Plasmonic Photoemission from Single-Crystalline Silver

Abstract

Optical fields interacting with solids excite single particle quantum transitions and elicit collective screening responses that define their penetration, absorption, and reflection. The interplay of these interactions on the attosecond time scale defines how optical energy transforms to electronic, setting the limits of efficiency for processes such as solar energy harvesting or photocatalysis. Our understanding of light–matter interactions is primarily based on specifying the electronic structure of solids and initial particles or fields occupying well-defined states, and the outcome of their interaction culminating in photoelectron or photon emission and analysis, with scant ability to follow the transitional, ultrafast many-body interactions that define it. The optical properties of metals transubstantiate from metallic to dielectric when the real part of their dielectric response function, Re­[ε­(ω)], passes through zero: at low frequencies, Re­[ε­(ω)] < 0, and the collective free electron plasmonic response confers high reflectivity; at high frequencies, Re­[ε­(ω)] > 0, and the fields penetrate as charge-density or longitudinal plasmon waves. How such collective plasmonic responses decay on the femtosecond time scale into single particle excitations is cardinal to plasmonics, but not sufficiently well described by experiment or theory. We examine the spectroscopic signatures of the nonlinear single particle and collective excitations of the low index crystals of silver by nonlinear two-photon photoemission spectroscopy, at frequencies where the bulk dielectric response passes through zero. We find that the transition through zero dielectric region is reflected in the nonlinear photoemission spectra, and in particular, the bulk plasmons decay by giving rise to a non-Einsteinian plasmonic photoemission component. This response, where the energy of photoelectrons is not defined by the incoming photons, occurs when photons excite the longitudinal plasmons, which then decay by exciting photoelectrons selectively from the Fermi level. Such mode of plasmon decay into hot electrons is contrary to the general agreement, but confirms a theoretical prediction by J. J. Hopfield from 1965. Our experiment illuminates a more energy efficient optical-to-electronic energy flow in metals that so far has escaped scrutiny.

Quantitative Ultrafast Electron‐Temperature Dynamics in Photo‐Excited Au Nanoparticles

“…In particular, it is shown that under excitation with photon energies matching the bulk plasmon energy, the bulk plasmon decay preferentially excites photoelectrons from the Fermi level rather than producing a broad energy distribution of hot electrons and holes, a process referred to "plasmonic photoemission". [44] In our setup, with pump photon energy much lower than bulk plasmon energy, this phenomenon cannot be observed.…”

“…Concerning the nonthermal carriers, recent experiments report that the actual distribution of non-thermal carriers in a plasmonic metal [44] may differ from the calculated ones for example, in refs. [48] and [49].…”

“…[41,42] Theoretical models, on the other hand, are becoming more and more refined, yet cannot handle, so far, the complexity of real systems. [3,15,22,43,44] In this work, we report the measurement of the ultrafast electron-temperature (ϑ e ) dynamics within an ensemble of plasmonic gold NPs, laid onto a transparent conductive oxide support, thereby allowing to perform photoemission experiments while allowing a degree of electron confinement sufficient to preserve the LSP resonance. Experiments were performed in pump-probe configuration by photo-exciting the NPs close to the LSP resonance and collecting ultrafast timeresolved photoemission spectra (tr-PES).…”

Quantitative Ultrafast Electron-Temperature Dynamics in Photo-Excited Au Nanoparticles

“…[7,[51][52][53] for an overview): highly energetic nonthermal carriers and thermalized hot carriers, described in terms of the electronic temperature ϑ e . Concerning the nonthermal carriers, recent experiments report that the actual distribution of non-thermal carriers in a plasmonic metal [44] may differ from the calculated ones in e.g. [48] and [49].…”

“…The current understanding of ultrafast relaxation processes in plasmonic nanosystems rests upon ultrafast time-resolved optical and, to a lesser extent, electronic spectroscopies, [28,[37][38][39][40] that however mostly yield indirect information about the time-dependent electron-gas or ion-lattice temperature [41,42]. Theoretical models, on the other hand, are becoming more and more refined, yet cannot handle, so far, the complexity of real systems [3,15,22,43,44]. In this work, we report the measurement of the ultrafast electron-temperature (ϑ e ) dynamics within an ensemble of plasmonic gold NPs, laid onto a transparent conductive oxide support, thereby allowing to perform photoemission experiments while allowing a degree of electron confinement sufficient to preserve the LSP resonance.…”