Abstract
Fused silica optics often exhibit surface scratches after polishing that radically reduce their damage resistance at the wavelength of 351 nm in the nanosecond regime. Consequently, chemical treatments after polishing are often used to increase the damage threshold and ensure a safe operation of these optics in large fusion-scale laser facilities. Here, we investigate the reasons for such an improvement. We study the effect of an HF-based wet etching on scratch morphology and propose a simple analytic model to reflect scratch widening during etching. We also use a finite element model to evaluate the effect of the morphological modification induced by etching on the electric field distribution in the vicinity of the scratch. We evidence that this improvement of the scratch damage resistance is due to a reduction of the electric field enhancement. This conclusion is supported by secondary electron microscopy (SEM) imaging of damage sites initiated on scratches after chemical treatment.
Highlights
Fused silica optics often exhibit surface scratches after polishing that radically reduce their damage resistance at the wavelength of 351 nm in the nanosecond regime
Fused silica optics located in the final optic assembly of high-power fusion-class laser facilities, such as the National Ignition Facility[1], SG-III2 or the Megajoule laser[3], are subjected to fluence in the 5 to 14 J/cm[2] range at the wavelength of 351 nm, with pulse duration of a few nanoseconds
The intentionally scratched surfaces were characterized by optical microscopy after the Beilby layer removal and after deep wet etching using an HF/HNO3 mixture
Summary
Fused silica optics often exhibit surface scratches after polishing that radically reduce their damage resistance at the wavelength of 351 nm in the nanosecond regime. It is efficient to remove the Beilby layer and metallic impurities embedded in this layer and beyond[8,9], the concentration of which is higher in surface defects such as scratches[10] It changes the scratch morphology by opening narrow fractures, which lead to an improvement of the damage threshold[11,12]. After chemical etching of more than approximately one micrometer, the fused silica optics surface is almost free of metallic impurities (Ce, Al, Cu, Fe, etc.) induced by the polishing process[8,10] Such precursors are unlikely to be responsible for the damage initiation in etched scratches. Laser damage tests on etched scratches were performed to assess the predominance of field enhancement in the damage process
Sign-in/Register to view highlights & summary Sign-in/Register to access full text options 
Talk to us
Join us for a 30 min session where you can share your feedback and ask us any queries you have
Schedule a callDisclaimer: All third-party content on this website/platform is and will remain the property of their respective owners and is provided on "as is" basis without any warranties, express or implied. Use of third-party content does not indicate any affiliation, sponsorship with or endorsement by them. Any references to third-party content is to identify the corresponding services and shall be considered fair use under The CopyrightLaw.
