Showing 1–28 of 28 results for author: Cesar, D

Mutually phase-stable tunable attosecond soft X-ray attosecond pulses from a free-electron laser

Authors: River Robles, David Cesar, Taran Driver, Paris Franz, Veronica Guo, Gabriel Just, Kirk A. Larsen, Randy Lemons, Ming-Fu Lin, Razib Obaid, Nicholas Sudar, Jun Wang, Zhen Zhang, James Cryan, Agostino Marinelli

Abstract: We demonstrate the production of mutually phase-stable attosecond X-ray pulse pairs with tunable relative time delays and phases in a cascaded X-ray free-electron laser. We showcase the method in an experiment at the LCLS-II, in which a shaped electron beam is used in a split undulator configuration to generate the two attosecond pulses. We achieve mutual phase stability by reusing microbunching g… ▽ More We demonstrate the production of mutually phase-stable attosecond X-ray pulse pairs with tunable relative time delays and phases in a cascaded X-ray free-electron laser. We showcase the method in an experiment at the LCLS-II, in which a shaped electron beam is used in a split undulator configuration to generate the two attosecond pulses. We achieve mutual phase stability by reusing microbunching generated in the first undulator in order to seed the FEL process in the second at a detuned frequency. We measure controllable temporal delays between the two pulses directly in the time domain using angular streaking of photoelectrons, with a step size of 250 attoseconds. We then show that the behavior of the X-ray spectrum is consistent with phase stability between the two pulses, with a relative phase that can be easily tuned using inter-undulator phase shifters. This method is particularly well-suited to few to ten eV energy separations and sub to few femtosecond time delays, which are ideal for experiments in the soft X-ray regime for pushing the limits of our models for molecular dynamics and exerting direct coherent control over quantum systems. △ Less

Submitted 20 August, 2026; originally announced August 2026.

Wave propagation and hybridization of plasmonic modes in Maxwell-Chern-Simons pseudo-electrodynamics

Authors: S. Duque Cesar, M. J. Neves

Abstract: We investigate the propagation of classical plane waves and surface plasmon-polariton modes within the framework of pseudo-electrodynamics (PED) supplemented by a non-local Chern-Simons (CS) topological term. Starting from the dimensionally reduced action, we derive the decoupled second-order wave equations for the electromagnetic field, and for the gauge-potential. In vacuum, we show that the top… ▽ More We investigate the propagation of classical plane waves and surface plasmon-polariton modes within the framework of pseudo-electrodynamics (PED) supplemented by a non-local Chern-Simons (CS) topological term. Starting from the dimensionally reduced action, we derive the decoupled second-order wave equations for the electromagnetic field, and for the gauge-potential. In vacuum, we show that the topological CS parameter works as a dynamical mass generator, modifying the strict transversality of plane waves, and introducing a distinct energy \textit{gap} in the dispersion relation. Considering a planar conducting medium that satisfies the Ohm law, the topological mass induces a novel hybridization mechanism between the Transverse Electric (TE), and Transverse Magnetic (TM) modes, a feature entirely absent in the conventional planar plasmonics. We analyze the asymptotic limits of this system, obtaining the exact real solutions in the lossless reactive Drude regime, and deriving the complex refractive index through a quasi-local approximation in the dissipative regime. △ Less

Submitted 20 July, 2026; originally announced July 2026.

Comments: 6 pages, 3 figures

Broadband hard X-ray attosecond pulses from extremely chirped electron beams

Authors: River Robles, Veronica Guo, David Cesar, Paris Franz, Aliaksei Halavanau, Alberto Lutman, Takahiro Sato, Sanghoon Song, Nicholas Sudar, Yanwen Sun, Zhen Zhang, Diling Zhu, Agostino Marinelli

Abstract: Attosecond pulses from free-electron lasers have opened the doors to atomic site-specific studies of bound electronic dynamics on their natural, sub-femtosecond timescales. Key to their success has been electron beam shaping techniques enabling the generation of sub-femtosecond current spikes with peak currents on the order of 10 kA. We demonstrate in an RF linac the generation of current spikes w… ▽ More Attosecond pulses from free-electron lasers have opened the doors to atomic site-specific studies of bound electronic dynamics on their natural, sub-femtosecond timescales. Key to their success has been electron beam shaping techniques enabling the generation of sub-femtosecond current spikes with peak currents on the order of 10 kA. We demonstrate in an RF linac the generation of current spikes with extreme chirps on the order of 350 MeV/micron, directly competitive with the chirps expected from beam-driven plasma wakefield accelerators. Leveraging chirp-taper compensation, we use these highly chirped beams to generate hard X-ray attosecond pulses with bandwidths exceeding 30 eV, a factor of two beyond previous demonstrations. We simultaneously present the first explicit experimental evidence of chirp-taper compensation in an attosecond XFEL, finding that optimal tapering improves the bandwidth and pulse energy by factors of two and five, respectively, for our conditions. In addition to the immediate utility of such broadband hard X-ray pulses, electron beams with such extreme chirps can be utilized for unique new experimental modalities by performing further compression after the undulators. Such post-lasing compression can enable subsequent superradiant light emission at longer wavelengths, or direct excitation of quantum systems with the beam's intense space-charge field for unique attosecond pump-probe possibilities. △ Less

Submitted 19 August, 2026; v1 submitted 10 April, 2026; originally announced April 2026.

