Imaging metallic multilayer structures through ultrafast optically driven excited electron transport

The transport of excited electrons in gold films excited by femtosecond las er pulses is observed to contain both a ballistic and an interactive compon ent and is, accordingly, well outside thermal equilibrium. In a specific ge ometry of ultrathin metallic multilayers the effective driving force for th e transport of excited electrons across the film is a linear concentration gradient of nonequilibrium electrons between excited and nonexcited volumes . The ballistic component is observed to traverse up to 400 nm through the film at near the Fermi velocity. The interactive component undergoes variou s scattering events and requires a correspondingly longer time to traverse the film. We demonstrate that these phenomena can be used to 'image' metall ic interlayers. Specifically, we report on the transport of nonequilibrium electrons through multilayers composed of AuTiAu films. The results are com pared to those for a gold film deposited next to the multilayer film. The e xperimental observation of the fractional change in reflectivity is depende nt on which component is being investigated.