arXiv subjects

Peter Baum

Publications and source records attributed to Peter Baum.

15 recordsLinked to original sources

arXiv2026

Localized Photon Absorption in a Single-Crystalline Material

The absorption of light is one of the most fundamental processes in condensed-matter physics and optics. Here we investigate under which conditions laser light is absorbed by a crystalline material as an electromagnetic wave with delocalized properties or rather as photons that cause discrete, localized, nanometer-sized consequences. We excite the first-order phase transition of vanadium dioxide with laser pulses of sufficient frequency to overcome the band gap but with insufficient pulse energy to overcome the latent heat. According to Maxwell's equations and Bloch theory, no transition should occur, because nowhere in the material is enough energy. Nevertheless, we observe with ultrafast electron diffraction a disordered crystal geometry with nanometer-sized spots of switched material that grow and diminish with time. The amount of localized spots matches approximately to the number of photons in the absorbed laser wave. Two optical experiments substantiate this phenomenon, and simulations reproduce all measurements results. We discuss whether crystals defects, temperature, or a genuine wavefunction collapse can explain the discovered phenomenon. Practically, the reported absorption mechanism enables local consequences at substantially higher energy than average and provides insight into symmetry breaks and non-thermal fluctuations within complex materials.

physics.optics↗

arXiv2026

Anisotropic light-electron-phonon coupling and ultrafast carrier separation in ferroelectric BaTiO$_3$

Ferroelectric materials with built-in electric fields are useful for ultrafast electronics and conversion of light into electrical energy, yet the interaction of carrier motion with ultrafast relaxation processes remains nontrivial. Combining ultrafast electron diffraction with electron microscopy of electromagnetic fields, we capture ultrafast lattice dynamics and nanometer-scale carrier transport in ferroelectric BaTiO$_3$. We discover that BaTiO$_3$ reacts to light with an anisotropic electron-phonon coupling that depends on the optical polarization of the excitation light. Excited electrons relax two times faster into phonons when the optical electric field aligns to the ferroelectric symmetry break. Furthermore, ultrafast electron electrometry captures the motion and separation of photo-excited electron-hole pairs in the presence of the ferroelectric field. These combined results provide insight into the tangled and anisotropic interaction of photons with phonons and the ferroelectric field, producing phonons and voltages in a stepwise reaction path.

cond-mat.mtrl-sci↗

arXiv2025

Two-electron quantum walks can probe entanglement and decoherence in an electron microscope

Classical physics is often a good approximation for quantum systems composed of many interacting particles, although wavepacket dispersion and scattering processes continuously induce delocalization and entanglement. According to decoherence theory, an entangled ensemble can appear classical when only a subset of all particles is observed. This emergence of macroscopic phenomena from quantum interactions is, for example, relevant for phase transitions, quantum thermalization, hydrodynamics, spin liquids, or time crystals. However, entanglement and decoherence in free electrons have not yet been explored, although the electron is a fundamental elementary particle with extraordinary technological relevance. Here, we investigate the degree of coherence and entanglement in a free-space electron gas in the beam of an ultrafast electron microscope. We introduce a two-electron quantum walk that transforms the quantum state into different bases for quantum state tomography of entangled or partially entangled electron-electron pairs. We apply this novel diagnostic to study quantum effects in short pulses of hundreds of electrons under strong Coulomb correlation. We observe a high contrast interference in the electron-electron correlations but no significant signs of electron-electron entanglement which we explain by limited purity of the initial states and decoherence effects from unmeasured reservoir electrons. The ability to characterize quantum states of multiple free electrons may allow verification of electron-electron entanglement for use in fundamental studies and quantum electron microscopy.

quant-ph↗

arXiv2025

Roadmap for Quantum Nanophotonics with Free Electrons

Over the past century, continuous advancements in electron microscopy have enabled the synthesis, control, and characterization of high-quality free-electron beams. These probes carry an evanescent electromagnetic field that can drive localized excitations and provide high-resolution information on material structures and their optical responses, currently reaching the sub-{\aa}ngstr\"om and few-meV regime. Moreover, combining free electrons with pulsed light sources in ultrafast electron microscopy adds temporal resolution in the sub-femtosecond range while offering enhanced control of the electron wave function. Beyond their exceptional capabilities for time-resolved spectromicroscopy, free electrons are emerging as powerful tools in quantum nanophotonics, on par with photons in their ability to carry and transfer quantum information, create entanglement within and with a specimen, and reveal previously inaccessible details on nanoscale quantum phenomena. This Roadmap outlines the current state of this rapidly evolving field, highlights key challenges and opportunities, and discusses future directions through a collection of topical sections prepared by leading experts.

cond-mat.mes-hall↗

arXiv2024

Canalized Light Creates Directional and Switchable Surface Structures in Vanadium Dioxide

