Anne-Sophie Morlens

A high-intensity highly coherent soft X-ray femtosecond laser seeded by a high harmonic beam

Synchrotrons have for decades provided invaluable sources of soft X-rays, the application of which has led to significant progress in many areas of science and technology. But future applications of soft X-rays—in structural biology, for example—anticipate the need for pulses with much shorter duration (femtoseconds) and much higher energy (millijoules) than those delivered by synchrotrons. Soft X-ray free-electron lasers should fulfil these requirements but will be limited in number; the pressure on beamtime is therefore likely to be considerable. Laser-driven soft X-ray sources offer a comparatively inexpensive and widely available alternative, but have encountered practical bottlenecks in the quest for high intensities. Here we establish and characterize a soft X-ray laser chain that shows how these bottlenecks can in principle be overcome. By combining the high optical quality available from high-harmonic laser sources (as a seed beam) with a highly energetic soft X-ray laser plasma amplifier, we produce a tabletop soft X-ray femtosecond laser operating at 10 Hz and exhibiting full saturation, high energy, high coherence and full polarization. This technique should be readily applicable on all existing laser-driven soft X-ray facilities.

Submicrometer digital in-line holographic microscopy at 32 nm with high-order harmonics

Soft-x-ray digital in-line microscopic holography is achieved using a fully coherent high-order harmonic source emitting at 32 nm. Combination of commercial-grade soft-x-ray optics and a back-illuminated CCD detector allows a compact and versatile holographic setup. Different experimental geometries have been tested by imaging calibrated 50 nm tips and 1 microm wires. Spatial resolution of 800 nm is measured with magnifications ranging from 30 to 110 and a numerical aperture around 0.01. Finally, the potentiality of three-dimensional numerical reconstruction from a single hologram acquisition is shown experimentally.

Compression of attosecond harmonic pulses by extreme-ultraviolet chirped mirrors

In the race toward attosecond pulses, for which high-order harmonics generated in rare gases are the best candidates, both the harmonic spectral range and the spectral phase have to be controlled. We demonstrate that multilayer extreme-ultraviolet chirped mirrors can be numerically optimized and designed to compensate for the intrinsic harmonic chirp that was recently discovered and that is responsible for temporal broadening of pulses. A simulation shows that an optimized mirror is capable of compressing the duration from approximately 260 to 90 as. This new technique is an interesting solution because of its ability to cover a wider spectral range than other technical devices that have already been proposed to overcome the chirp of high harmonics.

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.

Study of XUV beam splitter flatness for use on a Michelson interferometer

A XUV Michelson interferometer has been developed by LIXAM/CEA/LCFIO and has been tested as a Fourier-transform spectrometer for measurement of X-ray laser line shape. The observed strong deformation of the interference fringes limited the interest of such an interferometer for plasma probing. Because the fringe deformation was coming from a distortion of the beam splitter (5 × 5 mm 2 open aperture, about 150 nm thick), several parameters of the multilayer deposition used for the beam splitter fabrication have been recently optimized. The flatness has been improved from 80 nm rms obtained by using the ion beam sputtering technique, to 20 nm rms by using the magnetron sputtering technique. Over 3 × 3 mm 2 , the beam splitter has a flatness better than 4 nm rms.

Progress in optical-field-ionization soft X-ray lasers at LOA

We give on overview of recent advances in collisionally pumped optical field-ionization soft X-ray lasers developed at LOA. Saturated amplification has been achieved on the 5d-5p transition in Xe^8+ at 41.8 nm, and on the 4d-4p transition in Kr^8+ at 32.8 nm. We demonstrate a significant increase of the energy output from the Xe^8+ laser driven within two types of wave-guide. Finally, we present results of a pioneering work aimed to set up and characterize the first true soft X-ray laser chain.

High order harmonics wavefront measurement and optimization

We present a full optimization of the high harmonics wave-front thanks to the use of a soft x-ray Hartmann sensor. The sensor was calibrated using high harmonics source with a λ/50 accuracy. We observed relatively good high harmonics wave-front, two times the diffraction-limit, with astigmatism as the dominant aberration for any interaction parameters. By slightly clipping the unfocused beam, it is possible to produce a diffraction-limited beam containing about 90% of the incident energy. The influence of high harmonic generation parameters was also studied in particularly the influence of the infra-red wave-front. In particular we studied the correlation between the infrared wave-front use to create high harmonics and the high harmonic wave-front. We also report wave-front measurements of a high order harmonic beam into an x-ray laser plasma amplifier at 32.8 nm.

Characterization of an OFI seeded soft x-ray laser

Since the first seeding of an OFI soft x-ray laser in 2004, we progressed towards the full characterization of the output beam. The final is to be able to deliver to users well-known beam. Temporal as well as spatial parameters have been measured for different conditions of amplification. We observed a strong enhancement of the spatial coherence due to the amplification process with a far-field pattern exhibiting an airy-like shape. The gain zone having strong discontinuity behaves like a hard pinhole. Spatial filtering has been also observed on the wave front (δ/5 root-mean-square, rms, before seeding and δ/20 rms after amplification). Temporal coherence has been studied thanks to the use of a Fourier- Transform spectrometer. Spectral widths, δδ/δ, around 10-5 have been measured for different plasma lengths or gas pressures. Departure from Gaussian shape has been clearly observed on the spectral line for some cases.

XUV optics for attosecond applications

Among X-ray and extreme ultraviolet light sources able to produce shorter and shorter, coherent and intense pulses, High order harmonics generated in rare gases are currently the unique way to generate attosecond pulses. However, the manipulation and transport of attosecond pulses require the development of dedicated optics for reaching specific characteristics in terms of amplitude but also in terms of spectral phase control. We present here a multilayer design for chirp compensation of attosecond pulses. We also present an application of these multilayers mirrors for attosecond train pulse holography experiment with high harmonics. This experience took benefit of both temporal and spatial phase properties of high harmonics. A resolution of 750 nm has been achieved by using a 350 as train pulse for the reference wave constituted of four consecutive harmonics (λ=28 nm to λ=41 nm). This new method will allow making ultra fast movies with attosecond resolution of transient phenomena with quasi-3D resolution.

Status and Prospects on Soft X-Ray Lasers Seeded by a High Harmonic Beam at LOA

Thanks to the most recent works on x-ray laser and on high order harmonics (HHG), it is now possible to produce an energetic beam having at the same time the required optical properties. The solution consists in seeding the XRL amplifier medium with another beam (HHG). This experiment was successfully realized in LOA. We studied seeding of two x-ray laser transitions, 4d-4p at 32.8 nm in Kr8+ and 5d-5p 41.8 nm in Xe8+. The amplifying medium is generated by focussing a high energy circularly polarized, 35 fs 10 Hz Ti: sapphire laser system in a few mm cell filled with gas (xenon or krypton). We succeeded to increase from a factor 10 to 200 the HHG energy, without deteriorating their optical qualities. The resulting beam was polarized, coherent and we estimate the output energy to be about 0.5 eJ.