Researcher portrait : Eleonore BARTHELEMY

What are thin films in optics?

A domestic mirror, or that of a conventional telescope, does not reflect light perfectly. A significant part of the incident light is absorbed or diffused by the materials of which the mirror is made (most often: a glass substrate on which a thin layer of metal, usually aluminum, is deposited). In advanced astronomy, the exceptional sensitivity of the instruments requires optics (mirrors, filters, etc.) with better properties, such as high reflectivity over a wide range of wavelengths, or anti-reflective treatment. Such performance is possible, for instance, by replacing the metal layer with a special optical coating, namely a stack of thin dielectric layers.

Specialized in the deposition of this type of thin film, the Lyon Laboratory of Advanced Materials (Laboratoire des Matériaux Avancées de Lyon, LMA) is renowned for making the most perfect mirrors in the world. This level of excellence is due, in particular, to state-of-the-art technologies, including ion beam sputtering (IBS) and, since 2013, IAD, financed by the LabEX LIO.

 What is IAD?

Ion-assisted deposition (IAD), is a technique that uses ion beam-assisted evaporation to deposit an optical coating on a substrate very accurately. The principle is as follows: the materials are evaporated so as to fasten themselves in thin layers on the substrate which is above. An ion beam bombards these components to compact them and to avoid any roughness.

In which types of project(s) does LMA use IAD?

The IAD equipment was installed at the LMA to act in concert with the very powerful, but very expensive, IBS equipment which was famously used to manufacture the very high reflectivity mirrors of the VIRGO experiment. These mirrors enabled the existence of gravitational waves to be demonstrated.

 Optics for the world's largest telescopes

 Thanks to IAD, the LMA is expanding the scope of traditional astronomy, in particular by fabricating optics intended for large terrestrial observatories devoted to unraveling the secrets of the universe. Its deposition equipment makes it possible to work with particularly large substrates (up to 80 cm in diameter).
 For example, the machine was used to manufacture anti-reflective coated lenses for the ESPRESSO instrument installed on the VLT telescope in Chile, and dichroic (semi-reflective) mirrors for the PFS project.

Financing by the LabEX LIO has also brought the LMA closer to the Lyon Astrophysical Research Center (Centre de Recherche Astrophysique de Lyon (CRAL)), with whom they are collaborating on the 4MOST instrument that will be installed on the VISTA telescope in Chile.