The thin film optical constants are key parameters to carry out optical simulation or optimization of multilayer mirrors with high efficiency. However, for most materials, different sets of optical constants can be found in the literature especially in the EUV range, as these parameters are not as well-known in the EUV as in the visible or wavelength range. In this work, we have used several reflectance and transmittance measurements in the wavelength range from 10 nm to 60 nm. Different optical constant files have been tested and compared with the IMD simulation software. We will present some experimental spectra and theoretical simulations to highlight the existing problem on the reliability of optical constants sets and to discuss potential solutions. We focus our research on a few materials of particular interest in the EUV range such as aluminum, aluminum oxide, molybdenum, zirconium, magnesium, silicon carbide, and boron carbide. These analyses lead us to select the most reliable and accurate optical constants set, or to create the best one from the concatenation of existing data for each material of interest.
Observations in the UV and EUV allow many diagnostics of the outer layers of the stars and the Sun so that more and more space telescopes are developed to operate in this fundamental spectral range. However, absorption by residual contaminants coming from polymers outgassing causes critical effects such as loss of signal, spectral shifts, stray light… Thus, a cleanliness and contamination control plan has to be defined to mitigate the risk of damage of sensitive surfaces. In order to specify acceptable cleanliness levels, it is paramount to improve our knowledge and understanding of contamination effects, especially in the UV/EUV range. Therefore, an experimental study has been carried out in collaboration between CNES and IAS, in the frame of the development of the Extreme UV Imager suite for the ESA Solar Orbiter mission; this instrument consists of two High Resolution Imagers and one Full Sun Imager designed for narrow pass-band EUV imaging of the solar corona, and thus very sensitive to contamination. Here, we describe recent results of performance loss measured on representative optical samples. Six narrow pass-band filters, with a multilayer coating designed to select the solar Lyman Alpha emission ray, were contaminated with different amounts of typical chemical species. The transmittance spectra were measured between 100 and 200 nm under high vacuum on the SOLEIL synchrotron beam line. They were compared before and after contamination, and also after a long exposure of the contaminated area to EUV-visible radiations.
Since more than 20 years, Laboratoire Charles Fabry and Institut d’Astrophysique Spatiale are involved in development
of the EUV multilayer coating for solar imaging. Previous instruments, such as the SOHO EIT and STEREO EUVI
telescopes, employed the Mo/Si multilayer coatings, which offered at that time the best efficiency and stability. We
present here recent results of the development of highly efficient EUV multilayers coatings at 17.4 nm and 30.4 nm for
the Solar Orbiter mission. New multilayer structures, based on a combination of three materials including aluminum,
have been optimized both theoretically and experimentally. We have succeeded to reduce interfacial roughness of Albased
multilayers down to 0.5 nm via optimization of the multilayer design and the deposition process. The EUV peak
reflectance of Al/Mo/SiC and Al/Mo/B4C multilayer coatings reaches 56% at 17.4 nm, the highest value reported up to
now for this wavelength. We have also optimized specific bi-periodic structures that possess two reflection bands in the
EUV range with high spectral selectivity. The EUV reflectivity of these Al-based dual-band coatings are compared with
the Si/Mo/B4C baseline coating for Solar Orbiter. Since the stability of reflecting multilayer coating is an important issue
for space missions, we have also studied the temporal stability as well as the resistivity of the coatings to thermal cycling
and to proton irradiation. Experimental results confirm that Al/Mo/SiC and Al/Mo/B4C multilayer coatings are good
candidates for the Solar Orbiter EUV imaging telescopes.
We report on further development of reflective multilayer coatings containing aluminum as low absorbing material for the extreme ultra-violet (EUV) applications, in particular for solar physics. Optimizations of the multilayer design and deposition process have allowed us to produce Al-based multilayers having relatively low interface roughness and record EUV reflectances in the range from 17 to 40 nm. The peak reflectance values of 56 % at 17.5 nm, 50 % at around 21 nm, and 42 % at 32 nm were achieved with new three-material multilayers Al/Mo/SiC and Al/Mo/B4C at near-normal incidence. We observe a good temporal stability of optical parameters of the multilayers over the period of 4 years. Moreover, the multilayer structure remains stable upon annealing at 100 °C in air during several weeks. We will discuss the optical properties of more complex Al-based systems with regard to the design of multilayer coatings that reflect more than one wavelength and reject some others within the spectral range from 17 to 40 nm. Such multichannel systems with enhanced reflectance and selectivity would provide a further advance in optical performance and compactness of EUV solar imaging instruments. We will discuss general aspects of design, optimization and fabrication of single- and multi-channel multilayer mirrors made with the use of aluminum. We will present recent results on the EUV reflectivity of multilayer coatings based on the Al/Mo/SiC and Al/Mo/B4C material combinations. Al-based multilayer systems are proposed as optical coatings in EUV telescopes of future space missions and in other EUV applications.
Membranes a few hundred nanometers thick are used in EUV optics to make, for example, beams splitters or passband
filters. Despite their necessity in numerous applications these components are, because of their thinness, extremely
fragile and their implementation in space instruments is always difficult. The authors are developing thin film filters for
the Full Sun Imager, one of the EUV telescopes on board the Solar Orbiter mission with objectives of high optical
efficiency and mechanical strength. These filters are specifically designed to isolate one or the other of the two
passbands (17.4 and 30.4 nm) reflected by the telescope's dual band mirror coating. In this paper we present the optical
properties of the prototype components.
We report on further development of three-material multilayer coatings made with a use of aluminum for the extreme
ultra-violet (EUV) applications such as solar physics, high-order harmonic generation or synchrotron radiation. It was
found that an introduction of refractory metal in Al-based periodic stack helps to reduce significantly an interfacial
roughness and provides for a higher theoretical reflectance in the spectral range from 17 to 40 nm. The normal incidence
reflectivity as high as 55 % at 17 nm, 50 % at 21 nm and 42 % at 30 nm was achieved with the new Al/Mo/SiC and
Al/Mo/B4C multilayer mirrors, which have been optimized, fabricated and characterized with x-rays and synchrotron
radiation. A good temporal and thermal stability of the tri-component Al-based multilayers has been observed over 3