F. Tissandier

An optimized kHz two-colour high harmonic source for seeding free-electron lasers and plasma-based soft x-ray lasers

Free-electron lasers (FEL) and plasma-based soft x-ray lasers (PSXL) have been recently evolving very fast from the vacuum ultraviolet to the soft x-ray region. Once seeded with high harmonics, these schemes are considered as the next generation soft x-ray light sources delivering ultrashort pulses with high temporal and spatial coherence. Here, we present a detailed experimental study of a kHz two-colour high harmonic generation performed in various gases and investigate its potential as a suitable evolution of the actual seeding sources. It turns out that this double harmonic content source is highly tuneable, controllable and delivers intense radiation (measured here with a calibrated photodiode) with only one order of magnitude difference in the photon yield from 65 to 13?nm. Then, first and foremost, injections could be achieved at wavelengths shorter than what was previously accessible in FEL and PSXL and/or additional energy could be extracted. Also, such a strong and handy seed could allow the saturation range of FEL devices to be greatly extended to shorter wavelengths and would bring higher spectral as well as intensity stabilities in this spectral zone.

Aberration-free laser beam in the soft x-ray range.

By seeding an optical-field-ionized population-inverted plasma amplifier with the 25th harmonic of an IR laser, we have achieved what we believe to be the first aberration-free laser beam in the soft x-ray spectral range. This laser emits within a cone of 1.34 mrad(1/e(2)) at a repetition rate of 10 Hz at a central wavelength of 32.8 nm. The beam exhibits a circular profile and wavefront distortions as low as lambda/17. A theoretical analysis of these results shows that this high beam quality is due to spatial filtering of the seed beam by the plasma amplifier aperture.

Fourier-limited seeded soft x-ray laser pulse

We present what we believe to be the first measurement of the spectral properties of a soft x-ray laser seeded by a high-order harmonic beam. Using an interferometric method, the spectral profile of a seeded Ni-like krypton soft x-ray laser (32.8 nm) generated by optical field ionization has been experimentally determined, and the shortest possible pulse duration has been deduced. The source exhibits a Voigt spectral profile with an FWHM of 3.1+/-0.3 mA, leading to a Fourier-transform pulse duration of 4.7 ps. This value is comparable with the upper limit of the soft x-ray pulse duration determined by experimentally investigating the gain dynamics, from which we conclude that the source has reached the Fourier limit. The measured bandwidth is in good agreement with the predictions of a radiative transfer code, including gain line narrowing and saturation rebroadening.

Aberration-free high-harmonic source generated with a two-colour field

Imaging experiments of ultrafast phenomena of matter at nanometre-scale require intense, short pulse duration and diffraction-limited soft–X-ray beams, nowadays almost only provided by free-electron lasers. Here, we focused on a table-top soft–X-ray source, which fulfils these fundamental criteria and in addition presents high temporal coherence, the high harmonics generated with a two-colour field (ω+2ω). These harmonics revealed to be free from aberration just by slightly spatially filtering the laser used for generation (ω). Indeed, the measured wavefront distortions, equal to λ/17 RMS at 44u2009nm, correspond to a diffraction-limited beam. This behaviour is explained by an additional spatial nonlinear filtering effect of the driving laser wavefront, induced here in our particular but simple geometrical configuration of generation by the 2ω component.

Characterization of Zn X-Ray Laser at PALS Centre, Its Applications in Dense Plasma Probing and Astrophysics

This report presents the results from experiments at PALS Centre using a Zn X-ray laser with the pulse length of 0.15 ns and the wavelength of 21.2 nm, working in single or double pass regime with the output energy of 0.4 mJ or 4 mJ per pulse, respectively. The current X-ray laser was experimentally examined to obtain its temporal coherence and spectral width using a path-difference interferometer. The double pass regime shows that QSS plasma based source-amplifier is promising for “short” fs soft X-ray pulses. The X-ray laser is commonly used for user’s experiments. Its advantages can be shown in applications such as probing of dense plasmas (up to 2.5×1024 cm−3) or single shot experiments (4×1014 photons/pulse). The simple technique based on Talbot effect was used to obtain the gradients of electron densities of line plasmas produced under conditions corresponding to XRL’s amplifiers operating in TCE and QSS regime. To investigate radiative shock wave in laboratory is challenging in aspects of the optimization of experimental parameters. Due to the high electron density (1022 cm−3) produced in the gas medium propagated by the shock wave, the velocity of the shock wave, and the absorption losses on optical elements, it is necessary to use the energetic single shot probe.