Advanced Control of Electron Beams: Tailoring X-ray Production with Programmable Laser Shaping

Authors: Jack Hirschman, Randy Lemons, Hao Zhang, Razib Obaid, River Robles, Paris Franz, Benjamin Mencer, Nicole Neveu, Matthew Britton, David Cesar, Nicolas Sudar, Zhen Zhang, Justin Baker, Chad Pennington, Kurtis Borne, Taran Driver, Kirk A. Larsen, Veronica Guo, Yuantao Ding, Gabriel Just, Feng Zhou, James Cryan, Joseph Robinson, Ryan Coffee, Agostino Marinelli , et al. (1 additional authors not shown)

Abstract: Leveraging the full scientific capabilities of next-generation high-repetition-rate free-electron lasers requires programmable control over electron-beam properties at their source. The photoinjector drive laser defines the electron beam's initial six-dimensional phase-space distribution, yet has historically been limited to Gaussian or static flat-top profiles, with most manipulation occurring do… ▽ More Leveraging the full scientific capabilities of next-generation high-repetition-rate free-electron lasers requires programmable control over electron-beam properties at their source. The photoinjector drive laser defines the electron beam's initial six-dimensional phase-space distribution, yet has historically been limited to Gaussian or static flat-top profiles, with most manipulation occurring downstream. Here we demonstrate software-programmable ultraviolet pulse shaping at the LCLS-II photoinjector as a source-level actuator that complements traditional accelerator controls. Using a coupled architecture combining dispersion-controlled nonlinear frequency conversion with spatial-light-modulator spectral shaping, we generate user-defined temporal structures and observe their imprint on electron bunches through high-resolution time-domain diagnostics. Laser-imposed multi-peaked modulation persists through acceleration, magnetic compression, and undulator transport with shot-to-shot repeatability, producing clearly resolved current structure in the compressed beam. Variance-based reconstruction from transverse deflecting cavity measurements reveals structured X-ray emission profiles exhibiting temporal features consistent with the programmed laser waveform. By providing rapid, software-controlled reconfiguration of electron-beam initial conditions, this source-level control approach establishes a programmable upstream actuator for future adaptive optimization and autonomous facility operation at high-repetition-rate light sources. △ Less

Submitted 16 March, 2026; originally announced March 2026.

Upstream Laser-based Longitudinal Enhancement of Relativistic Photoelectrons

Authors: Hao Zhang, Randy Lemons, Jack Hirschman, Nicole Neveu, Nicolas Sudar, River Robles, Paris Franz, David Cesar, Zihan Zhu, Mathew Britton, Kurtis Borne, Zhen Zhang, Kirk A. Larsen, Benjamin Mencer, Justin Baker, Chad Pennington, Razib Obaid, Yuantao Ding, Ryan Coffee, Gabriel Just, Feng Zhou, Ji Qiang, James Cryan, Joseph Robinson, Agostino Marinelli , et al. (1 additional authors not shown)

Abstract: Controlling the longitudinal phase space of high-brightness relativistic electron beams is crucial for advancing a broad spectrum of charged-particle-based instrumentation and scientific frontiers. A generalized method for achieving this control involves manipulating the photoemission laser's temporal distribution at the picosecond level, a long-standing technical challenge. Recent developments in… ▽ More Controlling the longitudinal phase space of high-brightness relativistic electron beams is crucial for advancing a broad spectrum of charged-particle-based instrumentation and scientific frontiers. A generalized method for achieving this control involves manipulating the photoemission laser's temporal distribution at the picosecond level, a long-standing technical challenge. Recent developments in laser shaping have enabled the creation of high-power, picosecond-scale symmetrical and asymmetrical temporal profiles, capable of fine-tuning complex space-charge dynamics and external field effects in relativistic charged-particle beams. Here, we demonstrate that rather than deviations from theorized, idealized laser distributions, a controlled asymmetry can be harnessed to counteract accelerator-induced distortions. By implementing spatiotemporal shaping of the ultraviolet photocathode laser at the LCLS-II superconducting injector, we achieve deterministic control over the longitudinal phase space without downstream corrections. We find that this optical asymmetry induces a self-linearizing effect across both low (40 pC) and high (80 pC) charge regimes, effectively suppressing nonlinear compression and energy chirp. Consequently, this approach is expected to preserve a low emittance comparable to that of ideal flattop or regular Gaussian profiles, while delivering superior current uniformity and shot-to-shot stability. These results establish spatiotemporal laser shaping as a compact, generalizable tool for directly optimizing beam brightness at the source. △ Less

Submitted 6 January, 2026; originally announced January 2026.

Beam Halo Formation via Longitudinal-Transverse Coupling in Continuous-Wave Photoinjectors

Authors: Zhen Zhang, Yuantao Ding, David Cesar, Feng Zhou, Ji Qiang, Zhirong Huang

Abstract: Beam halo formation poses a critical challenge for high-repetition-rate continuous-wave (CW) free-electron lasers (FELs), directly affecting beam quality and machine protection, as observed during the LCLS-II commissioning. We identify and experimentally validate a previously unrecognized three-step mechanism for halo generation in the photoinjector, arising from coupled longitudinal-transverse dy… ▽ More Beam halo formation poses a critical challenge for high-repetition-rate continuous-wave (CW) free-electron lasers (FELs), directly affecting beam quality and machine protection, as observed during the LCLS-II commissioning. We identify and experimentally validate a previously unrecognized three-step mechanism for halo generation in the photoinjector, arising from coupled longitudinal-transverse dynamics in the low-energy beam. Theoretical analysis reveals that (i) the RF buncher induces an energy-radius correlation, (ii) velocity bunching transforms this correlation into hollowed density structures in the bunch head and tail, and (iii) differential overfocusing of these hollowed regions by downstream focusing forms the observed halo. This mechanism is confirmed by particle-in-cell simulations and direct experimental measurements, including controlled formation of a core-ring profile via solenoid tuning. The results establish the physical origin of the halo and demonstrate a mitigation via buncher compression tuning that reduces halo and downstream loss, supporting sustained high-repetition-rate FEL operation. △ Less

Submitted 7 November, 2025; originally announced November 2025.

