Femtosecond All-Optical Control of Plasmons: Hot-Electron versus Phonon Relaxation

The optical Kerr nonlinearity of plasmonic metals provides enticing prospects for developing reconfigurable and ultracompact all-optical modulators. In nanostructured metals, the coherent coupling of electromagnetic radiations to plasmons creates a nonequilibrium distribution of electrons at an elevated temperature that gives rise to significant Kerr optical nonlinearities. Although enhanced nonlinear responses of metals enable the optical control of light, the intrinsically slow relaxation dynamics of photoexcited carriers, primarily governed by electron-phonon interactions, impedes sub-picosecond modulation speeds. Here, we demonstrate femtosecond (~ 190 fs) all-optical modulation in plasmonic systems via the activation of relaxation pathways for hot-electrons at the interface of metals and electron acceptor materials. We show that the relaxation kinetics and the optical nonlinearity can be tuned by leveraging the spectral response of the plasmonic system in the linear regime. Our findings introduce a generic scheme for achieving sub-picosecond modulation speeds in plasmonic systems, suitable for the ultrafast control of the intensity, polarization, and phase of light upon exchange of energetic hot carriers.