Attosecond dynamics of nano-localized fields probed by photoelectron spectroscopy

This work focuses on the interaction of few-cycle laser pulses with nanosystems. Special emphasis is placed on the spatio-temporal evolution of the induced near-fields. Measurements on carrier-envelope-phase (CEP) controlled photoemission from isolated SiO2 nanospheres are taken by single-shot velocity map imaging (VMI) combined with CEP tagging. The obtained photoelectron spectra show a pronounced dependence on the CEP and extend to unexpectedly high energies. Comparison with numerical simulations identify the additional Coulomb forces of the liberated electron cloud as an effective additional acceleration mechanism for distinct trajectories. For larger spheres, an asymmetry in the field distribution is classically predicted. This asymmetry is also observed in the photoelectron momentum distributions. The mapping between position and momentum space in the VMI approach are investigated by analyzing the correlation of the photoelectron's birth and detection position. In a second set of experiments, photoemission at intensities exceeding 10^14 W/cm^2 from isolated nanospheres of different composition (SiO2, ZrO2, TiO2, Si, Au) is examined by stereo time-of-flight spectroscopy. It is found that the measured cutoff energies scale non-linearly with laser intensity depending on the material properties of the nanosystem. A trend towards a unified behavior for high intensities is observed indicating a drastic change in optical properties within the duration of the few-cycle laser pulse. The charge carrier generation mechanisms that could lead to such a transient effect are discussed. For a better understanding of the interaction of few-cycle fields with nanosystems, a direct access to the temporal evolution of (plasmonic) near-fields is highly desirable. The efforts on the realization of nanoplasmonic attosecond streaking spectroscopy are presented. Numerical simulations are used to identify the influence of the inhomogeneous near-field distributions on the streaking process. First experimental results obtained from Au nanotips show clear streaking features of sub-micron localized near-fields. The near-field oscillations are found to be phase offset as compared to reference measurements. The exact origin of the streaking features of the Au tip and possible improvements of the experimental approach are discussed.Im Mittelpunkt dieser Arbeit steht die Wechselwirkung von ultrakurzen Laserpulsen mit Nanosystemen wobei besonderes Augenmerk auf die örtlichen und zeitlichen Eigenschaften der erzeugten Nahfelder gelegt wird. Zur direkten und indirekten Bestimmung der Nahfeldentwicklung und -verteilung wird auf verschiedene Formen der Elektronenspektroskopie zurückgegriffen. Zum einen wird die Photoemission von isolierten SiO2 Nanokugeln mit Hilfe der Velocity-Map-Imaging-Methode bei gleichzeitiger Bestimmung der Träger-Einhüllenden-Phase der ultrakurzen Laserpulse gemessen. Die Impulsspektren zeigen eine starke Abhängigkeit von der Feldentwicklung des Laserfeldes und erstrecken sich zu unerwartet hohen Energien. Mit Hilfe numerischer Simulationen kann gezeigt werden, dass photoionisierte Elektronen eine hochdynamische Ladungsverteilung an der Oberfläche erzeugen, welche für eine zusätzliche Beschleunigung für ausgewählte Elektronentrajektorien verantwortlich ist. Messungen an Nanokugeln mit verschiedener Größe zeigen, dass die durch Propagationseffekte erzeugte asymmetrische Feldverteilung direkt auf die Impulsprojektionen übertragen wird. Die Korrelation zwischen Orts- und Impulsraum der Photoelektronen und eine mögliche Rekonstruktion der Feldverteilung an der Oberfläche werden diskutiert. Mit weiteren Experimenten an einem Stereo-Flugzeitspektrometer wird die Photoemission von Nanoteilchen unterschiedlicher Zusammensetzung (SiO2, ZrO2, TiO2, Si, Au) bei hohen Intensitäten oberhalb von 10^14W/cm^2 untersucht. Diese zeigen eine nichtlineare Abhängigkeit der höchsten Elektronenenergien von der Intensität. Die Gesetzmäßigkeit aller Materialien konvergiert, was ein starkes Indiz für eine drastische Änderung der optischen Eigenschaften noch während des Laserpulses ist. Die verfügbaren theoretischen Modelle zur Erzeugung freier Ladungsträger, die zu einem solchen transienten Effekt führen können, werden diskutiert. Zeitaufgelöste Messungen der Nahfeldoszillationen an Nanoteilchen würden ein tiefgreifenderes Verständnis und Charakterisierung der kollektiven Elektronendynamik ermöglichen. Die Anwendung von Attosekundenpulsen zu diesem Zweck wird diskutiert wobei besonderes Augenmerk auf die inhomogene örtliche Verteilung der Felder an Nanostrukturen gelegt wird. Erste experimentelle Resultate zur Messung der Nahfeldoszillationen an Gold-Nanospitzen werden präsentiert. Die Ergebnisse zeigen einen deutlichen Phasenversatz zu Referenzmessungen. Die örtliche Herkunft des Signals und mögliche Verbesserungen des Experiments werden aufgezeigt

