arXiv subjects

Matt Zepf

Publications and source records attributed to Matt Zepf.

At least 19 recordsLinked to original sources

arXiv2026

A homodyne detection scheme for all-optical photon-photon scattering experiments using 2D detectors

Low signal-to-noise ratios are a common problem in experiments attempting to measure photon-photon scattering. In the optical regime, where petawatt lasers with femtosecond pulse durations are used, the large beam sizes cause the major contribution of the background to be spread over up to 100 ps in arrival time, whereas the signal is confined to the femtosecond scale. We present a balanced homodyne measurement scheme, which exploits this property to suppress the background. By interfering the signal with a short reference pulse, the measurement becomes effectively gated to the pulse duration and is therefore only sensitive to the co-timed part of the light, reducing the effective background by 3-4 orders of magnitude. Additionally, increasing the reference pulse energy increases the amplitude of the measured quantity without changing the intrinsic signal-to-noise ratio. Using this property, other external noise sources can be made negligible by boosting the measured quantity above the noise floor. Using two-dimensional detectors further enhances the scheme by improving sensitivity and enabling self-referenced single-pulse measurements. In addition, an evaluation procedure based on maximum-likelihood estimation is presented and demonstrated. The robustness and performance of this scheme are demonstrated on simulated data, where a more than 100-fold reduction of measurement time compared to conventional photon-counting methods under realistic conditions is found.

hep-ex↗

arXiv2026

Theory of Relativistic Surface Plasmon Excitation on Smooth Surface by High-Intensity Laser

We present a classical theory of relativistic surface plasmon (RSP) excitation at a smooth plasma-vacuum interface driven by either a ponderomotive force or an electric field of an intense laser pulse. Starting from Maxwell equations coupled to a cold-fluid plasma response, we derive a general driven wave equation for the RSP and solve it analytically. We show that an infinite planar surface enforces conservation of the in-plane wavevector. A finite longitudinal interaction length or axial modulation supplies a finite kz spectrum, while cylindrical curvature replaces one continuous transverse in-plane wavenumber by a discrete azimuthal mode index m. This partially relaxes the planar in-plane constraint, while axial phase matching remains controlled by the longitudinal spectrum of the drive. The excitation strength is controlled by the overlap between the drive and the surface eigenfield, which is determined by the surface geometry. This provides a general principle for controlling RSP excitation. We also show that relativistic effects can substantially modify the dielectric response and can be preliminarily verified by particle-in-cell simulations. Within the local relativistic dielectric model, the overlap-normalised planar source saturates at large a0, and cylindrical curvature partially alleviates this reduction before strong surface softening develops. The role of surface geometry is analysed. A cylindrical surface can sustain an on-axis accelerating field, enabling highly nonlinear wakefield generation for particle acceleration. In addition, the cylindrical geometry imposes a precise mode-selection rule that provides intrinsic control over RSP excitation. Axisymmetric ponderomotive drive selects fundamental mode m=0. A linearly polarised laser field selects a superposition of m=+1 and m=-1 modes, and a circularly polarised laser field selects a single helical mode.

physics.plasm-ph↗

arXiv2026

Resonant Excitation of Surface Plasmon for Wakefield Acceleration by Beating GW Lasers on Smooth Cylindrical Surface

We present a theoretical and numerical study of resonant surface-plasmon (SP) excitation driven by the beating of two co-propagating laser pulses on a smooth cylindrical plasma-vacuum interface. Analytical expressions for the SP dispersion relation, field amplitude, geometric coupling factor, and resonance conditions are derived and validated by fully three-dimensional particle-in-cell simulations. We reveal that curvature-induced geometric effects can substantially modify the SP dispersion and enable resonant matching by laser beat waves. This is inaccessible in planar geometries or with a single laser. Under matched resonance conditions, a high-amplitude SP-based wakefield can be generated by a few gigawatt lasers, placing this mechanism within reach of state-of-the-art fibre lasers. It therefore opens a route toward portable laser-driven plasma wakefield accelerators.

