Ultrafast transport and relaxation of hot plasmonic electrons in metal-dielectric heterostructures

Owing to the ultrashort timescales of ballistic electron transport, relaxation dynamics of hot nonequilibrium electrons is conventionally considered local. Utilizing propagating surface plasmon-polaritons (SPs) in metal-dielectric heterostructures, we demonstrate that both local (relaxation) and nonlocal (transport) hot electron dynamics contribute to the transient optical response. The data obtained in two distinct series of pump-probe experiments demonstrate a strong increase in both nonthermal electron generation efficiency and nonlocal relaxation timescales at the SP resonance. We develop a simple kinetic model incorporating a SP excitation, where both local and nonlocal electron relaxation in metals are included, and analyze nonequilibrium electron dynamics in its entirety in the case of collective electronic excitations. Our results elucidate the role of SPs in nonequilibrium electron dynamics and demonstrate rich perspectives of ultrafast plasmonics for tailoring spatiotemporal distribution of hot electrons in metallic nanostructures.

Figure 1

The electron dynamics upon ultrafast laser excitation of a thick metal film. Left: Nonthermal population (the shaded area) is created within the optical penetration depth $\zeta$. Transport of hot electrons leads to the in-depth redistribution (dashed and dash-dotted lines) and nonlocal relaxation of their population. Right: Thermalization of hot electrons (local relaxation) results in the temperature rise. The inset illustrates energy flow from the nonequilibrium electrons (orange) to the thermalized electrons (blue) and lattice (green). The top inset shows the laser-induced creation of the nonthermal electrons population.

Figure 2

(a) False color transmittivity map of the plasmonic Au-garnet grating. The low transmittivity area illustrates the SP dispersion, as highlighted with the white dashed line. The spectral width of the incident radiation is on the order of 50 nm. (b) Time-resolved transmittivity variations for the SP-resonant case (p-polarization, ${\lambda }_{\mathrm{p}}=1.3\mu \mathrm{m}$) and two nonresonant cases. The red solid lines are the fit curves using Eq. (2). The inset: Schematics of the pump-probe experiments.

Figure 3

(a) The amplitude of the pump-induced transmittance variations is strongly increased at ${\lambda }_p=1.3\mu \mathrm{m}, {\theta }_p={24}^{\text{o}}$, corresponding to the SP excitation. The solid lines and shades are guides to the eye. (b) Spectral dependence of the hot electrons transport (${\tau }_1$, bottom) and thermalization (${\tau }_2$, top) timescales. The solid red line indicates the pronounced enhancement of ${\tau }_1$ at the SP resonance (${\lambda }_{\mathrm{SP}}=1.3\mu \mathrm{m}$).

Figure 4

The nonlocal relaxation time ${\tau }_1$ as a function of the in-plane pump wave-vector component $k_{\mathrm{in}}$, obtained from the experiments with varied ${\lambda }_p$ (black, ${\theta }_p={24}^{\text{o}}$) and ${\theta }_p$ (red, ${\lambda }_p=1.3\mu \mathrm{m}$). The remarkable agreement of the two data sets confirms the SP resonance-driven ${\tau }_1$ behavior illustrated with the shaded area.