Ultrafast Dissipative Localization of Electronic Energy in AuPt Superlattices
Controlling the spatial distribution of absorbed optical energy is central to nanoscale photothermal chemistry, plasmonics, and ultrafast materials control. Here, we show that a metallic AuPt superlattice concentrates electronic energy in Pt within a few hundred femtoseconds, regardless of the initial energy distribution between the two constituents. Ultrafast X-ray diffraction follows this energy redistribution through the amplitude of a coherent 570 GHz superlattice phonon driven by the stress imbalance at the AuPt interfaces. Despite the nearly homogeneous absorption at 400 nm, the observed lattice motion is identical to that produced by 800 nm excitation, which is absorbed predominantly in Pt. This dissipation driven localization of energy arises from the large electronic heat capacity of Pt and rapid electronic transport through the superlattice, providing a route to femtosecond control of nanoscale energy distributions.