Nonlinear absorption of femtosecond laser on a metal surface embedded by metallic nanoparticles

Metal surface with nanosize roughness is modeled as a plane surface embedded with clusters. A high power femtosecond laser pulse normally incident on the metal surface undergoes field enhancement when laser frequency equals the frequency of surface charge oscillations of the clusters. The enhanced field causes increased absorption of the laser pulse. The model suggests that the increase in absorption decreases with the increase of the laser power. The results are in line with recent observations.