arXiv subjects

Martin Kozák

Publications and source records attributed to Martin Kozák.

At least 19 recordsLinked to original sources

arXiv2026

Coherent regime of Kapitza-Dirac effect with electrons

Electron matter waves coherently diffract when passing through a periodic structure of light formed by two interfering light waves. In this so-called Kapitza-Dirac effect, the electron momentum changes due to absorption and emission of photons via stimulated Compton scattering. Until now, the effect has only been observed with low energy electrons due to the small momentum of a visible photon compared to the momentum of high energy electron leading to diffraction angles of 10^(-4) rad or smaller. We report on the observation of the Kapitza-Dirac effect in a scanning electron microscope using high energy (20 and 30 keV) electrons with de-Broglie wavelengths of 9 pm and 7 pm, respectively. The photon sidebands in the electron transverse momentum spectrum are detected in the convergent beam diffraction geometry using spatial filtering. As the coupling strength between the electrons and the light field increases, the sideband populations exhibit coherent, reversible oscillations among diffraction orders. The effect can serve as a coherent electron beam-splitter or a phase-plate in various types of electron microscopes.

quant-ph↗

arXiv2026

Transverse Modulation of Continuous Electron Beams by a Structured Optical Cavity

Compact aberration correction at large semi-angles remains a central challenge in electron microscopy. Here we propose a phase plate for continuous electron beams based on the ponderomotive interaction with an intracavity standing wave in a near-concentric Fabry--Pérot resonator. For electrons propagating along the cavity axis, a standing-wave with Laguerre-Gaussian mode of topological charge $l=1$ imprints an annular phase shift that is opposite in sign to the third-order spherical aberration of a conventional electron lens. For a $5~\mathrm{keV}$ beam with a convergence semi-angle of $25~\mathrm{mrad}$, we fully compensate the third-order spherical aberration of an objective lens with $C_\mathrm{s}=1~\mathrm{mm}$, yielding the transverse width of the corrected probe in the focus of $σ_\mathrm{p}=1.4~\mathrmÅ$. The accessible transverse electron phases are set by the supported resonator modal basis and its coherent superpositions.

physics.optics↗

arXiv2026

Independent amplitude and phase control using a single phase-only SLM

Liquid-crystal spatial light modulators (SLMs) are widely used for programmable wavefront control but are typically operated as phase-only devices, limiting applications that require independent amplitude and phase shaping. Here we demonstrate full complex-field modulation using a single phase-only SLM by implementing two sequential modulation planes on different regions of the same device. The phase retardance introduced by the first SLM region is converted into amplitude modulation by a polarizer placed in the beam path, while the second region compensates the associated phase offset and imposes the required phase distribution. The field from the first region is imaged onto the second, enabling complex-field synthesis without a second modulator. We validate the approach by generating Bessel--Gaussian beams, helical-phase fields, and arbitrary focal-plane intensity patterns. This single-SLM platform provides a compact route to programmable complex wavefront engineering for structured illumination, holography, and electron--light interaction experiments.

physics.optics↗

arXiv2026

Imaging the transverse component of optical near-fields in resonant photonic structures

We report on imaging the optical near-fields in resonant periodic photonic structures with nanometer resolution using ultrafast 4D scanning transmission electron microscopy (U4DSTEM). In particular, U4DSTEM is applied to visualize the transverse component of the Lorentz force of a synchronous near-field mode excited by an infrared femtosecond pulse in a periodic silicon nanostructure designed for photonic acceleration of electrons. Our results show that in addition to the accelerating/decelerating force acting on the electrons in the longitudinal direction along the electron propagation, the structures can be efficiently used for transverse electron streaking at optical frequencies when excited by light with polarization perpendicular to the electron trajectory. The measured spatial profile of the excited near-field mode intensity is consistent with the numerical simulations performed using finite-difference time domain technique.

physics.optics↗

arXiv2026

Quantum Optical Electron Pulse Shaper

Coherent control of ultrafast quantum phenomena benefits from pulse-shaping capabilities allowing to modulate the envelope and instantaneous phase of optical fields on femtosecond time scales. While such control is available for optical fields, an analogy of a pulse shaper for freely propagating electrons is lacking. In this study, we theoretically demonstrate a method that enables near arbitrary light-based shaping of electron wave packets in the time domain. The method is based on the quantum phase modulation of electron waves by coherent light with time-dependent frequency leading to generation of spectrally separated electron energy side bands with shaped time-energy profiles and envelopes. Our results show that few femtosecond time durations can be achieved without additional spectral broadening of the electron wave packet, allowing one to reach the combination of high time, spatial, and spectral resolutions in ultrafast imaging and diffraction experiments with pulsed electron beams.

physics.optics↗

arXiv2026

Coherent electronic Raman excitation of valley-orbit split states of phosphorus dopants in silicon

