  Harnessing short-lived photoexcited electron-hole pairs in metal
nanostructures has the potential to define a new phase of optoelectronics,
enabling control of athermal mechanisms for light harvesting, photodetection
and photocatalysis. To date, however, the spatiotemporal dynamics and transport
of these photoexcited carriers have been only qualitatively characterized.
Plasmon excitation has been widely viewed as an efficient mechanism for
generating non-thermal hot carriers. Despite numerous experiments, conclusive
evidence elucidating and quantifying the full dynamics of hot carrier
generation, transport, and injection has not been reported. Here, we combine
experimental measurements with coupled first-principles electronic structure
theory and Boltzmann transport calculations to provide unprecedented insight
into the internal quantum efficiency, and hence internal physics, of hot
carriers in photoexcited gold (Au)-gallium nitride (GaN) nanostructures. Our
results indicate that photoexcited electrons generated in 20 nm-thick Au
nanostructures impinge ballistically on the Au-GaN interface. This discovery
suggests that the energy of hot carriers could be harnessed from metal
nanostructures without substantial losses via thermalization. Measurements and
calculations also reveal the important role of metal band structure in hot
carrier generation at energies above the interband threshold of the plasmonic
nanoantenna. Taken together, our results advance the understanding of excited
carrier dynamics in realistically-scaled metallic nanostructures and lay the
foundations for the design of new optoelectronic devices that operate in the
ballistic regime.