Comments: 7 pages, 5 figures

Nonlinear reversal of photo-excitation on the attosecond time scale improves ultrafast x-ray diffraction images

Authors: Anatoli Ulmer, Phay J. Ho, Bruno Langbehn, Stephan Kuschel, Linos Hecht, Razib Obaid, Simon Dold, Taran Driver, Joseph Duris, Ming-Fu Lin, David Cesar, Paris Franz, Zhaoheng Guo, Philip A. Hart, Andrei Kamalov, Kirk A. Larsen, Xiang Li, Michael Meyer, Kazutaka Nakahara, Robert G. Radloff, River Robles, Lara Rönnebeck, Nick Sudar, Adam M. Summers, Linda Young , et al. (6 additional authors not shown)

Abstract: The advent of isolated and intense sub-femtosecond X-ray pulses enables tracking of quantummechanical motion of electrons in molecules and solids. The combination of X-ray spectroscopy and diffraction imaging is a powerful approach to visualize non-equilibrium dynamics in systems beyond few atoms. However, extreme x-ray intensities introduce significant electronic damage, limiting material contras… ▽ More The advent of isolated and intense sub-femtosecond X-ray pulses enables tracking of quantummechanical motion of electrons in molecules and solids. The combination of X-ray spectroscopy and diffraction imaging is a powerful approach to visualize non-equilibrium dynamics in systems beyond few atoms. However, extreme x-ray intensities introduce significant electronic damage, limiting material contrast and spatial resolution. Here we show that newly available intense subfemtosecond (sub-fs) x-ray FEL pulses can outrun most ionization cascades and partially reverse x-ray damage through stimulated x-ray emission in the vicinity of a resonance. In our experiment, we compared thousands of coherent x-ray diffraction patterns and simultaneously recorded ion spectra from individual Ne nanoparticles injected into the FEL focus. Our experimental results and theoretical modeling reveal that x-ray diffraction increases and the average charge state decreases in particles exposed to sub-fs pulses compared to those illuminated with 15-femtosecond pulses. Sub-fs exposures outrun most Auger decays and impact ionization processes, and enhance nonlinear effects such as stimulated emission, which cycle bound electrons between different states. These findings demonstrate that intense sub-fs x-ray FEL pulses are transformative for advancing high-resolution imaging and spectroscopy in chemical and material sciences, and open the possibilities of coherent control of the interaction between x-rays and complex specimen beyond few atoms. △ Less

Submitted 24 June, 2025; originally announced June 2025.

Experimental demonstration of attosecond hard X-ray pulses

Authors: Ichiro Inoue, River Robles, Aliaksei Halavanau, Veronica Guo, Thomas M. Linker Andrei Benediktovitch, Stasis Chuchurka, Matthew H. Seaberg, Yanwen Sun, Diling Zhu, David Cesar, Yuantao Ding, Vincent Esposito, Paris Franz, Nicholas S. Sudar, Zhen Zhang, Taito Osaka, Gota Yamaguchi, Yasuhisa Sano, Kazuto Yamauchi, Jumpei Yamada, Uwe Bergmann, Matthias F. Kling, Claudio Pellegrini, Makina Yabashi, Nina Rohringer , et al. (2 additional authors not shown)

Abstract: We present the first direct experimental confirmation of attosecond pulse generation in the hard X-ray regime with a free-electron laser. Our experiment is based on measurements of a nonlinear optical phenomenon known as amplified spontaneous emission (ASE) from 3d transition metals. By analyzing the yield of the collective X-ray fluorescence induced by ultrashort pulses at the Linac Coherent Ligh… ▽ More We present the first direct experimental confirmation of attosecond pulse generation in the hard X-ray regime with a free-electron laser. Our experiment is based on measurements of a nonlinear optical phenomenon known as amplified spontaneous emission (ASE) from 3d transition metals. By analyzing the yield of the collective X-ray fluorescence induced by ultrashort pulses at the Linac Coherent Light Source, we identify the generation of attosecond pulses and shot-to-shot fluctuations in their duration, ranging from 100 as to 400 as. The observed product of bandwidth and pulse duration for 100 as pulses is approximately 2 fs$\cdot$eV, indicating the generation of nearly transform-limited pulses. Our results extend the photon energy reach of attosecond techniques by one order of magnitude, providing the ability to simultaneously probe matter on the time-scales of electronic phenomena and with atomic spatial resolution. Furthermore, attosecond hard X-ray pulses can outrun the fastest radiation damage processes, paving the way to single-shot damage-free X-ray measurements. △ Less

Submitted 9 June, 2025; originally announced June 2025.

Remarks on classical pseudo-electrodynamics

Authors: S. Duque Cesar, M. J. Neves

Abstract: Classical studies as the conservation laws and the radiation fields are investigated in the pseudo-electrodynamics. We explore the action symmetry under infinitesimal transformations to obtain the energy-momentum, the Belinfante-Rosenfeld, and the general angular momentum tensors for this nonlocal planar electrodynamics. Through the results such as the retarded potentials and fields generated by a… ▽ More Classical studies as the conservation laws and the radiation fields are investigated in the pseudo-electrodynamics. We explore the action symmetry under infinitesimal transformations to obtain the energy-momentum, the Belinfante-Rosenfeld, and the general angular momentum tensors for this nonlocal planar electrodynamics. Through the results such as the retarded potentials and fields generated by a point particle in an arbitrary motion, we study the radiation of an electric dipole and it radiated power in 1+2 dimensions. In addition, we propose a way to introduce magnetic monopoles in pseudo-electrodynamics, in which the solutions and conservation laws are also presented. △ Less

Submitted 28 June, 2025; v1 submitted 10 January, 2025; originally announced January 2025.

