Ultrafast strong-field photoemission from plasmonic nanoparticles

Abstract

We demonstrate the ultrafast generation of electrons from tailored metallic nanoparticles and unravel the role of plasmonic field enhancement in this process by comparing resonant and off-resonant particles, as well as different particle geometries. We find that electrons become strongly accelerated within the evanescent fields of the plasmonic nanoparticles and escape along straight trajectories with orientations governed by the particle geometry. These results establish plasmonic nanoparticles as versatile ultrafast, nanoscopic sources of electrons.

Figure 1

Scheme of photoemission from plasmonic nanoparticles. (a) A femtosecond laser pulse excites an array of virtually identical gold nanorods from below. The laser wavelength is matched with the plasmon resonance, and the polarization is aligned along the long axis of the nanorod (see arrow in panel b). In the strong plasmonic fields, electrons become photoemitted and ponderomotively accelerated and are finally analyzed by time-of-flight spectrometry. (b) Simulation results for semiclassical simple-man’s model (same simulation parameters as in Figure 4). The lines pointing away from the nanorod report the electron trajectories; the colors indicate the final kinetic energies. The colored surface region (front right) on the rod shows the field enhancement for plane wave excitation of the laser.

Figure 2

Measured optical spectra and electron kinetic energy distributions for nanorods and bowtie nanoparticles. (a) Measured extinction spectra for nanorods with dimensions of (b) 120 × 87 × 40 nm³, (c) 152 × 87 × 40 nm³, and (d) 183 × 87 × 40 nm³, which are blue-shifted, in resonance, and red-shifted with respect to the excitation bandwidth centered at λ_exc = 805 nm (see dashed box). Spectra are offset for clarity. The bottom curve in panel a reports the spectrum for a bowtie structure with 90 nm width, 40 nm height, and 260 nm length (20 nm gap). (f) Electron spectra for different particle geometries and for a laser peak intensity of 25.1 GW/cm². The data below 3 eV are of limited validity due to instrumental restrictions of the time-of-flight spectrometer.

Figure 3

Electron spectra as a function of peak intensity for (a) resonant rod, (b) red-shifted rod, (c) blue-shifted rod, and (d) resonant bowtie structure. (e) Cutoff energies ε_cut, where the electron distribution f(ε) lies within 1 and 5% of its maximum value above 3 eV (range of cutoff energies according to errorbar), as a function of laser intensity. The circles show for comparison also ε_cut obtained from f(ε_cut) = f(ε_cut/2) × 10^–2, according to the prescription of ref (19). The solid and dashed lines show simulation results for the resonant rod and bowtie nanoparticle, respectively. The influence of different cutoff angles θ_cut = 6–10° (accounting for the acceptance cone of the electron spectrometer) on the cutoff energies is indicated by the shaded areas.

Figure 4

Simulation results. (a) The upper part (above dashed line) reports the electron trajectories, with colors chosen according to the final kinetic energies of the electrons. The color on the particle surface corresponds to the final kinetic energy of the electron photoemitted from the respective spot. The lower part shows the absolute value of the field enhancement f = E_ind/E_ext on the nanoparticle surface. (b) Same as panel a but for bowtie particle. (c) Time dependence of electron energies for electrons originating from the spots indicated by the symbols in the inset. For all simulations the laser intensity is set to 35 GW/cm², corresponding to a maximal field strength of E_ext ≈ 0.5 V/nm (Keldysh parameter of 0.66 for field enhancement of 50), and the electrons are photoexcited at the maximum of E_ext. (d) Simulated electron distribution for nanorod and for different excitation powers (dashed lines). The solid lines show spectra for the electrons photoexcited at the largest field strengts (Keldysh parameter γ < 2). In the simulations we assume a Gaussian envelope for the exciting laser pulse and use θ_cut = 10°.