Instituto de Óptica “Daza de Valdés”

Unveiling the effects of the surface and in-depth nanostructure on the far-UV optical reflectance of thin fluoride multilayer coatings

Óptica para el espacio e iones de alta energía (GOLDION)

Madrid / November 3, 2023

Researchers from the Optics Institute of the CSIC, the Materials Microanalysis Center of the Autonomous University and the NASA Goddard Space Center have recently published in the magazine Applied Surface Science the results of work aimed at optimizing for the first time a new reflective optical coating that can be used in optical devices operating in the far ultraviolet.

This improvement in the coatings is very relevant for future NASA space missions, such as the so-called “Habitable Worlds Observatory” that want to capture all the light that arrives from the infrared to the high ultraviolet (LUVOIR). One of the instruments proposed for these new missions is the LUMOS (LUVOIR Ultraviolet Multi-Object Spectrograph), designed to revolutionize high-resolution spectroscopy and whose imaging channel needs better filters in the far ultraviolet, as the authors highlight.
Animation of the deployment plan of the 15 m LUVOIR-A observatory. /
To achieve this, it is essential to have mirrors and optical filters with very high reflectance and very sensitive to changes in wavelength. Multilayer fluoride coatings are currently the best option, but need to be improved for the next generation of space missions.
In this work, the research team has characterized, by means of atomic force microscopy and ion spectrometry, the roughness of the layer, the grain size and the mixing problems between layers of the coatings formed by alternating layers of aluminum fluoride and aluminum fluoride. lanthanum, and have compared it to traditional magnesium fluoride and lanthanum systems.
The results showed that aluminum and lanthanum coatings have fewer structural defects. In theory, the more bilayers used, the more the reflectance will increase, but surface roughness and other defects also increase. In the study, it was found that the optimal number was 15 bilayers that with the AlF3/LaF3 multilayer coatings managed to reflect 87% of the ultraviolet light with a wavelength of 121.6 nm compared to 75% with the multilayer coatings of MgF2/LaF3.
This work lays the foundation for optimizing the design of ultraviolet optical mirrors and filters that will be essential for NASA’s next generation of large space telescopes, accelerating scientific progress in this crucial but still underexplored part of the electromagnetic spectrum.
In addition to the new generation of space telescopes, other technological fields can benefit from advances in far-ultraviolet reflective coatings. These include excimer laser optics, which is a type of ultraviolet laser. Far-UV coatings may also be useful in fields such as thermonuclear fusion reactors for hydrogen characterization and also in the microchip manufacturing industry using 193 lithography.
The design of mirrors and optical filters capable of efficiently reflecting far ultraviolet light today poses a great technological challenge. In this part of the electromagnetic spectrum, below 200 nm wavelength, most materials strongly absorb radiation due to the high energies involved. This greatly reduces the availability of substances with low absorption that can be used as UV mirrors. At these wavelengths, quantum effects are relevant in the light-matter interaction, making it very difficult to accurately predict and control the refractive indices and other characteristics of each new material. This complicates the design of new multilayer coatings, where nanometric control of layer thicknesses is required.

IO-CSIC Communication

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