Comments: 18 pages, no figures

Journal ref: International Journal of Modern Physics A, 40, No 16, 2550044 (2025)

Attosecond Coherent Electron Motion in a Photoionized Aromatic Molecule

Authors: Taran Driver, Zhaoheng Guo, Erik Isele, Gilbert Grell, Marco Ruberti, Jordan T. ONeal, Oliver Alexander, Sandra Beauvarlet, David Cesar, Joseph Duris, Douglas Garratt, Kirk A. Larsen, Siqi Li, Přemysl Kolorenč, Gregory A. McCracken, Daniel Tuthill, Zifan Wang, Nora Berrah, Christoph Bostedt, Kurtis Borne, Xinxin Cheng, Louis F. DiMauro, Gilles Doumy, Paris L. Franz, Andrei Kamalov , et al. (28 additional authors not shown)

Abstract: In molecular systems, the ultrafast motion of electrons initiates the process of chemical change. Tracking this electronic motion across molecules requires coupling attosecond time resolution to atomic-scale spatial sensitivity. In this work, we employ a pair of attosecond x-ray pulses from an x-ray free-electron laser to follow electron motion resulting from the sudden removal of an electron from… ▽ More In molecular systems, the ultrafast motion of electrons initiates the process of chemical change. Tracking this electronic motion across molecules requires coupling attosecond time resolution to atomic-scale spatial sensitivity. In this work, we employ a pair of attosecond x-ray pulses from an x-ray free-electron laser to follow electron motion resulting from the sudden removal of an electron from a prototypical aromatic system, para-aminophenol. X-ray absorption enables tracking this motion with atomic-site specificity. Our measurements are compared with state-of-the-art computational modeling, reproducing the observed response across multiple timescales. Sub-femtosecond dynamics are assigned to states undergoing non-radiative decay, while few-femtosecond oscillatory motion is associated with electronic wavepacket motion in stable cation states, that will eventually couple to nuclear motion. Our work provides insight on the ultrafast charge motion preceding and initiating chemical transformations in moderately complex systems, and provides a powerful benchmark for computational models of ultrafast charge motion in matter. △ Less

Submitted 3 November, 2024; originally announced November 2024.

"Beam `a la carte": laser heater shaping for attosecond pulses in a multiplexed x-ray free-electron laser

Authors: Siqi Li, Zhen Zhang, Shawn Alverson, David Cesar, Taran Driver, Paris Franz, Erik Isele, Joseph P. Duris, Kirk Larsen, Ming-Fu Lin, Razib Obaid, Jordan T O'Neal, River Robles, Nick Sudar, Zhaoheng Guo, Sharon Vetter, Peter Walter, Anna L. Wang, Joseph Xu, Sergio Carbajo, James P. Cryan, Agostino Marinelli

Abstract: Electron beam shaping allows the control of the temporal properties of x-ray free-electron laser pulses from femtosecond to attosecond timescales. Here we demonstrate the use of a laser heater to shape electron bunches and enable the generation of attosecond x-ray pulses. We demonstrate that this method can be applied in a selective way, shaping a targeted subset of bunches while leaving the remai… ▽ More Electron beam shaping allows the control of the temporal properties of x-ray free-electron laser pulses from femtosecond to attosecond timescales. Here we demonstrate the use of a laser heater to shape electron bunches and enable the generation of attosecond x-ray pulses. We demonstrate that this method can be applied in a selective way, shaping a targeted subset of bunches while leaving the remaining bunches unchanged. This experiment enables the delivery of shaped x-ray pulses to multiple undulator beamlines, with pulse properties tailored to specialized scientific applications. △ Less

Submitted 2 April, 2024; originally announced April 2024.

Spectrotemporal shaping of attosecond x-ray pulses with a fresh-slice free-electron laser

Authors: River R. Robles, Kirk A. Larsen, David Cesar, Taran Driver, Joseph Duris, Paris Franz, Douglas Garratt, Veronica Guo, Gabriel Just, Randy Lemons, Ming-Fu Lin, Razib Obaid, Nicholas Sudar, Jun Wang, Zhen Zhang, James Cryan, Agostino Marinelli

Abstract: We propose a scheme allowing coherent shaping, i.e., controlling both the amplitude and phase, of attosecond x-ray pulses at free-electron lasers. We show that by seeding an FEL with a short coherent seed that overfills the amplification bandwidth, one can shape the Wigner function of the pulse by controlling the undulator taper profile. The examples of controllable pulse pairs and trains, as well… ▽ More We propose a scheme allowing coherent shaping, i.e., controlling both the amplitude and phase, of attosecond x-ray pulses at free-electron lasers. We show that by seeding an FEL with a short coherent seed that overfills the amplification bandwidth, one can shape the Wigner function of the pulse by controlling the undulator taper profile. The examples of controllable pulse pairs and trains, as well as isolated spectrotemporally shaped pulses with very broad bandwidths are examined in detail. Existing attosecond XFELs can achieve these experimental conditions in a two-stage cascade, in which the seed is generated by a short current spike in an electron bunch and shaped in an unspoiled region within the same bunch. We experimentally demonstrate the production and control of phase-stable pulse trains using this method at the Linac Coherent Light Source II. △ Less

Submitted 10 February, 2025; v1 submitted 4 March, 2024; originally announced March 2024.

