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ISSIS imager has been thought as an open purpose instrument within the World Space Observatory (WSO) international space mission. The highest priorities of ISSIS, an instrument to be developed by Spain, are to guarantee high spatial resolution and high sensitivity down to the far ultraviolet (FUV). The paper displays the capacities of GOLD for multilayer deposition and FUV reflectometry, among other metrologies, for ISSIS optical elements.

12 pags., 8 figs., 2 tabs., Novel narrowband multilayer coatings efficient at a wavelength as short as 100 mn are presented, which pushes shortwards the existing limit of reported narrowband multilayers. Such limit had been established at ~120 nm, close to the MgF2 cutoff wavelength. The new multilayers combine layers of Al, LiF, and SiC, in an Al/LiF/SiC/LiF multilayer design (four layers, starting with the innermost layer). Among these materials, Al and LiF are deposited by evaporation and SiC by ion-beam-sputtering. In addition to a high, narrow peak close to H Lyman β (102.6 nm), these multilayers simultaneously provide a very small reflectance at H Lyman α (121.6 nm). This combined performance is demanded in space instrumentation for astrophysics and solar physics observations among others, where imaging the sky at the important diagnostic spectral line of Lyman β line requires rejecting the frequently much more intense background at Lyman α line. Such is the case for solar corona observations at Lyman β, which is masked by the strong Lyman α line. The multilayer peak is placed close to another important diagnostic tool: the OVI doublet at 103.2 and 103.8 nm. The target of small reflectance at 121.6 nm was seen to be the most critical. The best strategy in multilayer preparation was to prepare it with such minimum reflectance at slightly shorter wavelengths so that the coating evolved to shift it longwards over time. Multilayers kept a remarkable 102.6 nm/121.6 nm reflectance ratio over time in spite of some performance degradation. Hence, a multilayer coating aged of 4 years kept a reflectance of 43% at 102.6 nm and 0.2% at 121.6 nm., Spanish Programa Estatal de Investigación Científica y Técnica de Excelencia, Secretaría de Estado de Investigación, Desarrollo e Innovación (AYA2013-42590-P, ESP2016-76591-P

GOLD (Grupo de Optica de Laminas Delgadas) is devoted to the development of novel coatings with challenging performance in the far and the extreme ultraviolet (FUV-EUV, 50–200 nm). One of the main goals of this research is to provide the communities of astronomy, solar physics and atmospheric physics with coatings with high reflectance or transmittance at a target wavelength or band, and high rejection of the out-of-band at this complicated spectral range. Above the transparency cutoff of MgF2 (115 nm), transmittance filters based on Al/MgF2 multilayers have been developed peaked at wavelengths as short as 124 nm, with a peak transmittance of 27% and a FWHM of 12 nm for a non-aged coating. Below 115 nm, a research on reflectance filters has recently started with very promising results on filters peaked at the 83.4 nm OII spectral line. Fresh filters with 27% peak reflectance at normal incidence and a FWHM of 14 nm have been obtained. Furthermore, the peak reflectance wavelength of these filters can be tuned by rotation. A filter peaked at 83 nm at normal incidence will shift to ∼73 nm at 30 deg from the normal and to ∼58 nm at 45 deg. These novel reflective filters based on Al, Yb and SiO must still demonstrate stability over time.