physics.plasm-ph↗

arXiv2025

Simulating vacuum birefringence with a diffractive beam propagation code

Ninety years after their prediction, quantum vacuum nonlinearities in macroscopic electromagnetic fields still await a direct experimental verification in the laboratory. A particularly promising route towards their first measurement is the collision of counter-propagating laser beams in a pump-probe type experiment. Here, the key challenge is to separate the small quantum vacuum signal at the oscillation frequency of the probe that is mainly emitted in the vicinity of its forward cone from the large probe background. While quantitatively accurate predictions of the associated quantum vacuum signals are available, to date there is no framework that combines these predictions with a diffractive beam propagation code. Such codes are designed to holistically model optical experiments and can reliably account for diffraction and absorption losses of optical devices, like lenses and apertures. The latter inevitably influence and modify both the induced signal and background components prior to their detection in experiment. The present work addresses this topical issue and reports on the first implementation of a quantum vacuum signals emission module in an established diffractive beam propagation toolkit designed for the realistic modelling of optical experiments.

physics.optics↗

arXiv2025

Back-reflection in dipole fields and beyond

Quantum reflection is a fascinating signature of the quantum vacuum that emerges from inhomogeneities in the electromagnetic fields. In pursuit of the prospective real-world implementation of quantum reflection in the back-reflection channel, we provide the first numerical estimates for the light-by-light scattering with dipole pulses, which are known to provide the tightest focusing of light possible. For an all-optical setup with a dipole pump and Gaussian probe of the same frequency, we find that the dominant signal signature is related mainly to the back-reflection channel from 4-wave mixing. Focusing on this, we study the particular case of a multiple focusing pulses configuration (belt configuration) as an approximation to the idealized dipole pulse. Using Bayesian optimization methods, we determine optimal parameters that maximize the detectability of a discernible back-reflection signal. Our study indicates that the optimization favors a three-beam collision setup, which we further investigate both numerically and analytically.

quant-ph↗

arXiv2025

Determination of few femtosecond to attosecond electron bunch durations using a passive plasma lens

Determining the pulse duration of femtosecond electron bunches is challenging and often experimentally invasive. An effective method for measuring the duration based on the time-dependent variations in electron beam divergence induced by a passive plasma lens is described. Reconstruction of the temporal shape of the electron bunch down to $c \cdot dt=10$ nm ($\sim 30$ as) without external RF-cavities or multi-octave spanning spectrometer is shown numerically. Experimental data from a $\sim$ 3fs electron bunch demonstrates practical applicability of this method. While this approach can be used with any high current electron beam, it is particularly well matched to laser-driven and particle-driven wakefield accelerators and also accommodates electron beams with a time-dependent beam-energy (eg. 'chirp').

physics.acc-ph↗

arXiv2025

The Darkfield Approach to Measuring Vacuum Birefringence and Light-by-Light Couplings -- A Proof-of-Principle Experiment

Vacuum fluctuations give rise to effective nonlinear interactions between electromagnetic fields. These generically modify the characteristics of light traversing a strong-field region. X-ray free-electron lasers constitute a particularly promising probe, due to their brilliance, the possibility of precise control and favourable frequency scaling. However, the nonlinear vacuum response is very small even when probing a tightly focused high-intensity laser field with XFEL radiation and direct measurement of light-by-light scattering of real photons and the associated fundamental physics constants of the quantum vacuum has not been possible to date. Achieving a sufficiently good signal-to-background separation is key to a successful quantum vacuum experiment. To master this challenge, a darkfield detection concept has recently been proposed. Here we present the results of a proof-of-principle experiment validating this approach at the High Energy Density scientific instrument of the European X-Ray Free Electron Laser.

hep-ex↗

arXiv2024

Convergent trajectories of relativistic electrons interacting with lasers in plasma waves

The dynamics of relativistic electrons interacting with a laser pulse in a plasma wave has been investigated theoretically and numerically based on the classical Landau-Lifshitz equation. There exists a convergent trajectory of electrons when the energy gain of electrons via direct laser acceleration can compensate the energy loss via radiation. An electron beam initially around the convergent trajectory evolves into the trajectory, making its occupied phase space volume decrease exponentially while mean energy remain the same. This mechanism can be used for cooling relativistic electron beams especially those produced in plasma-based acceleration.