Attosecond X-ray Core-level Chronoscopy of Aromatic Molecules

Authors: Jia-Bao Ji, Zhaoheng Guo, Taran Driver, Cynthia S. Trevisan, David Cesar, Xinxin Cheng, Joseph Duris, Paris L. Franz, James Glownia, Xiaochun Gong, Daniel Hammerland, Meng Han, Saijoscha Heck, Matthias Hoffmann, Andrei Kamalov, Kirk A. Larsen, Xiang Li, Ming-Fu Lin, Yuchen Liu, C. William McCurdy, Razib Obaid, Jordan T. ONeal, Thomas N. Rescigno, River R. Robles, Nicholas Sudar , et al. (10 additional authors not shown)

Abstract: Attosecond photoemission or photoionization delays are a unique probe of the structure and the electronic dynamics of matter. However, spectral congestion and spatial delocalization of valence electron wave functions set fundamental limits to the complexity of systems that can be studied and the information that can be retrieved, respectively. Using attosecond X-ray pulses from LCLS, we demonstrat… ▽ More Attosecond photoemission or photoionization delays are a unique probe of the structure and the electronic dynamics of matter. However, spectral congestion and spatial delocalization of valence electron wave functions set fundamental limits to the complexity of systems that can be studied and the information that can be retrieved, respectively. Using attosecond X-ray pulses from LCLS, we demonstrate the key advantages of measuring core-level delays: the photoelectron spectra remain atom-like, the measurements become element specific and the observed scattering dynamics originate from a point-like source. We exploit these unique features to reveal the effects of electronegativity and symmetry on attosecond scattering dynamics by measuring and calculating the photoionization delays between N-1s and C-1s core shells of a series of aromatic azabenzene molecules. Remarkably, the delays increase with the number of nitrogen atoms in the molecule and reveal multiple resonances. We identify two previously unknown mechanisms regulating the associated attosecond dynamics, namely the enhanced confinement of the trapped wavefunction with increasing electronegativity of the atoms and the decrease of the coupling strength among the photoemitted partial waves with increasing symmetry. This study demonstrates the unique opportunities opened by measurements of core-level photoionization delays for unraveling attosecond electron dynamics in complex matter. △ Less

Submitted 27 May, 2025; v1 submitted 27 February, 2024; originally announced February 2024.

Experimental Demonstration of Attosecond Pump-Probe Spectroscopy with an X-ray Free-Electron Laser

Authors: Zhaoheng Guo, Taran Driver, Sandra Beauvarlet, David Cesar, Joseph Duris, Paris L. Franz, Oliver Alexander, Dorian Bohler, Christoph Bostedt, Vitali Averbukh, Xinxin Cheng, Louis F. DiMauro, Gilles Doumy, Ruaridh Forbes, Oliver Gessner, James M. Glownia, Erik Isele, Andrei Kamalov, Kirk A. Larsen, Siqi Li, Xiang Li, Ming-Fu Lin, Gregory A. McCracken, Razib Obaid, Jordan T. ONeal , et al. (25 additional authors not shown)

Abstract: Pump-probe experiments with sub-femtosecond resolution are the key to understanding electronic dynamics in quantum systems. Here we demonstrate the generation and control of sub-femtosecond pulse pairs from a two-colour X-ray free-electron laser (XFEL). By measuring the delay between the two pulses with an angular streaking diagnostic, we characterise the group velocity of the XFEL and demonstrate… ▽ More Pump-probe experiments with sub-femtosecond resolution are the key to understanding electronic dynamics in quantum systems. Here we demonstrate the generation and control of sub-femtosecond pulse pairs from a two-colour X-ray free-electron laser (XFEL). By measuring the delay between the two pulses with an angular streaking diagnostic, we characterise the group velocity of the XFEL and demonstrate control of the pulse delay down to 270 as. We demonstrate the application of this technique to a pump-probe measurement in core-excited para-aminophenol. These results demonstrate the ability to perform pump-probe experiments with sub-femtosecond resolution and atomic site specificity. △ Less

Submitted 26 January, 2024; originally announced January 2024.

Comments: 55 pages, main manuscript (5 figures) + supplementary materials (25 figures), 30 figures total. Submitted to Nature Photonics

Efficient prediction of attosecond two-colour pulses from an X-ray free-electron laser with machine learning

Authors: Karim K. Alaa El-Din, Oliver G. Alexander, Leszek J. Frasinski, Florian Mintert, Zhaoheng Guo, Joseph Duris, Zhen Zhang, David B. Cesar, Paris Franz, Taran Driver, Peter Walter, James P. Cryan, Agostino Marinelli, Jon P. Marangos, Rick Mukherjee

Abstract: X-ray free-electron lasers are sources of coherent, high-intensity X-rays with numerous applications in ultra-fast measurements and dynamic structural imaging. Due to the stochastic nature of the self-amplified spontaneous emission process and the difficulty in controlling injection of electrons, output pulses exhibit significant noise and limited temporal coherence. Standard measurement technique… ▽ More X-ray free-electron lasers are sources of coherent, high-intensity X-rays with numerous applications in ultra-fast measurements and dynamic structural imaging. Due to the stochastic nature of the self-amplified spontaneous emission process and the difficulty in controlling injection of electrons, output pulses exhibit significant noise and limited temporal coherence. Standard measurement techniques used for characterizing two-coloured X-ray pulses are challenging, as they are either invasive or diagnostically expensive. In this work, we employ machine learning methods such as neural networks and decision trees to predict the central photon energies of pairs of attosecond fundamental and second harmonic pulses using parameters that are easily recorded at the high-repetition rate of a single shot. Using real experimental data, we apply a detailed feature analysis on the input parameters while optimizing the training time of the machine learning methods. Our predictive models are able to make predictions of central photon energy for one of the pulses without measuring the other pulse, thereby leveraging the use of the spectrometer without having to extend its detection window. We anticipate applications in X-ray spectroscopy using XFELs, such as in time-resolved X-ray absorption and photoemission spectroscopy, where improved measurement of input spectra will lead to better experimental outcomes. △ Less

Submitted 26 March, 2024; v1 submitted 23 November, 2023; originally announced November 2023.

