29 research outputs found
Nanoscale diffractive probing of strain dynamics in ultrafast transmission electron microscopy
The control of optically driven high-frequency strain waves in nanostructured
systems is an essential ingredient for the further development of
nanophononics. However, broadly applicable experimental means to quantitatively
map such structural distortion on their intrinsic ultrafast time and nanometer
length scales are still lacking. Here, we introduce ultrafast convergent beam
electron diffraction (U-CBED) with a nanoscale probe beam for the quantitative
retrieval of the time-dependent local distortion tensor. We demonstrate its
capabilities by investigating the ultrafast acoustic deformations close to the
edge of a single-crystalline graphite membrane. Tracking the structural
distortion with a 28-nm/700-fs spatio-temporal resolution, we observe an
acoustic membrane breathing mode with spatially modulated amplitude, governed
by the optical near field structure at the membrane edge. Furthermore, an
in-plane polarized acoustic shock wave is launched at the membrane edge, which
triggers secondary acoustic shear waves with a pronounced spatio-temporal
dependency. The experimental findings are compared to numerical acoustic wave
simulations in the continuous medium limit, highlighting the importance of
microscopic dissipation mechanisms and ballistic transport channels
Nanoscale mapping of ultrafast magnetization dynamics with femtosecond Lorentz microscopy
Novel time-resolved imaging techniques for the investigation of ultrafast
nanoscale magnetization dynamics are indispensable for further developments in
light-controlled magnetism. Here, we introduce femtosecond Lorentz microscopy,
achieving a spatial resolution below 100 nm and a temporal resolution of 700
fs, which gives access to the transiently excited state of the spin system on
femtosecond timescales and its subsequent relaxation dynamics. We demonstrate
the capabilities of this technique by spatio-temporally mapping the
light-induced demagnetization of a single magnetic vortex structure and
quantitatively extracting the evolution of the magnetization field after
optical excitation. Tunable electron imaging conditions allow for an
optimization of spatial resolution or field sensitivity, enabling future
investigations of ultrafast internal dynamics of magnetic topological defects
on 10-nanometer length scales
High-purity free-electron momentum states prepared by three-dimensional optical phase modulation
We demonstrate the quantized transfer of photon energy and transverse
momentum to a high-coherence electron beam. In an ultrafast transmission
electron microscope, a three-dimensional phase modulation of the electron
wavefunction is induced by transmitting the beam through a laser-illuminated
thin graphite sheet. This all-optical free-electron phase space control results
in high-purity superpositions of linear momentum states, providing an
elementary component for optically programmable electron phase plates and beam
splitters
Coulomb-correlated electron number states in a transmission electron microscope beam
We demonstrate the generation of Coulomb-correlated pair, triple and
quadruple states of free electrons by femtosecond photoemission from a
nanoscale field emitter inside a transmission electron microscope. Event-based
electron spectroscopy allows a spatial and spectral characterization of the
electron ensemble emitted by each laser pulse. We identify distinctive energy
and momentum correlations arising from acceleration-enhanced interparticle
energy exchange, revealing strong few-body Coulomb interactions at an energy
scale of about two electronvolts. State-sorted beam caustics show a discrete
increase in virtual source size and longitudinal source shift for few-electron
states, associated with transverse momentum correlations. We observe
field-controllable electron antibunching, attributed primarily to transverse
Coulomb deflection. The pronounced spatial and spectral characteristics of
these electron number states allow filtering schemes that control the
statistical distribution of the pulse charge. In this way, the fraction of
specific few-electron states can be actively suppressed or enhanced,
facilitating the preparation of highly non-Poissonian electron beams for
microscopy and lithography, including future heralding schemes and correlated
multi-electron probing
Probing chirality with inelastic electron-light scattering
Circular dichroism spectroscopy is an essential technique for understanding
molecular structure and magnetic materials, but spatial resolution is limited
by the wavelength of light, and sensitivity sufficient for single-molecule
spectroscopy is challenging. We demonstrate that electrons can efficiently
measure the interaction between circularly polarized light and chiral materials
with deeply sub-wavelength resolution. By scanning a nanometer-sized focused
electron beam across an optically-excited chiral nanostructure and measuring
the electron energy spectrum at each probe position, we produce a
high-spatial-resolution map of near-field dichroism. This technique offers a
nanoscale view of a fundamental symmetry and could be employed as "photon
staining" to increase biomolecular material contrast in electron microscopy