In this study, we demonstrate coherent optical excitation of the electronic Raman transition between the $1s\left(A_1\right)$ and $1s\left(E\right)$ split states of phosphorus donor in crystalline silicon. The dynamics of the generated wavepacket is characterized in the time domain using a degenerate pump-probe technique with mid-infrared femtosecond pulses via transient polarization anisotropy of the probe pulse. In addition, we study the role of resonantly excited carriers, and we show that the amplitude and coherence time of the electronic wavepacket depend on the pre-excited carrier density. Further, we demonstrate that under certain conditions, the Raman-type excitation changes to displacive impulsive excitation, which allows us to address the Raman-forbidden transition between $1s\left(A_1\right)$ and $1s\left(T_1\right)$.

cond-mat.mes-hall↗

arXiv2025

Light-based electron aberration corrector

Achieving atomic resolution in electron microscopy has historically been hindered by spherical aberration, a fundamental limitation of conventional electron lenses. Its correction typically requires complex assemblies of electromagnetic multipoles. Here, we demonstrate that spherical aberration in a cylindrically symmetric electron lens can be fully compensated via interaction with a shaped light field. By analyzing distortions in high-magnification point-projection electron images of optical standing waves, we quantify the spherical aberration before and after light-induced correction. This approach introduces a new paradigm for optical control in electron beam shaping and opens a pathway towards compact and tunable light-based aberration correctors for high-resolution electron microscopy.

physics.optics↗

arXiv2025

Ultrafast 4D scanning transmission electron microscopy for imaging of localized optical fields

Ultrafast electron microscopy aims for imaging transient phenomena occurring on nanoscale. One of its goals is to visualize localized optical and plasmonic modes generated by coherent excitation in the vicinity of various types of nanostructures. Such imaging capability was enabled by photoninduced near-field optical microscopy, which is based on spectral filtering of electrons inelastically scattered due to the stimulated interaction with the nearfield. Here, we report on the development of ultrafast four-dimensional (4D) scanning transmission electron microscopy, which allows us to image the transverse components of the optical near-field while avoiding the need of electron spectral filtering. We demonstrate that this method is capable of imaging the integrated Lorentz force generated by optical near-fields of a tungsten nanotip and the ponderomotive potential of an optical standing wave with a spatial resolution of 21 nm.

physics.optics↗

arXiv2025

Temporal characterization of femtosecond electron pulses inside ultrafast scanning electron microscope

In this work, we present the implementation of all-optical method for directly measuring electron pulse duration in an ultrafast scanning electron microscope. Our approach is based on the interaction of electrons with the ponderomotive potential of an optical standing wave and provides a precise in situ technique to characterize femtosecond electron pulses at the interaction region across a wide range of electron energies (1-30 keV). By using single-photon photoemission of electrons by ultraviolet femtosecond laser pulses from a Schottky emitter we achieve electron pulse durations ranging from 0.5 ps at 30 keV to 2.7 ps at 5.5 keV under optimal conditions where Coulomb interactions are negligible. Additionally, we demonstrate that reducing the photon energy of the femtosecond pulses used for photoemission from 4.8 eV (257.5 nm) to 2.4 eV (515 nm) decreases the initial energy spread of emitted electrons, leading to significantly shorter pulse durations, particularly at lower electron energies.

physics.optics↗

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-ångström 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↗

arXiv2025

Attosecond control of solid-state high harmonic generation using ω-3ω fields

High harmonic spectra generated in condensed matter carry the fingerprints of sub-cycle electronic motion and the energy structure of the studied system. Here we show that tailoring the waveform of mid-infrared driving light by using a coherent combination with its third harmonic frequency allows to control the time of electron tunneling to the conduction band within each half-cycle of the fundamental wave with attosecond precision. We introduce an experimental scheme in which we simultaneously monitor the modulation of amplitude and emission delays of high harmonic radiation and the excited electron population generated in crystalline silicon as a function of the relative phase between the $ω$-3$ω$ fields. We observe that the mutual $ω$-3$ω$ phase required for the maximum yield of high harmonic generation is shifted by approximately $π/2$ with respect to the phase leading to maximal generated carrier population. The observed emission delays of high harmonic photons of up to few hundred attoseconds scale with the time delay and with the ratio between the electric field amplitudes of the two-color fields. These results reveal the connection between electron tunneling and high harmonic emission processes in solids.

physics.optics↗

arXiv2025

Light-based Chromatic Aberration Correction of Ultrafast Electron Microscopes

We propose and theoretically demonstrate a technique that allows one to compensate for chromatic aberrations of traditional electron lenses in ultrafast electron microscopes. The technique is based on space- and time-dependent phase modulation of a pulsed electron beam using interaction with a shaped pulsed ponderomotive lens. The energy-selective focal distance is reached by combining the electron temporal chirp with the time-dependent size of the effective potential, with which the electrons interact. As a result, chromatic aberration can be reduced by up to a factor of seven. This approach paves the way for advanced transverse and longitudinal wavefront shaping of electrons in free space.

