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Thermomechanical stress degradation of metal mirror surfaces under pulsed laser irradiation

Progressive degradation of the optical surface of metal mirrors in pulsed laser applications can result from cumulative plastic deformation under thermo-mechanical stress caused by relatively small temperature excursions during irradiation. An analysis of the transient stress-strain behavior of the near-surface region shows that elastic response is maintained below a limiting surface temperature rise (ΔT y ), which depends on the yield stress, modulus of elasticity, Poisson's ratio, and coefficient of thermal expansion of the metal. This temperature rise defines the threshold for first-pulse plastic yield at the surface. An elastic-plastic model of metal behavior shows that the increment of plastic strain accumulated during each successive laser pulse will be proportional to the temperature rise above 2 AT. This plastic strain will appear at the surface as slip bands and intergranular slip that progressively degrades the optical quality of the surface. The surface temperature rise of metal mirrors under pulsed laser irradiation depends on both the metal optical and thermal properties as well as the laser pulse parameters. The peak temperature excursion produced by a constant-flux (rectangular) pulse is expressed in terms of the optical absorptance, pulse fluence, pulse duration, thermal conductivity, density, and specific heat. Using this relationship, the pulse fluence threshold (F y ) for first-pulse plastic yield is derived. The threshold for multipulse accumulation of plastic deformation is 2 F y . Numerical values of these thresholds are calculated for copper under CO 2 laser irradiation. The appearance of well-defined slip bands on the surface of copper mirrors after irradiation by a mode-locked CO 2 laser pulse burst has been experimentally demonstrated. The theoretical quantification of thermomechanical stress degradation presented here is used to calculate the expected behavior of copper under the reported experimental conditions, and these results are correlated with the available experimental data.