Experimental and Ab Initio Ultrafast Carrier Dynamics in Plasmonic Nanoparticles

Abstract

Ultrafast pump-probe measurements of plasmonic nanostructures probe the nonequilibrium behavior of excited carriers, which involves several competing effects obscured in typical empirical analyses. Here we present pump-probe measurements of plasmonic nanoparticles along with a complete theoretical description based on first-principles calculations of carrier dynamics and optical response, free of any fitting parameters. We account for detailed electronic-structure effects in the density of states, excited carrier distributions, electron-phonon coupling, and dielectric functions that allow us to avoid effective electron temperature approximations. Using this calculation method, we obtain excellent quantitative agreement with spectral and temporal features in transient-absorption measurements. In both our experiments and calculations, we identify the two major contributions of the initial response with distinct signatures: short-lived highly nonthermal excited carriers and longer-lived thermalizing carriers. DOI: 10.1103/PhysRevLett.118.087401 Plasmonic hot carriers provide tremendous opportunities for combining efficient light capture with energy conversion [1][2][3][4][5] and catalysis [6,7] at the nanoscale [8][9][10]. The microscopic mechanisms in plasmon decays across various energy, length, and time scales are still a subject of considerable debate, as seen in recent experimental [11,12] and theoretical literature [13][14][15][16]. The decay of surface plasmons generates hot carriers through several mechanisms, including direct interband transitions, phonon-assisted intraband transitions, and geometry-assisted intraband transitions, as we have shown in previous work [17,18].Dynamics of hot carriers are typically studied via ultrafast pump-probe measurements of plasmonic nanostructures using a high-intensity laser pulse to excite a large number of electrons and measure the optical response as a function of time using a delayed probe pulse [11,[19][20][21][22][23][24][25]. Various studies have taken advantage of this technique to investigate electron-electron scattering, electron-phonon coupling, and electronic transport [20,22,[26][27][28][29][30][31][32]. Figure 1 shows a representative map of the differential extinction cross section as a function of pump-probe delay time and probe wavelength. With an increase in electron temperature, the real part of the dielectric function near the resonant frequency becomes more negative, while the imaginary part increases [33]. This causes the resonance to broaden and blueshift at short times as the electron temperature rises rapidly, and then to narrow and shift back over longer times as electrons cool down, consistent with previous observations [34]. Taking a slice of the map at one probe wavelength reveals the temporal behavior of the electron relaxation [ Fig. 1(b)], whereas a slice of the map at one time gives the spectral response, as shown in Fig. 1(a) for a set of times relative to the delay time with maximum signal, t max ¼ 700 fs.Conventional analyses of pu...

Transport of hot carriers in plasmonic nanostructures

“…Experimentally, this spatial inhomogeneity on the tens to hundreds of nanometers is vital to understand hot carrier imaging, photodetection, photovoltaic and photochemical energy conversion, which all involve carrier collection into a semiconductor or molecule. [54,55] However, optical probes of the carrier response (which we can predict quantitatively using our first-principles framework as shown previously [53]) cannot sense this inhomogeneity due to the diffraction limit, Aluminum is less efficient at collecting low energy carriers due to a shorter electron-phonon scattering mean free path, but enhances high-energy electron collection compared to gold because of increased generation of (and comparable mean free paths for) high-energy electrons. and instead measure a spatially-averaged result.…”

Effects of Interlayer Coupling on Hot‐Carrier Dynamics in Graphene‐Derived van der Waals Heterostructures

“…[23] With an ab initio framework for calculating optical response and electronphonon interactions, we previously evaluated mechanisms of hot carrier generation and relaxation in plasmonic metals, [24,25] and identified their signatures in ultrafast pump-probe measurements. [26,27] In particular, the small mean free paths of higher energy carriers helped elucidate the efficiency limits in plasmonic energy conversion devices and potential strategies to overcome them. [13] Here, we investigate the dynamics of hot carriers in graphene and in graphene-derived vdW heterostructures to explore their potential for efficient hot carrier extraction.…”

Quantum plasmons with optical-range frequencies in doped few-layer graphene

“…[35]). This takes into account the detailed electronic structure effects such as the response of electrons far from the Dirac point, as well as scattering against both acoustic and optical phonons, including umklapp and intervalley processes [35][36][37][38]. Doping, that is, a change in the position of the Fermi level E F changes the value of τ , and hence calculations were carried out for several different values of E F ranging from the neutral (undoped) value to 1.5 eV above it (see the Supplemental Material [34] for details of formulation and [35] for values of τ ).…”