Comments: 14 pages, 8 figures

Ultrafast quantum dynamics driven by the strong space charge field of a relativistic electron beam

Authors: D. Cesar, A. Acharya, J. P. Cryan, A. Kartsev, M. F. Kling, A. M. Lindenberg, C. D. Pemmaraju, A. D. Poletayev, V. S. Yakovlev, A. Marinelli

Abstract: In this article, we illustrate how the Coulomb field of a highly relativistic electron beam can be shaped into a broadband pulse suitable for driving ultrafast and strong-field physics. In contrast to a solid-state laser, the Coulomb field creates a pulse which can be intrinsically synchronized with an x-ray free electron laser (XFEL), can have a cutoff frequency which is broadly tunable from THz… ▽ More In this article, we illustrate how the Coulomb field of a highly relativistic electron beam can be shaped into a broadband pulse suitable for driving ultrafast and strong-field physics. In contrast to a solid-state laser, the Coulomb field creates a pulse which can be intrinsically synchronized with an x-ray free electron laser (XFEL), can have a cutoff frequency which is broadly tunable from THz to EUV, and which acts on target systems as a "half-cycle" impulse. Explicit examples are presented to emphasize how the unique features of this excitation can be a tool for novel science at XFEL facilities like the LCLS. △ Less

Submitted 26 July, 2022; originally announced July 2022.

Comments: 13 pages, 6 figures

SHarD: A beam dynamics simulationcode for dielectric laser acceleratorsbased on spatial harmonic field expansion

Authors: Alexander Ody, Sophie Crisp, Pietro Musumeci, David Cesar, R. Joel England

Abstract: In order to demonstrate acceleration of electrons to relativistic scales by an on chipdielectric laser accelerator (DLA), a ponderomotive focusing scheme capable of capturingand transporting electrons through nanometer-scale apertures over extended interactionlengths has been proposed. We present a Matlab-based numerical code (SHarD) utilizinga spatial harmonic expansion of the fields within the d… ▽ More In order to demonstrate acceleration of electrons to relativistic scales by an on chipdielectric laser accelerator (DLA), a ponderomotive focusing scheme capable of capturingand transporting electrons through nanometer-scale apertures over extended interactionlengths has been proposed. We present a Matlab-based numerical code (SHarD) utilizinga spatial harmonic expansion of the fields within the dielectric structure to simulate theevolution of the beam phase space distribution in this scheme. The code can be usedto optimize key-parameters for the accelerator performance such as the final energy,transverse spot size evolution and total number of electrons accelerated through currentlyfabricated structures. Eventually, the simulation model will be applied to inform thephase mask profile to be added to a pulse front tilt drive laser pulse using a liquid crystalmask in the experimental setup being assembled at UCLA Pegasus Laboratory. △ Less

Submitted 23 July, 2021; originally announced July 2021.

Comments: 12 pages, 4 figures, 1 table, Manuscript Accepted at NIM-A

Electron beam shaping via laser heater temporal shaping

Authors: D. Cesar, A. Anakru, S. Carbajo, J. Duris, P. Franz, S. Li, N. Sudar, Z. Zhang, A. Marinelli

Abstract: Active longitudinal beam optics can help FEL facilities achieve cutting edge performance by optimizing the beam to: produce multi-color pulses, suppress caustics, or support attosecond lasing. As the next generation of superconducting accelerators comes online, there is a need to find new elements which can both operate at high beam power and which offer multiplexing capabilities at Mhz repetition… ▽ More Active longitudinal beam optics can help FEL facilities achieve cutting edge performance by optimizing the beam to: produce multi-color pulses, suppress caustics, or support attosecond lasing. As the next generation of superconducting accelerators comes online, there is a need to find new elements which can both operate at high beam power and which offer multiplexing capabilities at Mhz repetition rate. Laser heater shaping promises to satisfy both criteria by imparting a programmable slice-energy spread on a shot-by-shot basis. We use a simple kinetic analysis to show how control of the slice energy spread translates into control of the bunch current profile, and then we present a collection of start-to-end simulations at LCLS-II in order to illustrate the technique. △ Less

Submitted 8 June, 2021; v1 submitted 7 June, 2021; originally announced June 2021.

Comments: 12 pages, 11 figures

An Approach to Gender Pay Equity Analysis Using Bayesian Hierarchical Regression

Authors: Diana Cesar

Abstract: Diversity and inclusion, or D and I, is a topic that sparks the interest of companies, research groups, and individuals alike. Recently in the United States, renewed focus has been placed on fair and equitable pay practices, which are a key component of promoting diversity in the workplace. Despite the increased demand for reliable pay equity analysis, the challenges of conducting this type of ana… ▽ More Diversity and inclusion, or D and I, is a topic that sparks the interest of companies, research groups, and individuals alike. Recently in the United States, renewed focus has been placed on fair and equitable pay practices, which are a key component of promoting diversity in the workplace. Despite the increased demand for reliable pay equity analysis, the challenges of conducting this type of analysis on industry data have not been adequately addressed. This paper explains a few limitations of current approaches to pay equity analysis by gender and improves on them with a Bayesian hierarchical regression model. Using global workforce data from a large U.S. semiconductor company, Micron Technology, Inc., the paper demonstrates how the model provides a holistic view of gender pay equity across the organization, while overcoming issues more common in industry data, such as small sample size and poor gender representation. When compared to a prior analysis of Micron's U.S. workforce, this approach decreased the amount of manual review required, enabling decision makers to finalize pay adjustments across a workforce of 31,738 people within four weeks of receiving preliminary model results. △ Less

Submitted 21 December, 2020; originally announced December 2020.

