F. Bridou

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.

Study of normal incidence of three-component multilayer mirrors in the range 20-40 nm.

We study theoretically and experimentally the increase of normal incidence reflectivity generated by addition of a third material in the period of a standard periodic multilayer, for wavelengths in the range 20 to 40 nm. The nature and thickness of the three materials has been optimized to provide the best enhancement of reflectivity. Theoretical reflectivity of an optimized B4C/Mo/Si multilayer reaches 42% at 32 nm. B4C/Mo/Si multilayers have been deposited with a magnetron sputtering system and a reflectivity of 34% at 32 nm has been measured on a synchrotron radiation source.

Ion-beam-deposited Mo/Si multilayers for EUV imaging applications in astrophysics

Imaging of the solar corona by selecting Fe IX (λ=17.1nm,), Fe XII (λ=19.5nm), Fe XV (λ=28.4nm) and He II (λ=30.4nm) emission lines with a Ritchey-Chretien telescope requires to coat the optics with multilayers having a high accuracy in their layer thicknesses, a high reflectivity and an optimal bandpass. Multilayers were simulated in order to determine the most adequate formula for each wavelength channel. Mo/Si coatings were deposited by using the ion beam sputtering technique in a high vacuum chamber equipped with a micro balance and an in-situ reflectometer. The multilayers were studied by grazing angle reflectometry at 0.1541nm, and their reflectances around the operating wavelengths were measured on the SA62 IAS/LURE beam line of the SuperACO synchrotron facility located at Orsay. In addition, aging versus time and behavior of the multilayers under a rapid thermal annealing were investigated. Performances of the ion-beam deposited multilayers have been improved compared to the Mo/Si coatings obtained in the past by the e-beam evaporation technique for the SOHO mission Extreme UV Imaging Telescope (EIT). The EUVI telescopes for the STEREO mission are being proceduced by depositing these new generation of multilayers onto primary and secondary mirrors. The reflectivity measurements on a telescope are presented.

X-ray–ultraviolet beam splitters for the Michelson interferometer

With the aim of realizing a Michelson interferometer working at 13.9 nm, we have developed a symmetrical beam splitter with multilayers deposited on the front and back sides of a silicon nitride membrane. On the basis of the experimental optical properties of the membrane, simulations have been performed to define the multilayer structure that provides the highest reflectivity-transmission product. Optimized Mo-Si multilayers have been successfully deposited on both sides of t he membrane by use of the ion-beam sputtering technique, with a thickness-period reproducibility of 0.1 nm. Measurements by means of synchrotron radiation at 13.9 nm and at an angle of 45 degrees provide a reflectivity of 14.2% and a transmission of 15.2% for a 60% s-polarized light, close to the simulated values. Such a beam splitter has been used for x-ray laser Michelson interferometry at 13.9 nm. The first interferogram is discussed.

Large field double Kirkpatrick–Baez microscope with nonperiodic multilayers for laser plasma imaging

The double Kirkpatrick–Baez (DBA) microscope is derived from the grazing x-ray Kirkpatrick–Baez (KB) microscope. The KB is compound of two concave spherical mirrors working at grazing incidence and in an energy range about 100 eV–10 keV. The combination of two similar mirrors in the DKB increases the useful field. This device only requires spherical mirrors, more easy to manufacture (precision and roughness) than aspherical ones. In order to image a laser plasma source in a large field of view and within a bandpass of 0.6 keV around 3.4 keV, a KB optic covered with multilayers is developed. In fact, a compromise has to be found between the resolution of the optic (better with a less grazing angle), and the reflectivity (better with more grazing angle). We have chosen to keep the average grazing incidence on the four mirrors around 2°–3° just as for a first uncoated DKB made in our laboratory. This allows us to keep the same radius of curvature for the mirrors. At this energy multilayers are needed, due to...

Simultaneous characterization of the electro-optic, converse-piezoelectric, and electroabsorptive effects in epitaxial (Sr,Ba)Nb2O6 thin films

Implementation of the linear electro-optic (EO) effect in thin film waveguides is expected to allow drastic reductions in the drive voltage, power, and dimensions of devices devoted to light modulation. It should also enable the realization of electrically tunable photonic crystal devices. In this paper we introduce a method which eliminates systematically the sources of the unreliability which strongly affects thin film EO characterization. Based on a Fabry–Perot reflective configuration, the method enables characterizing simultaneously the EO, converse-piezoelectric, and electroabsorptive effects in a film. It provides the magnitude and sign of each of the involved coefficients, and allows accounting for the whole of experimental data versus angle of incidence for both transverse-electric and transverse-magnetic polarizations. At λ=633 nm and room temperature, the results obtained with an epitaxial strontium barium niobate (SrxBa1−xNb2O6, x=0.60) ferroelectric thin film, are: r13=+8.5±1.3 pm/V, r33=+38....

