Scientists from the Thin Film Optics Group of the Optics Institute, the LABMET of the Carlos III University of Madrid and the Catholic University of America and NASA Goddard Space Center have just published an article in the magazine Optical Materials Express in which they study for the first time the ideal manufacturing conditions to achieve the coating necessary for the operation of optical instruments that "see" the far ultraviolet and thus give us new information about the universe that surrounds us.

Far UV

Astronomical observations in the far ultraviolet (with wavelengths λ = 100-200 nm) are key to reveal fundamental information for astrophysics, solar physics and the physics of the atmosphere.
For example LUMOS is the multi-Object ultraviolet spectrograph of the mission of the new LUVOIR space telescope that competes to become the next NASA flagship mission, and has as missions the study of host stars of exoplanets, of water jets of water in the satellites of the outer solar system and the tomography of circungalctic halos, among others.

Coatings for far ultraviolet

The next generation of space instruments calls for the development of mirrors with efficient dielectric coatings in the far ultraviolet.
Far ultraviolet astronomy has been severely hampered by the performance of its optical components, along with detectors, which ultimately limit the sensitivity of space telescopes and their instruments. The high absorption of this type of light by the materials makes it difficult to manufacture the optical elements of a telescope, such as a mirror. This, together with the need for precise knowledge of the optical constants of materials in this part of the spectrum are among the main factors limiting the optical performance of coatings.

These coatings are based on multiple very thin layers of MgF2 and LaF3 (fluorides are very transparent to far ultraviolet) in which the light is reflected or transmitted in all the layer changes creating constructive or destructive interferences and that thanks to its design they achieve that it ends up reflecting only the wavelength that the instrument needs.
However, the deposition procedure to obtain coatings with optimal performance has not been fully detailed so far.

This work investigates the effect of the number of layers and the temperature of the substrate during deposition on the reflectance of the coating, and its evolution over time. The coatings maintained a reflectance above 90% after nearly a year of storage in a desiccator.
If the coating is deposited at a high temperature, it expands more than most substrates, and when cooling it can crack; on the other hand, if the coating is deposited on a substrate at room temperature, the layers tend to be more porous. Such porosity could fill with water vapor molecules or other contaminants when exposed to the atmosphere, which can increase the absorption of far ultraviolet light and therefore decrease the performance of the coating.

The study found a compromise deposition temperature of ~240° C and 13 bilayers for optimal reflectance. Above this deposition temperature, the stress increased, which resulted in an extension of the cracked area and a slight increase in roughness and decrease in reflectance in this light. Layers that were deposited at room temperature and then annealed resulted in reflectance and stress similar to that of coatings hot-laid for the same temperature.

Other fields of research can also benefit from the development of more efficient far ultraviolet light mirrors, such as space observations for solar physics and atmospheric physics or other fields involving far ultraviolet radiation, such as excimer laser optics (ultraviolet light laser), lines of petawatt laser light, thermonuclear fusion reactors and the semiconductor industry.

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This work is a collaboration between the Institute of Optics, the LABMET of the Carlos III University and the Catholic University of America - NASA Goddard Space Center