Far UV-enhanced Al mirrors with a Ti seed film

7 pags., 4 figs.A Ti seed film is investigated towards improving the far UV reflectance of Al/MgF2 mirrors. Samples were initially coated with a Ti film in half of the area and they were later coated in the full area with an Al film and protected with MgF2. All materials were deposited by evaporation. Samples were prepared with the MgF2 layer deposited either at room temperature (RT) or at 225°C. A 3-nm thick Ti seed film was seen to significantly increase the reflectance of Al/MgF2 mirrors at the well-known reflectance dip centered at ∼160 nm; this was attributed to a reduction of short-range surface roughness at the Al/MgF2 interface, which is responsible for radiation absorption through surface-plasmon (SP) coupling. SP absorption was more efficiently reduced with a Ti seed film on samples fully deposited at RT. A Ti seed film as thin as 1 nm provided the largest SP absorption reduction, and the SP dip was almost completely removed.Agencia Estatal de Investigación (PID2019-105156GB-I00).Peer reviewe

New tools for optical constant calculation and analysis

Conferencia invitada. -- 304. PTB-Seminar, Helmholtz-Building of the Berlin-Charlottenburg, October 19th - 20th, 2017Peer Reviewe

Improved broadband and narrowband far UV coatings

237th Meeting Online Planner of the American Astronomical Society, 12-15 January 2021, virtually anywhereNASA Ultraviolet & visible light (UV-Vis) SIG and Technology Interest Group (TIG

Far-UV reflectance and stress of narrowband AlF3/LaF3 multilayers

14 pags., 9 figs., 3 tabs.Upcoming space instrumentation, such as LUMOS (LUVOIR Ultraviolet MultiObject Spectrograph) in LUVOIR (Large Ultraviolet Optical Infrared Surveyor) mission, demands efficient narrowband coatings centered in the far UV (FUV). Narrowband FUV coatings can be prepared with all-dielectric multilayers (MLs) based on two fluorides. This research evaluates the performance of AlF3/LaF3 FUV MLs prepared by thermal evaporation and compares this performance with MgF2/LaF3 MLs, which were previously investigated. FUV reflectance, stress, and the influence of substrate materials have been investigated, along with ML stability over time when stored in a desiccator. Coatings were deposited both on fused silica and on CaF2 crystals, two common optical substrates. AlF3/LaF3 MLs exhibited reduced stress compared with MgF2/LaF3 MLs, resulting in a larger thickness threshold before crack generation. This enables preparing MLs with more layers and hence with higher performance. AlF3/LaF3 MLs underwent lower reflectance decay over time compared with MgF2/LaF3 MLs. Fresh MLs centered at ~160 nm displayed a peak reflectance close to 100%, and most of the AlF3/LaF3 MLs kept a reflectance of 99% after several months of storage. The bandwidth of AlF3/LaF3 MLs for a given number of layers was found to be somewhat larger than for MgF2/LaF3 MLs.Agencia Estatal de Investigación (BES-2017-081909, ESP2016- 76591-P, PID2019-105156GB-I00); Ministerio de Economía, Industria y Competitividad, Gobierno de España (BES-2017-081909)

Optimization of the deposition parameters of MgF2/LaF3 narrowband reflective FUV multilayers

14 pags., 7 figs., 3 tabs.The development of efficient dielectric coatings in the far UV (FUV) is demanded for upcoming space instrumentation, such as LUMOS (LUVOIR Ultraviolet Multi-Object Spectrograph) in LUVOIR mission, among other applications. Multilayers (MLs) based on MgF2 and LaF3 have been developed in the last decades for the 157-nm and 193-nm lithography, demonstrating excellent optical properties. Yet, the deposition procedure to obtain coatings with optimal performance has not been fully detailed in the open literature, such as the dependence of ML performance with deposition and post-deposition temperature. This research investigates the effect of the substrate deposition temperature of MgF2/LaF3 ML coatings prepared by thermal evaporation on FUV reflectance, stress, roughness, as well as the performance of the coatings and their evolution over time. The relatively higher expansion coefficient of these two fluorides in comparison with fused silica substrates results in a large tensile stress for coatings deposited at high temperature and later cooled down to room temperature. Such stress may result in coating cracking and delamination. A compromise deposition temperature of ∼240°C and 13 bilayers was found for optimal ML reflectance peaked at ∼160 nm before cracks are generated. Above this deposition temperature, stress increased, which resulted in an extension of the cracked area and in a slight roughness increase and FUV reflectance decrease. MLs that were deposited at room temperature and later annealed resulted in a similar reflectance and stress to those of hot-deposited coatings for a given temperature.Ministerio de Economía, Industria y Competitividad, Gobierno de España (BES-2017-081909, ESP2016-76591-P, PID2019-105156GB-I00).Peer reviewe

