Future space missions will operate in very harsh and extreme environments. Optical and electronics components need to be optimized and qualified in view of such operational challenges. This work focuses on the effect of low alpha particles irradiation on coatings. Low energy He<sup>+</sup> (4 keV and 16 keV) ions have been considered in order to simulate in laboratory the irradiation of solar wind (slow and fast components) alpha particles. Mono- and proper bi-layers coatings have been investigated. The experimental tests have been carried out changing doses as well as fluxes during the irradiation sessions. Optical characterization in the UV-VIS spectral range and superficial morphological analysis have performed prior and after irradiation.
The scientific goals required to the next-generation space missions lead the development of innovative instrumentation, conceived to operate in increasingly harsh environments. Optical coatings are among the sub-systems which can highly suffer the agents in such environments. In particular, as recently demonstrated, the accelerated ions and particles can potentially jeopardize the coatings optical performances, with a consequent degradation of the overall functionality of an instrument. Despite its importance, this issue is still poorly investigated. In fact, the fragmentary knowledge of the space environments and the low number of previous ground testing experiments complicates the definition of clear procedures to investigate the behavior of the optical coatings in space. A systematic approach devoted to identify a methodology for the validation of optical coatings under ions irradiation is presented. Monte Carlo simulations are used to evaluate the effects induced by different ion species and energies on both layers and multilayers of different materials, getting an accurate overview of the main criticalities. Such results are then used to plan representative irradiation experiments and the subsequent analysis procedures needed for a proper characterization of the exposed samples. In this paper, a summary of the experiments performed so far is presented. Thanks to these studies we have identified three main damage mechanisms which can be used to explain most of the degradation effects observed when an optical coating is irradiated with low energy particles. A brief discussion of such mechanisms is reported.
Metallic films of palladium (Pd) and palladium-tin (Pd-Sn) have been deposited by evaporation technique. They were used as sensitive material for optical sensor by measuring the variation of absorbance. All samples were then oxidized by annealing at 500°C in low vacuum atmosphere. All the films were investigated by X-Ray Diffraction (XRD), Atomic Force Microscopy (AFM) to observe the influence of the structure and morphology on the optical properties of the films, carrying useful information for the sensing properties of the different sensing materials. Furthermore, the sensing performances were tested by monitoring the variation on the optical absorbance induced during the absorption / desorption of hydrogen gas. While the use of Pd for gas sensing has been widely covered for electrical and SPR sensors, this work aims to extend our comprehension of the optical sensing behavior, especially in absorbance-mode, of the thin films of PdO, Pd-Sn and PdO-SnO<sub>2</sub>.
Space exploration is linked to the development of increasingly innovative instrumentation, able to withstand the operation environment, rich in ion particles and characterized by high temperatures. Future space missions such as JUICE and SOLAR ORBITER will operate in a very harsh and extreme environment-. Electrons and ions are considered among the causes of potential damage of the optical instrumentation and components. Development of hard coatings capable to preserve their optical properties is pivotal. Different coating materials have been exposed to ion irradiation in particle accelerators. Change in optical performances has been observed in the extreme ultraviolet and visible spectral region and structural properties have been analyzed by different techniques. The knowledge of the damage mechanisms and thresholds allows the selection of more promising candidate materials to realize the optical components for the new frontiers space missions.
Low energy ions coming from the quite solar wind are considered among the causes of potential damage of the optical instrumentation and components on board of ESA Solar Orbiter. Predictions of space radiation parameters are available for instruments on board of such mission. Accelerators are commonly used to reproduce the particle irradiation on a spacecraft during its lifetime at the ground level. By selecting energies and equivalent doses it is possible to replicate the damage induced on space components. Implantation of Helium ions has been carried out on different single layer thin films at LEI facility at Forschungszentrum Dresden-Rossendorf varying the total dose. Profile of the implanted samples has been experimentally recovered by SIMS measurements. The change in reflectance performances of such coatings has been experimentally evaluated and modelled. The outcomes have been used to verify the potential impact on the METIS instrument and to drive the optimization of the M0 mirror coating..
