arXiv subjects

Alexander Schröder

Publications and source records attributed to Alexander Schröder.

6 recordsLinked to original sources

arXiv2026

Vibrational high-harmonics and period-doubling bifurcation probed by time-resolved electron diffraction

Nanoscale mechanical oscillators exhibit a plethora of nonlinear phenomena with promising applications for the sensing and clocking of processes down to atomic length scales. Oscillator dynamics are typically probed by electrical or optical means, providing only limited access to the spatial profile of the oscillator motion. Here, we introduce event-based convergent beam electron diffraction for the spatio-temporal mapping of nanoscale mechanical resonators in ultrafast transmission electron microscopy. Employing an optically driven silicon membrane resonator at various driving strengths, we gain access to nonlinear processes with increasing complexity, ranging from a simple Duffing behavior to nonlinear multimode coupling and period-doubling bifurcations. The time-resolved diffraction probing approach supports a spatial resolution down to a few nanometers and a temporal resolution of 5 ns and provides quantitative information on the local membrane bending. Because the diffraction signal responds to local displacement gradients, which become more pronounced as resonators shrink, this approach offers a route toward probing nonlinear nanomechanics at the atomic scale.

physics.optics↗

arXiv2025

All-optical stochastic switching of magnetization textures in Fe$_3$Sn$_2$

The all-optical control of magnetization at room temperature broadens the scope of applications of spin degrees-of-freedom in data storage, spintronics, and quantum computing. Topological magnetic spin structures, such as skyrmions, are of particular interest due to their particle-like properties, small size and inherent stability. Controlling skyrmion states without strong magnetic fields or large current densities would create new possibilities for their application. In this work, we utilize femtosecond optical pulses to alter the helicity of the spin configuration in dipolar skyrmions formed in the kagome magnet Fe$_3$Sn$_2$ in the absence of an external magnetic field and at room temperature. In situ Lorentz transmission electron microscopy is used to visualize the stochastic, light-induced switching process of chiral Néel caps, while the internal Bloch component of the dipolar skyrmions remain unchanged. In addition to this switching process, we observe the interconversion between type I skyrmionic and type II bubble configurations depending on the external magnetic field and illumination conditions. To corroborate the spin states and the light-induced magnetization dynamics, micromagnetic modelling and simulations of the resulting electron phase shift maps are conducted to elucidate the spin rearrangement induced by individual femtosecond optical pulses.

physics.optics↗

arXiv2024

Laser-driven cold-field emission source for ultrafast transmission electron microscopy

Ultrafast transmission electron microscopy (UTEM) has emerged as a versatile technique for the time-resolved imaging of nanoscale dynamics on timescales down to few-hundred attoseconds but the temporal and spatial resolutions are still limited by the coherence properties of pulsed electron sources. Here, we report the development of a novel laser-driven linear cold-field electron emitter integrated in a state-of-the-art UTEM system. Illuminating the sharp tungsten emitter tip with a UV light pulse generates ultrashort femtosecond electron pulses of 220 fs pulse duration, with energy widths as low as 360 meV. The photoelectron emitter demonstrates exceptional spatial coherence, achieving focal spot sizes down to 2 $\mathring {\mathrm A}$ and a peak normalized brightness exceeding 6.7 $\times 10^{13}$ A/m$^2$sr. With an order-of-magnitude improvement compared to previously employed laser-driven Schottky field emitters, the present development opens up the field of ultrafast atomic-scale electron probing.

physics.ins-det↗

arXiv2024

Visualizing Standing Light Waves in Continuous-Beam Transmission Electron Microscopy

The phase-resolved imaging of confined light fields by homodyne detection is a cornerstone of metrology in nano-optics and photonics, but its application in electron microscopy has been limited so far. Here, we report the mapping of optical modes in a waveguide structure by illumination with femtosecond light pulses in a continuous-beam transmission electron microscope. Multi-photon photoemission results in a remanent charging pattern which we image by Lorentz microscopy. The resulting image contrast is linked to the intensity distribution of the standing light wave and quantitatively described within an analytical model. The robustness of the approach is showcased in a wider parameter range and more complex sample geometries including micro- and nanostructures. We discuss further applications of light-interference-based charging for electron microscopy with in-situ optical excitation, laying the foundation for advanced measurement schemes for the phase-resolved imaging of propagating light fields.

cond-mat.mes-hall↗

arXiv2023

Improving the temporal resolution of event-based electron detectors using neural network cluster analysis

Novel event-based electron detector platforms provide an avenue to extend the temporal resolution of electron microscopy into the ultrafast domain. Here, we characterize the timing accuracy of a detector based on a TimePix3 architecture using femtosecond electron pulse trains as a reference. With a large dataset of event clusters triggered by individual incident electrons, a neural network is trained to predict the electron arrival time. Corrected timings of event clusters show a temporal resolution of 2 ns, a 1.6-fold improvement over cluster-averaged timings. This method is applicable to other fast electron detectors down to sub-nanosecond temporal resolutions, offering a promising solution to enhance the precision of electron timing for various electron microscopy applications.

physics.ins-det↗

arXiv2014

The Impact of Oceanic Heat Transport on the Atmospheric Circulation: a Thermodynamic Perspective

The present study investigates how global thermodynamic properties of the climate system are affected by the changes in the intensity of the imposed oceanic heat transport in an atmospheric general circulation model in aqua-planet configuration. Increasing the poleward oceanic heat transport results in an overall increase in the surface temperature and a decrease in the equator-to-pole surface temperature difference as a result of the ice-albedo feedback. Following the classical ansatz by Stone, the atmospheric heat transport changes in such a way that the total poleward heat transport remains almost unchanged. We also find that the efficiency of the climate machine, the intensity of the Lorenz energy cycle and the material entropy production of the system decline with increased oceanic heat transport which suggests that the climate system becomes less efficient and turns into a state of reduced entropy production, as the enhanced oceanic transport performs a stronger large-scale mixing between geophysical fluids with different temperature, thus reducing the availability in the climate system and bringing it closer to a state of thermal equilibrium.

physics.ao-ph↗