J. A. Aznárez

Far‐ultraviolet absolute reflectometer for optical constant determination of ultrahigh vacuum prepared thin films

An absolute reflectometer is described, which can determine the optical constants of thin films prepared and maintained in ultrahigh vacuum (UHV) in the 30–200 nm far‐ultraviolet (FUV) region. The optical constants are calculated with Fresnel equations from reflectance measurements as a function of the incidence angle, which can be continuously varied from near‐normal to grazing incidence. Corrections for the surface roughness effects of both the deposited film and the substrate on the reflectance in the calculation of optical constants were incorporated. The surface roughness was determined by atomic force microscopy. An innovative feature of the instrument is an oxidation chamber containing an atomic oxygen source attached to the reflectometer. Inside this chamber the FUV reflectance degradation of materials, caused by oxidation processes in low earth orbits, can be simulated on freshly prepared UHV samples. Another improvement of the reflectometer is the capability of performing the reflectance measure...

Far-ultraviolet reflectance measurements and optical constants of unoxidized aluminum films

The far-UV reflectance of thin unoxidized aluminum films prepared and maintained in ultra-highvacuum conditions was measured versus the angle of incidence, and the complex refractive index was obtained from those measurements on several wavelengths from 82.6 to 113.5 nm. Measurements were made on two perpendicular planes of incidence to deal with the unknown of the polarization state of the radiation beam. The surface roughness was characterized by atomic force microscopy. The refractive index is obtained for the first time, to our knowledge, from direct optical measurements in this spectral range. Current results match well the former values in the literature that were calculated through the Kramers-Kronig analysis by using in the above interval reflectances estimated from electron-energy-loss spectra and from optical measurements on surfaces of unstated roughness.

Far-UV reflectance of UHV-prepared Al films and its degradation after exposure to O 2

The far-ultraviolet reflectance of aluminum films prepared and maintained under ultrahigh-vacuum conditions was measured for wavelengths ranging from 82.6 to 120.0 nm. The degradation of the reflectance after exposure to controlled doses of molecular oxygen was also studied. The degradation rate proved to be higher for wavelengths less than 120 nm than previous measurements made at 121.6 nm.

Optical constants of aluminum films in the extreme ultraviolet interval of 82–77 nm

Measurements of the extreme ultraviolet (EUV) reflectance of unoxidized aluminum films versus the angle of incidence in the interval of 82-77 nm, just below the aluminum plasma wavelength (83 nm), are presented. The continuum of helium was used as a radiation source for the first time in EUV reflectometry, to our knowledge. The surface roughness of substrates and samples was characterized by atomic force microscopy. The complex refractive index of unoxidized aluminum was obtained from reflectance measurements at each wavelength for the first time in this spectral range. Data on the refractive index, the dielectric constant, and the energy loss function in the above interval are shown together with our previous data obtained in an interval of 82.6-113.5 nm. Current results on the refractive index show a good match with the data in the literature calculated through the Kramers-Kronig analysis, the largest differences being in the imaginary part of the refractive index at the shortest wavelengths.

Degradation of far ultraviolet reflectance of aluminum films exposed to atomic oxygen. In-orbit coating application

Abstract The drop in reflectance at the interval 82.6–174.4 nm of ultra high vacuum prepared aluminum coatings when exposed to controlled doses of atomic oxygen with average energy 0.17 eV has been measured by the first time. We show that atomic oxygen produces a much stronger effect on the far ultraviolet (FUV) reflectance of aluminum that molecular oxygen: for the same relative drop in reflectance of 40% at 82.6 nm, the required exposure to molecular oxygen is about 5000 times larger than to atomic oxygen. These measurements allowed us to calculate a higher limit of the useful lifetime of an aluminum mirror placed in a low earth orbit (LEO, 200 to 700 km altitude; oxygen atoms with energy of 5 eV). This limit was found to be as short as a few hours. To avoid the oxidation by atomic oxygen a high altitude orbit should be used. A more practical alternative may be found by placing the aluminum mirror behind a wakeshield device, a well known proposal, in a LEO. In this case, the flux of oxygen atoms impinging on the mirror would be strongly reduced and consequently the lifetime of the mirror greatly increased. We have calculated that the energy of oxygen atoms impinging on the shielded mirror would range from 0.07 to 1.8 eV. As the energy of oxygen atoms in our experiment is within that interval of energies, a plausible estimate of the lifetime expected for an in-orbit aluminum coating behind a wakeshield can be derived from our experimental measurements. For a wakeshield operating as a free flyer at a 300 km altitude orbit an aluminum mirror would suffer a negligible drop in its FUV reflectance after a time as long as 20 years, which is a stimulating prospective.

Narrowband filters and broadband mirrors for the spectral range from 50 to 200 nm

A summary of the optical coatings for the spectral range from 50 to 200 nm that can be prepared at GOLD is presented. This spectral range, named here 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/MgF2, SiC, B and Al/MgF2/SiC cover the FUV/EUV spectral range from 50 to 200 nm. Above the transparency cutoff of MgF2 (115 nm), transmittance filters based on Al/MgF2 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 reflectance filters has recently started with very promising 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.

Life prolongation of far ultraviolet reflecting aluminum coatings by periodic recoating of the oxidized surface

Abstract In-orbit operating optical instruments provided with pure aluminum coatings are expected to promote great advances in far ultraviolet (FUV) astronomy. A difficulty to be overcome however, is the oxidizability of aluminum coatings in low earth orbits (LEO), where the residual atmosphere is constituted mainly by atomic oxygen. It is here demonstrated that the lifetime of an aluminum coating oxidized by limited exposures to atomic oxygen can be substantially extended by periodic recoating of the oxidized aluminum surface with new aluminum coatings. Recoating thicknesses of 30 and 60 nm are shown to completely recover the near normal FUV reflectance in the case of a mirror exposed to doses of atomic oxygen which caused decreases in the reflectance of about 13% and 30%, respectively, at 104.8 nm. After ten recoatings the reflectance of the primary aluminum film was completely recovered. Accordingly the lifetime of an aluminum coating placed in a LEO could be increased by an order of magnitude or more.

Characterization of linear polarizers in the wavelength range 100-150 nm (VUV) for solar physics applications

VUV polarimetry has been recognized as one of the most powerful diagnostic tools for remote sensing of the solar corona, with the potential of providing accurate space resolved information on magnetic activity through observation of resonance lines of the most abundant species. In an on-going collaboration between our groups from Spain and Italy, a program to design, build and characterize optical components for the VUV region has been activated. In particular, using the beamline BEAR at the synchrotron facility Elettra in Trieste (Italy) we have characterized some thin film reflecting linear polarizers, designed and optimized for the study of polarimetric properties of the HI Ly-alpha at 121.6 nm. The characterizations are performed from 100 to 150 nm at different angles of incidence (40 – 80 deg). Some polarizers have shown excellent performances with an average reflectivity R ≈ 34% and a modulation factor exceeding 95%. The calibration of several samples is reported and aging effects on some old samples is discussed. One of the calibrated sample will be used for the evaluation of the performances of a new fast calibration set-up facility for VUV.

Narrowband multilayer mirrors for the extreme ultraviolet spectral range of 50 to 95 nm

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.

Coatings with high 102.6-to-121.6 nm reflectance ratio

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.