quant-ph↗

arXiv2025

Ultrafast room-temperature valley manipulation in silicon and diamond

Some semiconductors have more than one degenerate minimum of the conduction band in their band structure. These minima-known as valleys-can be used for storing and processing information, if it is possible to generate a difference in their electron populations. However, to compete with conventional electronics, it is necessary to develop universal and fast methods for controlling and reading the valley quantum number of the electrons. Even though selective optical manipulation of electron populations in inequivalent valleys has been demonstrated in two-dimensional crystals with broken time-reversal symmetry, such control is highly desired in many technologically important semiconductor materials, including silicon and diamond. We demonstrate an ultrafast technique for the generation and read-out of a valley-polarized population of electrons in bulk semiconductors on subpicosecond timescales. The principle is based on the unidirectional intervalley scattering of electrons accelerated by an oscillating electric field of linearly polarized infrared femtosecond pulses. Our results are an advance in the development of potential room-temperature valleytronic devices operating at terahertz frequencies and compatible with contemporary silicon-based technology.

cond-mat.mes-hall↗

arXiv2025

High harmonic generation in monolayer MoS2 controlled by resonant and near-resonant pulses on ultrashort time scales

We report on experimental investigation of nonperturbative high harmonic generation (HHG) in monolayer MoS2 in the ultraviolet spectral region driven by mid-infrared light. We study how the HHG is influenced by pre-excitation of the monolayer using resonant and near-resonant pulses in a pump-probe-like scheme. The resonant light creates high density exciton population. Due to ultrafast dephasing caused by electron-electron scattering, the HHG is suppressed in the presence of pre-excited carriers. In the case of near-resonant excitation with photon energy below the exciton transition, the dynamics of the observed suppression of the HHG yield contains a fast component which is a consequence of momentum scattering at carriers, which are excited by two-photon transition when the two pulses temporally overlap in the sample. This interpretation is supported by comparison of the experimental data with theoretical calculations of two-photon absorption spectrum of MoS2 monolayer. This work demonstrates a possibility to control HHG in lowdimensional materials on ultrashort timescales by combining the driving strong-field pulse with a weak near-resonant light.

physics.optics↗

arXiv2025

Analysis of electron spectra dynamics in a moving periodical ponderomotive potential

The interaction between freely propagating electrons and light waves is typically described using an approximation in which we assume that the electron velocity remains approximately the same during the interaction. In this article we analytically describe the dynamics of electrons in an interaction potential generated by an optical beat wave beyond this regime and find a structure of sharp electron distribution peaks that periodically alternate in the energy/momentum spectrum. In the classical description we analytically solve the nonlinear equation of motion, which is an analogy to the mathematical pendulum. While addressing the problem using quantum mechanics, we first use a parabolic approximation of the interaction potential and then we also study the evolution of the electron wavepacket in an infinite periodical potential. Using numerical simulations we show the classical and quantum evolution of the electron spectra during the interaction for different conditions and experimental settings.

quant-ph↗

arXiv2025

Electron vortex beams for chirality probing at the nanoscale

In this work we propose a method for probing the chirality of nanoscale electromagnetic near fields utilizing the properties of a coherent superposition of free-electron vortex states in electron microscopes. Electron beams optically modulated into vortices carry orbital angular momentum, thanks to which they are sensitive to the spatial phase distribution and topology of the investigated field. The sense of chirality of the studied specimen can be extracted from the spectra of the electron beam with nanoscale precision owing to the short picometer de Broglie wavelength of the electron beam. We present a detailed case study of the interaction of a coherent superposition of electron vortex states and the optical near field of a golden nanosphere illuminated by circularly polarized light as an example, and we examine the chirality sensitivity of electron vortex beams on intrinsically chiral plasmonic nanoantennae.

physics.optics↗

arXiv2025

Design for light-based spherical aberration correction of ultrafast electron microscopes

We theoretically demonstrate that ponderomotive interactions near the electron cross-over can be used for aberration correction in ultrafast electron microscopes. Highly magnified electron shadow images from Si$_3$N$_4$ thin films are utilized to visualize the distortions induced by spherical aberrations. Our simulations of electron-light interactions indicate that spherical aberrations can be compensated resulting in an aberration free angle of \SI{8.1}{mrad}. For achieving the necessary light distribution, we use a gradient descent algorithm to optimize Zernike polynomials and shape the light beam into a modified Gaussian and Laguerre-Gaussian beam.

physics.optics↗

arXiv2025

Monochromatization of Electron Beams with Spatially and Temporally Modulated Optical Fields

Inelastic interaction between coherent light with constant frequency and free electrons enables periodic phase modulation of electron wave packets leading to periodic side-bands in the electron energy spectra. In this Letter we propose a generalization of the interaction by considering linearly chirped electron wave packets interacting with chirped optical fields. We theoretically demonstrate that when matching the chirp parameters of the electron and light waves, the interaction leads to partial monochromatization of the electron spectra in one of the energy side-bands. Depending on the coherence time of the electrons, the electron spectrum may be narrowed down by a factor of 5-times with 26% of the electron distribution in the monochromatized energy band. This approach will improve the spectral resolution and reduce color aberrations in ultrafast imaging experiments with free electrons.

physics.optics↗