  Hybrid plasmonic devices involve a nanostructured metal supporting localized
surface plasmons to amplify light-matter interaction, and a non-plasmonic
material to functionalize charge excitations. Application-relevant epitaxial
heterostructures, however, give rise to ballistic ultrafast dynamics that
challenge the conventional semiclassical understanding of unidirectional
nanometal-to-substrate energy transfer. We study epitaxial Au nanoislands on
WSe$_2$ with time- and angle-resolved photoemission spectroscopy and
femtosecond electron diffraction: this combination of techniques resolves
material, energy and momentum of charge-carriers and phonons excited in the
heterostructure. We observe a strong non-linear plasmon-exciton interaction
that transfers the energy of sub-bandgap photons very efficiently to the
semiconductor, leaving the metal cold until non-radiative exciton recombination
heats the nanoparticles on hundreds of femtoseconds timescales. Our results
resolve a multi-directional energy exchange on timescales shorter than the
electronic thermalization of the nanometal. Electron-phonon coupling and
diffusive charge-transfer determine the subsequent energy flow. This complex
dynamics opens perspectives for optoelectronic and photocatalytic applications,
while providing a constraining experimental testbed for state-of-the-art
modelling.
