Nonlinear evolution of aluminum surface relief under multiple femtosecond laser irradiation

The electromagnetic field interference distribution on the aluminum surface during formation of laser-induced periodic surface structures (ripples) under femtosecond laser irradiation is studied. The nonlinear dependence of optical feedback on the geometric parameters of the ripples is shown to play a key role in nonlinear evolution of the relief with increasing number of pulses. The strongest optical feedback is observed for periods of ripples in the range of $\Lambda=0.65\lambda$–$0.75\lambda$ at relief modulation of $h=0.15\lambda$–$0.2\lambda$, where $\lambda$ is the laser wavelength. On the basis of the developed approach, we explain why the frequently observed femtosecond laser-induced ripples have similar periods on materials with a high value of the imaginary part and strongly negative real part of permittivity.