Materials with switchable nanostructured surfaces enable optical and electronic functionalities beyond those of natural materials. Here we report the creation of self-organized, re-writable, laser-induced surface structures in single-crystalline vanadium dioxide. We discover anisotropic features caused by canalized surface plasmon polaritons that can only propagate along one crystal axis. The nanostructures remain single-crystalline and preserve the raw material's metal-to-insulator transition, enabling femtosecond switching by temperature or light.

cond-mat.mes-hall↗

arXiv2024

Femtosecond electron beam probe of ultrafast electronics

The need for ever-faster information processing requires extremely small devices operating at frequencies approaching the terahertz and petahertz regimes. For the diagnostics of such devices, researchers need a spatiotemporal tool that surpasses the device under test in speed and spatial resolution and therefore, such a tool cannot be provided by electronics itself. Here we show how ultrafast electron beam probe with terahertz-compressed electron pulses can directly sense local electro-magnetic fields in electronic devices with femtosecond, micrometre and millivolt resolution under normal operation conditions. We analyse the dynamical response of a coplanar waveguide circuit and reveal the impulse response, signal reflections, attenuation and waveguide dispersion directly in the time domain. The demonstrated measurement bandwidth reaches 10 THz and the sensitivity to electric fields is tens of millivolts or -20 dBm. Femtosecond time resolution and the potential to directly integrate our technique into existing electron-beam inspection devices in semiconductor industry makes our femtosecond electron beam probe a promising tool for research and development of next-generation electronics at unprecedented speed and size.

physics.app-ph↗

arXiv2023

Field-induced rocking curve effects in attosecond electron diffraction

Recent advances in electron microscopy trigger the question whether attosecond electron diffraction can resolve atomic-scale electron dynamics in crystalline materials in space and time. Here we explore the physics of the relevant electron-lattice scattering process in the time domain. We drive a single-crystalline silicon membrane with the optical cycles of near-infrared laser light and use attosecond electron pulses to produce electron diffraction patterns as a function of delay. For all Bragg spots, we observe time-dependent intensity changes and position shifts that are correlated with a time delay of 0.5-1.2 fs. For single-cycle excitation pulses with strong peak intensity, the correlations become nonlinear. Origin of these effects are local and integrated beam deflections by the optical electric and magnetic fields at the crystal membrane that modify the diffraction intensities in addition to the atomic structure factor dynamics by time-dependent rocking-curve effects. However, the measured time delays and symmetries allow to disentangle both effects. Future attosecond electron diffraction and microscopy experiments need to be based on these results.

physics.atom-ph↗

arXiv2023

Determination of the nearest-neighbor interaction in VO$_2$ via fractal dimension analysis

The Ising model is one of the simplest and most well-established concepts to simulate phase transformations in complex materials. However, its most central constant, the interaction strength J between two nearest neighbors, is hard to obtain. Here we show how this basic constant can be determined with a fractal dimension analysis of measured domain structures. We apply this approach to vanadium dioxide, a strongly correlated material with a first-order insulator-to-metal phase-transition with enigmatic properties. We obtain a nearest-neighbor interaction of 13.8 meV, a value close to the thermal energy at room temperature. Consequently, even far below the transition temperature, there are spontaneous local phase-flips from the insulating into the metallic phase. These fluctuations explain several measured anomalies in VO$_2$, in particular the low thermal carrier activation energy and the finite conductivity of the insulating phase. As a method, our fractal dimension analysis links the Ising model to macroscopic material constants for almost any first-order phase transition.

cond-mat.stat-mech↗

arXiv2022

Spatiotemporal attosecond control of electron pulses via subluminal terahertz waveforms

Shaping electron beams with the cycles of light provides femtosecond and attosecond time resolution in electron microscopy and enables fundamental quantum-coherent measurements. However, efficient light-electron control requires a prolonged interaction between the two beams for cascaded transfer of photon energy and momentum to the freely propagating electrons. Here we report the use of traveling evanescent terahertz waves to achieve velocity matching and thereby high acceleration gradients both in space and in time. With experiment and simulations, we demonstrate attosecond streaking, temporal pulse compression, acceleration and spatial focusing of sub-relativistic electron pulses with a single evanescent-wave element under the control of selected terahertz delays and phases. Based on these results, we propose to use a symmetric arrangement with two evanescent terahertz waves to generate isolated attosecond electron pulses in a beam with realistic parameters. These results establish subluminal terahertz waves as a promising tool for ultrafast electron pulse control.

physics.optics↗

arXiv2021

Free-electron tomography of few-cycle optical waveforms

Ultrashort light pulses are ubiquitous in modern research, but the electromagnetic field of the optical cycles is usually not easy to obtain as a function of time. Field-resolved pulse characterization requires either a nonlinear-optical process or auxiliary sampling pulses that are shorter than the waveform under investigation, and pulse metrology without at least one of these two prerequisites is often thought to be impossible. Here we report how the optical field cycles of laser pulses can be characterized with a field-linear sensitivity and no short probe events. We let a free-space electron beam cross with the waveform of interest. The randomly arriving electrons interact by means of their elementary charge with the optical waveform in a linear-optical way and reveal the optical cycles as the turning points in a time-integrated deflection histogram on a screen. The sensitivity of the method is only limited by the emittance of the electron beam and can reach the level of thermal radiation and vacuum fluctuations. Besides overturning a common belief in optical pulse metrology, the idea also provides practical perspectives for in-situ characterization and optimization of optical waveforms in higher-harmonics experiments, ultrafast transmission electron microscopes, laser-driven particle accelerators, free-electron lasers or generally any experiments with waveform-controlled pulses in a vacuum environment.

physics.optics↗

arXiv2021

Photo-switchable nanoripples in Ti3C2Tx MXene

MXenes are two-dimensional materials with a rich set of remarkable chemical and electromagnetic properties, the latter including saturable absorption and intense surface plasmon resonances. To fully harness the functionality of MXenes for applications in optics, electronics and sensing, it is important to understand the interaction of light with MXenes on atomic and femtosecond dimensions. Here, we use ultrafast electron diffraction and high-resolution electron microscopy to investigate the laser-induced structural dynamics of Ti3C2Tx nanosheets. We find an exceptionally fast lattice response with an electron-phonon coupling time of 230 femtoseconds. Repetitive femtosecond laser excitation transforms Ti3C2Tx through a structural transition into a metamaterial with deeply sub-wavelength nanoripples that are aligned with the laser polarization. By a further laser illumination, the material is reversibly photo-switchable between a flat and rippled morphology. The resulting nanostructured MXene metamaterial with directional nanoripples is expected to exhibit an anisotropic optical and electronic response as well as an enhanced chemical activity that can be switched on and off by light.

cond-mat.mtrl-sci↗

arXiv2020

Single-cycle optical control of beam electrons

Single-cycle optical pulses with a controlled electromagnetic waveform allow to steer the motion of low-energy electrons in atoms, molecules, nanostructures or condensed-matter on attosecond dimensions in time. However, high-energy electrons under single-cycle light control would be an enabling technology for beam-based attosecond physics with free-electron lasers or electron microscopy. Here we report the control of freely propagating keV electrons with an isolated optical cycle of mid-infrared light and create a modulated electron current with a peak-cycle-specific sub-femtosecond structure in time. The evident effects of the carrier-envelope phase, amplitude and dispersion of the optical waveform on the temporal composition, pulse durations and chirp of the free-space electron wavefunction demonstrate the sub-cycle nature of our control. These results create novel opportunities in laser-driven particle acceleration, seeded free-electron lasers, attosecond space-time imaging, electron quantum optics and wherever else high-energy electrons are needed with the temporal structure of single-cycle light.

physics.optics↗

arXiv2019

Asymmetric nonlinear optics of a polar chemical bond

A dielectric material's response to light is macroscopically described by electric displacement fields due to polarization and susceptibility, but the atomistic origin is light-cycle-driven motion of electron densities in the restoring forces of the atomic environment. Here we report how the macroscopic nonlinear-optical response of a heteronuclear crystal relates to the alignment and orientation of its chemical bonds. Substantial nonlinear emission is only observed if the electric field of an optical single-cycle pulse points from the less electronegative to the more electronegative element and not vice versa. This asymmetry is a consequence of the unbalanced real-space motion of valence charges along the direction of the bonds. These results connect a material's chemical structure to the optical response and may facilitate the comprehension and design of novel materials for applications in optics and lasers on basis of the atoms and how they connect.

physics.optics↗

arXiv2018

Attosecond Control of Electron Beams at Dielectric and Absorbing Membranes

Ultrashort electron pulses are crucial for time-resolved electron diffraction and microscopy of fundamental light-matter interaction. In this work, we study experimentally and theoretically the generation and characterization of attosecond electron pulses by optical-field-driven compression and streaking at dielectric or absorbing interaction elements. The achievable acceleration and deflection gradient depends on the laser-electron angle, the laser's electric and magnetic field directions and the foil orientation. Electric and magnetic fields have similar contributions to the final effect and both need to be considered. Experiments and theory agree well and reveal the optimum conditions for highly efficient, velocity-matched electron-field interactions in longitudinal or transverse direction. We find that metallic membranes are optimum for light-electron control at mid-infrared or terahertz wavelengths, but dielectric membranes are excel in the visible/near-infrared regimes and are therefore ideal for the formation of attosecond electron pulses.

physics.optics↗

arXiv2013

A tip-based source of femtosecond electron pulses at 30keV

We present a nano-scale photoelectron source, optimized towards ultrashort pulse durations and well-suited for time-resolved diffraction experiments. A tungsten tip, mounted in a suppressor-extractor electrode configuration, allows the generation of 30 keV electron pulses with an estimated pulse duration of 37 fs at the gun exit. We infer the pulse duration from particle tracking simulations, which are in excellent agreement with experimental measurements of the electron-optical properties of the source. We furthermore demonstrate femtosecond laser-triggered operation. Besides the short electron pulse duration, a tip-based source is expected to feature a large transverse coherence as well as a nanometric emittance.

physics.acc-ph↗