importance of nanoscale morphology and layer ordering in the optical modeling of HHG in ultrathin films and provide a practical framework for interpreting experimental data.