Franck Delmotte

EUVI: the STEREO-SECCHI extreme ultraviolet imager

The Extreme Ultraviolet Imager (EUVI) is part of the SECCHI instrument suite currently being developed for the NASA STEREO mission. Identical EUVI telescopes on the two STEREO spacecraft will study the structure and evolution of the solar corona in three dimensions, and specifically focus on the initiation and early evolution of coronal mass ejections (CMEs). The EUVI telescope is being developed at the Lockheed Martin Solar and Astrophysics Lab. The SECCHI investigation is led by the Naval Research Lab. The EUVI’s 2048 x 2048 pixel detectors have a field of view out to 1.7 solar radii, and observe in four spectral channels that span the 0.1 to 20 MK temperature range. In addition to its view from two vantage points, the EUVI will provide a substantial improvement in image resolution and image cadence over its predecessor SOHO-EIT, while complying with the more restricted mass, power, and volume allocations on the STEREO mission.

Polarization control of high order harmonics in the EUV photon energy range

We report the generation of circularly polarized high order harmonics in the extreme ultraviolet range (18-27 nm) from a linearly polarized infrared laser (40 fs, 0.25 TW) focused into a neon filled gas cell. To circularly polarize the initially linearly polarized harmonics we have implemented a four-reflector phase-shifter. Fully circularly polarized radiation has been obtained with an efficiency of a few percents, thus being significantly more efficient than currently demonstrated direct generation of elliptically polarized harmonics. This demonstration opens up new experimental capabilities based on high order harmonics, for example, in biology and materials science. The inherent femtosecond time resolution of high order harmonic generating table top laser sources renders these an ideal tool for the investigation of ultrafast magnetization dynamics now that the magnetic circular dichroism at the absorption M-edges of transition metals can be exploited.

Design and characterization of extreme-ultraviolet broadband mirrors for attosecond science.

A novel multilayer mirror was designed and fabricated based on a recently developed three-material technology aimed both at reaching reflectivities of about 20% and at controlling dispersion over a bandwidth covering photon energies between 35 and 50 eV. The spectral phase upon reflection was retrieved by measuring interferences in a two-color ionization process using high-order harmonics produced from a titanium: sapphire laser. We demonstrate the feasibility of designing and characterizing phase-controlled broadband optics in the extreme-ultraviolet domain, which should facilitate the manipulation of attosecond pulses for applications.

Structural properties of Al/Mo/SiC multilayers with high reflectivity for extreme ultraviolet light

We present the results of an optical and chemical, depth and surface study of Al/Mo/SiC periodic multilayers, designed as high reflectivity coatings for the extreme ultra-violet (EUV) range. In comparison to the previously studied Al/SiC system, the introduction of Mo as a third material in the multilayer structure allows us to decrease In comparison to the previously studied Al/SiC system with a reflectance of 37% at near normal incidence around 17 nm, the introduction of Mo as a third material in the multilayer structure allows us to decrease the interfacial roughness and achieve an EUV reflectivity of 53.4%, measured with synchrotron radiation. This is the first report of a reflectivity higher than 50% around 17 nm. Time-of-flight secondary ion mass spectrometry (ToF-SIMS) and x-ray photoelectron spectroscopy (XPS) measurements are performed on the Al/Mo/SiC system in order to analyze the individual layers within the stack. ToF-SIMS and XPS results give evidence that the first SiC layer is partially oxidized, but the O atoms do not reach the first Mo and Al layers. We use these results to properly describe the multilayer stack and discuss the possible reasons for the difference between the measured and simulated EUV reflectivity values.

Longitudinal coherence measurements of a transient collisional x-ray laser

We present what is to our knowledge the first longitudinal coherence measurement of a transient inversion collisional x-ray laser. We investigated the picosecond output of a Ni-like Pd x-ray laser at 14.68 nm generated by the COMET laser facility at the Lawrence Livermore National Laboratory. Interference fringes were generated with a Michelson interferometer setup in which a thin multilayer membrane was used as a beam splitter. We determined the longitudinal coherence for the 4dS01→4pP11 lasing transition to be ~400μm (1/e half-width) by changing the length of one interferometer arm and measuring the resultant variation in fringe visibility. The inferred gain-narrowed linewidth of ~0.29pm is a factor of 4 less than previously measured in quasi-steady-state x-ray laser schemes.

Control of the attosecond synchronization of XUV radiation with phase-optimized mirrors

We report on the advanced amplitude and phase control of attosecond radiation allowed by specifically-designed multilayer XUV mirrors. We first demonstrate that such mirrors can compensate for the intrinsic chirp of the attosecond emission over a large bandwidth of more than 20 eV. We then show that their combination with metallic foils introduces a third-order dispersion that is adjustable through the mirrors incidence angle. This results in a controllable beating allowing the radiation to be shaped from a single to a series of sub-100 as pulses.

Molecular frame photoemission in dissociative ionization of H2 and D2 induced by high harmonic generation femtosecond XUV pulses

We report the first results of molecular frame photoelectron emission for dissociative photoionization (DPI) of H2 and D2 molecules induced by a spectrally filtered single high harmonic of a few femtosecond duration, using coincident electron-ion velocity vector correlation techniques. For the studied photon energies around 32 eV, where the resonant excitation of the Q1 and Q2 doubly excited states occurs, autoionization and nuclear dynamics are coupled on a few femtosecond timescale, giving rise to quantum interferences. Molecular frame photoelectron angular distributions (MFPADs), traced as a function of the kinetic energy release of the atomic fragments, provide the most sensitive observables for such complex dynamics. These results compare well with recent spectrally resolved experiments using synchrotron radiation which are also reported. As a novel XUV light source running at multi-kHz repetition rate and synchronized with laser pulses, high-order harmonic generation (HHG) opens new possibilities for extending these investigations to time-resolved studies at the femtosecond scale.

Spatiotemporal distortions of attosecond pulses

In this article, we report that usual multilayer mirror configurations to focus attosecond pulses generate geometric aberrations and can significantly stretch pulses. The numerical simulations show that the effects can be strong enough to delay some parts of the pulses of an attosecond pulse train and make them interfere with the next pulses of the train. The influence of the numerical aperture on the pulse duration is also studied, showing that such effects can occur even with very low numerical apertures.

X-ray spectroscopic application of Cr∕Sc periodic multilayers

The use of Cr∕Sc multilayer interferential mirrors (MIMs) in optical systems such as x-ray microscopes or telescopes have been reported for the water window (between C K- and O K-absorption edges). However, their possibilities in spectroscopic application have never been described in the literature. The purpose of the paper is to report for the first time on the performances of Cr∕Sc MIMs as Bragg dispersive devices for the analysis in wavelength dispersive spectrometry of samples containing N or Sc atoms. The possibility to distinguish the chemical state of the emitting N or Sc atoms is evidenced by using Johan-type and double-crystal spectrometers.

High efficiency multilayer gratings for monochromators in the energy range from 500 eV to 2500 eV

Multilayer (ML) gratings have been prepared by coating shallow ion etched lamellar gratings with a Mo2C/B4C multilayer having a layer thickness close to the groove depth. It was shown that such a structure behaves as a 2D synthetic crystal and can reach very high efficiencies when the Bragg condition is satisfied. A ML coated grating has been characterized at the SOLEIL Metrology beamline between 700 and 1700 eV and a beamline of PTB at BESSY II between 1750 and 3500 eV. A peak diffraction efficiency of nearly 27 % was measured at 2200 eV. The measured efficiencies are well reproduced by numerical simulations made with the electromagnetic propagation code CARPEM, This grating will be used in the Deimos beamline at SOLEIL together with a matched multilayer mirror.