Temperature dependence of absorptance in laser damage of metallic mirrors: I Melting

Abstract

Naval Weapons Center damage thresholds for ultraclean metals (45 J/cm2, 100 ns, 10.6 μm, Cu) are a milestone in laser-damage studies, being reproducible, with no isolated-spot damage, and showing the first excellent agreement (∼ 15%) with a first-principles theory (developed here) for any metal or transparent material. An exact premelting temperature result for the case of optical absorptance A = Ai + A1Ts indicates that the surface-temperature rise Ts reduces the theoretical threshold (Itp)m for raising Ts to the melting temperature Tm by a factor of 3.6 to 1.6, typically. Raising Ts to Tm is a good damage criterion for 100-ns pulses, but not for single subnanosecond pulses. The previous scaling Itp∼tp1/2/A is invalid in general, but Itp∼tp1/2 is valid for (Itp)m even with A = Ai + A1Ts. Approximating a Gaussian I(t) pulse by a square pulse makes (Itp)m 17% too small in the constant-A approximation.