Densification and residual stress induced by CO2 laser-based mitigation of SiO2 surfaces

M. D. Feit; M. J. Matthews; T F. Soules; J. S. Stolken; R. M. Vignes; S. T. Yang; J. D. Cooke

doi:10.1117/12.867727

30 November 2010 Densification and residual stress induced by CO₂ laser-based mitigation of SiO₂ surfaces

M. D. Feit, M. J. Matthews, T F. Soules, J. S. Stolken, R. M. Vignes, S. T. Yang, J. D. Cooke

Author Affiliations +

Proceedings Volume 7842, Laser-Induced Damage in Optical Materials: 2010; 78420O (2010) https://doi.org/10.1117/12.867727
Event: Laser Damage Symposium XLII: Annual Symposium on Optical Materials for High Power Lasers, 2010, Boulder, Colorado, United States

Abstract

Knowing the ultimate surface morphology resulting from CO₂ laser mitigation of induced laser damage is important both for determining adequate treatment protocols, and for preventing deleterious intensification upon subsequent illumination of downstream optics. Physical effects such as evaporation, viscous flow and densification can strongly affect the final morphology of the treated site. Evaporation is a strong function of temperature and will play a leading role in determining pit shapes when the evaporation rate is large, both because of material loss and redeposition. Viscous motion of the hot molten material during heating and cooling can redistribute material due to surface tension gradients (Marangoni effect) and vapor recoil pressure effects. Less well known, perhaps, is that silica can densify as a result of structural relaxation, to a degree depending on the local thermal history. The specific volume shrinkage due to structural relaxation can be mistaken for material loss due to evaporation. Unlike evaporation, however, local density change can be reversed by post annealing. All of these effects must be taken into account to adequately describe the final morphology and optical properties of single and multiple-pass mitigation protocols. We have investigated, experimentally and theoretically, the significance of such densification on residual stress and under what circumstances it can compete with evaporation in determining the ultimate post treatment surface shape. In general, understanding final surface configurations requires taking all these factors including local structural relaxation densification, and therefore the thermal history, into account. We find that surface depressions due to densification can dominate surface morphology in the non-evaporative regime when peak temperatures are below 2100K.

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M. D. Feit, M. J. Matthews, T F. Soules, J. S. Stolken, R. M. Vignes, S. T. Yang, and J. D. Cooke "Densification and residual stress induced by CO₂ laser-based mitigation of SiO₂ surfaces", Proc. SPIE 7842, Laser-Induced Damage in Optical Materials: 2010, 78420O (30 November 2010); https://doi.org/10.1117/12.867727

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