physics.plasm-ph↗

arXiv2024

Numerical optimization of quantum vacuum signals

The identification of prospective scenarios for observing quantum vacuum signals in high-intensity laser experiments requires both accurate theoretical predictions and the exploration of high-dimensional parameter spaces. Numerical simulations address the first requirement, while optimization provides an efficient solution for the second one. In the present work, we demonstrate the potential of Bayesian optimization in maximizing photonic quantum vacuum signals on the example of two-beam collisions. This allows us to find the optimal waist sizes for beams with elliptic cross sections, and to identify the specific physical process leading to a discernible signal in a coherent harmonic focusing configuration scenario.

hep-ph↗

arXiv2024

Novel Signatures of Radiation Reaction in Electron-Laser Sidescattering

In this article we investigate novel signatures of radiation reaction via the angular deflection of an electron beam colliding at 90 degrees with an intense laser pulse. Due to the radiation reaction effect, the electrons can be deflected towards the beam axis for plane wave backgrounds, which is not possible in the absence of radiation reaction effects. The magnitude and size of the deflection angle can be controlled by tailoring the laser pulse shapes. The effect is first derived analytically using the Landau-Lifshitz equation, which allows to determine the important scaling behavior with laser intensity and particle energy. We then move on to full scale 3D Monte Carlo simulations to verify the effect is observable with present day laser technology. We investigate the opportunities for an indirect observation of laser depletion in such side scattering scenarios.

physics.plasm-ph↗

arXiv2024

A dark-field setup for the measurement of light-by-light scattering with high-intensity lasers

We put forward a concrete experimental setup allowing to measure light-by-light scattering in the collision of two optical high-intensity laser beams at state-of-the-art high-field facilities operating petawatt class laser systems. Our setup uses the same focusing optics for both laser beams to be collided and employs a dark-field approach for the detection of the single-photon-level nonlinear quantum vacuum response in the presence of a large background. Based on an advanced modeling of the colliding laser fields, we in particular provide reliable estimates for the prospective numbers of signal photons scattered into the dark-field for various laser polarizations.

physics.optics↗

arXiv2023

Phase-dependent bubble hosing and resonant amplification of betatron oscillation in few-cycle laser wakefield accelerator

Betatron oscillation of trapped electrons in laser-driven long-distance propagating plasma bubble has been investigated with the help of particle-in-cell simulations and theoretical analysis. Parametric oscillation of the trapped electron beam is identified as a result of bubble breathing which develops with the steepening and depletion of the laser pulse. It leads to parametric amplification of the betatron oscillation of the electron beam with the exponential growth of its amplitude. It results in severe degradation of the qualities of the electron beam and thus is important for long-scale laser wakefield acceleration.

physics.acc-ph↗

arXiv2022

Direct accessibility of the fundamental constants governing light-by-light scattering

Quantum field theory predicts the vacuum to exhibit a non-linear response to strong electromagnetic fields. This fundamental tenet has remained experimentally challenging and is yet to be tested in the laboratory. We present proof of concept and detailed theoretical analysis of an experimental setup for precision measurements of the quantum vacuum signal generated by the collision of a brilliant x-ray probe with a high-intensity pump laser. The signal features components polarised parallel and perpendicularly to the incident x-ray probe. Our proof-of-concept measurements show that the background can be efficiently suppressed by many orders of magnitude which should not only facilitate a detection of the perpendicularly polarised component of non-linear vacuum response, but even make the parallel polarised component experimentally accessible for the first time. Remarkably, the angular separation of the signal from the intense x-ray probe enables precision measurements even in presence of pump fluctuations and alignment jitter. This provides direct access to the low-energy constants governing light-by-light scattering.

hep-ph↗

arXiv2021

Intense harmonics with time-varying orbital angular momentum from relativistic plasma mirrors

In this work using three-dimensional particle-in-cell simulations and analytical considerations we demonstrate intense high-order plasma surface harmonics carrying a time-varying orbital angular momentum (OAM) -- the self-torque. We show that by using two laser beams with different OAMs $l_1$ and $l_2$ and a certain delay between each other and shooting them obliquely on an overdense plasma target, one can generate harmonics with OAM spanning $nl_1$ to $nl_2$, where $n$ is the order of the harmonic. Such intense self-torqued harmonics can offer new possibilities in ultrafast spectroscopy.

