15 research outputs found
Continuous-Wave Multiphoton Photoemission from Plasmonic Nanostars
Highly nonlinear optical processes, such as multiphoton photoemission,
require high intensities, typically achieved with ultrashort laser pulses and,
hence, were first observed with the advent of picosecond laser technology. An
alternative approach for reaching the required field intensities is offered by
localized optical resonances such as plasmons. Here, we demonstrate localized
multiphoton photoemission from plasmonic nanostructures under continuous-wave
illumination. We use synthesized plasmonic gold nanostars, which exhibit sharp
tips with structural features smaller than 5 nm, leading to
near-field-intensity enhancements exceeding 1000. This large enhancement
facilitates 3-photon photoemission driven by a simple continuous-wave laser
diode. We characterize the intensity and polarization dependencies of the
photoemission yield from both individual nanostars and ensembles. Numerical
simulations of the plasmonic enhancement, the near-field distributions, and the
photoemission intensities are in good agreement with experiment. Our results
open a new avenue for the design of nanoscale electron sources
Electronic and structural fingerprints of charge density wave excitations in extreme ultraviolet transient absorption spectroscopy
Femtosecond core-level transient absorption spectroscopy is utilized to
investigate photoinduced dynamics of the charge density wave in 1T-TiSe2 at the
Ti M2,3 edge (30-50 eV). Photoexcited carriers and phonons are found to
primarily induce spectral red-shifts of core-level excitations, and a carrier
relaxation time and phonon heating time of approximately 360 fs and 1.0 ps are
extracted, respectively. Pronounced oscillations in delay-dependent absorption
spectra are assigned to coherent excitations of the optical phonon
(6.0 THz) and the charge density wave amplitude mode (3.3 THz). By
comparing the measured spectra with time-dependent density functional theory
simulations, we determine the directions of the momentary atomic displacements
of both coherent modes and estimate their amplitudes. This work presents a
first look on charge density wave excitations with table-top core-level
transient absorption spectroscopy, enabling simultaneous access to electronic
and lattice excitation and relaxation
Ultrafast high-harmonic nanoscopy of magnetization dynamics
Light-induced magnetization changes, such as all-optical switching, skyrmion
nucleation, and intersite spin transfer, unfold on temporal and spatial scales
down to femtoseconds and nanometers, respectively. Pump-probe spectroscopy and
diffraction studies indicate that spatio-temporal dynamics may drastically
affect the non-equilibrium magnetic evolution. Yet, direct real-space magnetic
imaging on the relevant timescale has remained challenging. Here, we
demonstrate ultrafast high-harmonic nanoscopy employing circularly polarized
high-harmonic radiation for real-space imaging of femtosecond magnetization
dynamics. We observe the reversible and irreversible evolution of nanoscale
spin textures following femtosecond laser excitation. Specifically, we map
quenched magnetic domains and localized spin structures in Co/Pd multilayers
with a sub-wavelength spatial resolution down to 16 nm, and strobosocopically
trace the local magnetization dynamics with 40 fs temporal resolution. Our
approach enables the highest spatio-temporal resolution of magneto-optical
imaging to date. Facilitating ultrafast imaging with an extreme sensitivity to
various microscopic degrees of freedom expressed in chiral and linear
dichroism, we envisage a wide range of applications spanning magnetism, phase
transitions, and carrier dynamics.Comment: 14 pages, 4 figure
Nanotip-based photoelectron microgun for ultrafast LEED
We present the design and fabrication of a micrometer-scale electron gun for the
implementation of ultrafast low-energy electron diffraction from surfaces. A multi-step
process involving photolithography and focused-ion-beam nanostructuring is used to assemble and
electrically contact the photoelectron gun, which consists of a nanotip photocathode in a
Schottky geometry and an einzel lens for beam collimation. We characterize the low-energy
electron pulses by a transient electric field effect and achieve pulse durations of 1.3 ps
at an electron energy of 80 eV. First diffraction images in a backscattering geometry (at 50
eV electron energy) are shown