Disentangling the Temporal Dynamics of Nonthermal Electrons in Photoexcited Gold Nanostructures

Abstract

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.

Time-Resolved Spectral Densities of Nonthermal Electrons in Gold

“…Our calculations indicate that differences in the electron density during the thermalization process can persist for more than a picosecond. Note that there exist several approaches, which include the dynamics of nonthermal electrons in a TTM-like description. − Qualitative and quantitative comparisons with and in between these different models and our Boltzmann approach would be very interesting in future.…”

Hot carrier spatio-temporal inhomogeneities in ultrafast nanophotonics

“…This behaviour however, utterly unrelated to the local pattern of radiation dissipation, can be understood by considering that, in the most general case, different pump wavelengths correspond to: (i) a different amount of energy released by the light pulse and absorbed by the system, which is dictated by the value of A 0 (λ p ); (ii) a different modulation of the interband permittivity term arising from the non-thermal portion of out-of-equilibrium electrons, N, the spectral dispersion of which depends on the pump photon energy (i.e. on λ p ) [64].…”

Plasmon-induced hot carrier dynamics and utilization

“…Then, energy transfer from the hot electrons into cool electrons and lattice happens initiated by electron–electron and electron–phonon scattering until a thermalized Fermi–Dirac distribution profile is established. This process, called electron thermalization, is usually completed in less than 500 fs [40,41] . After thermalization, the ratio of high-energy electrons able to surmount the Schottky barrier is usually negligible.…”