Ultrafast nanooptics
Attosecond probing of localized surface plasmon fields (Kleineberg)
Attosecond probing of localized surface plasmon fields (Kleineberg)
Localized Surface Plasmons (LSP) as coherent collective electron excitations manifest itself as nano-localized optical field enhancement, which undergoes ultrafast attosecond dynamics due to the inherently large spectral bandwidth of LSP resonances.
To date, a direct experimental observation of this optical field dynamics on a nanometer spatial and attosecond temporal scale has not been accomplished. It is the goal of this project to set the experimental pre-requisites for attosecond optical field nanoscopy by applying few-cycle optical pump pulses for resonant excitation and attosecond-timed photoelectron emission induced by single attosecond XUV pulses for probing. The interaction of the liberated valence-band photoelectrons with the instantaneous localized surface plasmon field results in a change of the kinetic photoelectron energy, thus carrying a "snapshot fingerprint" of the ultrafast LSP field. Time-of-flight photoelectron emission microscopy is used as a detector to record the spatial location as well as kinetic energy of the emitted photoelectrons. The measurements are accomplished by a unique experimental setup of a high repetition rate (10 kHz) High Harmonic Generation source providing single attosecond XUV pulses at ~ 90 eV photon energy, an interferometric pump probe delay stage with 800 nm/4 fsec few cycle pump pulses (400 nm SHG pulses optional), and a UHV-ToF PEEM system with two complimentary imaging energy filters and detectors (see Figure above).
First measurements of attosecond XUV-PEEM images and ToF spectra on Au/Si nanostructures have indicated, that space charge effects, which occur when liberating more than one electron per pulse and that would spoil spatial and energy resolution, could be efficiently suppressed for the fast Au-3d valence band electrons by providing XUV pulses of low energy and at high repetition rate, while space charge effects prevail for slow secondary electrons. Thus, energy-filtered PEEM is an essential pre-requisite for the successful implementation of attosecond nanoplasmonic field microscopy (Figure left).
Future scientific topics on the temporal dynamics of localized surface plasmons to be addressed by the project include, but are not limited to the following questions:
a) How fast does the localized optical field beating evolve during the plasmon dephasing time and how can it be described by theoretical models?
b) How does electronic coherence on a nanoscale evolve in multiphoton excitation on an attosecond time scale ?
c) How does size shape and material properties of plasmonic nanostructures affect the field dynamics and how can
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HHG PEEM images of Au/Si nanostructures (left) and selected are ToF electron spectra |
nanostructures be optimized for ultrafast field dynamics?
d) How can plasmonic field dynamics be controlled on an attosecond time scale by the carrier envelope phase of few-cycle optical pulses?
