Detection and analysis of photo-acoustic emission in Direct Laser Interference Patterning

Tobias Steege,Andrés Fabián Lasagni,Sabri Alamri,Tim Kunze

doi:10.1038/s41598-021-93927-w

Abstract

Functional laser texturing by means of Direct Laser Interference Patterning is one of the most efficient approaches to fabricate well-defined micro textures which mimic natural surfaces, such as the lotus effect for self-cleaning properties or shark skin for reduced friction. While numerous technical and theoretical improvements have been demonstrated, strategies for process monitoring are yet to be implemented in DLIP, for instance aiming to treat complex and non-plane surfaces. Over the last 35 years, it has been shown that the sound pressure generated by a laser beam hitting a surface and producing ablation can be detected and analysed using simple and commercially available transducers and microphones. This work describes the detection and analysis of photo-acoustic signals acquired from airborne acoustic emission during DLIP as a direct result of the laser–material interaction. The study includes the characterization of the acoustic emission during the fabrication of line-like micro textures with different spatial periods and depths, the interpretation the spectral signatures deriving from single spot and interference ablation, as well as a detailed investigation of the vertical extent of the interference effect based on the ablated area and its variation with the interference period. The results show the possibility to develop an autofocusing system using only the signals from the acoustic emission for 3D processing, as well as the possibility to predict deviations in the DLIP processing parameters.

Highlights

Laser based tools for micromachining and surface functionalization provide today enormous potentials for the processing of various materials
These monitoring approaches are nowadays slowly adapted into functional laser texturing, especially for already well-established approaches such as Direct Laser Writing (DLW), where a single laser beam is scanned over the material surface, creating microstructures through the ablation process[9]
The Direct Laser Interference Patterning (DLIP) setup employed in this work is a compact optical module consisting of diffractive, refractive and focusing optics, by which a main infrared laser beam can be split into two sub-beams and microscopic line-like patterns with variable size can be generated

Summary

Introduction

Laser based tools for micromachining and surface functionalization provide today enormous potentials for the processing of various materials. In the field of laser welding and drilling, process monitoring includes the analysis of the mid- and long wave infrared radiation via infrared cameras or pyrometers for investigating the heat development, CCD and CMOS cameras to capture process lighting, as well as microphones for recording the process noise[8] These monitoring approaches are nowadays slowly adapted into functional laser texturing, especially for already well-established approaches such as Direct Laser Writing (DLW), where a single laser beam is scanned over the material surface, creating microstructures through the ablation process[9]. Due to the nature of its working principle, microstructures which tend not to reflect light (e.g. due to surface oxidation or large structure depth) can be hardly detected through this optical method Complementary to this technique, scatterometry can be utilized as quality monitoring for surfaces functionalised through DLIP28. It has to be mentioned that all the above-mentioned and explored process monitoring approaches for DLIP, are based mainly on the evaluation of the quality of a periodic pattern and do not gain any information from the ablation process itself

Methods

Results

Conclusion