Luis Rodríguez-de Marcos

Reflective and transmissive broadband coating polarizers in a spectral range centered at 121.6 nm

Polarimetry is a powerful tool for the interpretation of the role of the coronal plasma in the energy transfer processes from the inner parts of the Sun to the outer space. One of the key lines for observations is H I Lyman α (121.6 nm) among few spectral lines in the far ultraviolet (FUV), and hence efficient linear polarizers at this line are demanded. New designs based on (Al/MgF2)n multilayer coatings have been developed to obtain the smallest possible reflectance in the parallel plane of polarization (Rpar) with a simultaneous high reflectance in the perpendicular plane of polarization (Rper). Samples stored in nitrogen for ~8–17 months resulted in efficient polarizers at 121.6 nm, with Rpar ~ 0.01–0.017 and Rper ~ 0.69–0.725. The designs with a number n = 3–4 bilayers of Al/MgF2 result in a wider spectral range of efficient linear polarizers, compared to what can be obtained with n = 2. Coatings following various designs with good polarizing performance in a 7–8 nm wide FUV range were prepared. For the first time, a transmissive coating polarizer has been developed for this range, which has the benefit that it involves no deviation of the beam; it is based on another design of (Al/MgF2)3 multilayer coating. The transmissive polarizer has a good transmittance ratio between the two polarization components and, even though its figure of merit is not as high as that of the reflective polarizers, it incorporates filtering properties to reject wavelengths both below and above 121.6 nm; this property might enable a polarimeter for solar physics with an improved global figure of merit if a filter to isolate the H I Lyman α line could be avoided.

Optical constants of SrF2 thin films in the 25–780-eV spectral range

The transmittance and the optical constants of SrF2 thin films, a candidate material for multilayer coatings operating in the extreme ultraviolet and soft x-rays, have been determined in the spectral range of 25–780 eV, in most of which no experimental data were previously available. SrF2 films of various thicknesses were deposited by evaporation onto room-temperature, thin Al support films, and their transmittance was measured with synchrotron radiation. The transmittance as a function of film thickness was used to calculate the extinction coefficient k at each photon energy. A decrease in density with increasing SrF2 film thickness was observed. In the calculation of k, this effect was circumvented by fitting the transmittance versus the product of thickness and density. The real part of the refractive index of SrF2 films was calculated from k with Kramers-Kronig analysis, for which the measured spectral range was extended both to lower and to higher photon energies with data in the literature combined ...

In-band and out-of-band reflectance calibrations of the EUV multilayer mirrors of the atmospheric imaging assembly instrument aboard the Solar Dynamics Observatory

Experimental multilayer reflectance data on flight mirrors and witnesses for three extreme ultraviolet (EUV) channels of the Atmospheric Imaging Assembly (AIA) instrument aboard NASA’s Solar Dynamics Observatory are presented and compared to theoretical models. The relevance of these results to the performance of the AIA instrument is discussed.

Self-consistent optical constants of sputter-deposited B 4 C thin films

The optical constants of ion-beam-sputtered B₄C films have been measured by ellipsometry in the 190-950 nm range. The set of data has been extended toward both shorter and longer wavelengths with data in the literature, along with interpolations and extrapolations, in order to obtain a self-consistent set of data by means of Kramers-Krönig analysis. All data correspond to films that were deposited by sputtering on nonheated substrates, and hence they are expected to be amorphous. The B₄C bandgap was calculated as a fitting parameter of Tauc equations for indirect transitions using the present optical constants. Good global accuracy of the data was estimated through the use of various sum rules. The consistent data set includes the visible to the extreme UV (EUV); this large spectrum of characterization will enable the design of multilayer coatings that combine a relatively high reflectance in parts of the EUV with a desired performance at a secondary range, such as the visible.

Transmittance and optical constants of Lu films in the 3–1800 eV spectral range

The optical constants n and k of lutetium (Lu) films were obtained in the 3–1800 eV range from transmittance measurements performed at room temperature. These are the first experimental optical constant data of Lu in the whole range. Thin films of Lu with various thicknesses were deposited by evaporation in ultrahigh vacuum conditions and their transmittance was measured in situ. Lu films were deposited onto grids coated with a thin, C support film. Transmittance measurements were used to obtain the extinction coefficient k of Lu films. The refractive index n of Lu was calculated with Kramers–Kronig analysis. k data were extrapolated both on the high and on the low-energy sides by using experimental and calculated k values available in the literature. Lu, similar to other lanthanides, has a low-absorption band below the O2,3 edge onset; the lowest absorption was measured at ∼25.1 eV. Therefore, Lu is a promising material for filters and multilayer coatings in the energy range below the O2,3 edge in which ...

Transmittance and optical constants of Sr films in the 6–1220 eV spectral range

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 ava...

Self-consistent optical constants of MgF 2 , LaF 3 , and CeF 3 films

Various fluorides are materials in nature that extend their transparency range to the shortest wavelengths in the far ultraviolet (FUV, 100nm<λ<200 nm). These are relevant materials to prepare multilayer coatings in the FUV for demanding applications such as space instrumentation for astrophysics, solar physics and atmosphere physics, as well as free electron lasers, plasma diagnostics, synchrotron radiation, lithography, spectroscopy, etc. Multilayer design requires an optical constant of the coating materials. Multilayers optimally alternate two transparent materials with contrasting refractive indices. The optical constants of a low-index material, MgF2, and of two high-index materials, LaF3 and CeF3, have been determined in a wide spectral range and are presented here. Thin films of MgF2, LaF3, and CeF3 were deposited by boat evaporation onto substrates at 523 K. Transmittance, reflectance, and ellipsometry measurements were performed in ranges jointly covering the 30-950-nm spectral range. This range was extended with literature data and extrapolations to obtain self-consistent optical constants using the Kramers-Kronig (KK) analysis. An iterative, double KK analysis procedure (successive reflectance-phase and k-n KK analyses) was carried out to obtain a self-consistent set of optical constants per material. With the final data sets, the experimental measurements were satisfactorily reproduced. Global self-consistency of the data sets was successfully evaluated through sum rules; additionally, local self-consistency at each photon energy range was also evaluated through a novel sum-rule method which involves window functions. The new sets of optical constant extend the data availability mainly to the FUV and beyond, particularly for CeF3, where few data had been reported.

Multilayer coatings for the far and extreme ultraviolet

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.

Optimization of MgF 2 -deposition temperature for far UV Al mirrors

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 MgF2-protected Al reflectance through depositing MgF2 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 MgF2 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. MgF2 and Al roughness as well as MgF2 density were analyzed by x-ray grazing incidence reflectometry. A noticeable reduction in both Al and MgF2 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.

Multilayer reflective polarizers for the far ultraviolet

Polarimetry in the far ultraviolet (FUV) is a powerful tool for the interpretation of the role of the coronal plasma in the energy transfer processes from the inner parts of the Sun to the outer space. FUV polarimetry from space provides more accurate observations on the kinetics of the features and on local magnetic fields through the Doppler and Hanle resonant electron scattering effects. Particularly interesting lines for FUV polarimetry are H Lyman α (121.6 nm) and β (102.6 nm), along with OVI lines at 103.2 and 103.8 nm. One key element to perform polarimetry measurements at these wavelengths is the need of efficient polarizers. A limitation of the available polarizers, such as crystal plates of MgF2 and LiF working at Brewster angle, is their moderate reflectance at the non-extinguished component of the electric field, which results in a modest polarizer efficiency.