10 pags., 8 figs., Conference Volume 9912. Advances in Optical and Mechanical Technologies for Telescopes and Instrumentation II, Edinburgh, United Kingdom, June 26, 2016, Conferencia programada dentro de Advances in Optical and Mechanical Technologies for Telescopes and Instrumentation, part of SPIE Astronomical Telescopes + Instrumentation., Exploitation of far ultraviolet (FUV, 100-200 nm) observations extends to most areas of modern astronomy, from detailed observations of Solar System objects, the interstellar medium, exoplanets, stars and galaxies, to studies of crucial cosmological relevance. Despite several developments in recent decades, yet many observations are not possible due to technical limitations, of which one of the most important is the lack of optical coatings with high throughput. Development and optimization of such efficient FUV coatings have been identified in several roadmap reports as a key goal for future missions. The success of this development will ultimately improve the performance of nowadays feasible optical instruments and will enable new scientific imaging capabilities. GOLD’s research is devoted to developing novel coatings with enhanced performance for space optics. Several deposition systems are available for the deposition of multilayer coatings. A deposition system was developed to deposit FUV coatings to satisfy space requirements. It consists of a 75-cm-diameter deposition chamber pumped with a cryo-pump and placed in an ISO-6 clean room. This chamber is available for deposition by evaporation of top-requirement coatings such as Al/ MgF2 mirrors or (Al/MgF2)n multilayer coatings for transmittance filters. A plan to add an Ion-Beam-Sputtering system in this chamber is under way. In this and other chambers at GOLD the following FUV coatings can be prepared: Transmittance filters based on (Al/MgF2)n multilayer coatings. These filters can be designed to have a peak at the FUV spectral line or band of interest and a high peak-to-visible transmittance ratio. Filters can be designed with a peak transmittance at a wavelength as short as 120 nm and with a transmittance in the visible smaller than 10-5. Narrowband reflective coatings peaked close to H Lyman β (102.6 nm) with a reflectance at H Lyman α (121.6 nm) two orders of magnitude below the one at 102.6 nm. Other potential spectral lines at which these coatings could be peaked are the OVI doublet (103.2, 103.8 nm). Narrowband reflective mirrors based on (MgF2/LaF3)n multilayers peaked at a wavelength as short as 120 nm. Target wavelengths include lines of high interest for space observations, such as H Lyman α (121.6 nm), OI (130.4 and 135.6 nm), CIV (154.8, 155.1 nm), among others. Coating-based linear polarizers tuned at H Lyman α (121.6 nm) both based on reflectance or on transmittance. Reflective polarizers present a high efficiency. Transmissive polarizers have a more modest peak performance compared to reflective polarizers; however, they involve spectral filtering properties to reject the long FUV and even more the near UV to the IR, which turn them competitive compared to reflective polarizers. In this communication we present a summary of our research on the above FUV coatings developed at GOLD. © (2016) COPYRIGHT Society of Photo-Optical Instrumentation Engineers (SPIE)., We acknowledge support by Spanish Programa Estatal de Investigación Científica y Técnica de Excelencia, Secretaría de Estado de Investigación, Desarrollo e Innovación, project number AYA2013-42590-P.

15 pags.; 13 figs.; 3 tabs., Space observations in the far ultraviolet (FUV, 100-200 nm) are aimed at providing essential information for astrophysics, solar physics, and atmosphere physics. There are key spectral lines and bands in the FUV for the above disciplines. Despite various developments in the recent decades, yet many observations are not possible due to technical limitations, of which one of the most important is the lack of efficient optical coatings. Hence for solar physics applications there are needs of narrowband coatings for key wavelengths such as H Lyman ß (102.6 nm) and OVI lines (103.2 and 103.8 nm). For atmosphere physics, narrowband coatings are required for observations at spectral lines such as OI (135.6 nm) and at the N2 Lyman-Birge-Hopfield band (LBH, 127-240 nm). In solar corona observations, often the intensities of the target lines are weak, and this radiation may be masked by more intense lines, such as H Lyman ¿ at 121.6 nm. Until now, no narrowband multilayers peaked in the ~100-105 nm range have been reported, which is due to the absorption of materials at these wavelengths. When efficient narrowband coatings are not possible, an option is the use of coatings with high reflectance at the target wavelength and simultaneously low reflectance at the undesired wavelength, such as Lyman ¿. We have developed novel multilayers to address this target, with combinations of these materials: Al, LiF, SiC and C. We developed multilayers based on the following three systems, Al/LiF/SiC, Al/LiF/SiC/C, and Al/LiF/SiC/LiF. Their reflectance was measured both when fresh and after some storage in a desiccator. Al/LiF/SiC and Al/LiF/SiC/C systems displayed a high Lyman ß/Lyman ¿ reflectance ratio when fresh, although they resulted in an undesired reflectance increase at Lyman ¿ for the aged samples and the reflectance ratio Lyman ß/Lyman ¿ became small; this behavior turned these systems useless for the present application. The most promising multilayers were the ones based on the Al/LiF/SiC/LiF system, which resulted in a good performance and a limited evolution after months of storage in a desiccator. Five samples based on the Al/LiF/SiC/LiF system were prepared and measured in the 50-190 nm spectral range. These samples resulted in high reflective and narrowband coatings peaked at 100-101 nm, with a promising reflectance ratio Lyman ß/Lyman ¿ when fresh. Some efficiency degradation was observed after the sample storage in a desiccators; however all samples retained a narrowband performance over time and a high Lyman ß/Lyman ¿ ratio. The same system can be designed to be an efficient narrowband coating peaked in the target spectral range and not constrained to a specific performance at Lyman ¿. Hence an 8-month aged sample exhibited a reflectance as high as 61% at the peak wavelength of 100.9 nm, at near-normal incidence, the highest experimental reflectance reported in this range for a narrowband coating. We have also prepared narrowband transmission coatings tuned either at 135.6 nm or at the center of the LBH band (~160 nm), with the requirement to strongly reject the out-of-band, particularly the visible and close ranges. The coatings were based on (Al/MgF2)n multilayers, with n ranging between 3 and 4. The coatings were successful at rejecting the visible, with a transmittance lower than 10-5. The transmittance at the peak was 0.087 for coatings stored in a desiccators for 13 days. Keywords: Multilayers, Extreme Ultraviolet, Far Ultraviolet, Solar physics, Atmosphere physics, Filters, Space optics, Mirrors *

