Laser interaction with metallic surfaces

Direct real-time measurements of a target material's optical properties with subnanosecond resolution may provide crucial, process-revealing signatures in following the interaction of an intense laser beam with a metal as the surface progressively undergoes heating, plastic deformation, slip, vaporization, ejection of liquid metal, plasma formation, etc. Three classes of physical processes have been proposed to account for a substantial decrease in reflectance observed during the interaction of an intense laser pulse with a metal surface: (1) deformation of the surface, (2) plasma formation, and (3) a nonlinear process causing enhanced absorption within the metal. Thus far we may conclude that specular reflectance is a sensitive indicator of surface deformation. Total reflectance measurements, on the other hand, indicate that until the surface temperature of a metal target reaches the vicinity of the boiling point, the total reflectance does not differ significantly from that given by a Drude-type free-electron model. The reflectivity decrease of a Drude model for a metal heated from room temperature to a liquid at the boiling point is not large enough to account for the substantial reflectance decrease observed experimentally.