Generation of terawatt, attosecond pulses from relativistic transition radiation

Authors: Xinlu Xu, David B. Cesar, Sébastien Corde, Vitaly Yakimenko, Mark J. Hogan, Chan Joshi, Agostino Marinelli, Warren B. Mori

Abstract: When a fs duration and hundreds of kA peak current electron beam traverses the vacuum and high-density plasma interface a new process, that we call relativistic transition radiation (R-TR) generates an intense $\sim100$ as pulse containing $\sim$ TW power of coherent VUV radiation accompanied by several smaller fs duration satellite pulses. This pulse inherits the radial polarization of the incide… ▽ More When a fs duration and hundreds of kA peak current electron beam traverses the vacuum and high-density plasma interface a new process, that we call relativistic transition radiation (R-TR) generates an intense $\sim100$ as pulse containing $\sim$ TW power of coherent VUV radiation accompanied by several smaller fs duration satellite pulses. This pulse inherits the radial polarization of the incident beam field and has a ring intensity distribution. This R-TR is emitted when the beam density is comparable to the plasma density and the spot size much larger than the plasma skin depth. Physically, it arises from the return current or backward relativistic motion of electrons starting just inside the plasma that Doppler up-shifts the emitted photons. The number of R-TR pulses is determined by the number of groups of plasma electrons that originate at different depths within the first plasma wake period and emit coherently before phase mixing. △ Less

Submitted 24 July, 2020; originally announced July 2020.

Comments: 5 pages, 4 figures

Journal ref: Phys. Rev. Lett. 126, 094801 (2021)

Temporal magnification for streaked ultrafast electron diffraction and microscopy

Authors: David Cesar, Pietro Musumeci

Abstract: One of the frontiers of modern electron scattering instrumentation is improving temporal resolution in order to enable the observation of dynamical phenomena at their fundamental time-scales. We analyze how a radiofrequency cavity can be used as an electron longitudinal lens in order to produce a highly magnified temporal replica of an ultrafast process, and, in combination with a deflecting cavit… ▽ More One of the frontiers of modern electron scattering instrumentation is improving temporal resolution in order to enable the observation of dynamical phenomena at their fundamental time-scales. We analyze how a radiofrequency cavity can be used as an electron longitudinal lens in order to produce a highly magnified temporal replica of an ultrafast process, and, in combination with a deflecting cavity, enable streaked electron images of optical-frequency phenomena. We present start-to-end simulations of an MeV electron beamline for two variations of this idea (a `magnifying-glass' and a `point-projection' configuration) showing the feasibility for an electron probe to achieve single shot 1.4 fs(rms) temporal resolution. △ Less

Submitted 14 January, 2019; originally announced January 2019.

Comments: 9 pages, 6 figures

All Optical Control of Beam Dynamics in a DLA

Authors: David Cesar, Pietro Musumeci, R. Joel England

Abstract: Dielectric laser acceleration draws upon nano-fabrication techniques to build photonic structures for high gradient electron acceleration. At the small spatial scales characteristic of these structures conventional accelerator techniques become ineffective at stabilizing the beam dynamics. Instead we propose a scheme to stabilize the motion by directly modulating the drive laser, in analogy to a r… ▽ More Dielectric laser acceleration draws upon nano-fabrication techniques to build photonic structures for high gradient electron acceleration. At the small spatial scales characteristic of these structures conventional accelerator techniques become ineffective at stabilizing the beam dynamics. Instead we propose a scheme to stabilize the motion by directly modulating the drive laser, in analogy to a radio-frequency-quadrupole. Here we present a design for a programmable `lattice' being built at UCLA's Pegasus laboratory. The accelerator accepts an unmodulated 3.5 MeV electron beam and then bunches and accelerates the beam by 1.5 MeV over a distance of 2 cm. △ Less

Submitted 14 January, 2019; originally announced January 2019.

Comments: 5 pages, 4 figures, Proceedings of the Advanced Accelerator Concepts (AAC) 2018

Enhanced energy gain in a dielectric laser accelerator using a tilted pulse front laser

Authors: D. Cesar, J. Maxson, P. Musumeci, X. Shen, R. J. England, K. P. Wootton, S. Tan

Abstract: Using an 800 nm, 45 fs pulse-front-tilted laser we demonstrate a record 315 keV energy gain in a dual grating dielectric laser accelerator (DLA) and average accelerating gradients of 560 MV/m over 0.5 mm. These results open a new regime in DLA characterized by significant evolution of the beam distribution in the longitudinal phase space, corresponding to >1/4 of a synchrotron oscillation. By tilt… ▽ More Using an 800 nm, 45 fs pulse-front-tilted laser we demonstrate a record 315 keV energy gain in a dual grating dielectric laser accelerator (DLA) and average accelerating gradients of 560 MV/m over 0.5 mm. These results open a new regime in DLA characterized by significant evolution of the beam distribution in the longitudinal phase space, corresponding to >1/4 of a synchrotron oscillation. By tilting the laser wavefront we control the resonant velocity of the DLA and observe a net energy gain, indicating that a tapered optical phase could be used to achieve very high energy gain. △ Less

Submitted 6 April, 2018; v1 submitted 2 April, 2018; originally announced April 2018.

Comments: 5 pages, 5 figures

Optical design for increased interaction length in a high gradient dielectric laser accelerator

Authors: D. Cesar, J. Maxson, P. Musumeci, X. Shen, R. J. England, K. P. Wootton

Abstract: We present a methodology for designing and measuring pulse front tilt in an ultrafast laser for use in dielectric laser acceleration. Previous research into dielectric laser accelerating modules has focused on measuring high accelerating gradients in novel structures, but has done so only for short electron-laser coupling lengths. Here we demonstrate an optical design to extend the laser-electron… ▽ More We present a methodology for designing and measuring pulse front tilt in an ultrafast laser for use in dielectric laser acceleration. Previous research into dielectric laser accelerating modules has focused on measuring high accelerating gradients in novel structures, but has done so only for short electron-laser coupling lengths. Here we demonstrate an optical design to extend the laser-electron interaction to 1mm. △ Less

Submitted 3 January, 2018; originally announced January 2018.

