arXiv subjects

Till Domröse

Publications and source records attributed to Till Domröse.

7 recordsLinked to original sources

arXiv2026

Ultrafast order-selective electron imaging and spectroscopy

Ultrafast pump-probe spectroscopies in energy and momentum have become indispensable for probing non-equilibrium dynamics in quantum materials, revealing pathways in ultrafast energy conversion and light-induced phase transitions. Different modalities uniquely access the coupled lattice, charge, and spin degrees of freedom that underpin the functionality of these materials. However, investigating technologically relevant nanoscale devices also requires high spatial resolution. Although electron microscopy provides nanometer-scale imaging, it frequently lacks the simultaneous ultrafast temporal resolution and spectroscopic specificity needed for such studies. In this perspective, we review recent advances in dark-field electron and photoelectron microscopy, focusing on two complementary techniques: femtosecond photoelectron momentum microscopy and ultrafast transmission electron microscopy. Their application enables comprehensive insights into the ultrafast dynamics of the electron, lattice, and spin subsystems. These order-selective electron imaging and spectroscopy techniques open broad scientific opportunities for a microscopic understanding of non-equilibrium phenomena in quantum materials, nanostructures and devices.

cond-mat.mtrl-sci↗

arXiv2025

Megahertz cycling of ultrafast structural dynamics enabled by nanosecond thermal dissipation

Light-matter interactions are of fundamental scientific and technological interest. Ultrafast electron microscopy and diffraction with combined femtosecond-nanometer resolution elucidate the laser-induced dynamics in structurally heterogeneous systems. These measurements, however, remain challenging due to the brightness limitation of pulsed electron sources, leading to an experimental trade-off between resolution and contrast. Larger signals can most directly be obtained by higher repetition rates, which, however, are typically limited to a few kHz by the thermal relaxation of thin material films. Here, we combine nanometric electron-beam probing with sample support structures tailored to facilitate rapid specimen cooling. Optical cycling of a charge-density wave transformation enables quantifying the mean temperature increase induced by pulsed laser illumination. Varying the excitation fluence and repetition rate, we gauge the impact of excitation confinement and efficient dissipation on the heat diffusion in different sample designs. In particular, a thermally optimized support can dissipate average laser intensities of up to 200 $μW/μm^2$ within a few nanoseconds, allowing for reversible driving and probing of the CDW transition at a repetition rate of 2 MHz. Sample designs tailored to ultrafast measurement schemes will thus extend the capabilities of electron diffraction and microscopy, enabling high-resolution investigations of structural dynamics.

cond-mat.mtrl-sci↗

arXiv2024

Femtosecond trimer quench cycled at megahertz rates in the unconventional charge-density wave material $1\textit{T'}-\text{TaTe}_2$

Ultrafast optical switching of materials properties is of great relevance both for future technological applications as well as gaining fundamental physical insights to microscopic couplings and nonequilibrium phenomena. Transition-metal dichalcogenides (TMDCs) combine photo-sensitivity with strong correlations, furthering rich phase diagrams and enhanced tunability. Owing to its chemical composition, $1\textit{T'}-\text{TaTe}_2$ exhibits an electronically and structurally unique set of charge-density waves (CDWs), featuring increased conductivity and a reduced prominence of amplitude modes. Compared to other charge-ordered TMDCs, only very few studies addressed the ultrafast response of this material to optical excitation. In particular, the question whether such unconventional properties translate to unusual quench dynamics remains largely unresolved. Here, we investigate the structural dynamics in $1\textit{T'}-\text{TaTe}_2$ by means of ultrafast nanobeam electron diffraction. The experiments are carried out with a tailored sample design that allows for excitation at 2$\,$MHz repetition rate, higher than any structural phase transformation probed thus far. Harnessing the enhanced resolution and sensitivity of this approach, we reveal a strongly directional cooperative atomic motion during the one-dimensional quench of the low-temperature trimer lattice. These dynamics are completed within less than 500$\,$fs, substantially faster than reported previously. In striking contrast, the periodic lattice distortion of the room-temperature phase is unusually robust against high-density electronic excitation. In conjunction with the known sensitivity of $1\textit{T'}-\text{TaTe}_2$ to chemical doping, we thus expect the material to serve as a versatile platform for tunable structural control by optical stimuli.

cond-mat.mtrl-sci↗

arXiv2023

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.

cond-mat.mes-hall↗

arXiv2022

Light-induced hexatic state in a layered quantum material

The tunability of materials properties by light promises a wealth of future applications in energy conversion and information technology. Strongly correlated materials such as transition-metal dichalcogenides (TMDCs) offer optical control of electronic phases, charge ordering and interlayer correlations by photodoping. Here, we find the emergence of a transient hexatic state in a TMDC thin-film during the laser-induced transformation between two charge-density wave (CDW) phases. Introducing tilt-series ultrafast nanobeam electron diffraction, we reconstruct CDW rocking curves at high momentum resolution. An intermittent suppression of three-dimensional structural correlations promotes a loss of in-plane translational order characteristic of a hexatic intermediate. Our results demonstrate the merit of tomographic ultrafast structural probing in tracing coupled order parameters, heralding universal nanoscale access to laser-induced dimensionality control in functional heterostructures and devices.

cond-mat.mtrl-sci↗

arXiv2020

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.

cond-mat.mtrl-sci↗

arXiv2016

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 9 Å focused beam diameter, 200 fs pulse duration and 0.6 eV 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.

cond-mat.mes-hall↗