Manuela Vidal-Dasilva

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...

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.

Transmittance and optical constants of ca films in the 4-1000 eV spectral range

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 grid-supported 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-Krönig relations. This is the first self-consistent data set on Ca covering a wide spectral range including the EUV.

Coating development for the far and extreme ultraviolet based on material characterization

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 B4C, 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 MgF2.

Narrowband coatings for the 100-105 nm range

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.

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.

Multilayer coatings for optics in the extreme ultraviolet

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.