Marcel Möller

Ultrafast transmission electron microscopy using a laser-driven field emitter: Femtosecond resolution with a high coherence electron beam

We present the development of the first ultrafast transmission electron microscope (UTEM) driven by localized photoemission from a field emitter cathode. We describe the implementation of the instrument, the photoemitter concept and the quantitative electron beam parameters achieved. Establishing a new source for ultrafast TEM, the Göttingen UTEM employs nano-localized linear photoemission from a Schottky emitter, which enables operation with freely tunable temporal structure, from continuous wave to femtosecond pulsed mode. Using this emission mechanism, we achieve record pulse properties in ultrafast electron microscopy of 9Å focused beam diameter, 200fs pulse duration and 0.6eV energy width. We illustrate the possibility to conduct ultrafast imaging, diffraction, holography and spectroscopy with this instrument and also discuss opportunities to harness quantum coherent interactions between intense laser fields and free-electron beams.

Light-Induced Metastable Magnetic Texture Uncovered by in situ Lorentz Microscopy

Magnetic topological defects, such as vortices and Skyrmions, can be stabilized as equilibrium structures in nanoscale geometries and by tailored intrinsic magnetic interactions. Here, employing rapid quench conditions, we report the observation of a light-induced metastable magnetic texture, which consists of a dense nanoscale network of vortices and antivortices. Our results demonstrate the emergence of ordering mechanisms in quenched optically driven systems, which may give a general access to novel magnetic structures on nanometer length scales.

Nanoscale magnetic imaging using circularly polarized high-harmonic radiation

We introduce laboratory-scale magneto-optical imaging with sub–50-nm resolution using 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 exceptional 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.

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.

Highly coherent femtosecond electron pulses for ultrafast transmission electron microscopy

We describe the implementation and detailed characterization of a laser-triggered field-emitter electron source integrated into a modified transmission electron microscope. Specifically, localized linear photoemission from the front facet of a tip-shaped ZrO/W(100) Schottky emitter is employed, yielding electron pulses with a spectral bandwidth of 0.6 eV and pulse durations down to 200 fs (full-width-at-half-maximum). Furthermore, transverse electron beam properties are characterized for a range of TEM illumination conditions by caustic measurements of the focused beam in the sample plane of the electron microscope, demonstrating a beam emittance down to 1.8 nm mrad, photoelectron probe sizes below 1 nm and a degree of transverse coherence exceeding 10 %. The electron pulse properties achieved here enable ultrafast high-resolution phase-contrast imaging and Lorentz microscopy, electron holography and spatially-resolved electron spectroscopy, allowing for a comprehensive mapping of ultrafast processes in nanoscale systems.

Nanoscale magnetic imaging using a compact high-harmonic source

Compact sources based on high harmonic generation (HHG) offer experimental access to ultrafast dynamics, high-resolution imaging and spectroscopy, and also provide for a simultaneous probing of element-specific spin and charge carrier dynamics. Here, we extend the capabilities of compact EUV sources to the nanoscale imaging of magnetic structures by using circularly polarized harmonics in conjunction with Fourier transform holography.