Thermal and mechanical response of gold films during nanosecond laser pulse heating

Free electrons in metal films absorb laser light and then transfer the absorbed photon energy to the metal lattice through electron-lattice collisions, which can result in lattice heating, thermal stress, melting, and evaporation. This work studies nanosecond laser heating of gold films both theoretically and experimentally. A two-step radiation heating model is utilized to characterize transient temperatures of the electron system and the lattice system. Results show that in the nanosecond regime electrons and the lattice are in thermal equilibrium and the classical Fourier heat conduction model is applicable. Microstructures and morphology of films before and after laser pulse heating are characterized with optical and electron microscopes. Two different types of thermal and mechanical responses of gold films are observed. For thin films, thermal stress plays a significant role in laser-film interactions, which can lead to structure changes of films at a temperature much lower than the melting point. For thick films, structure changes are due mainly to melting.