Observation of multi-directional energy transfer in a hybrid plasmonic-excitonic nanostructure

TL;DR

This study reveals complex, multi-directional energy transfer dynamics in a hybrid plasmonic-excitonic nanostructure, challenging traditional unidirectional models and highlighting ultrafast interactions relevant for optoelectronic applications.

Contribution

It provides the first experimental observation of multi-directional energy exchange in epitaxial Au/WSe2 heterostructures using advanced time- and angle-resolved spectroscopy.

Findings

Efficient sub-bandgap photon energy transfer to the semiconductor.

Metal remains cold until exciton recombination heats nanoparticles.

Energy exchange occurs on sub-100 femtosecond timescales.

Abstract

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.