Frederik Böhle

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).

Carrier-envelope-phase stable, high-contrast, double chirped-pulse-amplification laser system

We present the first carrier-envelope-phase stable chirped-pulse amplifier (CPA) featuring high temporal contrast for relativistic intensity laser-plasma interactions at 1 kHz repetition rate. The laser is based on a double-CPA architecture including cross-polarized wave (XPW) filtering technique and a high-energy grism-based compressor. The 8 mJ, 22 fs pulses feature 10⁻¹¹ temporal contrast at -20  ps and a carrier-envelope-phase drift of 240 mrad root mean square.

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.

Generation of 3-mJ, 4-fs CEP-Stable Pulses by Long Stretched Flexible Hollow Fibers

3-mJ, 4-fs CEP-stable pulses were generated by spectral broadening of circularly polarized 8mJ pulses in a differentially pumped 2-m-long hollow fiber. The pulses were characterized by SHG d-scan method.

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.

High-charge relativistic electron bunches from a kHz laser-plasma accelerator

We report on electron wakefield acceleration in the resonant bubble regime with few-millijoule near-single-cycle laser pulses at a kilohertz repetition rate. Using very tight focusing of the laser pulse in conjunction with microscale supersonic gas jets, we demonstrate a stable relativistic electron source with a high charge per pulse up to 24 pC/shot. The corresponding average current is 24 nA, making this kilohertz electron source useful for various applications.

Relativistic electron beams driven by single-cycle laser pulses at kHz repetition rate (Conference Presentation)

Laser-plasma accelerators are usually driven by 100-TW class laser systems with rather low repetition rates. However, recent years have seen the emergence of laser-plasma accelerators operating with kHz lasers and energies lower than 10 mJ. The high repetition-rate is particularly interesting for applications requiring high stability and high signal-to-noise ratio but lower energy electrons. For example, our group recently demonstrated that kHz laser-driven electron beams could be used to capture ultrafast structural dynamics in Silicon nano-membranes via electron diffraction with picosecond resolution. In these first experiments, electrons were injected in the density gradients located at the plasma exit, resulting in rather low energies in the 100 keV range. The electrons being nonrelativistic, the bunch duration quickly becomes picosecond long. Relativistic energies are required to mitigate space charge effects and maintain femtosecond bunches. In this paper, we will show very recent results where electrons are accelerated in laser-driven wakefields to relativistic energies, reaching up to 5 MeV at kHz repetition rate. The electron energy was increased by nearly two orders of magnitude by using single-cycle laser pulses of 3.5 fs, with only 2.5 mJ of energy. Using such short pulses of light allowed us to resonantly excite high amplitude and nonlinear plasma waves at high plasma density, ne=1.5-2×1020 cm-3, in a regime close to the blow-out regime. Electrons had a peaked distribution around 5 MeV, with a relative energy spread of ~30 %. Charges in the 100’s fC/shot and up to pC/shot where measured depending on plasma density. The electron beam was fairly collimated, ~20 mrad divergence at Full Width Half Maximum. The results show remarkable stability of the beam parameters in terms of beam pointing and electron distribution. 3D PIC simulations reproduce the results very well and indicate that electrons are injected by the ionization of Nitrogen atoms, N5+ to N6+, leading to the formation of an electron bunch of 1 fs duration. The interaction of single-cycle pulses with the plasma also leads to new physical effects. We have observed experimental evidence that plasma dispersion cannot be neglected in this regime. This is due to the extremely broad bandwidth of the laser, extending from 400 nm to 1000 nm, and to the high electron density. Therefore, the acceleration process is optimal when small positive chirps are introduced: the negative dispersion of the plasma then causes the re-compression of the laser pulse inside the plasma. Simulations indicate that this help localizing the injection process, leading to single femtosecond electron bunch. Such a kHz femtosecond electron source will pave to way to numerous innovative applications, such as sub-10 fs electron diffraction, radiolysis of water with unprecedented resolution or the generation of femtosecond X-ray at kHz.

CEP-stable, multi-mJ, 4.3 fs pulses from long stretched flexible hollow fibers

CEP-stable 4.3fs pulses of 3mJ energy were generated by spectral broadening of circularly polarized 8mJ pulses in a differentially pumped 2-m-long hollow fiber. The pulses were characterized by a single-shot SHG FROG.