# Monitoring of Evolving Laser Induced Periodic Surface Structures
## 2 Experimental  
Experiments have been performed at the Max-Born-Institute High Field Ti:Sapphire laser. In the current work we investigated nanostructuring of 1 \(\upmu\)m thick titanium and Si\({}_{3}\)N\({}_{4}\) foils within a proton acceleration experiment. Details for the laser acceleration experiments are described elsewhere [27]. Here, we concentrate only on the nanostructuring part. The pulse duration of the p-polarized laser is about (30-35) fs. The experimental setup is shown in Figure 1.  
After carefully adjusting the target position, such that the focal plane of the off-axis parabolic mirror (OAP) lies in the target plane (focal distance = 15 cm), an aperture is introduced into the beam path and reduces the beam diameter to \(\sim\)6 mm. The energy on target was carefully measured and could be varied by changing a laser amplifier delay. Typically, we used energies of about 10-30 \(\upmu\)J. With the reduction of beam diameter, the focal spot size increases to FWHM of about 25 \(\upmu\)m and the spatial profile shows the typical Airy pattern. Under these conditions we were able to generate LIPSS in the central part of the focal spot. The intensity in the Airy-rings was not sufficiently high in order to induce ripple formation. LIPSS were characterized both by scanning electron microscopy (SEM) and atomic force microscopy (AFM). Typical results are shown in Figure 2. SEM and AFM also revealed that the initial surface quality of the samples differs significantly. While Si\({}_{3}\)N\({}_{4}\) has a nearly perfect optical surface, the titanium foils are relatively rough. In agreement with others, LIPSS in Ti have a periodicity of \(\sim\)600-800 nm, a height of \(\sim\)200 nm [3; 32] and an orientation perpendicular to the polarization vector of the laser field. However, in contrast to those works, relatively low pulse numbers (5-50) and relatively high fluences (2-6 J/cm\({}^{2}\)) were applied. Furthermore, our laser pulse duration was shorter and the experiments were performed in vacuum. LIPSS on Si\({}_{3}\)N\({}_{4}\) have a lower periodicity of \(\sim\)250 nm and a lower height of about 50 nm but the same orientation with respect to the laser field polarization vector. Occasionally, we have also observed larger structures in Si\({}_{3}\)N\({}_{4}\) but those structures are usually restricted to a small part of the structured region. Here, the periodicity is again about 800 nm and the height is about 140 nm. Our setup comprises a backreflection imaging system: Backscattered light is collected and collimated by the same off-axis parabolic mirror that focuses the laser pulse onto the target. Part of this backscattered light leaks through a dielectric mirror (M2 in Figure 1) and is refocused onto a CCD camera (PIXIS with (25 \(\upmu\)m)\({}^{2}\) quadratic pixels) by a lens (focal distance of 150 cm). Thus, the imaging system has a magnification of 10\(\times\). Mirror M2 may also be replaced by a beam splitter. In our particular setup the mirror was required to deliver highest possible pulse energies to the sample for the laser acceleration experiments.  
Figure 1: Experimental setup. Backscattered light from the target is collected by an off-axis parabola (OAP). Part of it is leaking through a mirror (M2) and is focused by a lens onto a CCD camera.  
Figure 2: SEM (**a**) and AFM images of nanostructured titanium foil (**b**) and AFM image of nanostructured Si\({}_{3}\)N\({}_{4}\) (**c**).  
With the onset of LIPSS generation, our backreflection changes dramatically: a secondary diffraction pattern (in addition to the Airy pattern due to the aperture) appears with LIPSS (cf. Figure 3). The fringe distance is 3-4 px, corresponding to 75-100 \(\upmu\)m in the image plane. We do not observe a significant difference between the fringe patterns for titanium or Si\({}_{3}\)N\({}_{4}\) samples.