28 research outputs found
Sub-Terahertz Nearfields for Electron-Pulse Compression
The advent of ultrafast science with pulsed electron beams raised the need in
controlling the temporal features of the electron pulses. One promising
suggestion is the nano-selective driving of quantum optics with
multi-electrons, which can occur if laser-structured consecutive electrons are
bunched within the decoherence time of the quantum system. Compressing electron
pulses with terahertz (THz) radiation formed in optical nonlinear crystals is
an attractive methodology to generate the rapidly varying electric fields
necessary for electron compression, with an inherent temporal locking to
laser-triggered electrons. Sub-THz are especially appealing as they can
compress longer (picosecond-) pulses, however, the generation of such low
frequencies is inefficient and their focusability is fundamentally limited by
diffraction. This work proposes electron-pulse compression with sub-THz fields
directly in the vicinity of their dipolar origin, thereby avoiding their
mediation through radiation. We analyze the merits of such nearfields for
electron compression and find that it is particularly advantageous for small
numerical apertures and micro-joule-level optical pulses, and hence can be
implemented within the tight constraints of standard electron microscopes and
work at high repetition rates. We target temporal compression within low-eV
electron microscopes but the new paradigm we suggest offers a realistic
approach for controlling electron pulses spatially and temporally in many
experiments, opening the path of flexible multi-electron manipulation for
analytic and quantum sciences.Comment: 21 pages, 3 figure
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
Generation of bright phase-matched circularly-polarized extreme ultraviolet high harmonics
Circularly-polarized extreme ultraviolet and X-ray radiation is useful for analysing the structural, electronic and magnetic properties of materials. To date, such radiation has only been available at large-scale X-ray facilities such as synchrotrons. Here, we demonstrate the first bright, phase-matched, extreme ultraviolet circularly-polarized high harmonics source. The harmonics are emitted when bi-chromatic counter-rotating circularly-polarized laser pulses field-ionize a gas in a hollow-core waveguide. We use this new light source for magnetic circular dichroism measurements at the M-shell absorption edges of Co. We show that phase-matching of circularly-polarized harmonics is unique and robust, producing a photon flux comparable to linearly polarized high harmonic sources. This work represents a critical advance towards the development of table-top systems for element-specific imaging and spectroscopy of multiple elements simultaneously in magnetic and other chiral media with very high spatial and temporal resolution. Circularly-polarized radiation in the extreme ultraviolet (EUV)and soft X-ray spectral regions has proven to be extremelyuseful for investigating chirality-sensitive light–matter inter-actions. It enables studies of chiral molecules using photoelectron circular dichroism1, ultrafast molecular decay dynamics2, the direct measurement of quantum phases (for example, Berry’s phase and pseudo-spin) in graphene and topological insulators3,4 and reconstruction of band structure and modal phases in solids5
Nanoscale Magnetic Imaging using Circularly Polarized High-Harmonic Radiation
This work demonstrates nanoscale magnetic imaging using bright circularly
polarized high-harmonic radiation. We utilize the magneto-optical contrast of
worm-like magnetic domains in a Co/Pd multilayer structure, obtaining
quantitative amplitude and phase maps by lensless imaging. A
diffraction-limited spatial resolution of 49 nm is achieved with iterative
phase reconstruction enhanced by a holographic mask. Harnessing the unique
coherence of high harmonics, this approach will facilitate quantitative,
element-specific and spatially-resolved studies of ultrafast magnetization
dynamics, advancing both fundamental and applied aspects of nanoscale
magnetism.Comment: Ofer Kfir and Sergey Zayko contributed equally to this work.
Presented in CLEO 2017 (Oral) doi.org/10.1364/CLEO_QELS.2017.FW1H.