Long-lived modulation of plasmonic absorption by ballistic thermal injection

Light–matter interactions that induce charge and energy transfer across interfaces form the foundation for photocatalysis 1 , 2 , energy harvesting 3 and photodetection 4 , among other technologies. One of the most common mechanisms associated with these processes relies on carrier injection. However, the exact role of the energy transport associated with this hot-electron injection remains unclear. Plasmon-assisted photocatalytic efficiencies can improve when intermediate insulation layers are used to inhibit the charge transfer 5 , 6 or when off-resonance excitations are employed 7 , which suggests that additional energy transport and thermal effects could play an explicit role even if the charge transfer is inhibited 8 . This provides an additional interfacial mechanism for the catalytic and plasmonic enhancement at interfaces that moves beyond the traditionally assumed physical charge injection 9 – 12 . In this work, we report on a series of ultrafast plasmonic measurements that provide a direct measure of electronic distributions, both spatially and temporally, after the optical excitation of a metal/semiconductor heterostructure. We explicitly demonstrate 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 free electrons in the semiconductor, without an equivalent spatiotemporal transfer of charge. Further, we demonstrate that this ballistic thermal injection mechanism can be utilized as a unique means to modulate plasmonic interactions. These experimental results are well-supported by both rigorous multilayer optical modelling and first-principle ab initio calculations. An energy transduction mechanism across metal/semiconductor interfaces, which relies on electron–electron energy transfer rather than the transport of charge, is demonstrated through ultrafast infrared spectroscopy. This ballistic thermal injection process allows for extended modulation of plasmonic absorption in epsilon-near-zero media.