Stability of Thin Films and Stress from Ultrafast Laser-Induced Nanograting Structures

Abstract

Ultrafast laser stress figuring (ULSF) is a non-contact technique for correcting low spatial frequency surface figure error of thin transparent mirror substrates, even after coatings are applied. Ultrafast laser pulses focused into the substrate material generate stress from nanograting structures formed within the bulk substrate, creating bending moments. The stability of ultrafast laser generated modifications (nanogratings) has been studied via stress birefringence, but the stability of the stress state produced by nanogratings has not been thoroughly investigated. For ULSF to be used as a post-coating figuring process, both the ultrafast laser generated stress and the film stress must be stable. This thesis presents the results of three experiments to measure the stability of ultrafast laser-generated stress and stress in thin films. The first experiment was an isochronal annealing study up to 1000 °C on uncoated fused silica and Corning ultra-low expansion (ULE) glass figured with astigmatism, which was found to be stable up to 500 °C. We measured only small changes in astigmatism after each thermal cycle, and the direction of relaxation may have depended on the material composition local to the modifications, coupled with thermal expansion mismatch and point defect erasure. After thermal cycling beyond 500 °C, stress either increased due to densification or decreased due to nanograting erasure. The second experiment was a long-term room temperature stability study on ULSF-flattened, coated mirrors. The small changes in power measured after 2-3 years are likely due to the coating’s sensitivity to differences in ambient temperature between measurements. In the third experiment, fused silica substrates coated with an antireflective film were annealed and then thermally cycled to investigate coating stress relaxation. High temperature annealing produced the most stable substrates without significantly increasing reflectance. Finally, the results of all three experiments are summarized with recommendations for new avenues to measure stability and ultrafast laser generated stress.