Observations in the far ultraviolet (FUV) at wavelengths in the ~100-105 nm range, which include 102.6 nm (H Lyman β), 103.2 and 103.8 nm (O VI lines), are expected to unveil fundamental information for solar physics and astrophysics. Often the intensity of those lines is weak, and they may be masked by more intense lines, such as H Lyman α at 121.6 nm for observations of the solar corona. Narrowband multilayers peaked in the ~100-105 nm have not been available because of the absorption of materials at these wavelengths along with a strong influence of contamination in this range. When efficient narrowband coatings are not possible, an option is the use of coatings with high reflectance at the target wavelength and simultaneously low reflectance at the undesired wavelength, such as 121.6 nm. High-reflective-narrowband coatings peaked at 100-105 nm have been developed. We have designed a four-layer system (Al/LiF/SiC/LiF) that results in a high H Lyman β-to-Lyman α reflectance ratio. Three samples with slightly different film thicknesses were prepared and measured in the 50-190 nm spectral range. All samples showed a promising reflectance ratio when fresh; however, some sample ageing was observed after months of storage in a desiccator, probably due to the effect of reaction with water vapor among other contaminants at the outermost layer (LiF). All samples retained a narrowband performance over time. The reflectance at 121.6 nm, which was very low on fresh samples, typically increased over time, although keeping a high 102.6-to-121.6-nm reflectance ratio. The same system results in an efficient narrowband coating peaked in the target spectral range. We measured a reflectance as high as 63% at the peak wavelength of 100.3 nm, at near-normal incidence, the highest experimental reflectance reported in this range for a narrowband coating.

CARMENES (Calar Alto high-Resolution search for M dwarfs with Exo-earths with Near-infrared and optical Echelle Spectrographs) is a next-generation instrument for the 3.5m telescope at the Calar Alto Observatory, built by a consortium of eleven Spanish and German institutions. The CARMENES instrument consists of two separate echelle spectrographs covering the wavelength range from 0.55 μm to 1.7 μm at a spectral resolution of R = 82, 000, fed by fibers from the Cassegrain focus of the telescope. Both spectrographs are housed in temperature-stabilized vacuum tanks, to enable a long-term 1 m/s radial velocity precision employing a simultaneous calibration with Th-Ne and U-Ne emission line lamps. CARMENES has been optimized for a search for terrestrial planets in the habitable zones (HZs) of low-mass stars, which may well provide our first chance to study environments capable of supporting the development of life outside the Solar System. With its unique combination of optical and near-infrared ´echelle spectrographs, CARMENES will provide better sensitivity for the detection of low-mass planets than any comparable instrument, and a powerful tool for discriminating between genuine planet detections and false positives caused by stellar activity. The CARMENES survey will target 300 M dwarfs in the 2014 to 2018 time frame.

