70 research outputs found
Density Matrix Reconstructions in Ultrafast Transmission Electron Microscopy: Uniqueness, Stability, and Convergence Rates
In the recent paper [17] the first experimental determination of the density
matrix of a free electron beam has been reported. The employed method leads to
a linear inverse problem with a positive semidefinite operator as unknown. The
purpose of this paper is to complement the experimental and algorithmic results
in the work mentioned above by a mathematical analysis of the inverse problem
concerning uniqueness, stability, and rates of convergence under different
types of a-priori information
Ultrafast nano-imaging of the order parameter in a structural phase transition
Understanding microscopic processes in materials and devices that can be
switched by light requires experimental access to dynamics on nanometer length
and femtosecond time scales. Here, we introduce ultrafast dark-field electron
microscopy, tailored to map the order parameter across a structural phase
transition. We track the evolution of charge-density wave domains in 1T-TaS2
after ultrashort laser excitation, elucidating relaxation pathways and domain
wall dynamics. The unique benefits of selective contrast enhancement will
inspire future beam shaping technology in ultrafast transmission electron
microscopy.Comment: Main text, supplementary materials, and five movie
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
Nonlinear Spectroscopy and All-Optical Switching of Femtosecond Soliton Molecules
The emergence of confined structures and pattern formation are exceptional
manifestations of concurring nonlinear interactions found in a variety of
physical, chemical and biological systems[1]. Optical solitons are a hallmark
of extreme spatial or temporal confinement enabled by a variety of
nonlinearities. Such particle-like structures can assemble in complex stable
arrangements, forming "soliton molecules"[2,3]. Recent works revealed
oscillatory internal motions of these bound states, akin to molecular
vibrations[4-8]. These observations beg the question as to how far the
"molecular" analogy reaches, whether further concepts from molecular
spectroscopy apply in this scenario, and if such intra-molecular dynamics can
be externally driven or manipulated. Here, we probe and control such ultrashort
bound-states in an optical oscillator, utilizing real-time spectroscopy and
time-dependent external perturbations. We introduce two-dimensional
spectroscopy of the linear and nonlinear bound-state response and resolve
anharmonicities in the soliton interaction leading to overtone and sub-harmonic
generation. Employing a non-perturbative interaction, we demonstrate
all-optical switching between distinct states with different binding
separation, opening up novel schemes of ultrafast spectroscopy, optical logic
operations and all-optical memory.Comment: 3 figure
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
Structure and Non-Equilibrium Heat-Transfer of a Physisorbed Molecular Layer on Graphene
The structure of a physisorbed sub-monolayer of 1,2-bis(4-pyridyl)ethylene
(bpe) on epitaxial graphene is investigated by Low-Energy Electron Diffraction
and Scanning Tunneling Microscopy. Additionally, non-equilibrium heat-transfer
between bpe and the surface is studied by Ultrafast Low-Energy Electron
Diffraction. Bpe arranges in an oblique unit cell which is not commensurate
with the substrate. Six different rotational and/or mirror domains, in which
the molecular unit cell is rotated by 28{\pm}0.1{\deg} with respect to the
graphene surface, are identified. The molecules are weakly physisorbed, as
evidenced by the fact that they readily desorb at room temperature. At liquid
nitrogen temperature, however, the layers are stable and time-resolved
experiments can be performed. The temperature changes of the molecules and the
surface can be measured independently through the Debye-Waller factor of their
individual diffraction features. Thus, the heat flow between bpe and the
surface can be monitored on a picosecond timescale. The time-resolved
measurements, in combination with model simulations, show the existence of
three relevant thermal barriers between the different layers. The thermal
boundary resistance between the molecular layer and graphene was found to be
2{\pm}1{\cdot}10-8 K m2 W-1
Few-nm tracking of magnetic vortex orbits and their decay with ultrafast Lorentz microscopy
Transmission electron microscopy is one of the most powerful techniques to
characterize nanoscale magnetic structures. In light of the importance of fast
control schemes of magnetic states, time-resolved microscopy techniques are
highly sought after in fundamental and applied research. Here, we implement
time-resolved Lorentz imaging in combination with synchronous radio-frequency
excitation using an ultrafast transmission electron microscope. As a model
system, we examine the current-driven gyration of a vortex core in a 2
m-sized magnetic nanoisland. We record the trajectory of the
vortex core for continuous-wave excitation, achieving a localization precision
of 2nm with few-minute integration times. Furthermore, by tracking the
core position after rapidly switching off the current, we find a temporal
hardening of the free oscillation frequency and an increasing orbital decay
rate attributed to local disorder in the vortex potential
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