Ph. Zeitoun

Imaging and quality assessment of high-harmonic focal spots

We present a direct method of studying the focusability of an intense, short-pulse extreme-ultraviolet (XUV) beam obtained by high-harmonic generation. We perform near-field imaging of the focal spot of five high-harmonic orders strongly focused by a broadband toroidal mirror. To visualize the focal spot directly, we image the fluorescence induced by an XUV beam on a cerium-doped YAG crystal on a visible CCD camera. We can thus measure the harmonic spot size on a single image, together with the Strehl ratio, to evaluate the quality of focusing. Such techniques should become instrumental in optimizing the focusing conditions and reaching intensities required for exploring attosecond nonlinear optics in the XUV range.

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.

Optimization of soft x-ray amplifier by tailoring plasma hydrodynamics.

Plasma-based soft x-ray lasers have the potentiality to generate high-energy, highly coherent, short pulse beam. Thanks to their high density, plasmas created by interaction of intense laser with solid target should store the highest amount of energy among every plasma amplifiers. However, to date output energy from solid amplifiers remains as low as 60 nJ [1]. For 30 m micrometer focal line width, we demonstrated with the 2D hydrodynamic code with radiation transport in AMR ARWEN [2] that deleterious hydrodynamic effects, as the lateral expansion and thermal conduction, reduce the amplification surface and the gain coefficient. Thus, carefully tailoring the plasma shape is crucial for extracting energy stored in the plasma. With 1 mm wide plasma, energy as high as 20 J in sub-ps pulse is achievable [3]. With such tailored plasma, pumping efficiency has been increased by nearly a factor of 10 as compared to former plasma amplifiers.

Demonstration of a spatial filtering amplifier for high-order harmonics.

We report what is to our knowledge the first demonstration of spatial filtering of a high-order harmonic beam into a soft-x-ray laser plasma amplifier at 32.8 nm. After amplification the seed energy is enhanced by a factor of 50, and the beam profile of the amplified beam exhibits an Airy-like shape due to the spatial filtering by the optical field ionized plasma. Moreover, the transverse coherence of the spatially filtered amplified beam is strongly enhanced, resulting in the generation of a peak coherent power of 0.9 x 10(5) to 1.8 x 10(5) W.

INVESTIGATION OF STRONG ELECTRIC-FIELD INDUCED SURFACE PHENOMENA BY SOFT X-UV LASER INTERFEROMETRY

Abstract Transient perturbed niobium surface states are observed for the first time during a long continued action of the applied DC-field owing to surface imaging by the means of an X–UV laser interferometry technique.

The influence of prepulse level on the 3p-3s XUV laser output from Ne-like ions of Zn, Cu and Ni

Abstract We have studied the effect of prepulses in enhancing the efficiency of generating ASE beams in soft X-ray laser plasma amplifiers based on pumping Ne-like ions. Slab targets were irradiated with a weak prepulse followed by a main plasma heating pulse of nanosecond duration. Time-integrated: time and spectrally resolved and time and angularly resolved lasing emissions on the 3p-3s ( J = 0–1) XUV lasing lines of Ne-like Ni, Cu and Zn at wavelengths 232 A, 221 A and 212 A respectively have been monitored. Measurements were made for pre-pulse/main-pulse intensity ratios from 10 −5 –10 −1 and for pump delay times of 2 ns and 4.5 ns. Zinc is shown to exhibit a peak in output intensity at ∼ 2 × 10 −3 pre-pulse fraction for a 4.5 ns pump delay, with a main pulse pump intensity of ∼ 1.3 × 10 13 W cm −2 on a 20 mm target. The Zn lasing emission had a duration of ∼ 240 ps and this was insensitive to prepulse fraction. The J = 0–1 XUV laser output for nickel and copper increased monotonically with prepulse fraction, with copper targets showing least sensitivity to either prepulse level or prepulse to main pulse delay. Under the conditions of the study, the pre-pulse level was observed to have no significant influence on the output intensity of the 3p-3s ( J = 2−1) lines of any of the elements investigated.

Three-dimensional Maxwell-Bloch calculation of the temporal profile of a seeded soft x-ray laser pulse

We present three-dimensional modeling of amplification of a high-order harmonic seed by a soft x-ray laser plasma. The time-dependent evolution of the x-ray signal is determined from a fully dynamic Maxwell-Bloch calculation. At high seed intensities, a simplified one-dimensional calculation leads to strong Rabi-like temporal oscillations of the output signal. However, such oscillations have not been observed experimentally. Our calculations demonstrate that this is due to spatial non-uniformities in the plasma gain that cause the Rabi oscillations to dampen dramatically. Large amplitude Rabi-like oscillations are expected to appear only in long and uniform plasma. Such targets require optimized guiding techniques.

Fourier transform holography with high harmonic spectra for attosecond imaging applications

We demonstrate a method of using a Fourier holographic technique to utilize attosecond soft x-ray pulses to image nanometer-scale objects. A discrete frequency comb of laser-generated high-order harmonics, yielding a train of attosecond pulses, has been used to record spatially and spectrally resolved images. The individual wavelengths were also combined to form a single image, albeit with lower spatial resolution, demonstrating the applicability of the method to using isolated attosecond pulses with continuous bandwidths.

High gain‐production efficiency and large brightness X‐UV laser at Palaiseau

A large gain has been measured for the J=0–1 line of neonlike Zn at λ=21.2 nm. The time evolutions and the localization of emission zones of the J=0–1 and J=2−1 lines are compared. It is shown that a train of very small prepulses before the main pulse has an important role in the J=0=1 emission. A half‐cavity has been successfully used to attain a nearly saturated intensity with a 2 cm long plasma. The X‐UV pulse energy is of 400 μJ, the laser power of 5 MW. The driving laser is the 0.4 KJ, 600 ps laser of LULI.

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.