Bessel spatial profile of a soft x-ray laser beam

We report far-field profile measurements of an optical-field-ionized high-order harmonic-seeded soft x-ray laser. We show that the beam transverse profile can be controlled between a regular Gaussian shape and a Bessel profile exhibiting several rings via the infrared laser pump intensity. These experimental data are supported by a complete numerical modeling including a two-dimensional plasma amplifier simulation and a two-level soft x-ray amplification using a Maxwell–Bloch treatment. This model takes into account the experimental high-order harmonic wavefront and intensity before it is numerically amplified.

Femtosecond soft x-ray laser by temporal ionization gating

Emerging applications of coherent soft x-ray sources, notably in biology, require high energy and ultrashort pulse duration in the femtosecond-scale to probe the ultra-fast dynamics of matter in the nanometer scale. Alongside current efforts to provide high brilliance x-ray coherent sources with X-ray free electron lasers, significant potential lays in the realization of compact and relatively cheap ultra-intense x-ray coherent sources. Plasma-based soft x-ray lasers turn out to be good candidates since they can emit a large number of photon (up to 1015 per pulse) within a narrow linewidth and exhibit high-quality optical properties once seeded with high-harmonic sources. However, the duration of these sources has been limited so far to the picosecond range consequently restricting the field of possible applications.

Development and applications of x-ray lasers at PALS Centre

The results of development and applications of the secondary sources at PALS Center will be presented. Currently the iodine system and the Ti:Sapphire system are operating at the PALS Center as driving lasers for generation of secondary sources. The iodine system with net energy of 1kJ is used for QSS X-ray lasing schemes. The most robust and most energetic QSS scheme with this driver is the Ne-like Zn X-ray laser, which is working here as standard user beamline for diverse applications. Recent experiment devoted to temporal coherence measurement shows possibility to amplify short duration X-rays. The second system with high rep rate is Ti: Sapphire laser chain with peak power 20TW. This laser system is used for generation high order harmonics and transient collisionally excited X-ray lasers.

An intense kHz and aberration-free two-colour high harmonic source for seeding FEL and XRL

Free-electron lasers (FEL) and soft X-ray lasers (SXRL) have been recently evolving very fast in the extreme-ultraviolet to soft X-ray region. Once seeded with high harmonics (HH), these light sources deliver amplified emissions with properties which are, for most of them, directly linked to the injected HH beam, e.g. the ultrashort pulse duration for FEL and the high temporal and spatial degree of coherence for both. Here we present a detailed experimental study of a kHz twocolour (fundamental + second harmonic) HH generation and investigate its potential as a suitable evolution of the actual seeding sources. It turns out that this source (both odd and even harmonics) is highly tuneable, and delivers intense radiations with only one order of magnitude difference in the photon yield from 65 nm to 13 nm. We also observed an astonishing “aberration-free” character of these harmonics (aberration below λ/17 rms, λ=44 nm). Then, the implementation of this technique on seeded FEL and XRL would allow amplification to be achieved at wavelengths shorter than previously accessible.

Advances in laser driven soft x-ray lasers at LOA

We report spatial and spectral characterization an optical-field-ionized high-order harmonic-seeded soft-x-ray laser. We show that it can be controlled between a regular Gaussian shape and a Bessel profile exhibiting several rings via the IR laser pump intensity. The temporal coherence and spectral linewidth of both the seeded and unseeded soft-x-ray lasers were experimentally measured using a varying path difference interferometer. It showed that the high-order harmonic is subject to a strong spectral narrowing during its propagation in the plasma amplifier without rebroadening at saturation. Also, we present a new method to generate ultra-short x-ray laser pulses by using the laser-driven betatron source to photo-pump inner-shell transitions.