Observations in the far ultraviolet (FUV) at wavelengths below ~125 nm, which include the H Lyman series and the spectral lines of many other important species, are expected to unveil fundamental information for solar physics and astrophysics. Among these, observations of the solar corona at 102.6 nm H Lyman β are of high interest, but they may be masked by the strong H Lyman α at 121.6 nm. This goal has been addressed here through the development of novel multilayer coatings with high reflectance at 102.6 nm and at the same time a low reflectance at 121.6 nm; the latter wavelength is mostly absorbed. An efficient reflection/rejection coating is not straightforward because of the lack of high-transmission materials in the short FUV. We have designed and prepared novel multilayers with combinations of the following materials: Al, LiF, SiC and C. Various combinations were found to display a high reflectance ratio at 102.6/121.6 nm when fresh. Some of them resulted in an undesired reflectance increase at 121.6 nm for the samples aged for a few weeks. The most promising multilayers are based on Al/LiF/SiC/LiF (starting with the innermost layer), which resulted in a good performance and a small evolution after months of storage in a desiccator. At the same time, these multilayers may the most efficient reflective narrowband coatings that have been developed with a peak wavelength in the ~100-130 nm.

Strontium (Sr) is a material with low-absorption bands in the extreme ultraviolet (EUV), which makes it a potential candidate for band pass filters and multilayer coatings. Yet, a better knowledge of the optical properties of Sr is required for these developments. The optical constants n and k of Sr thin films have been obtained in the 6-1220 eV range from transmittance measurements performed at room temperature. These are the first experimental optical constant data of Sr in most of the range. Thin films of Sr with various thicknesses were deposited by evaporation in ultrahigh vacuum conditions, and their transmittance was measured in situ. Sr films were deposited onto grids coated with a thin C support film. Transmittance measurements were used to directly obtain the extinction coefficient k of Sr films. The refractive index n of Sr was calculated with Kramers-Kronig analysis. For this, k data were extrapolated both on the high-and on the low-energy sides by using experimental and calculated k data available in the literature. It was found that, similar to other alkaline-earth metals, Sr has a low absorption band in the EUV, which for Sr is below its N-2,N-3 edge, with a minimum at similar to 18.5 eV, a range where most materials in nature have a large absorption. A second spectral range of interest for the low absorption of Sr is below its M-4,M-5 edge at 132 eV. In spite of these remarkable properties, Sr is a very reactive material and the stability of coatings encompassing Sr may be an issue. Good consistency of the data resulted from the application of f and inertial sum rules.

Little development on coatings has been available in the 50-120-nm spectral range until recently. One main reason for this is the large absorption of most materials in nature in this range. Our group has followed a research towards the development of novel coatings for this spectral range. This research has been based on the search and characterization of new materials mainly with low absorption. For many materials we have performed their optical characterizations in a large spectral range to reduce common inconsistencies that arise when combinations of data from different sources are used. We summarize our research on the characterization of many lanthanides, among other materials. Lanthanides are particularly interesting because they have a relatively low absorption in the spectral range of interest. Self-consistent characterization of other materials, such as SiC and B 4 C, has been performed for their interest as candidate materials for coatings involving the EUV to the visible. The discovery in lanthanides of a wealth of materials with relatively low absorption has enabled the development of multilayers based on the low absorption of Yb and Eu lanthanides. This resulted in the first narrowband multilayers with a peak wavelength in the 70 to 100 nm. We also report recent research on the development of multilayers with a peak reflectance above 100 nm; these multilayers address two targets: a) narrowband performance; b) zero reflectance at a wavelength slightly longer (such as 121.6 nm) than the peak wavelength. As for a), a promising preliminary result is obtained with a narrowband multilayer peaked at 101 nm. Regarding b), multilayers with a high reflectance at 102.6 nm and a low reflectance at 121.6 nm were prepared and they displayed a successful performance when measured in situ (not exposed to the atmosphere); however, the minimum at 121.6 nm was lost after a short exposure to air. The latter research is still underway and we plan to experiment with new designs. Our group has also prepared efficient narrowband transmittance coatings peaked at wavelengths longer than 120 nm. They are based on the classical combination of Al and MgF 2 .

We present the development of novel coatings for the far and extreme ultraviolet (FUV-EUV). In the EUV above ~50 nm, the strong absorption of materials has precluded the development of narrowband coatings. An extensive research has been performed on the search and characterization of new materials with low absorption; the lanthanide series has been found to be a source of materials with relatively low absorption in the range of interest. The discovery of a wealth of materials with relatively low EUV absorption is basic to develop efficient multilayers, particularly with narrowband properties. In this way, we have developed multilayers based on Yb, Al, and SiO with narrowband performance in the 50-92 nm range; these are first narrowband coatings peaked above 70 nm. Our recent research on multilayers based on Eu, Al, and SiO provide promising results, with an increase in the peak reflectance versus Yb/Al/SiO multilayers, along with a peak wavelength that can be extended up to ~100 nm. For applications where FUV-EUV narrowband coatings have not been able to be prepared, we can design multilayers that address specific purposes, such as maximizing the reflectance ratio at two wavelengths or bands. Our first goal in this direction is the development of coatings with high 102.6 nm/ 121.6 nm reflectance ratio. Calculations predict that a high reflectance at Lyman β with a good rejection at Lyman α can be obtained through multilayer coatings. We are at the beginning of experimental research for this goal.