Extreme Ultraviolet (EUV) multilayer (ML) technology has been intensively applied in many scientific and technological fields such as solar physics and photolithography. More recently, the advent of free electron lasers (FEL) emitting bright sub-ps pulses with very high quality in term of intensity stability, coherence and temporal shape has encouraged the usage of multilayer coatings also in the transport and manipulation of FEL radiation. In fact, conventional single layers coated mirrors provide negligible reflectance in the EUV spectral range whereas ML mirrors can reach high efficiency at normal incidence without affecting the pulses characteristics. Such optical elements have been also exploited at FERMI@ELETTRA FEL where novel multilayer coatings specifically conceived for pump and probe experiment and ultrafast absorption spectroscopy have been designed. The main results are reported.
Metallic nanostructures are widely studied because of their peculiar optical properties. They possess characteristic
absorbance spectra with a peak due to plasmonic resonance. This feature is directly dependent on the nanostructures
shape, size, distribution and environment surrounding them. This makes them good candidates for a variety of
applications, such as localized surface plasmon resonance sensing (LSPR), surface-enhanced Raman scattering (SERS)
and photovoltaics. A well established technique used to create nanoisland on flat substrates is performing a thermal
treatment after the deposition of a thin metal film. While the most widely investigated metal in this context is gold, we
have extended our investigation to palladium, which is interesting for sensing applications because it has an excellent
hydrogen absorption ability. The morphological properties of the nanoisland depend mainly on the starting thickness of
the deposited layer and on the annealing parameters, temperature and duration. The deposition and annealing process has
been investigated, and the resulting samples has been tested optically and morphologically in order to optimize the
structures in view or their application for sensing purposes.
Graphene–metals interfaces are investigated in many subject areas both applicative and speculative. The interest mainly
stems from the possibility for CVD synthesis of large area graphene on metals. In this case the metal acts as a catalyst for
complete dehydrogenetaion of hydrocarbon precursors that leaves carbon behind at the surface. Such bilayer are also
very appealing for surface plasmon resonance devices, since graphene acts both as a protective layer and biorecognition
element. Several pairs of graphene–metal interfaces have been studied in terms of SPR performance and physicalchemical
properties at the interface. With regard to this last aspect, NEXAFS spectroscopy is a powerful method to study
single-, double-, and few- layers graphene and to illustrate any evolution of the electronic states.
Single layer thin films have been exposed to low energy alpha particles (4keV). Implanted doses are equivalent to those accumulated in 1, 2, 4 and 6 years of ESA Solar Orbiter mission operation. Two ions fluences have been considered. In order to change the total dose accumulated, for each ion flux the time of exposure was varied. Reflectance in the visible spectral range has been measured prior and after implantation. Results show no significant change in performances in gold and palladium, while a small decrease in performances is observed in iridium. The implantation rate does not seem to affect the experiment.
Probing of Hermean Exosphere By Ultraviolet Spectroscopy (PHEBUS) is a dual channels spectrometer working in the Extreme UltraViolet (EUV) and Far UltraViolet (FUV) range. It will be on board of ESA BepiColombo cornerstone mission and it will be devoted to investigate the composition, the dynamic, the formation and the feeding mechanisms of Mercury’s exosphere system. A consistent interpretation of the observational data collected by PHEBUS requires a deeply knowledge of its radiometric behavior. The Mueller’s matrix formalism can be adopted to derive an accurate radiometric model able to takes into account also the polarization state of the source observed by PHEBUS. Moreover, this theoretical model can be further verified and refined during an experimental ground calibration campaign. In this work we present the radiometric model derived for PHEBUS spectrometer together with some results obtained during the Flight Model (FM) ground calibration which is still ongoing. In particular, the obtained results employing this approach show that this is a complete and versatile method to perform the radiometric calibration of a generic space instrument.
In this work, three TiO2 thin films with thicknesses of 22.7, 48.5 and 102.9 nm were grown on Si (100) substrates by the technique of electron beam evaporation. The films were deposited at a substrate temperature of 150°C with a deposition rate of 0.3 - 0.5 A/sec. The films thicknesses were characterized by spectroscopic ellipsometry and profilometry. The surface roughness was measured by AFM obtaining RMS of less than 0.7nm. Investigations performed by XPS method have shown that stoichiometric TiO<sub>2</sub> was obtained on all the samples with no suboxide presences. Reflectance measurements of the samples were performed in EUV and SX spectral regions from 25.5 to 454.2eV using synchrotron radiation. Analyzing the refractive index N=n+<i>i</i>k of TiO<sub>2</sub> thin films, optical constants (n,k) in this energy range were both determined by fitting the Fresnel equations with least-square fitting methods.
