Title: Ultrafast laser micromachining for stress-based figure correction of thin mirrors
Speaker: Prof. Heng Zuo, PhD, Department of Mechanical Engineering University of New Mexico
Abstract: The fabrication of many high-resolution thin-shell mirrors for future space telescopes remains challenging, especially for revolutionary mission concepts like NASA’s Lynx X-ray Observatory. It is generally harder to fabricate thin mirrors to the exact shape than thicker ones, and the coatings deposited onto mirror surfaces to increase the reflectivity typically have high intrinsic stress which deforms the mirrors further. On the other hand, rapid developments of ultrafast laser technologies in the past three decades have enabled high-accuracy high-throughput material processing and structuring with micrometer resolution. In this talk, I will discuss our efforts in figure correction and stress compensation of thin-shell mirrors by implementing ultrafast laser micromachining with stress-based figuring technique. We employ a laser to selectively remove regions of a stressed film that is grown onto the back surface of the mirror, to modify the stress states of the mirror. By using simple optical setups with a scanning X-Y stage, we have shown that both equibiaxial and general biaxial stress fields can be generated with laser micromachined features on thin flat silicon mirrors with silicon oxide films. We also built a finite element model to simulate the laser-induced stresses and resulting shape changes of thin mirrors due to the periodic patterning of a stressed film on silicon substrates using ultrafast laser ablation, and the numerical predictions compare reasonably well with curvature measurements of the patterned troughs achieving > 82% quantitative agreement. I will also present strength testing analysis to show how this process minimally affects the strength of mirrors treated with ultrafast laser micromachining. These developments are beneficial to the high-throughput figure correction of thin-shell mirrors for space-based telescopes and other types of thin substrates used in the semiconductor industry.