The optical constants of erbium (Er) films were obtained in the 3:25-1580 eV range from transmittance measurements performed at room temperature. Thin films of Er were deposited by evaporation in ultra high vacuum conditions and their transmittance was measured in situ. Substrates consisted of a thin C film supported on a grid. Transmittance measurements were used to obtain the extinction coefficient k of the Er films. The refractive index n of Er was calculated using the Kramers-Krönig analysis. k data were extrapolated both on the high- and low-energy parts of the spectrum by using experimental data and calculated k values available in the literature. Er, similar to other lanthanides, has a low-absorption band below the O2,3 edge onset; the smallest absorption was measured at ~22:5 eV. Therefore, Er is a promising material for filters and multilayer coatings in the energy range below the O2,3 edge, in which materials typically have an absorption stronger than at other energies. Good consistency of the data resulted from the application of f and inertial sum rules. © 2011 Optical Society of America., This work was also supported by the National Programme for Space Research, Subdirección General de Proyectos de Investigación, Ministerio de Ciencia y Tecnología (project AYA2008-06423-C03-02/ ESP, AYA2009-14070, and AYA2010-22032). A. M. Malvezzi acknowledges the partial support of the Fondo d’Ateneo per la Ricerca of the Università di Pavia. M. Fernández-Perea is thankful to Consejo Superior de Investigaciones Científicas (Spain) for funding under the Programa I3P (reference I3PBPD2004), partially supported by the European Social Fund. M. Vidal-Dasilva acknowledges financial support from an Formación de Personal Investigador BES-2006-14047 fellowship.

The strong absorption of materials in the extreme ultraviolet (EUV) above ~50 nm has precluded the development of efficient coatings. The development of novel coatings with improved EUV performance is presented. An extensive research was performed on the search and characterization of new materials with low absorption or high reflectance. Lanthanide series was found to be a source of materials with relatively low absorption in this range, where most materials in nature present a strong absorption. Other materials, such as SiO and B, have been found to have interesting properties for applications on EUV coatings. As a result, novel multilayers based on Yb, Al, and SiO have been developed with narrowband performance in the 50-92 nm range. In some cases, the difficulty of developing narrowband coatings in the EUV can be overcome by designing multilayers that address specific purposes, such as maximizing and/or minimizing the reflectance at two or more wavelengths or bands. In this direction, we are working towards the development of coatings that combine a relatively high reflectance in a desired EUV band with a low reflectance in another band, for applications in which the presence of the latter radiation may mask a weak EUV radiation source.

A new type of multilayer coatings with narrowband reflection properties and peaked in the ∼50-92 nm spectral range has been developed. Multilayers are based on Yb, Al, and SiO films and they have been prepared by thermal evaporation. Efficient multilayers based on Yb and Al, with an SiO protective layer were prepared, but they developed a dendrite structure, which was attributed to the reactivity between Al and Yb. Multilayers based on Yb and Al, with both SiO protective and barrier layers, resulted in efficient reflective filters, with no observable dendrite growth. The peak reflectance of aged multilayers was of the order of ∼0.20, with bandwidths in the range of 12 to 22 nm FWHM. © 2009 Optical Society of America., This work was supported by the National Programme for Space Research, Subdirección General de Proyectos de Investigación, Ministerio de Ciencia y Tecnología, project number ESP2005-02650 and AYA2008-06423-C03-02/ESP. M. F. is thankful to Consejo Superior de Investigaciones Científicas (Spain) for funding under the Programa I3P (Ref. I3P-BPD2004), partially supported by the European Social Fund; at present she is at Lawrence Livermore National Laboratory, 7000 East Avenue, Livermore, California 94550, USA. M. V. is thankful to Ministerio de Educación y Ciencia for funding under the FPI BES-2006-14047 fellowship.