The interest in graphene–like materials involves many research areas, including the development of biosensors devices. We have recently studied the use of graphene/metal bilayer for surface plasmon resonance (SPR) equipment devoted to detection of chemical processes and biomolecules recognition. The dual role of graphene is to protect the metal layer underneath and to enhance the bioaffinity by adsorbing biomolecules with carbon–based ring structures. Depending on the application, it may be necessary laser and chemical treatments of graphene to improve the performances of the whole device. The processing effects will be investigated by near edge X-ray absorption fine structure (NEXAFS) spectroscopy. The use of synchrotron light is mandatory for NEXAFS analysis since a continuous EUV source of selected polarization is required. The ideas, the analysis and the results are the subjects of this work.
The European Space Agency mission Solar Orbiter (SOLO) is dedicated to the study of the solar atmosphere and heliosphere. As a part of the payload, the instrument METIS (Multi Element Telescope for Imaging and Spectroscopy) will provide images of the corona, both in the visible range and at the hydrogen Lyman-α emission line (121.6 nm). The realization of optical coatings, based on Al and MgF2, able to reflect/transmit such spectral components is, therefore, necessary. Since optical characteristics of materials in the vacuum ultraviolet range are not well studied and vary greatly with the realization process, we implemented a study of their properties in different deposition conditions. This is aimed to the realization of a custom designed filter able to transmit the 121.6 nm wavelength while reflecting visible light, and thus separating visible from ultraviolet light paths in the METIS instrument.
The practical use of graphene and graphene oxide beyond the research laboratories is strictly related to the fine tuning of
new methodologies for processing and mass–production purposes. The photoreduction processing is an innovative route
allowing exquisite control of the optoelectronic properties of graphene–like materials irradiated by coherent and
incoherent light. We have investigated the effects induced by a mercury lamp on graphene/palladium bilayer; the change
on the optical properties of the sample has been detected by using a surface plasmon resonance setup. The analysis, the
perspectives and the preliminary results are shown thereafter.
An optical system for the generation of a beam with a variable and controllable polarization status has been designed,
realized and tested. The system is based on an interferometric set up, consisting of a splitting system, a phase delay
system and a recombination system. By controlling the optical path, it is possible to obtain every polarization status:
linear, elliptical and circular. The system can be realized with an all reflective scheme and it can work in a wide spectral
band of the electromagnetic spectrum, from the near-infrared down to the extreme ultraviolet. The system can be
integrated in different optical setups in order to enhance their versatility, such as in laser devices, optical
instrumentations, synchrotron lines or free electron lasers beam transport system. Finally it can be also used to test
optical device and for calibration of optical components.
Future solar missions will investigate the Sun from very close distances and optical components are
constantly exposed to low energy ions irradiation. Single layer thin films as well as extreme ultraviolet
multilayer coatings have been exposed to low energy alpha particles (4keV). In order to change the total dose
accumulated, for each ion fluency the time of exposure was varied. The experiment was carried out
considering typical doses accumulated during the ESA Solar Orbiter mission. Results show that ion
implantation affects the performances of both single and multilayer coatings.
The phase delay induced by multilayer (ML) mirrors is an important feature in many fields such as attosecond pulses
compression, photolithography or in pump and probe experiments performed with Free Electron Laser (FEL) pulses. The
experimental characterization of the ML phase delay can be obtained by the standing wave distribution measurement (by
using Total Electron Yield (TEY) signal) combined to reflectance measurement. In this work, a ML structure with
aperiodic capping-layers was designed and deposited for FEL applications and their reflectance and phase delay was
characterized. The method adopted allows to retrieve the ML phase delay by using the TEY signals taken at different
working configurations and it doesn’t require the comparison with a bulk reference sample. The results obtained are
presented and discussed.