86 research outputs found
Erzeugung optischer DurchbrĂĽche bei hoher numerischer Apertur : numerische Simulationen zur Submikrometer-Manipulation transparenter Materialien und biologischer Zellen mit ultrakurzen Laserpulsen
[no abstract
Photoemission electron microscopy of localized surface plasmons in silver nanostructures at telecommunication wavelengths
We image the field enhancement at Ag nanostructures using femtosecond laser
pulses with a center wavelength of 1.55 micrometer. Imaging is based on
non-linear photoemission observed in a photoemission electron microscope
(PEEM). The images are directly compared to ultra violet PEEM and scanning
electron microscopy (SEM) imaging of the same structures. Further, we have
carried out atomic scale scanning tunneling microscopy (STM) on the same type
of Ag nanostructures and on the Au substrate. Measuring the photoelectron
spectrum from individual Ag particles shows a larger contribution from higher
order photoemission process above the work function threshold than would be
predicted by a fully perturbative model, consistent with recent results using
shorter wavelengths. Investigating a wide selection of both Ag nanoparticles
and nanowires, field enhancement is observed from 30% of the Ag nanoparticles
and from none of the nanowires. No laser-induced damage is observed of the
nanostructures neither during the PEEM experiments nor in subsequent SEM
analysis. By direct comparison of SEM and PEEM images of the same
nanostructures, we can conclude that the field enhancement is independent of
the average nanostructure size and shape. Instead, we propose that the
variations in observed field enhancement could originate from the wedge
interface between the substrate and particles electrically connected to the
substrate
Filamentation without intensity clamping
We present measurements of the supercontinuum emission (SCE) from ultrashort Ti:Saph laser pulse filamentation in air in a tightly focused geometry. The spectral broadening of SCE indicates that peak intensities exceed the clamping value of a few 1013 W/cm2 obtained for filamentation in a loose focusing geometry by at least one order of magnitude. We provide an interpretation for this regime of filamenation without intensity clamping
Attosecond electron-spin dynamics in Xe 4d photoionization
The photoionization of xenon atoms in the 70-100 eV range reveals several
fascinating physical phenomena such as a giant resonance induced by the dynamic
rearrangement of the electron cloud after photon absorption, an anomalous
branching ratio between intermediate Xe states separated by the spin-orbit
interaction and multiple Auger decay processes. These phenomena have been
studied in the past, using in particular synchrotron radiation, but without
access to real-time dynamics. Here, we study the dynamics of Xe 4d
photoionization on its natural time scale combining attosecond interferometry
and coincidence spectroscopy. A time-frequency analysis of the involved
transitions allows us to identify two interfering ionization mechanisms: the
broad giant dipole resonance with a fast decay time less than 50 as and a
narrow resonance at threshold induced by spin-flip transitions, with much
longer decay times of several hundred as. Our results provide new insight into
the complex electron-spin dynamics of photo-induced phenomena
Spatio-temporal coupling of attosecond pulses
The shortest light pulses produced to date are of the order of a few tens of
attoseconds, with central frequencies in the extreme ultraviolet range and
bandwidths exceeding tens of eV. They are often produced as a train of pulses
separated by half the driving laser period, leading in the frequency domain to
a spectrum of high, odd-order harmonics. As light pulses become shorter and
more spectrally wide, the widely-used approximation consisting in writing the
optical waveform as a product of temporal and spatial amplitudes does not apply
anymore. Here, we investigate the interplay of temporal and spatial properties
of attosecond pulses. We show that the divergence and focus position of the
generated harmonics often strongly depend on their frequency, leading to strong
chromatic aberrations of the broadband attosecond pulses. Our argumentation
uses a simple analytical model based on Gaussian optics, numerical propagation
calculations and experimental harmonic divergence measurements. This effect
needs to be considered for future applications requiring high quality focusing
while retaining the broadband/ultrashort characteristics of the radiation
Carrier-envelope phase dependent high-order harmonic generation with a high-repetition rate OPCPA-system
We study high-order harmonic generation with a high-repetition rate (200 kHz), few-cycle, driving laser, based on optical parametric chirped pulse amplification. The system delivers carrier-envelope phase stable, 8 fs, 10 ÎĽJ pulses at a central wavelength of 890 nm. High-order harmonics, generated in a high-pressure Ar gas jet, exhibit a strong CEP-dependence over a large spectral range owing to excellent stability of the driving laser pulses. This range can be divided into three spectral regions with distinct CEP influence. The observed spectral interference structures are explained by an analytical model based upon multiple pulse interferences.Marie Curie Research Training Network ATTOFELEuropean Research CouncilKnut and Alice Wallenberg foundationSwedish Foundation for Strategic ResearchSwedish Research Counci
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