The transmittance of dc magnetron-sputtered Mg thin films was measured in the 25-1300 eV spectral range. Freestanding Mg films protected with Al layers were characterized ex situ. Transmittance measurements were used to obtain the extinction coefficient k of Mg films. The obtained k values along with the data available in the literature, and with interpolations and extrapolations for the rest of the spectrum, were used to obtain the real part of the index of refraction n by the Kramers-Krönig analysis. Sum-rule tests indicated a good consistency of the data. © 2010 American Institute of Physics.

The spectral range of 50 to 115 nm in the extreme ultraviolet (EUV) is characterized by the high absorption and low normal incidence reflectance of most materials, which make difficult the development of high reflectance multilayer mirrors at normal incidence angles. The availability of efficient mirrors would have a great impact on the performance of EUV space telescopes and other EUV instruments at these wavelengths. In order to obtain high normal-incidence reflectance coatings, it is necessary to find materials with absorption as low as possible at 50 - 115 nm. Recently, lanthanides and close elements have attracted the attention of researchers because of their relatively low absorption at bands in the EUV, and several studies on the optical constants of lanthanides in the EUV have been published. As a result of these investigations we have identified those lanthanides which better match the low absorption requirement at wavelengths in the 50 - 115 nm range. In this work we present the use of the lanthanide Yb combined with other materials and protective capping layers in multilayers designed to have a reflectance maximum at a wavelength selected within the spectral range of 50 to 95 nm. Experimental results for the case of a multilayer composed of Yb, Al and SiO layers confirm the adequacy of this approach, providing a peak reflectance of 0.276 at 80 nm with FWHM of 14.5 nm for samples not exposed to the atmosphere. A decrease in peak reflectance from 0.276 to 0.209 was observed after 2 years of storage in a dessicator.

8 págs.; 11 figs., The transmittance of silicon monoxide films prepared by thermal evaporation was measured from 7.1 to 800 eV and used to determine the optical constants of the material. SiO films deposited onto C-coated microgrids in ultrahigh vacuum conditions were measured in situ from 7.1 to 23.1 eV. Grid-supported SiO films deposited in high vacuum conditions were characterized ex situ from 28.5 to 800 eV. At each photon energy, transmittance, and thickness data were used to calculate the extinction coefficient k. The obtained k values combined with data from the literature, and with interpolations and extrapolations in the rest of the electromagnetic spectrum provided a complete set of k values that was used in a Kramers-Kronig analysis to obtain the real part of the index of refraction, n. Two different sum-rule tests were performed that indicated good consistency of the data. © 2009 American Institute of Physics., This work was supported by the National Programme for Space Research, Subdirección General de Proyectos de Investigación, Ministerio de Ciencia y Tecnología, Project Nos. ESP2002-01391 and ESP2005-02650. This work was also performed under the auspices of the U.S. Department of Energy by the University of California Lawrence Berkeley National Laboratory under Contract No. DE-AC03-76F00098 and by the University of California Lawrence Livermore National Laboratory under Contract No. DE-AC52- 07NA27344. M.F.-P. is thankful to Consejo Superior de Investigaciones Científicas Spain for funding under the Programa I3P Contract No. I3P-BPD2004, partially supported by the European Social Fund. M.V.-D. acknowledges financial support from a FPI Contract No. BES-2006-14047 fellowship.

The optical constants of thulium Tm films were obtained in the 2.75-1600 eV range from transmittance measurements performed at room temperature. Thin films of Tm were deposited by evaporation in ultrahigh vacuum conditions and their transmittance was measured in situ. Tm films were deposited onto grids coated with a thin, C support film. Transmittance measurements were used to obtain the optical extinction coefficient k of Tm films. The refractive index n of Tm was calculated using the Kramers-Krönig analysis. k data were extrapolated both on the high and the low energy sides by using experimental and calculated extinction coefficient values available in the literature. Tm, similar to other lanthanides, has a low-absorption band below the O2,3 edge onset; the lowest absorption was measured at 23 eV. Therefore, Tm is a promising material for filters and multilayer coatings in the energy range below the O2,3 edge in which materials typically have an absorption stronger than away. Good consistency of the data was obtained through f and inertial sum rules.

