Abstract
The creation of energetic electrons through plasmon excitation of nanostructures before thermalization has been proposed for a wide number of applications in optical energy conversion and ultrafast nanophotonics. However, the use of “nonthermal” electrons is primarily limited by both a low generation efficiency and their ultrafast decay. We report experimental and theoretical results on the use of broadband plasmonic nanopatch metasurfaces comprising a gold substrate coupled to silver nanocubes that produce large concentrations of hot electrons, which we measure using transient absorption spectroscopy. We find evidence for three subpopulations of nonthermal carriers, which we propose arise from anisotropic electron–electron scattering within sp-bands near the Fermi surface. The bimetallic character of the metasurface strongly impacts the physics, with dissipation occurring primarily in the gold, whereas the quantum process of hot electron generation takes place in both components. Our calculations show that the choice of geometry and materials is crucial for producing strong ultrafast nonthermal electron components.
Highlights
The creation of energetic electrons through plasmon excitation of nanostructures before thermalization has been proposed for a wide number of applications in optical energy conversion and ultrafast nanophotonics
We address the issue of enhancing nonthermal electron generation with the aid of a plasmonic nanopatch metasurface geometry that creates strong electromagnetic hot spots
We find excellent agreement between experiments and simulations (Fig. 3b, c), and show an inverse dependence of the quadrupolar and gap plasmon resonance wavelengths with the spacer thickness as a result of coupling to the underlying Au film as shown in Supplementary Fig. 421, 26
Summary
The creation of energetic electrons through plasmon excitation of nanostructures before thermalization has been proposed for a wide number of applications in optical energy conversion and ultrafast nanophotonics. What are usually termed “hot” electrons fall into two disparate populations of excited carriers: “nonthermal” electrons with an initial stepwise distribution extending up to the photon energy from the Fermi level and possessing an indefinable temperature, and “thermal” electrons with a quasi-equilibrated Fermi–Dirac distribution and an electronic temperature elevated above that of the surrounding lattice Due to their relatively low energies, thermal electrons are ineffective for carrier injection and slow to relax. The broad distribution of scattering rates is a function of their energy; as nonthermal electrons scatter closer to the Fermi level their relaxation rate slows due to Pauli exclusion effects[17] This is generally described in the context of Fermi liquid theory (FLT), which predicts an e–e scattering rate proportional to (E–EF)[2]. The random orientation of the nanocubes aE h pump EF sp -band d -band
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