The means by which the primary energy input of a femtosecond laser pulse at a surface is propagated in time and space, how it can be influenced, and how the propagating energy can be decoupled to yield an energy transfer to a nanostructure has important ramifications for many future applications. After the creation of the initial excitation by a femtosecond laser pulse, several qualitatively different propagation mechanisms are possible. The size, shape, defect density, and the chemical composition of the medium for the propagation will be decisive which of these mechanisms will be favoured.

In larger particles, propagation can proceed at the surface of the particles via surface plasmon polaritons. Surface plasmon polariton waves (SPPs), i.e., propagating charge density waves (transversal magnetic), are practically light waves that are confined to the surface of a dielectric wave medium and are of particular importance when energy must be propagated to, from, or across a nanostructure. SPPs can easily be created by light, they can be guided along a surface, and they can be converted back into light at a different location on the surface.

In this project of the SPP 1391, Two Photon Photoemission (2PPE) is used to study the ultrafast properties of plasmon waves at surfaces. 2PPE PEEM of SPPs relies on plasmon enhanced photoemisison and provides a method to view a surface in the light of plasmon resonances. 2PPE PEEM was also recently used to directly observe the propagation of a SPP wave at the surface of a small Ag island in a pump-probe experiment. In such experiments, the propagation of the SPP with half of the speed of light is observed as a systematic shift of a characteristic (beat) pattern as a function of the delay time between pump-pulse and probe-pulse.

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