Ultrafast Dynamics of Hot Electrons in Nanostructures: Distinguishing the Influence on Interband and Plasmon Resonances

It is often necessary to distinguish between the plasmon oscillations arising from confinement in nanostructures and interband transitions characteristic of the bulk. Grating nanostructures are an ideal platform to achieve this goal, since they provide nanoscale confinement in one direction while maintaining macroscopic properties in the other. It is thereby possible to distinguish between confined plasmons and interband transitions by the choice of polarization of the pump and the probe pulses. Here, we report transient absorption (TA) experiments on gold gratings with a 500 nm period pumped with a short pulse centered at 680 nm, reasonably far from the plasmon resonance. The ensuing dynamics of the hot electrons generated by the pump are monitored by broad band visible probe pulses polarized both parallel and perpendicular to the grating lines. We observe a significant difference in the spectral response of the two probe polarizations, while the temporal responses are quite similar. Furthermore, the response of gold nano-gratings qualitatively does not depend on the pump polarization. These results indicate that, regardless of polarization, the pump creates hot electrons in the metal that influence both interband and confined plasmon absorptions. The time-scales of the TA signal, therefore, is indicative of the cooling of the hot electrons as they equilibrate with the lattice. Furthermore, we performed similar experiments on TiO2-covered gratings and found that it is consistent with the previous work on electron injection into TiO2, with a response that depends on the pump polarization. These results provide a clearer understanding of the interplay between interband and confined plasmons in photoexcited metal nanostructures.