FEMTOSECOND LASER-HEATING OF MULTILAYER METALS .1. ANALYSIS

Multi-layer metal films such as metallic coatings on metal substrate a re important elements in modern engineering applications. Specifically , gold-coated metal mirrors are widely used in high-power laser system s. This work studies microscopic energy deposition and transport proce sses during short-pulse laser heating of multi-layer metals: the absor ption of radiation energy by free electrons and the energy exchange be tween electrons and the lattice. The results show that multi-layer met als present very different thermal responses from single-layer metals during the heating process. In a gold and chromium multi-layer film, a lthough laser energy is absorbed by free electrons in the top gold coa ting layer, most of the absorbed energy is converted into lattice ener gy not in the gold layer but rather in the underlying chromium layer. The underlying chromium layer reduces the lattice-temperature rise of the top gold layer significantly during short-pulse laser heating, sug gesting a new way to increase the resistance of mirrors to thermal dam age in applications of high-power lasers.