A summary of the research performed on the optical characterization of Sc and of several lanthanides from the visible to the soft x-rays is presented. The low absorption of these materials mainly below the O 2,3 edge (L 2,3 edge for Sc) turns them promising materials for the realization of multilayer mirro rs in a spectral range in which most materials in nature absorb strongly. Thin-film samples with several thicknesses of the target material were deposited by evaporation over thin-film substrates in UHV, and their transmittance was measured in situ. A wide spectral range of direct characterization, along with extrapolations to longer and shorter wavelengths either using literature data (when available) or model predictions, enabled the development of consistent optical constants over the whole spectrum. An assortment of consistency sum rules has been used, and it was found that each of them highlights a gi ven spectral range, which may help evaluate the consistency of each part of the combined spectrum. Keywords: Extreme Ultraviolet, far ultraviolet, optical constants, absorption filters, multilayers, optical properties of thin films

A summary of the optical coatings for th e spectral range from 50 to 200 nm that can be prepared at Grupo de Optica de Laminas Delgadas is presented. This spectral range, named he re far and extreme ultraviolet (FUV/EUV), is characterized by the high absorption of almost all materials and the strong dependence of their optical properties on the deposition parameters and on the exposure of the samples to the atmosphere. Broadband mirrors and narrowband filters were prepared in an ultra high vacuum system where in situ FUV/EUV reflectometry is available. In this work we summarize the main aspects concerning in situ performance and ageing of these coatings . Broadband mirrors composed of Al/MgF 2 , SiC, B and Al/MgF 2 /SiC cover the FUV/EUV spectral range from 50 to 200 nm. Above the transparency cutoff of MgF 2 (115 nm), transmittance filters based on Al/MgF 2 multilayers have been developed peaked at wavelengths as short as 120 nm. An example is shown centered at 124 nm with a peak transmittance of 27% and a FWHM of 12 nm for a fresh coating. Below 115 nm, a research on re flectance filters has recently started with very promisi ng results on filters peaked at ~83 nm, close to the OII spectral line. Fresh filters with 27% peak reflectance at normal incidence and a FWHM of 14 nm have been obtained. These novel reflectance filters based on Al, Yb and SiO must still demonstrate stability over time. The developed coatings have a potential application in the field of astronomical observations. Keywords: far ultraviolet, extreme ultraviolet, optical coatings, mirrors, filters, optical constants, thin films: optical properties

Boron films deposited by evaporation with an electron-beam were found to have a relatively high reflectance in the extreme ultraviolet with values similar to those of ion-beam-sputtered (IBS) SiC and IBS B4C . The largest reflectance was measured for an 11¿nm thick boron film. Some reflectance degradation was observed for boron films stored in a desiccator. Reflectance degradation varied from sample to sample and was found to be either similar to that of IBS SiC and IBS B4C or larger. © 2008 Optical Society of America, This work was supported by the National Programme for Space Research, Subdirección General de Proyectos de Investigación, Ministerio de Ciencia y Tecnología, project number ESP2005-02650. M. Fernández-Perea is thankful to Consejo Superior de Investigaciones Científicas (Spain) for funding under the Programa I3P (Ref. I3P-BPD2004) partially supported by the European Social Fund. We acknowledge the technical assistance of José M. Sánchez-Orejuela. M. Vidal-Dasilva is thankful to Ministerio de Educación y Ciencia for funding under the FPI BES-2006- 14047 fellowship.

The extinction coefficient of Pr, Eu and Tm thin films prepared by evaporation in ultra high vacuum has been obtained in the spectral ranges 4 − 1600, 8.3 – 1400 and 10 – 1400 eV, respectively. These data were calculated from experimental values of the transmittance of the films performed in situ, which means that the samples were not exposed to the atmosphere before and during their characterization. Several films of increasing thickness were deposited onto grids coated with a thin C support film. The results show that Pr, Eu and Tm, similar to other lanthanides, have a lowabsorption band right below the O2,3 edge onset, with lowest absorption measured at about 16.9, 16.7 and 23 eV, respectively. Therefore, these materials are promising for filters and multilayer coatings in the energy range below O2,3 edge in which materials typically present a strong absorption. In the cases of Pr and Eu the f sum rule was applied to the extinction coefficient data in the whole spectrum, that included the current data along with those of the literature and extrapolations. The obtained values of the number of electrons contributing to the optical properties of the materials were close to predictions, which shows the consistency of current data. In the case of Pr, the real part of the index of refraction was also calculated through the Kramers-Kronig analysis, and the consistency of the results was assessed by means of the inertial sum rule.