Comments: 5 pages, 4 figures, Submitted to NIMA for proceedings of EAAC 2017

Nonlinear response in high-field dielectric laser accelerators

Authors: D. Cesar, S. Custodio, J. Maxson, P. Musumeci, X. Shen, E. Threlkeld, R. J. England, A. Hanuka, I. V. Makasyuk, E. A. Peralta, K. P. Wootton, Z. Wu

Abstract: Laser powered dielectric structures achieve high-gradient particle acceleration by taking advantage of modern laser technology capable of producing electric fields in excess of 10GV/m. These fields can drive the bulk dielectric beyond its linear response, and break the phase synchronicity between the accelerating field and the electrons. We show how control of the pulse dispersion can be used to c… ▽ More Laser powered dielectric structures achieve high-gradient particle acceleration by taking advantage of modern laser technology capable of producing electric fields in excess of 10GV/m. These fields can drive the bulk dielectric beyond its linear response, and break the phase synchronicity between the accelerating field and the electrons. We show how control of the pulse dispersion can be used to compensate the effect of self-phase modulation and maximize the energy gain in the laser accelerator.In our experiment, a high brightness 8MeV e-beam is used to probe accelerating fields of 1.8GV/m in a 'grating-reset' dielectric structure illuminated by a 45fs laser pulse with a fluence of 0.7J/cm$^2$. △ Less

Submitted 7 July, 2017; originally announced July 2017.

Comments: 7 pages, 5 figures

Direct measurement of sub-10 fs relativistic electron beams with ultralow emittance

Authors: Jared Maxson, David Cesar, Giacomo Calmasini, Alexander Ody, Pietro Musumeci, David Alesini

Abstract: Ultralow emittance ($\leq20$ nm, normalized) electron beams with $10^5$ electrons per bunch are obtained by tightly focusing an ultrafast ($\sim$ 100 fs) laser pulse on the cathode of a 1.6 cell radiofrequency photoinjector. Taking advantage of the small initial longitudinal emittance, a downstream velocity bunching cavity is used to compress the beam to $<10$ fs rms bunch length. The measurement… ▽ More Ultralow emittance ($\leq20$ nm, normalized) electron beams with $10^5$ electrons per bunch are obtained by tightly focusing an ultrafast ($\sim$ 100 fs) laser pulse on the cathode of a 1.6 cell radiofrequency photoinjector. Taking advantage of the small initial longitudinal emittance, a downstream velocity bunching cavity is used to compress the beam to $<10$ fs rms bunch length. The measurement is performed using a thick high voltage deflecting cavity which is shown to be well-suited to measure ultrashort durations of bunching beams, provided that the beam reaches a ballistic longitudinal focus at the cavity center. △ Less

Submitted 9 March, 2017; v1 submitted 5 November, 2016; originally announced November 2016.

Journal ref: Phys. Rev. Lett. 118, 154802 (2017)

Demonstration of single-shot picosecond time-resolved MeV electron imaging using a compact permanent magnet quadrupole based lens

Authors: D. Cesar, J. Maxson, P. Musumeci, Y. Sun, J. Harrison, P. Frigola, F. H. O'Shea, H. To, D. Alesini, R. K. Li

Abstract: We present the results of an experiment where a short focal length (~ 1.3 cm) permanent magnet electron lens is used to image micron-size features of a metal sample in a single shot, using an ultra- high brightness ps-long 4 MeV electron beam from a radiofrequency photoinjector. Magnifcation ratios in excess of 30x were obtained using a triplet of compact, small gap (3.5 mm), Halbach-style permane… ▽ More We present the results of an experiment where a short focal length (~ 1.3 cm) permanent magnet electron lens is used to image micron-size features of a metal sample in a single shot, using an ultra- high brightness ps-long 4 MeV electron beam from a radiofrequency photoinjector. Magnifcation ratios in excess of 30x were obtained using a triplet of compact, small gap (3.5 mm), Halbach-style permanent magnet quadrupoles with nearly 600 T/m field gradients. These results pave the way to- wards single shot time-resolved electron microscopy and open new opportunities in the applications of high brightness electron beams. △ Less

Submitted 4 April, 2016; originally announced April 2016.

Comments: 5 pages, 6 figures

Journal ref: Phys. Rev. Lett. 117, 024801 (2016)

Exploring the transport properties of polytypic and twin-plane nanowires: from tunneling phase-time to spin-orbit interaction effects

Authors: M. Rebello Sousa Dias, L. Villegas-Lelovsky, L. Diago-Cisneros, L. K. Castelano, D. F. Cesar, G. E. Marques, V. Lopez-Richard

Abstract: The variety of nanowire crystal structures gave rise to unique and novel transport phenomena. In particular, we have explored the superlattice profile generated by strain field modulation in twinplane nanowires for the tuning of transport channels and the built-in spin-orbit potential profile of polytypic nanowires, in order to realize a spin filter. The Multicomponent Scattering Approach has been… ▽ More The variety of nanowire crystal structures gave rise to unique and novel transport phenomena. In particular, we have explored the superlattice profile generated by strain field modulation in twinplane nanowires for the tuning of transport channels and the built-in spin-orbit potential profile of polytypic nanowires, in order to realize a spin filter. The Multicomponent Scattering Approach has been used in terms of the Transfer Matrix Method to describe the phase-time of charge carriers. This system showed advantages for attaining conditions for the propagation of wave packets with negative group velocity. Moreover, the spin transport effect of a potential profile with volumetric spin-orbit bulk inversion asymmetry, as present on polytypic nanowires, was described through the Reverse Runge-Kutta Method. Using the peculiar symmetry of the excited states we have characterized a dominant spin dependence on structural parameters that results in effective spin filtering. △ Less

Submitted 17 March, 2015; originally announced March 2015.

Comments: 17 pages, 7 figures