Polarizers tuned at key far-UV spectral lines for space instrumentation

8 pags., 4 figs., 2 tabs. -- EUV and X-ray Optics: Synergy between Laboratory and Space V. -- SPIE Optics + Optoelectronics, 2017, Prague, Czech RepublicPolarimetry is a valuable technique to help us understand the role played by the magnetic field of the coronal plasma in the energy transfer processes from the inner parts of the Sun to the outer space. Polarimetry in the far ultraviolet (FUV: 100-200 nm), which must be performed from space due to absorption in terrestrial atmosphere, supplies fundamental data of processes that are governed by the Doppler and Hanle effects on resonantly scattered line-emission. To observe these processes there are various key spectral lines in the FUV, from which H I Lyman ¿ (121.6 nm) is the strongest one. Hence some solar physics missions that have been proposed or are under development plan to perform polarimetry at 121.6 nm, like the suborbital missions CLASP I (2015) and CLASP II (2018), and the proposed solar missions SolmeX and COMPASS and stellar mission Arago. Therefore, the development of efficient FUV linear polarizers may benefit these and other possible future missions. C IV (155 nm) and Mg II (280 nm) are other spectral lines relevant for studies of solar and stellar magnetized atmospheres. High performance polarizers can be obtained with optimized coatings. Interference coatings can tune polarizers at the spectral line(s) of interest for solar and stellar physics. Polarizing beamsplitters consist in polarizers that separate one polarization component by reflection and the other by transmission, which enables observing the two polarization components simultaneously with a single polarizer. They involve the benefit of a higher efficiency in collection of polarization data due to the use of a single polarizer for the two polarization components and they may also facilitate a simplified design for a space polarimeter. We present results on polarizing beamsplitters tuned either at 121.6 nm or at the pair of 155 and 280 nm spectral lines.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’); measurements were performed under ELETTRA proposal numbers 20115134, 20120059, 20125119, 20130201, 20135237, 20140171, 20150344, and 2015497. This work was also supported by the National Programme for Research, Subdirección General de Proyectos de Investigación, Ministerio de Ciencia e Innovación, project numbers AYA2013-42590-P and ESP2016-76591-P. We also acknowledge support by the Italian Ministry of University and Research (MIUR) under the Programmi di Ricerca Scientifica di Rilevante Interesse Nazionale—Bando 2012.Peer Reviewe

Enhanced Far-UV reflectance of Al mirrors protected with hot-deposited MgF2

9 pags., 6 figs., 1 tab. -- Event: SPIE Optical Systems Design, 2018, Frankfurt, GermanyMirrors based on Al protected with a MgF2 film provide high reflectance over a broad spectral range down to the wavelength of 120 nm in the Far UV (FUV). After more than 50 years since the development of this technology, a significant FUV reflectance enhancement has been obtained in the last years. Such enhancement originates mostly in the higher transparency of the MgF2 protective layer deposited on a hot Al-coated substrate. Research has been conducted at GOLD to measure the dependence of the FUV reflectance enhancement with MgF2 deposition temperature. A reflectance enhancement was found for freshly-prepared samples; moreover, the reflectance degradation over time of Al films protected with hot-deposited MgF2 was also smaller than for the coatings deposited at room temperature. A reflectance as high as 90% was measured at 121.6 nm (hydrogen Lyman ¿ line) for aged samples. A FUV reflectance enhancement was also obtained on samples fully deposited at room temperature and later annealed in vacuum. The reflectance of Al mirrors as a function of MgF2 deposition temperature, as well as of post-deposition annealed mirrors, and their stability over time is presented. Structural data on film roughness, density, and main crystal orientations for mirrors with a MgF2 film deposited both at room temperature and at 250°C are also presented.This work was supported by the National Programme for Research, Subdirección General de Proyectos de Investigación, Ministerio de Ciencia e Innovación, grants AYA2013-42590-P and ESP2016-76591-P.Peer Reviewe