EUV reflectivity and stability of tri-component Al- based multilayers

We report on further development of three-material multilayer coatings made with a use of aluminum for the extreme ultra-violet (EUV) applications such as solar physics, high-order harmonic generation or synchrotron radiation. It was found that an introduction of refractory metal in Al-based periodic stack helps to reduce significantly an interfacial roughness and provides for a higher theoretical reflectance in the spectral range from 17 to 40 nm. The normal incidence reflectivity as high as 55 % at 17 nm, 50 % at 21 nm and 42 % at 30 nm was achieved with the new Al/Mo/SiC and Al/Mo/B4C multilayer mirrors, which have been optimized, fabricated and characterized with x-rays and synchrotron radiation. A good temporal and thermal stability of the tri-component Al-based multilayers has been observed over 3 years.

High-efficiency B 4 C/Mo 2 C alternate multilayer grating for monochromators in the photon energy range from 0.7 to 3.4 keV

An alternate multilayer (AML) grating has been prepared by coating an ion etched lamellar grating with a B4C/Mo2C multilayer (ML) having a layer thickness close to the groove depth. Such a structure behaves as a 2D synthetic crystal and can reach very high efficiencies when the Bragg condition is satisfied. This AML coated grating has been characterized at the SOLEIL Metrology and Tests Beamline between 0.7 and 1.7 keV and at the four-crystal monochromator beamline of Physikalisch-Technische Bundesanstalt (PTB) at BESSY II between 1.75 and 3.4 keV. A peak diffraction efficiency of nearly 27% was measured at 2.2 keV. The measured efficiencies are well reproduced by numerical simulations made with the electromagnetic propagation code CARPEM. Such AML gratings, paired with a matched ML mirror, constitute efficient monochromators for intermediate energy photons. They will extend the accessible energy for many applications as x-ray absorption spectroscopy or x-ray magnetic circular dichroism experiments.

Polarization-dependent vacuum-ultraviolet reflectometry using elliptically polarized synchrotron radiation

In the laboratory of the Physikalisch-Technische Bundesanstalt (PTB) at the Berlin electron-storage ring BESSY II, a procedure has been developed to investigate the dependence of vacuum-ultraviolet reflection on polarization. It is based on characterizing the elliptically polarized synchrotron radiation at PTBs normal-incidence monochromator beamline for reflectometry by means of polarization-sensitive photodetectors. For this purpose, the polarization dependency in the detector responsivity was determined at a small, low, solid angle of acceptance for the synchrotron radiation, i.e., within the orbital plane of the storage ring where the degree of linear polarization is known to be almost 100%. Our method allows the polarization dependence of reflection samples to be measured with relative standard uncertainties ranging from 2.4% to 11% in the spectral range between 60 and 160 nm. The method has been applied to the optimization of polarizing mirrors at the Lyman-alpha wavelength of 121.6 nm.

New optical transmission device to produce high monochromatic and high polarized neutron beams based on the tunneling frustrated total reflection in neutron guides

It has been shown that the tunneling of neutrons can be used in classical neutron guides to extract high monochromatic and/or high polarized cold neutron beams. This can be achieved by implanting 58NiTi58Ni or 58NiCo58Ni Fabry-Perot interferometers under total reflection condition in the neutron guide. According to the computed results, this tunneling phenomenon of neutrons manifests itself by the existence of sharp resonances in the 58Ni total reflection plateau. These resonances are localized at different spectral positions λr± for the monochromatization and λr+, λr− for the polarization. The narrow spectral bandwidth of the tunneling transmitted beam can reach values of the order of Δλ ≈ 0.01 A and a strong transmissivity higher than 99%. The number and the spectral position of these resonances vary mainly with the characteristics of the spacer layer Ti or Co whereas their spectral bandwidths are imposed by the thickness of the reflecting 58Ni layers of the Fabry-Perot interferometer. To obtain low resonance orders, the use of a nonmagnetic substrate with a negative nuclear scattering length density and a perfect collimated polychromatic neutron beam is required.