*4.1. Self-Organizing Periodic Surface Features*

The development of self-organizing periodic surface structures of microscopic feature size represents a typical phenomenon in laser materials processing. The specific morphology of the originating surface features can be controlled by the irradiation conditions with particular influence of wavelength and polarization state of the laser beam as well as laser peak fluence, number of impinging pulses and respective irradiation dose, the latter quantifying the total optical energy irradiated to the substrates per laser spot area. The great impact of the processing conditions on the characteristic feature properties is exemplified in Figure 6 by a variety of microscopic surface features originating upon femtosecond laser irradiations on mirror-polished stainless steel substrates. Figure 6b illustrates schematically the direct-write laser processing method applied to produce such radial matrixes shown in Figure 6a,c for different laser peak fluence. As indicated in this scheme, the lines 1, 2, 3, ... , n to be processed are scanned from center to edge, while the rotating angle α between the individual lines increased steadily from one line to the next. The spatial distance between the pulses within a single scanned line is determined by the chosen pulse repetition frequency and respective scan speed. In Figure 6c, this was set to 200 kHz at 1 m/s and in Figure 6a and 100 kHz and 0.2 m/s, respectively.

**Figure 6.** SEM micrographs showing ripple (**a**) and cone-like protrusion (CLP) (**c**) surface textures produced on stainless steel; the direct-write scanning procedure for the radial matrices as used for tightly mapping a variable set of pulse overlaps (**b**); magnified view on low spatial frequency LIPSS (LSFL) (**d**), micro grooves and spikes (**e**) as well as CLPs (**f**) originating at low and high peak fluence pulses of 0.7 J/cm2 and 3.7 J/cm2.

In this way, the spatial line distance along the single processed lines enlarges continuously from matrix center to outer edge that is the main advantage of the applied radial matrix method. This in turn, comprehensively overviews the potentially emerging surface features as function of irradiated dose, because the line distance varies from highly overlapping in the center to completely separated pulses at matrix edge. By doing so, a number of complex micro-scale surface morphologies could be identified in Figure 6 that have already been reported for repetitive laser pulse irradiations [38–44]. In fact, the developed self-organizing periodic surface structures can be differentiated as follows:

