arXiv subjects

Sven Ebel

Publications and source records attributed to Sven Ebel.

8 recordsLinked to original sources

arXiv2026

Spectral Anisotropy in Transition Radiation from Biaxial Media

In anisotropic optical media, the electromagnetic response depends on the orientation of the optical field relative to the material's principal dielectric axes. While this direction dependence is well understood in conventional optics, it should also influence light-generation processes driven by free electrons. Here, we experimentally observe spectrally anisotropic transition radiation from biaxial van der Waals crystals. Using cathodoluminescence spectroscopy on germanium sulphide (GeS) and molybdenum oxydichloride (MoOCl$_2$) crystals, we show that the transition-radiation spectra differ along the principal in-plane optical axes. To describe this spectral anisotropy, we develop a thin-film transition-radiation model that reproduces the experimental observations. Our results demonstrate that transition radiation is a sensitive probe of the axis-dependent dielectric response of biaxial optical media and suggest that optical anisotropy can provide an additional degree of freedom for free electron-driven spectroscopy, radiation sources, and transition-radiation-based diagnostics.

physics.optics↗

arXiv2026

Controlling Electron-Beam-Induced Charging in Colloidal Quantum Dots

Colloidal quantum dots (QDs) are attractive nanoscale emitters, yet their cathodoluminescence (CL) response remains poorly understood and often unstable under electron-beam excitation, limiting CL spectroscopy and electron-beam-based device processing. Here, we investigate the CL mechanism and strategies to improve its stability using highly photostable, structurally homogeneous giant-shell CdSe/CdS QDs combined with in situ CL and photoluminescence (PL) measurements. By identifying distinct signatures of excited states in both lifetime and spectral measurements, we demonstrate that the CL response is governed by electron-beam-induced charging. Charge accumulation drives multiexciton generation even at relatively low currents, leading to a pronounced blueshift, shorter average lifetimes, and rapid cathodobleaching. To test this further, we employ indirect excitation to reach sub-pA currents beyond the limits of typical electron beams, showing that neutral-exciton emission can be partially recovered and cathodobleaching mitigated, although charging cannot be fully suppressed. Furthermore, by replacing long insulating ligands with shorter ones, we improve charge drainage and strongly suppress biexciton formation. Together, these results show that biexciton formation can be controlled by limiting charge accumulation, providing a practical route toward stable CL for spectroscopy, imaging, and electron-beam-compatible photonic devices.

cond-mat.mes-hall↗

arXiv2026

Velocity-tunable exciton-photon hybridization in cathodoluminescence

Exciton-photon hybridization is typically realised in geometrically defined optical cavities, where tunability is achieved by modifying either the cavity or the excitonic medium. Here we investigate transition-radiation interferences in suspended subwavelength films resembling a free-electron-defined resonance and explore their interaction with excitons in transition metal dichalcogenides. We demonstrate that these resonances hybridize with excitonic transitions and can be tuned continuously by varying the electron energy. The resulting detuning depends on both film thickness and electron velocity, establishing the latter as an external and continuous knob for exciton-photon coupling. This approach enables tunable hybridization without structural modification and provides a free-electron-driven nanoscale platform for studying exciton-light interactions.

physics.optics↗

arXiv2026

Stimulated interactions of low-energy free-electrons with light

Free-electron interactions with light and matter have long served as a cornerstone for exploring the quantum and ultrafast dynamics of material excitation. In recent years, this paradigm has evolved from a classical description of radiation and acceleration toward a fully quantum framework, transforming our understanding of light-matter interactions at the single-electron level. These advances have opened new opportunities in high-resolution imaging, ultrafast spectroscopy, interferometry, and the coherent shaping of electron wavepackets. This review surveys stimulated interactions between slow electrons and light, encompassing free-space and near-field mediated mechanisms. We discuss how free-space optical fields coherently modulate electron momentum and energy, and how near-field coupling in nanophotonic and plasmonic structures enables strong, phase-matched, efficient momentum exchange with the electron wavepacket. We further describe electron recoil, which is significant in the slow-electron regime, and temporal and spatial wavepacket shaping that enhances coupling efficiency and extends access to quantum-coherent regimes. Building on these foundations, we outline emerging frameworks including hybrid optical-electrostatic modulation, ponderomotive laser-based aberration correction, and optical electron interferometry. By unifying these developments, stimulated electron-light interactions provide a versatile route to precise beam control, quantum-state engineering, and tailored light-matter coupling, with implications for ultrafast spectroscopy, nanoscale metrology, attosecond pulse generation, electron-photon entanglement, and the creation of nonclassical states of light.

physics.optics↗

arXiv2025

Substrate-Assisted Cathodoluminescence

Electron-beam-induced luminescence typically relies on direct excitation by high energy primary electrons. Here, we explore properties of an alternative excitation approach where cathodoluminescence (CL) is driven by substrate-generated electrons rather than by the primary electron beam. Using color centers in diamond as sensitive and durable local probes, we investigate the spatial profiles of such indirect CL in different geometries and substrates. Photon-correlation experiments demonstrate increased synchronization of emitters at reduced currents, which we propose as a method for extracting the effective indirect excitation currents experienced by the emitters. This approach enables the estimation of remarkably low currents, down to 0.1 pA, highlighting the potential of substrate-assisted excitation for minimally invasive probing of sensitive emitters in CL microscopy.

physics.optics↗

arXiv2024

An atlas of photonic and plasmonic materials for cathodoluminescence microscopy

Cathodoluminescence (CL) microscopy has emerged as a powerful tool for investigating the optical properties of materials at the nanoscale, offering unique insights into the behavior of photonic and plasmonic materials under electron excitation. We introduce an atlas of bulk CL spectra and intensity for a broad range of materials used in photonics and plasmonics. Through a combination of experimental CL microscopy and Monte Carlo simulations, we characterize spectra and intensity of coherent and incoherent CL, electron penetration depth and energy deposition, offering a foundational reference for interpreting CL signals and understanding material behavior under electron excitation. Our atlas captures CL signals across a wide range of materials, offering valuable insight into intrinsic emission properties for informed material selection and device design in photonics and plasmonics.

physics.optics↗

arXiv2024

Structured free-space optical fields for transverse and longitudinal control of electron matter waves

Controlling free-electron momentum states is of high interest in electron microscopy to achieve momentum and energy resolved probing and manipulation of physical systems. Free-electron and light interactions have emerged as a powerful technique to accomplish this. Here, we demonstrate both longitudinal and transverse phase control of a slow electron wavepacket by extending the Kapitza-Dirac effect to spatially-structured pulsed laser beams. This extension enables both inelastic and elastic stimulated Compton scattering. The interaction reveals the formation of distinct electron transverse momentum orders, each demonstrating a comb-like electron energy spectrum. By exerting complete control over light parameters, including wavelength, field intensity, pulse duration, and spatial mode order, as well as their combinations, it is possible to coherently control the population of these electron energy-momentum states that are separated by a few meV energy and multiple photon momentum orders. This free-space electron-light interaction phenomenon possesses the capability to coherently control the energy and momentum of electron beams in electron microscopes. Moreover, it has the potential to facilitate the selective probing of various material excitations, including plasmons, excitons, and phonons, and performing Talbot-Lau matter-wave interferometry with transversely shaped electron beams.

quant-ph↗

arXiv2023

Inelastic Electron Scattering at a Single-Beam Structured Light Wave

In this work we demonstrate the inelastic scattering of slow-electron wavepackets at a propagating Hermite-Gaussian light beam. The pulsed Hermite-Gaussian beam thereby forms a ponderomotive potential for the electron with large enough momentum components, leading to the inelastic scattering of electrons and their bunching along the longitudinal direction. We show that the resulting energy-gain spectra after the interaction is strongly influenced by the self-interference of the electron in this ponderomotive potential. It is shown that this effect is observable for various optical wavelengths and intensities and further discuss how the variation of the electron velocity and the light intensity allow to control the energy modulation of the electron wavepacket. This effect opens up a new platform for manipulating the electron wavepacket by utilizing the vast landscape of structured electromagnetic fields.

physics.optics↗