Direct measurement of the extinction coefficient by differential transmittance

15 pags., 8 figs., 1 tab.A new procedure to measure the extinction coefficient k of film materials that are relatively transparent is presented. This procedure does not require the use of an optical-constant model or the knowledge of extra physical properties of the material, such as the specific heat capacity. It involves preparing a sample with two areas, at least one of them coated with the film, whereas the other may remain uncoated or may be coated with a different thickness of the same material. The differential transmittance between the two sample areas is shown to be proportional to k of the film material in the following measurement conditions: the incident light is p polarized and it impinges at the film material Brewster angle. The differential transmittance is obtained with a single measurement by making the light beam or the sample to oscillate with respect to one another and by using a lock-in amplifier; for normalization purposes, the transmittance in one of the sample areas is also measured. The proportionality factor between the normalized differential transmittance and k only involves the wavelength, the film thickness, and the Brewster angle. The knowledge of the film Brewster angle requires that the film refractive index (n) is measured beforehand; this can be performed with standard procedures, such as ellipsometry, since such techniques are efficient at measuring n of a transparent material, but are inefficient at measuring a small k. The procedure is exemplified with the calculation of k in the far ultraviolet of AlF3 films deposited by evaporation. The dependence of the uncertainty of k obtained with this procedure is analyzed in terms of the uncertainty of the film n, of wavelength, and of the degree of polarization of the incident beam. The selection of a substrate with similar n to the film material is also discussed. The uncertainties involved with the present procedure were analyzed for a specific example and an uncertainty of 2 × 10-5 in k calculation is considered feasible.NGL acknowledges, for her technician position, the grant PTA2021-019935-I funded by MCIN/AEI/ 10.13039/501100011033 and by ESF Investing in your future. Grant PDC2022-133788-I00 funded by MCIN/AEI/ 10.13039/501100011033 and by the European Union NextGenerationEU/PRTR. Grant PID2019-105156GBI00 funded by MCIN/AEI/10.13039/501100011033. LRM acknowledges the support received from the CRESST II cooperative agreement supported by NASA under award number 80GSFC21M0002. A patent application with the procedure here presented was filed to Spanish Oficina Española de Patentes y Marcas in 2021 under No. 202130475.Peer reviewe

Optimization of MgF2-deposition temperature for far UV Al mirrors

10 pags., 6 figs., tab. -- OCIS codes: (230.4040) Mirrors; (310.1515) Protective coatings; (260.7210) Ultraviolet, vacuum; (160.4670) Optical materials; (310.6860) Thin films, optical properties; (120.6085) Space instrumentation.Progress towards far UV (FUV) coatings with enhanced reflectance is invaluable for future space missions, such as LUVOIR. This research starts with the procedure developed to enhance MgF-protected Al reflectance through depositing MgF on a heated aluminized substrate [Quijada et al., Proc. SPIE 8450, 84502H (2012)] and it establishes the optimum deposition temperature of the MgF2 protective film for Al mirrors with a reflectance as high as ∼90% at 121.6 nm. Al films were deposited at room temperature and protected with a MgF2 film deposited at various temperatures ranging from room temperature to 350°C. It has been found that mirror reflectance in the short FUV range continuously increases with MgF deposition temperature up to 250°C, whereas reflectance decreases at temperatures of 300°C and up. The short-FUV reflectance of mirrors deposited at 250°C only slightly decreased over time by less than 1%, compared to a larger decay for standard coatings prepared at room temperature. Al mirrors protected with MgF2 deposited at room temperature that were later annealed displayed a similar reflectance enhancement that mirrors protected at high temperatures. MgF and Al roughness as well as MgF density were analyzed by x-ray grazing incidence reflectometry. A noticeable reduction in both Al and MgF roughness, as well as an increase of MgF2 density, were measured for films deposited at high temperatures. On the other hand, it was found a strong correlation between the protective-layer deposition temperature (or post-deposition annealing temperature) and the pinhole open area in Al films, which could be prevented with a somewhat thicker Al film.Secretaría de Estado de Investigación, Desarrollo e Innovación (AYA2013-42590-P, ESP2016-76591-P).Peer Reviewe

Temperature dependence of AlF3 protection on Far-UV Al mirrors

© The Author(s).More efficient and stable far ultraviolet (FUV) mirrors will enable future space observatories. Traditional FUV mirrors are based on MgF2-protected Al. AlF3 has been identified as a promising substitute for MgF2 to prevent Al oxidation. Hence, the reflectivity, stability, and morphology of AlF3-protected Al mirrors have been investigated as a function of deposition temperature of the AlF3 film. In this work, it is shown how AlF3 deposition temperature is an important parameter whose optimization ultimately yields valuable throughput enhancement and improved endurance to large storage periods. Al films were deposited at room temperature (RT) and AlF3 protective layers were deposited at temperatures ranging from RT to 350 °C. It was found that the optimum AlF3 deposition temperature was between 200 and 250 °C, yielding the largest FUV reflectance and a better stability of the mirrors, which had been stored in a desiccator environment. Increasing AlF3 deposition temperature resulted in an increase in film density, approaching bulk density at 250 °C. The morphology of Al and AlF3 films as a function of AlF3 deposition temperature was also investigated. The increase in the AlF3 deposition temperature resulted in a decrease of both Al and AlF3 surface roughness and in the growth of the grain width at the AlF3 outer surface. It also resulted in a trend for the prevalent (111) planes of Al nanocrystals to orient parallel to the coating surface.This research was funded by Secretaría de Estado de Investigación, Desarrollo e Innovación (ESP2016-76591-P).Peer reviewe