Disentangling the Temporal Dynamics of Nonthermal Electrons in Photoexcited Gold Nanostructures

The study of nonthermal electrons, generated upon photoexcitation of plasmonic nanostructures, plays a key role in a variety of contexts, from photocatalysis and energy conversion to photodetection and nonlinear optics. Their ultrafast relaxation and subsequent release of energy to a low energy distribution of thermalized hot electrons has been the subject of a myriad of papers, mostly based on femtosecond transient absorption spectroscopy (FTAS). However, the FTAS signal stems from a complex interplay of different contributions arising from both nonthermal and thermal electrons, making the disentanglement of the two a very challenging task, so far accomplished only in terms of numerical simulations. Here a combined approach is introduced, based on a post-processing of the FTAS measurements guided by a reduced semiclassical model, the so-called extended two-temperature model, which has allowed the purely nonthermal contribution to the pump-probe experimental map recorded for 2D arrays of gold nanoellipsoids to be isolated. This approach displays the intimate correlation between electron energy and probe photon energy on the ultrafast time-scale of electron thermalization. It also sheds new light on the ultrafast transient optical response of gold nanostructures, and will help the development of optimized plasmonic configurations for nonthermal electrons generation and harvesting.

Automated summary

This study focuses on the analysis of nonthermal electrons generated by plasmonic nanostructures upon light excitation, highlighting the challenge of separating nonthermal and thermal electron contributions in femtosecond transient absorption spectroscopy (FTAS). By introducing a combined approach involving post-processing of FTAS measurements guided by a simplified semiclassical model, the purely nonthermal contribution to pump-probe experimental data for 2D arrays of gold nanoellipsoids was successfully isolated. This approach reveals the close relationship between electron energy and probe photon energy during ultrafast electron thermalization, shedding new insights on the transient optical response of gold nanostructures and aiding in the development of optimized plasmonic configurations for nonthermal electron generation.