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Full spatiotemporal resolution of the evolution of plasmonic fields is a major goal in plasmonics in order to investigate the buildup and decay of collective electron phenomena. Here, we demonstrate few-femtosecond probing of plasmon transients uniquely combined with nm-scale sensitivity.
In this work, we use broadband femtosecond pulses made up of ~1.5 eV photons to generate photoelectrons in the strong fields of the laser focus. These electrons then probe the optical near-fields in a time-resolved and spatially highly selective manner, due to the fact that photoemission is much more probable, and the electrons can gain higher energy, where the local fields are high. In contrast to previous experiment, by filtering for a certain kinetic energy range of photoemitted electrons, we can limit the measurement for rescattering electrons resulting in a sub-nm surface sensitivity and selectivity for plasmonic hot spots.
To realize this novel concept, we illuminate plasmonic nanoparticles with time-delayed replicas of few-cycle pulses of a femtosecond laser oscillator, resulting in electron emission. The photoelectron spectra were recorded for a set of delays of the interferometer arms. By filtering for the highest electron kinetic energies, we could record autocorrelation functions of the hot-spot field evolution of plasmonic nanostructures.
Through the analysis of the resulting energy-resolved autocorrelation traces, we could demonstrate that even the plasmon oscillation decay after the ultrafast excitation is in the sub-10-fs range under these extreme conditions.
By establishing this ultrafast time-resolved characterization technique, the buildup and decay of collective electron oscillations can be investigated with unprecedented spatiotemporal resolution and plasmonic nanoparticles can be tailored for ultrafast optics applications such as near-field-enhanced high harmonic generation, near-field spectroscopy etc.
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