Attosecond nanoplasmonic streaking of localized fields near metal nanospheres

Collective electron dynamics in plasmonic nanosystems can unfold on timescales in the attosec- ond regime and the direct measurements of plasmonic near-field oscillations is highly desirable. We report on numerical studies on the application of attosecond nanoplasmonic streaking spectroscopy to the measurement of collective electron dynamics in isolated Au nanospheres. The plasmonic field oscillations are induced by a few-cycle NIR driving field and are mapped by the energy of photoemitted electrons using a synchronized, time-delayed attosecond XUV pulse. By a detailed analysis of the amplitudes and phase shifts, we identify the different regimes of nanoplasmonic streaking and study the dependence on particle size, XUV photoelectron energy and emission position. The simulations indicate that the near-fields around the nanoparticles can be spatio-temporally reconstructed and may give detailed insight into the build-up and decay of collective electron motion.Comment: Revised versio

Quenching of material dependence in few-cycle driven electron acceleration from nanoparticles under many-particle charge interaction

The excitation of nanoscale near-fields with ultrashort and intense laser pulses of well-defined waveform enables strongly spatially and temporally localized electron emission, opening up the possibility for the generation of attosecond electron pulses. Here, we investigate the electron photoemission from isolated nanoparticles of different materials in few-cycle laser fields at intensities where the Coulomb field of the ionized electrons and residual ions significantly contribute to the electron acceleration process. The dependences of the electron cut-off energy on the material’s dielectric properties and electron binding energy are investigated systematically in both experiments and semi-classical simulations. We find that for sufficiently high near-field intensities the material dependence of the acceleration in the enhanced near-fields is quenched by many-particle charge-interaction

High-harmonic and single attosecond pulse generation using plasmonic field enhancement in ordered arrays of gold nanoparticles with chirped laser pulses

Coherent XUV sources, which may operate at MHz repetition rate, could find applications in high-precision spectroscopy and for spatio-time-resolved measurements of collective electron dynamics on nanostructured surfaces. We theoretically investigate utilizing the enhanced plasmonic fields in an ordered array of gold nanoparticles for the generation of high-harmonic, extreme-ultraviolet (XUV) radiation. By optimization of the chirp of ultrashort laser pulses incident on the array, our simulations indicate a potential route towards the temporal shaping of the plasmonic near-field and, in turn, the generation of single attosecond pulses. The inherent effects of inhomogeneity of the local fields on the high-harmonic generation are analyzed and discussed. While taking the inhomogeneity into account does not affect the optimal chirp for the generation of a single attosecond pulse, the cut-off energy of the high-harmonic spectrum is enhanced by about a factor of two

Quenching of material dependence in few-cycle driven electron acceleration from nanoparticles under many-particle charge interaction

The excitation of nanoscale near-fields with ultrashort and intense laser pulses of well-defined waveform enables strongly spatially and temporally localized electron emission, opening up the possibility for the generation of attosecond electron pulses. Here, we investigate the electron photoemission from isolated nanoparticles of different materials in few-cycle laser fields at intensities where the Coulomb field of the ionized electrons and residual ions significantly contribute to the electron acceleration process. The dependences of the electron cut-off energy on the material's dielectric properties and electron binding energy are investigated systematically in both experiments and semi-classical simulations. We find that for sufficiently high near-field intensities the material dependence of the acceleration in the enhanced near-fields is quenched by many-particle charge-interaction.114Nsciescopu

Quenching of material dependence in few-cycle driven electron acceleration from nanoparticles under many-particle charge interaction

The excitation of nanoscale near-fields with ultrashort and intense laser pulses of well-defined waveform enables strongly spatially and temporally localized electron emission, opening up the possibility for the generation of attosecond electron pulses. Here, we investigate the electron photoemission from isolated nanoparticles of different materials in few-cycle laser fields at intensities where the Coulomb field of the ionized electrons and residual ions significantly contribute to the electron acceleration process. The dependences of the electron cut-off energy on the material’s dielectric properties and electron binding energy are investigated systematically in both experiments and semi-classical simulations. We find that for sufficiently high near-field intensities the material dependence of the acceleration in the enhanced near-fields is quenched by many-particle charge-interaction