  Energy and charge transfer across metal-semiconductor interfaces are the
fundamental driving forces for a broad range of applications, such as
computing, energy harvesting, and photodetection. However, the exact roles and
physical separation of these two phenomena remains unclear, particularly in
plasmonically-excited systems or cases of strong nonequilibrium. We report on a
series of ultrafast plasmonic measurements that provide a direct measure of
electronic distributions, both spatially and temporally, following optical
excitation of a metal-semiconductor heterostructure. For the first time, we
explicitly show that in cases of strong non-equilibrium, a novel energy
transduction mechanism arises at the metal/semiconductor interface. We find
that hot electrons in the metal contact transfer their energy to pre-existing
electrons in the semiconductor, without transfer of charge. These experimental
results findings are well-supported by both rigorous multilayer optical
modeling and first-principle, ab initio calculations.