"The optical constants of Eu films were obtained in the 8.3-1400 eV range from transmittance measurements performed at room temperature. Thin films of Eu were deposited by evaporation in ultrahigh vacuum conditions and their transmittance was measured in situ. Eu films were deposited onto grids coated with a thin C support film. The refractive index n of Eu was calculated using the Kramers-Kronig analysis. Data were extrapolated both on the high- and low-energy sides by using experimental and calculated extinction coefficient values available in the literature. Eu, similar to other lanthanides, has a low-absorption band just below the O-2,O-3 edge onset; the lowest absorption was measured at about 16.7 eV. Therefore, Eu is a promising material for filters and multilayer coatings in the energy range below the O-2,O-3 edge in which materials typically have a strong absorption. The consistency of the composite optical constants was tested with the f and inertial sum rules and found to be good."

"The optical constants of Pr films were obtained in the 4-1600 eV range from transmittance measurements performed at room temperature. Thin films of polycrystalline Pr were deposited by evaporation in ultrahigh vacuum conditions and their transmittance was measured in situ. Pr films were deposited onto grids coated with a thin, C support film. Transmittance measurements were used to obtain the optical extinction coefficient k of Pr films in the 4-1600 investigated photon energy range. The refractive index n of Pr was calculated using the Kramers-Kronig analysis. Data were extrapolated both on the high and low-energy sides by using experimental and calculated extinction coefficient data available from the literature. Pr, similar to other lanthanides, has a low-absorption band right below the O-2,O-3 edge onset; the lowest absorption was measured at about 17 eV. Therefore, Pr is a promising material for filters and multilayer coatings in the energy range below O-2,O-3 edge in which most materials have a strong absorption. A good consistency of the data were obtained through f and inertial sum rules. (C) 2008 American Institute of Physics."

The optical constants of electron-beam evaporated boron from 6.8 to 900 eV were calculated through transmittance measurements of boron thin films deposited onto carbon-coated microgrids or LiF substrates in ultrahigh-vacuum conditions. In the low-energy part of the spectrum the measurements were performed in situ on freshly deposited samples, whereas in the high-energy range the samples were exposed to the atmosphere before the measurements. The extinction coefficient was calculated directly from the transmittance data, and a Kramers-Kronig analysis that combined the current data with data from the literature was performed to determine the dispersive part of the index of refraction. Finally, two different sum-rule tests were performed that indicated the good consistency of the data.

8 págs.; 8 figs.; OCIS codes: (260.7200) Ultraviolet, extreme; (120.4530) Optical constants; (350.2450) Filters, absorption; (230.4170) Multilayers; (310.6860) Thin films, optical properties., © 2015 Optical Society of America. The low expected absorption of Ca in the extreme ultraviolet (EUV) makes it an attractive material for multilayers and filters because most materials in nature strongly absorb the EUV. Few optical constant data had been reported for Ca. In this research, Ca films of various thicknesses were deposited on gridsupported C films and their transmittance measured in situ from the visible to the soft x-rays. The measurement range contains M2,3 and L2,3 absorption edges. Transmittance measurements were used to obtain the Ca extinction coefficient k. A minimum k of 0.017 was obtained at ∼23 eV, which makes Ca a promising low-absorption material for EUV coatings. A second spectral range of interest for its low absorption is below the Ca L3 edge at ∼343 eV. Measured k data and extrapolations were used to calculate the refractive index n using Kramers.Kronig relations. This is the first self-consistent data set on Ca covering a wide spectral range including the EUV., We acknowledge support by the European Community—Research Infrastructure Action under the FP6 “Structuring the European Research Area” Programme (through the Integrated Infrastructure Initiative “Integrating Activity on Synchrotron and Free Electron Laser Science”) through proposal number Ref. 2007655. This work was also supported by the National Programme for Space Research, Subdirección General de Proyectos de Investigación, Ministerio de Ciencia y Tecnología, project numbers AYA2010-22032 and AYA2013-42590-P. L. Rodrí- guez-de Marcos and S. García-Cortés are thankful to Consejo Superior de Investigaciones Científicas (CSIC) for funding under the Programa JAE, partially supported by the European Social Fund. M. Vidal-Dasilva acknowledges financial support from an FPI fellowship number BES-2006-14047