Observations of the morphology of laser-induced damage in copper mirrors

Abstract

The results of multiple-pulse damage tests on copper mirrors using 1.7-ns CO2 lasers are reported. The measured reduction in the brightness reflectivity of the mirrors is shown to be correlated to the dramatic appearance of fine scale microstructure on the mirror surface. Scanning electron micrographs of this surface structure are presented and possible explanations of the effects are discussed.

Growth Twinning and Generation of High-Frequency Surface Nanostructures in Ultrafast Laser-Induced Transient Melting and Resolidification

“…Micro and nanostructural changes strongly impact surface properties such as optical reflectivity, hydrophobicity, hardness, wear and corrosion resistance, potentially impacting the use of the treated materials. − Laser-induced surface modification is the result of material response to the extreme conditions of rapid highly localized heating and cooling created in the surface region of irradiated targets by ultrafast laser irradiation. Under multipulse exposition, high densities of crystal defects and subsurface voids can be accumulated during successive thermomechanical loading of the material leading to irreversible surface damage and ablation. − …”

“…At low fluences, in the “submelting” regime, laser irradiation could cause material damage through thermal stresses . After repetitive irradiation, the accumulated damage manifests itself through the formation of slip bands, increase in the surface roughening, and corresponding reflectivity changes. , At higher fluences, a thin surface layer may undergo rapid melting followed by epitaxial resolidification, leading to the generation of a high density of crystal defects, such as vacancies, dislocations, stacking faults and twin boundaries. ,, Further increase in laser fluence can lead to more extensive microstructural changes and modifications of surface morphology, including the generation of subsurface voids trapped by rapidly advancing solidification front, formation of a nanocrystalline surface layer, or surface nanospikes featuring pentagonal twinned structural elements. , The microstructural changes in this regime can extend down to a substantial depth under the irradiated surface, with laser-induced shear stresses producing deformation twins and dislocations far below the region experiencing laser melting and ablation.…”

“…13 After repetitive irradiation, the accumulated damage manifests itself through the formation of slip bands, increase in the surface roughening, and corresponding reflectivity changes. 14,15 At higher fluences, a thin surface layer may undergo rapid melting followed by epitaxial resolidification, 23 leading to the generation of a high density of crystal defects, such as vacancies, dislocations, stacking faults and twin boundaries. 16,24,25 Further increase in laser fluence can lead to more extensive microstructural changes and modifications of surface morphology, including the generation of subsurface voids trapped by rapidly advancing solidification front, 17 formation of a nanocrystalline surface layer, or surface nanospikes featuring pentagonal twinned structural elements.…”

Modeling Laser Effects on the Final Optics in Simulated IFE Environments

“…This is especially true for high-quality metal mirrors. The most frequently cited data is for Cu and Ag (Koumvakalis, Lee and Bass, 1983b;Thomas, Harrison and Figueira, 1982), Mo (Becker, Ma and Walser, 1990b), and Cu & Al in (Jee, Becker and Walser, 1986). Here we listed some main experiments, which have been more quoted in the literature in Table (1) below.…”

Inertial-confinement fusion-reactor dry-wall study. Final report, 13 August 1981-31 March 1983. Report WAESD-TR-83-0010

“…The latter is particularly suited to the present problem not only because it permits measurements on a o scale of 2 X 10" sec, but also because it eliminates many of the problems encountered in cw holography.HOLOGRAPHIC INTERFEROMETRYA hologram is a recording, usually on a high resolution photographic plate, of the stationary interference pattern produced by the interference of light coming from a reference beam and that produced by the object in question, called the "scene" or "subject" beam. Because the hologram is itself an interference pattern, it is important to differentiate between the 0166R-15R:2 (S3926)15…”