Modeling laser-induced periodic surface structures: an electromagnetic approach

This thesis presents and discusses laser-induced periodic surface structures (LIPSSs), as well as a model explaining their formation. LIPSSs are regular wavy surface structures with dimensions usually in the submicrometer range, which can develop on the surface of many materials exposed to laser radiation. The most common type of LIPSSs, which can be produced with continuous wave lasers or pulsed lasers, have a periodicity close to the laser wavelength and a direction orthogonal to the polarization of the laser radiation. They are usually referred to as low spatial frequency LIPSSs (LSFLs). It is generally accepted that these LIPSSs are the result of the interaction of the laser radiation with the rough surface of the material. Since the early 2000s, with the increasing availability of picosecond and femtosecond laser sources, LIPSSs with a periodicity significantly smaller than the laser wavelength and an orientation either parallel or orthogonal to the polarization have been reported in literature. These LIPSSs, referred to as high spatial frequency LIPSSs (HSFLs), renewed the interest of researchers in the topic for mainly two reasons. First, from a practical point of view, HSFLs show a strong potential for surface nanostructuring due to their small dimensions. While, from a theoretical point of view, the electromagnetic theory adopted to explain LSFL formation fails at accounting for the formation of all HSFLs. Other LIPSSs with a periodicity larger than the laser wavelength and an orientation either parallel or orthogonal to the polarization, referred to as grooves, were also reported. In this thesis, it is shown that the formation of any kind of LIPSSs can be understood in the frame of an electromagnetic approach. The model, predicting the formation of LSFLs, HSFLs and grooves, is based on the time dependent Maxwell’s curl equations, which are solved numerically using the finite-difference time-domain method. The outcome is analyzed in the space domain, as well as the frequency domain, and compared to experimental results