# Monitoring of Evolving Laser Induced Periodic Surface Structures
## 6 Conclusions  
In this work we have described an optical setup which is capable to monitor the appearance of laser induced periodic surface structures. In particular, we have shown that the generation of LIPSS on titanium and Si\({}_{3}\)N\({}_{4}\) surfaces give rise to a characteristic fringe pattern in the backscattered light of the patterning laser. This fringe pattern was experimentally observed and very well reproduced by Lumerical simulations using finite difference time domain method. They clearly show that the fringe pattern is unambiguously related to the periodic surface structure. Nevertheless, some questions remain open. We could not yet unambiguously resolve the role of carrier density. And also the fact that the very different LIPSS periods in Si\({}_{3}\)N\({}_{4}\) and titanium lead to quite similar fringe pattern is surprising. The described backreflection diagnostics is therefore a very simple way to monitor the generation of LIPSS with sub- laser wavelength periodicity _during_ the structuring process. This on-line monitoring technique will allow fabricating and using nanostructured targets/samples within a single experimental step, that is, without time-demanding sample characterization. One very promising application could for instance be the use of nanostructured targets for high-repetition rate laser ion acceleration (e.g., in radiation therapy): ions are accelerated by laser pulses (e.g., at 10 Hz) making use of tape targets. In between the individual high energy laser pulses, a high repetition rate laser can structure the sample. Its backreflection gives information about the presence of the structure.  
Figure 8: New LIPSS-induced angular components to the backscattered light are focused in front of the original image plane. Interference between the original and LIPSS-induced beam in the detector plane gives rise to the experimentally observed fringe pattern.  
**Author Contributions:** Conceptualization, A.L., Z.P. and M.S.; methodology, A.L., Z.P. and M.S.; validation, A.L., Z.P. and M.S.; formal analysis, A.L. and Z.P.; investigation, A.L., Z.P. and M.S.; writing--original draft preparation, A.L.; writing--review and editing, A.L., Z.P. and M.S.; visualization, A.L. and Z.P.; funding acquisition, Z.P. and M.S.  
**Funding:** This research was funded by Deutsche Forschungsgemeinschaft (project--Relativistic Nano-Plasma Photonic SCHN953/2-1) and National Office for Research, Development and Innovation (FK 128077).  
**Acknowledgments:** We thank [PERSON] (University Oldenburg), [PERSON] (MBI), [PERSON] (MBI) and [PERSON] (BAM) for extensive and fruitful discussion. In particular, we thank [PERSON] for triggering our collaboration and [PERSON] for providing additional samples. MS acknowledges funding from the Deutsche Forschungsgemeinschaft (project--Relativistic Nano-Plasma Photonic SCHN953/2-1). ZP thanks the support from National Office for Research, Development and Innovation (FK 128077).  
**Conflicts of Interest:** The authors declare no conflict of interest. The funders had no role in the design of the study; in the collection, analyses, or interpretation of data; in the writing of the manuscript, or in the decision to publish the results.