Experimental evidence of dominating ultrafast diffusive energy transport by hot electrons in Cu

When the dimensions of structures shrink to the order of the inelastic mean free path of the energy-carrying quasi-particles the character of energy transport changes from diffusive to ballistic. However, the point of transition remainsa matter of debate. Here, we determine the dominant channel of energy transport through a nanoscale Cu layer asfunction of its thickness. The energy rapidly transferred across Cu via hot electrons from a photo-excited Pt layer intoa buried Ni detection layer translates into a rapid expansion of the Ni layer probed via ultrafast x-ray diffraction. Thenon-linear dependence of the strain amplitude on the fluence indicates that the transport becomes more efficient withincreasing fluence. We demonstrate that this fluence-dependent efficiency is an unambiguous indicator for diffusivetransport since a quasi-ballistic transport model fails to capture the experimental results. Following this approach, weidentify diffusive energy transport via hot electrons in nonequilibrium with the phonons to govern the spatial energydistribution for Cu layer thicknesses larger than twice the electronic inelastic mean free path.