Maïmouna Bocoum

Relativistic electron beams driven by kHz single-cycle light pulses

Laser-plasma acceleration(1,2) is an emerging technique for accelerating electrons to high energies over very short distances. The accelerated electron bunches have femtosecond duration(3,4), making them particularly relevant for applications such as ultrafast imaging(5) or femtosecond X-ray generation(6,7). Current laser-plasma accelerators deliver 100 MeV (refs 8-10) to GeV (refs 11, 12) electrons using Joule-class laser systems that are relatively large in scale and have low repetition rates, with a few shots per second at best. Nevertheless, extending laser-plasma acceleration to higher repetition rates would be extremely useful for applications requiring lower electron energy. Here, we use single-cycle laser pulses to drive high-quality MeV relativistic electron beams, thereby enabling kHz operation and dramatic downsizing of the laser system. Numerical simulations indicate that the electron bunches are only similar to 1 fs long. We anticipate that the advent of these kHz femtosecond relativistic electron sources will pave the way to applications with wide impact, such as ultrafast electron diffraction in materials(13,14) with an unprecedented sub-10 fs resolution(15).

Effect of the Laser Wave Front in a Laser-Plasma Accelerator

A high-repetition rate electron source is generated by tightly focusing kHz, few-mJ laser pulses into an underdense plasma. This high-intensity laser-plasma interaction leads to stable electron beams over several hours but with strikingly complex transverse distributions even for good quality laser focal spots. We find that the electron beam distribution is sensitive to the laser wave front via the laser midfield distribution rather than the laser focal spot itself. We are able to measure the laser wave front around the focus and include it in realistic particle-in-cell simulations demonstrating the role of the laser wave front on the acceleration of electrons. Distortions of the laser wave front cause spatial inhomogeneities in the midfield laser intensity and, consequently, the laser pulse drives an inhomogeneous transverse wakefield whose focusing and defocusing properties affect the electron distribution. These findings explain the experimental results and suggest the possibility of controlling the electron spatial distribution in laser-plasma accelerators by tailoring the laser wave front.

Relativistic-intensity near-single-cycle laser system at 1 kHz (Conference Presentation)

Controlled few-cycle light waveforms find numerous applications in attosecond science, most notably the production of isolated attosecond pulses in the XUV spectral region for studying ultrafast electronic processes in matter. Scaling up the pulse energy of few-cycle pulses could extend the scope of applications to even higher intensity processes, such as the generation of attosecond pulses with extreme brightness from relativistic plasma mirrors. Hollow-fiber compressors are widely used to produce few-cycle pulses with excellent spatiotemporal quality, whereby octave-spanning broadened spectra can be temporally compressed to near-single-cycle duration. In order to scale up the peak power of hollow-fiber compressors, the effective length and area mode of the fiber has to be increased proportionally, thereby requiring the use of longer waveguides with larger apertures. Thanks to an innovative design utilizing stretched flexible capillaries, we show that a stretched hollow-fiber compressor can generate pulses of TW peak power, the duration of which can be continuously tuned from the input seed laser pulse duration down to almost a single cycle (3.5fs at 750nm central wavelength) simply by increasing the gas pressure at the fiber end. The pulses are characterized online using an integrated d-scan device directly under vacuum. While the pulse duration and chirp are tuned, all other pulse characteristics, such as energy, pointing stability and focal distribution remain the same on target. This unique device makes it possible to explore the generation of high-energy attosecond XUV pulses from plasma mirrors using controllable relativistic-intensity light waveforms at 1kHz.

Surface plasma attosource beamlines at ELI-ALPS

ELI-ALPS, one of the three pillars of the Extreme Light Infrastructure (ELI) project, will be in a unique position to offer dedicated experimental platforms for ultrashort time-resolved investigations of strongly excited dynamical systems. The state-of-the-art surface plasma attosource (SPA) beamlines at ELI-ALPS are being designed and developed to enable new directions in plasma-based attoscience research. The SPA beamlines will be driven by ultrashort, high peak power, high repetition rate lasers based on the latest technology and are aimed to develop previously unavailable attoscience experimental platforms employing surface high-harmonic generation process. This endeavor involves research and development challenges and careful considerations. Here we discuss the physics of plasma attosources and their characteristics under such extreme conditions and the beamline functionalities that would facilitate these objectives. Finally, we delineate the initial research possibilities with these sophisticated instruments

Relativistic-intensity 1.3 optical cycle laser pulses at 1kHz from a stretched hollow-core-fiber compressor

Waveform-controlled few-cycle light pulses have become an important tool in contemporary ultrafast science not only because they provide high temporal resolution but also because they facilitate sub-cycle gating of highly nonlinear process [1]. Sub-cycle gating of relativistic laser-plasma interactions is of particular interest as it could be a potentially efficient source of high-energy attosecond XUV pulses [2] or MeV femtosecond electron bunches [3] for time-resolved applications. Achieving this requires near-single-cycle pulses with TW peak power and high spatiotemporal fidelity, so they can be focused down to relativistic intensity (>1018 W/cm2 for 800nm light).

Correlated emission of high-harmonics and fast electrons beams from plasma mirrors

We report for the first time on the correlated emission of high-harmonics and fast electrons from femtosecond plasma mirrors. We show that both processes cannot occur simultaneously for the same density gradient at sub-relativistic intensities.

Effect of the laser wavefront in a laser-plasma accelerator

A high-repetition rate electron source is generated by tightly focusing kHz, few-mJ laser pulses into an underdense plasma. This high-intensity laser-plasma interaction leads to stable electron beams over several hours but with strikingly complex transverse distributions even for good quality laser focal spots. We find that the electron beam distribution is sensitive to the laser wave front via the laser midfield distribution rather than the laser focal spot itself. We are able to measure the laser wave front around the focus and include it in realistic particle-in-cell simulations demonstrating the role of the laser wave front on the acceleration of electrons. Distortions of the laser wave front cause spatial inhomogeneities in the midfield laser intensity and, consequently, the laser pulse drives an inhomogeneous transverse wakefield whose focusing and defocusing properties affect the electron distribution. These findings explain the experimental results and suggest the possibility of controlling the electron spatial distribution in laser-plasma accelerators by tailoring the laser wave front.