physics.plasm-ph↗

arXiv2021

Experimental estimates of the photon background in a potential light-by-light scattering study

High power short pulse lasers provide a promising route to study the strong field effects of the quantum vacuum, for example by direct photon-photon scattering in the all-optical regime. Theoretical predictions based on realistic laser parameters achievable today or in the near future predict scattering of a few photons with colliding Petawatt laser pulses, requiring single photon sensitive detection schemes and very good spatio-temporal filtering and background suppression. In this article, we present experimental investigations of this photon background by employing only a single high power laser pulse tightly focused in residual gas of a vacuum chamber. The focal region was imaged onto a single-photon sensitive, time gated camera. As no detectable quantum vacuum signature was expected in our case, the setup allowed for characterization and first mitigation of background contributions. For the setup employed, scattering off surfaces of imperfect optics dominated below residual gas pressures of $1\times 10^{-4}$ mbar. Extrapolation of the findings to intensities relevant for photon-photon scattering studies is discussed.

hep-ex↗

arXiv2021

Conceptual Design Report for the LUXE Experiment

This Conceptual Design Report describes LUXE (Laser Und XFEL Experiment), an experimental campaign that aims to combine the high-quality and high-energy electron beam of the European XFEL with a powerful laser to explore the uncharted terrain of quantum electrodynamics characterised by both high energy and high intensity. We will reach this hitherto inaccessible regime of quantum physics by analysing high-energy electron-photon and photon-photon interactions in the extreme environment provided by an intense laser focus. The physics background and its relevance are presented in the science case which in turn leads to, and justifies, the ensuing plan for all aspects of the experiment: Our choice of experimental parameters allows (i) effective field strengths to be probed at and beyond the Schwinger limit and (ii) a precision to be achieved that permits a detailed comparison of the measured data with calculations. In addition, the high photon flux predicted will enable a sensitive search for new physics beyond the Standard Model. The initial phase of the experiment will employ an existing 40 TW laser, whereas the second phase will utilise an upgraded laser power of 350 TW. All expectations regarding the performance of the experimental set-up as well as the expected physics results are based on detailed numerical simulations throughout.

hep-ex↗

arXiv2020

Boosting quantum vacuum signatures by coherent harmonic focusing

We show that coherent harmonic focusing provides an efficient mechanism to boost all-optical signatures of quantum vacuum nonlinearity in the collision of high-intensity laser fields, thereby offering a promising route to their first experimental detection. Assuming two laser pulses of given parameters at our disposal, we demonstrate a substantial increase of the number of signal photons measurable in experiments where one of the pulses undergoes coherent harmonic focusing before it collides with the fundamental-frequency pulse. Imposing a quantitative criterion to discern the signal photons from the background of the driving laser photons and accounting for the finite purity of polarization filtering, we find that signal photons arising from inelastic scattering processes constitute a promising signature. By contrast, quasi-elastic contributions which are conventionally assumed to form the most prospective signal remain background dominated. Our findings may result in a paradigm shift concerning which photonic signatures of quantum vacuum nonlinearity are accessible in experiment.

hep-ph↗

arXiv2018

The Vacuum Emission Picture Beyond Paraxial Approximation

Optical signatures of the effective nonlinear couplings among electromagnetic fields in the quantum vacuum can be conveniently described in terms of stimulated photon emission processes induced by strong classical, space-time dependent electromagnetic fields. Recent studies have adopted this approach to study collisions of Gaussian laser pulses in paraxial approximation. The present study extends these investigations beyond the paraxial approximation by using an efficient numerical solver for the classical input fields. This new numerical code allows for a consistent theoretical description of optical signatures of QED vacuum nonlinearities in generic electromagnetic fields governed by Maxwell's equations in the vacuum, such as manifestly non-paraxial laser pulses. Our code is based on a locally constant field approximation of the Heisenberg-Euler effective Lagrangian. As this approximation is applicable for essentially all optical high-intensity laser experiments, our code is capable of calculating signal photon emission amplitudes in completely generic input field configurations, limited only by numerical cost.

physics.optics↗